3638 PART 15 Disorders Associated with Environmental Exposures

a frequent finding and can be addressed by glucose infusion. Since

peripheral vasoconstriction delays heat dissipation, repeated administration of discrete boluses of isotonic crystalloid for hypotension is

preferable to the administration of α-adrenergic agonists.

Evaporative cooling is frequently the most practical and effective

technique. Rapid cooling is essential in both CHS and EHS, and an

immediate improvement in vital signs and mental status may prove

valuable for diagnostic purposes. Cool water (15°C [60°F]) is sprayed

on the exposed skin while fans direct continuous airflow over the

moistened skin. Cold packs applied to the neck, axillae, and groin are

useful cooling adjuncts. If cardiac electrodes will not adhere, they can

be applied to the patient’s back.

Immersion cooling in ice-cold water is an alternative option in EHS

but can induce peripheral vasoconstriction and shivering. The initial

increase in temperature from peripheral vasoconstriction will rapidly

be overcome by the large conductive thermal transfer into cold water.

This technique presents significant monitoring and resuscitation challenges in many clinical settings. The safety of immersion cooling is

best established for young, previously healthy patients with EHS (but

not for those with CHS). To avoid hypothermic afterdrop (continued

cooling after immersion), active cooling should be terminated at

~38°–39°C (100.4°F–102.2°F).

Cooling with commercially available cooling blankets should not be

the sole technique used, since the rate of cooling is far too slow. Other

methods are less efficacious and rarely indicated, such as IV infusion

of cold fluids and cold irrigation of the bladder or gastrointestinal

tract. Cold thoracic and peritoneal lavage are efficient maneuvers but

are invasive and rarely necessary. Endovascular cooling also provides

effective cooling.

■ RESUSCITATION

Aspiration commonly occurs in heatstroke, and endotracheal intubation is usually necessary. Depolarizing agents should be avoided. The

metabolic demands are high, and supplemental oxygenation is essential

due to hypoxemia induced by thermal stress and pulmonary dysfunction. The oxyhemoglobin dissociation curve is shifted to the right.

Pneumonitis, pulmonary infarction, hemorrhage, edema, and acute

respiratory distress syndrome occur frequently in heatstroke patients.

Seizures are common and can occur during therapeutic cooling. Cold

induced tonic-clonic muscular rigidity mimics seizure activity.

The circulatory fluid requirements, particularly in CHS, may be

deceptively modest. Aggressive cooling and modest volume repletion

usually elevate the CVP to 12–14 mmHg. The reading, however, may

be deceptive. Many patients present with a thermally induced hyperdynamic circulation accompanied by a high cardiac index, low peripheral

vascular resistance, and an elevated CVP caused by right-sided heart

failure. In contrast, most patients with EHS require far more zealous

isotonic crystalloid resuscitation.

The hypotension that is initially common among patients with heatstroke results from both dehydration and high-output cardiac failure

caused by peripheral vasodilation. Inotropes causing α-adrenergic

stimulation (e.g., norepinephrine) can impede cooling by causing significant vasoconstriction. Vasoactive catecholamines such as dopamine or dobutamine may be necessary if the cardiac output remains

depressed despite an elevated CVP, particularly in patients with a

hyperdynamic circulation.

A wide variety of tachyarrhythmias are routinely observed on

presentation and usually resolve spontaneously during cooling. The

administration of atrial or ventricular antiarrhythmic medications is

rarely indicated during cooling. Anticholinergic medications (including atropine) inhibit sweating and should be avoided. With a cardiac

rhythm that sustains perfusion, electrical cardioversion of the hyperthermic myocardium should be deferred until the myocardium is

cooled. Significant shivering, discomfort, or extreme agitation is preferably mitigated with short-acting benzodiazepines, which are ideal

due to their renal clearance. On the other hand, chlorpromazine may

lower the seizure threshold, has anticholinergic properties, and can

exacerbate the hypotension or cause neuroleptic malignant syndrome.

With hepatic dysfunction, barbiturates should be avoided and seizures

treated with benzodiazepines.

Coagulopathies more commonly occur after the first day of illness. After cooling, the patient should be monitored for disseminated

intravascular coagulation, and replacement therapy with fresh-frozen

plasma and platelets should be considered.

There is no therapeutic role for antipyretics in the control of environmentally induced hyperthermia; these drugs block the actions of

pyrogens at hypothalamic receptor sites. Salicylates can further uncouple oxidative phosphorylation in heatstroke and exacerbate coagulopathies. Acetaminophen may further stress hepatic function. The safety

and efficacy of dantrolene are not established. Although aminocaproic

acid impedes fibrinolysis, it may cause rhabdomyolysis and is not recommended in heatstroke.

■ DISPOSITION

Most patients with minor heat-emergency syndromes (including heat

edema, heat syncope, and heat cramps) require only stabilization and

treatment with outpatient follow-up. Although there are no decision

rules to guide disposition choices in heat exhaustion, many of these

patients have multiple predisposing factors and comorbidities that will

require prolonged observation or hospital admission.

Essentially all patients with actual heatstroke require admission to a

monitored setting, and most require intensive care. There are reports

of very high survival rates of patients following prehospital immersion

cooling without intensive care. Most or all of these patients appear to

have had heat exhaustion. Many actual heatstroke patients also require

prolonged tracheal intubation, invasive hemodynamic monitoring,

and support for various degrees of multiorgan dysfunction syndrome.

The prognosis worsens if the initial core temperature exceeds 42°C

(107.6°F) or if there was a prolonged period during which the core

temperature exceeded this level. Other features of a negative prognosis

include acute renal failure, massively elevated liver enzymes, and significant hyperkalemia. As expected, the number of dysfunctional organ

systems also correlates directly with mortality risk.

■ FURTHER READING

Balmain BN et al: Aging and thermoregulatory control: The clinical

implications of exercising under heat stress in older individuals.

BioMed Res Int 2018:8306154, 2018.

Casa DJ et al: National Athletic Trainers’ Association position statement: Exertional heat illnesses. J Athl Train 50:986, 2015.

Hosokawa Y et al: Inconsistency in the standard of care-toward

evidence-based management of exertional heat stroke. Front Physiol

10:108, 2019.

Lawton EM et al: Review article: Environmental heatstroke and

long-term clinical neurological outcomes: A literature review of case

reports and case series 2000-2016. Emerg Med Australas 31:163,

2019.

Leon LR et al: Pathophysiology of heat-related illnesses, in Auerbach’s

Wilderness Medicine, 7th ed. PS Auerbach et al (eds): Philadelphia,

Elsevier, 2017, pp. 249–267.

Lipman GS et al: Wilderness Medical Society practice guidelines for

the prevention and treatment of heat-related illness. Wilderness

Environ Med 24:351, 2013.

Platt M et al: Heat illness, in Rosen’s Emergency Medicine: Concepts

and Clinical Practice, 9th ed. Walls RM et al (eds). Philadelphia,

Elsevier, 2018, pp. 1755–1764.

Genes, the Environment, and Disease PART 16

Principles of Human

Genetics

J. Larry Jameson, Peter Kopp

466

IMPACT OF GENETICS AND GENOMICS

ON MEDICAL PRACTICE

Human genetics refers to the study of individual genes, their role

and function in disease, and their mode of inheritance. Genomics

refers to an organism’s entire genetic information, the genome, and

the function and interaction of DNA within the genome, as well as

with environmental or nongenetic factors, such as a person’s lifestyle.

With the characterization of the human genome, genomics not only

complements traditional genetics in our efforts to elucidate the etiology and pathogenesis of disease, but it now plays a prominent and

continuously expanding role in diagnostics, prevention, and therapy

(Chap. 467). These transformative developments, originally emerging from the Human Genome Project, have been variably designated

genomic medicine, personalized medicine, or precision medicine. Precision medicine aims at customizing medical decisions to an individual

patient. For example, a patient’s genetic characteristics (genotype)

can be used to optimize drug therapy and predict efficacy, adverse

events, and drug dosing of selected medications (pharmacogenomics)

(Chap. 68). The characterization of the mutational profile of a malignancy allows the identification of driver mutations or overexpressed

signaling molecules, which then facilitates the selection of targeted

therapies. Genome-wide polygenic risk scores (PRS) for common

complex diseases are also beginning to emerge and potentially impact

disease prevention.

Genetics has traditionally been viewed through the window of

relatively rare single-gene diseases. These disorders account for

~10% of pediatric admissions and childhood mortality. Historically,

genetics has focused predominantly on chromosomal and metabolic

disorders, reflecting the long-standing availability of techniques to

diagnose these conditions. For example, conditions such as trisomy

21 (Down’s syndrome) or monosomy X (Turner’s syndrome) can be

diagnosed using cytogenetics. Likewise, many metabolic disorders

(e.g., phenylketonuria, familial hypercholesterolemia) are diagnosed

using biochemical analyses. The advances in DNA and RNA diagnostics have extended the field of genetics to include virtually all medical

specialties and have led to the elucidation of the pathogenesis of the

majority of monogenic disorders. In addition, it is apparent that virtually every medical condition has a genetic component. As is often

evident from a patient’s family history, many common disorders such

as hypertension, heart disease, asthma, diabetes mellitus, and mental

illnesses are significantly influenced by the genetic background. These

polygenic or multifactorial (complex) disorders involve the contributions of many different genes, as well as environmental factors that can

modify disease risk. Genome-wide association studies (GWAS) have

elucidated numerous disease-associated loci and are providing novel

insights into the allelic architecture of complex traits. These studies

have been facilitated by the availability of comprehensive catalogues

of human single nucleotide polymorphism (SNP) haplotypes (HapMap, International Genome Sample Resource/1000 Genomes Project).

Next-generation DNA sequencing (NGS) technologies have evolved

rapidly, and the cost of sequencing whole exomes (the exons within

the genome; whole exome sequencing [WES]) or genomes (whole

genome sequencing [WGS]) has plummeted. Comprehensive unbiased

sequence analyses are now frequently used to characterize individuals

with complex undiagnosed conditions or to determine the mutational

profile of advanced malignancies in order to select targeted therapies.

The routine assembly of diploid genomes, which can reveal a comprehensive spectrum of human genetic variation, will be possible in the

near future and should provide further insights into heritability and

disease mechanisms.

Cancer has a genetic basis because it results from acquired somatic

mutations in genes controlling growth, apoptosis, and cellular differentiation (Chap. 71). In addition, the development of many cancers

is associated with a hereditary predisposition. Characterization of the

genome (and epigenome) in various malignancies has led to fundamental new insights into cancer biology and reveals that the genomic

profile of mutations is in many cases more important in determining

the appropriate therapy than the organ in which the tumor originates.

The Cancer Genome Atlas (TCGA) initiative of the National Cancer

Institute and the National Human Genome Research Institute has

already characterized the genomic landscape of >30 malignancies.

TCGA consists of comprehensive analyses of genomic and proteomic

alterations and is providing fundamental new insights into the molecular pathogenesis of cancer. These data, together with comprehensive

catalogues of somatic mutations identified in human cancer, have

direct clinical ramifications that impact cancer taxonomy, as well as the

development and choice of targeted therapies.

Genetic and genomic approaches have proven invaluable for the

detection of infectious pathogens and are used clinically to identify

agents that are difficult to culture such as mycobacteria, viruses, and

parasites, or to track infectious agents locally or globally. In many

cases, molecular genetics has improved the feasibility and accuracy of

diagnostic testing and is beginning to open new avenues for therapy,

including gene and cellular therapies (Chap. 470). Molecular genetics

has also provided the opportunity to characterize the microbiome,

a new field that characterizes the population dynamics of bacteria,

viruses, and parasites that coexist with humans and other animals

(Chap. 471). Emerging data indicate that the microbiome has significant effects on normal physiology as well as various disease states, and

the field is now focusing on defining the mechanisms underlying these

interactions.

Molecular biology has significantly changed the treatment of human

disease. Peptide hormones, growth factors, cytokines, and vaccines

can be produced in large amounts using recombinant DNA and RNA

technology (e.g., mRNA vaccines against SARS-CoV-2). Targeted

modifications of recombinant peptides provide improved therapeutic

tools, as illustrated by genetically modified insulin analogues with

more favorable kinetics.

The astounding rate at which new genetic and genomic information

is being generated has led to major challenges for physicians, health

care providers, and basic investigators. Although many functional

aspects of the genome remain unknown, there are many clinical

situations where sufficient evidence exists for the use of genetic and

genomic information to optimize patient care and treatment. Much

genetic information resides in databases that provide easy access to

the expanding information about the human genome, genetic disease,

and genetic testing (Table 466-1). For example, several thousand

monogenic disorders are summarized in a large, continuously evolving

compendium, referred to as the Online Mendelian Inheritance in Man

(OMIM) catalogue (Table 466-1). The constant refinement of bioinformatics and new developments in big data analytics, together with the

widespread adoption of electronic health records (EHRs), are simplifying the access, analysis, and integration of this daunting amount of new

information. Importantly, genomic data can be integrated readily into

EHRs and thus impact clinical practice.

■ THE HUMAN GENOME

Structure of the Human Genome The Human Genome Project

was initiated in the mid-1980s as an ambitious effort to characterize the

entire human genome and culminated in the completion of the DNA

3640 PART 16 Genes, the Environment, and Disease

TABLE 466-1 Selected Databases Relevant for Genomics and Genetic Disorders

SITE URL COMMENT

National Center for Biotechnology

Information (NCBI)

http://www.ncbi.nlm.nih.gov/ Broad access to biomedical and genomic information, literature (PubMed),

sequence databases, software for analyses of nucleotides and proteins

Extensive links to other databases, genome resources, and tutorials

National Human Genome Research

Institute

http://www.genome.gov/ An institute of the National Institutes of Health focused on genomic and genetic

research; links providing information about the human genome sequence,

genomes of other organisms, and genomic research

Catalog of Published Genome-Wide

Association Studies

https://www.ebi.ac.uk/gwas/ Published high-resolution genome-wide association studies (GWAS)

Ensembl Genome browser http://www.ensembl.org Maps and sequence information of eukaryotic genomes

Online Mendelian Inheritance in Man http://www.ncbi.nlm.nih.gov/omim Online compendium of Mendelian disorders and human genes causing genetic

disorders

American College of Medical Genetics and

Genomics

http://www.acmg.net/ Extensive links to other databases relevant for the diagnosis, treatment, and

prevention of genetic disease

American Society of Human Genetics http://www.ashg.org Information about advances in genetic research, professional and public

education, and social and scientific policies

The Cancer Genome Atlas https://cancergenome.nih.gov/ Comprehensive, multidimensional characterization of the genomic and proteomic

landscape of malignancies with high public health impact

COSMIC Catalogue of Somatic Mutations

in Cancer

https://cancer.sanger.ac.uk/cosmic Comprehensive catalogue of somatic mutations in human cancer

Genetic Testing Registry https://www.ncbi.nlm.nih.gov/gtr/ International directory of genetic testing laboratories and prenatal diagnosis

clinics; reviews and educational materials

Genomes Online Database (GOLD) http://www.genomesonline.org/ Information on published and unpublished genomes

HUGO Gene Nomenclature http://www.genenames.org/ Gene names and symbols

GENECODE https://www.gencodegenes.org/ High-quality reference gene annotation and experimental validation for human and

mouse genomes

MITOMAP, a human mitochondrial genome

database

http://www.mitomap.org/ A compendium of polymorphisms and mutations of the human mitochondrial DNA

The International Genome Sample

Resource (IGSR)

http://www.internationalgenome.org Public catalogue of human variation and genotype data from numerous ethnic

groups

Human Genome Variation Society https://www.hgvs.org/ Collection and documentation of genomic variations including population

distribution and phenotypic associations

ENCODE http://www.genome.gov/10005107 Encyclopedia of DNA Elements; catalogue of all functional elements in the human

genome

Dolan DNA Learning Center, Cold Spring

Harbor Laboratories

http://www.dnalc.org/ Educational material about selected genetic disorders, DNA, eugenics, and

genetic origin

The Online Metabolic and Molecular

Bases of Inherited Disease (OMMBID)

http://ommbid.mhmedical.com Online version of the comprehensive text on the metabolic and molecular bases of

inherited disease

Online Mendelian Inheritance in Animals

(OMIA)

https://www.omia.org/home/ Online compendium of Mendelian disorders in animals

The Jackson Laboratory http://www.jax.org/ Information about murine models and the mouse genome

Mouse genome informatics http://www.informatics.jax.org Mouse genome informatics, potential mouse models of human disease, information

on phenotypic similarity between mouse models and human patients

Note: Databases are evolving constantly. Pertinent information may be found by using links listed in the few selected databases.

sequence for the last of the human chromosomes in 2006. The scope of

a whole genome sequence analysis can be illustrated by the following

analogy. Human DNA consists of ~3 billion base pairs (bp) of DNA

per haploid genome, which is nearly 1000-fold greater than that of the

Escherichia coli genome. If the human DNA sequence were printed

out, it would correspond to about 120 volumes of Harrison’s Principles

of Internal Medicine.

In addition to the human genome, the genomes of thousands of

organisms have been sequenced completely or partially (Genomes

Online Database [GOLD]; Table 466-1). They include, among others,

eukaryotes such as the mouse (Mus musculus), Saccharomyces cerevisiae, Caenorhabditis elegans, and Drosophila melanogaster; bacteria

(e.g., E. coli); and archaea, viruses, organelles (mitochondria, chloroplasts), and plants (e.g., Arabidopsis thaliana). Genomic information

of infectious agents has significant impact for the characterization

of infectious outbreaks and epidemics. Other ramifications arising

from the availability of genomic data include, among others, (1) the

comparison of entire genomes (comparative genomics); (2) the study

of large-scale expression of RNAs (functional genomics), proteins

(proteomics), or protein families (e.g., the kinome, the complete set of

protein kinases) to detect differences between various tissues in health

and disease; (3) the characterization of the variation among individuals by establishing catalogues of sequence variations and SNPs; and

(4) the identification of genes that play critical roles in the development

of polygenic and multifactorial disorders.

CHROMOSOMES The human genome is divided into 23 different chromosomes, including 22 autosomes (numbered 1–22) and the X and Y sex

chromosomes (Fig. 466-1). Adult cells are diploid, meaning they contain

two homologous sets of 22 autosomes and a pair of sex chromosomes.

Females have two X chromosomes (XX), whereas males have one X

and one Y chromosome (XY). As a consequence of meiosis, germ cells

(sperm or oocytes) are haploid and contain one set of 22 autosomes and

one of the sex chromosomes. At the time of fertilization, the diploid

genome is reconstituted by pairing of the homologous chromosomes

from the mother and father. With each cell division (mitosis), chromosomes are replicated, paired, segregated, and divided into two daughter

cells.

Principles of Human Genetics

3641CHAPTER 466

STRUCTURE OF DNA DNA is a double-stranded helix composed of

four different bases: adenine (A), thymidine (T), guanine (G), and

cytosine (C). Adenine is paired to thymidine, and guanine is paired

to cytosine, by hydrogen bond interactions that span the double helix

(Fig. 466-1). DNA has several remarkable features that make it ideal

for the transmission of genetic information. It is relatively stable, and

the double-stranded nature of DNA and its feature of strict base-pair

complementarity permit faithful replication during cell division. Complementarity also allows the transmission of genetic information from

DNA → RNA → protein (Fig. 466-2). mRNA is encoded by the socalled sense or coding strand of the DNA double helix and is translated

into proteins by ribosomes.

The presence of four different bases provides surprising genetic

diversity. In the protein-coding regions of genes, the DNA bases are

arranged into codons, a triplet of bases that specifies a particular

amino acid. It is possible to arrange the four bases into 64 different

triplet codons (43

). Each codon specifies 1 of the 20 different amino

acids, or a regulatory signal such as initiation and stop of translation.

Because there are more codons than amino acids, the genetic code is

degenerate; that is, most amino acids can be specified by several different codons. By arranging the codons in different combinations and

in various lengths, it is possible to generate the tremendous diversity of

primary protein structure.

DNA length is normally measured in units of 1000 bp (kilobases, kb)

or 1,000,000 bp (megabases, Mb). In the human genome, only ~1% of

DNA accounts for protein-coding sequences. The noncoding DNA has

multiple functional and structural roles including (1) sequences that

form introns; (2) regulatory elements (promoters, enhancers, silencers,

insulators); (3) sequences that generate RNAs that do not code for proteins; (4) centromeres and telomeres; (5) regions defining chromatin

structure and histone modifications; (6) various forms of repetitive

sequences of variable length; and (7) pseudogenes and regions without

currently discernible functional or structural roles (Fig. 466-1).

GENES A gene is a functional unit that is regulated by transcription

(see below) and encodes an RNA product, which is most commonly,

but not always, translated into a protein that exerts activity within

or outside the cell (Fig. 466-3). Historically, genes were identified

because they conferred specific traits that are transmitted from one

generation to the next. Now, they are frequently characterized based

on expression in various tissues (transcriptome). The size of genes is

quite broad; some genes are only a few hundred base pairs, whereas

others are extraordinarily large (2 Mb). The number of genes greatly

underestimates the complexity of genetic expression, because single

genes can generate multiple spliced messenger RNA (mRNA) products

(isoforms), which are translated into proteins that are subject to complex posttranslational modification such as phosphorylation. Exons

refer to the portion of genes that are eventually spliced together to form

mRNA. Introns refer to the spacing regions between the exons that

are spliced out of precursor RNAs during RNA processing. The gene

locus also includes regions that are necessary to control its expression

(Fig. 466-2). Current estimates predict roughly 20,000 protein-coding

genes in the human genome with an average of about four different

coding transcripts per gene. Remarkably, the exome only constitutes

1.14% of the genome. Of note, the number of transcripts is close to

200,000 and includes thousands of noncoding transcripts (RNAs of

various length such as microRNAs [miRNA] and long noncoding

RNAs [lncRNA]). These noncoding RNAs are involved in numerous

cellular processes such as transcriptional and posttranscriptional

regulation of gene expression, chromatin remodeling, and protein

trafficking, among others. Not surprisingly, aberrant expression and/or

mutations in these RNAs play a pathogenic role in numerous diseases.

SINGLE-NUCLEOTIDE POLYMORPHISMS Each individual has roughly

5 million sequence variants that differentiate one person from another.

Some of these variants have no impact on health, whereas others may

increase or lower the risk for developing a specific disease. Remarkably,

however, the primary DNA sequence of humans has ~99.9% similarity

compared to that of any other human. An SNP is a variation of a single base pair in the DNA. The identification of the ~10 million SNPs

estimated to occur in the human genome has generated a catalogue of

common genetic variants that occur in human beings from distinct

ethnic backgrounds (Fig. 466-3). SNPs are the most common type of

sequence variation and account for ~90% of all sequence variation.

They occur on average every 100–300 bases and are the major source

of genetic heterogeneity. SNPs that are in close proximity are inherited together (e.g., they are linked) and are referred to as haplotypes

(Fig. 466-4). Haplotype maps describe the nature and location of these

SNP haplotypes and how they are distributed among individuals within

and among populations, information that has been facilitating GWAS

designed to elucidate the complex interactions among multiple genes

and lifestyle factors in multifactorial disorders (see below). Moreover,

haplotype analyses are useful to assess variations in responses to medications (pharmacogenomics) and environmental factors, as well as the

prediction of disease predisposition.

COPY NUMBER VARIATIONS Copy number variations (CNVs) are relatively large genomic regions (1 kb to several Mb) that have been duplicated or deleted on certain chromosomes and hence alter the diploid

status of the DNA (Fig. 466-5). It has been estimated that 5–10% of the

genome can display CNVs. When comparing the genomes of two individuals, ~0.4–0.8% of their genomes differ in terms of CNVs scattered

throughout the genome. Of note, de novo CNVs have been observed

between monozygotic twins, who otherwise have identical genomes.

Some CNVs have no functional consequences, whereas others have

been associated with susceptibility or resistance to disease, and CNVs

also occur in cancer cells.

Histone

H1

Nucleosome

fiber

Metaphase

chromosome

Solenoid

q, long arm

p, short arm

Centromere

Supercoiled

chromatin

Telomere

Nucleosome core

Histone H2A, H2B, H4

Double-strand DNA

without histones

Adenine

H

H

H

O

O

P OO

O

O–

P OO

O

O–

O

H

H

H

H H

H

H

H

H

H H3C

H

Thymine

Cytosine Guanine

C C

C C

C

C

C

C

C C

C

C

C C C

C

N

N

T

A

C

G

G

C

A

T

N

N

N N

N

N N

N

N

C

C

C

N

N

N

N

FIGURE 466-1 Structure of chromatin and chromosomes. Chromatin is composed of

double-strand DNA that is wrapped around histone and nonhistone proteins forming

nucleosomes. The nucleosomes are further organized into solenoid structures.

Chromosomes assume their characteristic structure, with short (p) and long (q)

arms at the metaphase stage of the cell cycle.

3642 PART 16 Genes, the Environment, and Disease

CRE

Enhancer Silencer

RE CAAT TATA 1 2 3

1 2 3

Nuclear

receptor

Transcription

factor

RNA polymerase II

Nuclear

receptor

CBP TAF

GTF CREB CREB TBP

HAT

CoA

Steroids Ca Growth 2+ Cytokines

factors

Hormones

Light

UV-light

Mechanical stress

Regulation of Gene Expression

Transcription

mRNA Processing

Translation

Posttranslational Processing

Cytoplasm

Nucleus

DNA

hRNA

–COOH

5′ -Cap –Poly-A Tail

mRNA

Protein

1 2 3

NH2–

FIGURE 466-2 Flow of genetic information. Multiple extracellular signals activate intracellular signal cascades that result in altered regulation of gene expression through

the interaction of transcription factors with regulatory regions of genes. RNA polymerase transcribes DNA into RNA that is processed to mRNA by excision of intronic

sequences. The mRNA is translated into a polypeptide chain to form the mature protein after undergoing posttranslational processing. CBP, CREB-binding protein; CoA,

co-activator; COOH, carboxyterminus; CRE, cyclic AMP responsive element; CREB, cyclic AMP response element–binding protein; GTF, general transcription factors; HAT,

histone acetyl transferase; NH2, aminoterminus; RE, response element; TAF, TBP-associated factors; TATA, TATA box; TBP, TATA-binding protein.

Replication of DNA and Mitosis Genetic information in DNA

is transmitted to daughter cells under two different circumstances:

(1) somatic cells divide by mitosis, allowing the diploid (2n) genome

to replicate itself completely in conjunction with cell division; and (2)

germ cells (sperm and ova) undergo meiosis, a process that enables

the reduction of the diploid (2n) set of chromosomes to the haploid

state (1n).

Prior to mitosis, cells exit the resting, or G0

 state, and enter the cell

cycle. After traversing a critical checkpoint in G1

, cells undergo DNA

synthesis (S phase), during which the DNA in each chromosome is replicated, yielding two pairs of sister chromatids (2n → 4n). The process

of DNA synthesis requires stringent fidelity in order to avoid transmitting errors to subsequent generations of cells. Genetic abnormalities

of DNA mismatch/repair include xeroderma pigmentosum, Bloom’s

syndrome, ataxia telangiectasia, and hereditary nonpolyposis colon

cancer (HNPCC), among others. Many of these disorders strongly

predispose to neoplasia because of the rapid acquisition of additional

mutations (Chap. 71). After completion of DNA synthesis, cells enter

G2

 and progress through a second checkpoint before entering mitosis.

At this stage, the chromosomes condense and are aligned along the

equatorial plate at metaphase. The two identical sister chromatids, held

together at the centromere, divide and migrate to opposite poles of the

cell. After formation of a nuclear membrane around the two separated

sets of chromatids, the cell divides and two daughter cells are formed,

thus restoring the diploid (2n) state.

Assortment and Segregation of Genes During Meiosis Meiosis occurs only in germ cells of the gonads. It shares certain features

with mitosis but involves two distinct steps of cell division that reduce

the chromosome number to the haploid state. In addition, there is

active recombination that generates genetic diversity. During the first

cell division, two sister chromatids (2n → 4n) are formed for each chromosome pair and there is an exchange of DNA between homologous

paternal and maternal chromosomes. This process involves the formation of chiasmata, structures that correspond to the DNA segments that

cross over between the maternal and paternal homologues (Fig. 466-6).

Usually there is at least one crossover on each chromosomal arm;

recombination occurs more frequently in female meiosis than in male

meiosis. Subsequently, the chromosomes segregate randomly. Because

there are 23 chromosomes, there exist 223 (>8 million) possible combinations of chromosomes. Together with the genetic exchanges that

occur during recombination, chromosomal segregation generates tremendous diversity, and each gamete is genetically unique. The process

of recombination and the independent segregation of chromosomes

provide the foundation for performing linkage analyses, whereby one

attempts to correlate the inheritance of certain chromosomal regions

(or linked genes) with the presence of a disease or genetic trait (see

below).

After the first meiotic division, which results in two daughter cells

(2n), the two chromatids of each chromosome separate during a second meiotic division to yield four gametes with a haploid state (1n).

When the egg is fertilized by sperm, the two haploid sets are combined,

thereby restoring the diploid state (2n) in the zygote.

■ REGULATION OF GENE EXPRESSION

Regulation by Transcription Factors The expression of genes

is regulated by DNA-binding proteins that activate or repress transcription. The number of DNA sequences and transcription factors

that regulate transcription is much greater than originally anticipated.

Most genes contain at least 15–20 discrete regulatory elements within

300 bp of the transcription start site. This densely packed promoter

region often contains binding sites for ubiquitous transcription factors. However, factors involved in cell-specific expression may also

bind to these sequences. Key regulatory elements may also reside

at a large distance from the proximal promoter. The globin and the

Principles of Human Genetics

3643CHAPTER 466

Chromosome 7

p22.3

p22.1

p21.3

p21.1

p15.3

p15.1

p14.3

p14.1

p13

p12.3

p12.1

p1

q11.21

q11.22

q11.23

q21.11

p21.13

q21.3

q22.1

q22.3

q31.1

q31.2

q31.31

q31.33

q32.1

q33

q34

q35

q36.1

q36.3

1.2

Known Genes

(1260)

SNPs

(612,977)

CFTR Gene

116.90 Mb 116.94 Mb 116.98 Mb 117.02 Mb 117.06 Mb

200 Kb

20 Kb

SNPs

Intronic

Coding region, synonymous Coding region, nonsynonymous

Splice site

Coding region, frameshift

FIGURE 466-3 Chromosome 7 is shown with the density of single nucleotide polymorphisms (SNPs) and genes above. A 200-kb region in 7q31.2 containing the CFTR gene

is shown below. The CFTR gene contains 27 exons. Close to 2000 mutations in this gene have been found in patients with cystic fibrosis. A 20-kb region encompassing exons

4–9 is shown further amplified to illustrate the SNPs in this region.

FIGURE 466-4 The origin of haplotypes is due to repeated recombination events

occurring in multiple generations. Over time, this leads to distinct haplotypes. These

haplotype blocks can often be characterized by genotyping selected Tag single

nucleotide polymorphisms (SNPs), an approach that facilitates performing genomewide association studies (GWAS).

immunoglobulin genes, for example, contain locus control regions that

are several kilobases away from the structural sequences of the gene.

Specific groups of transcription factors that bind to these promoter

and enhancer sequences provide a combinatorial code for regulating

transcription. In this manner, relatively ubiquitous factors interact

with more restricted factors to allow each gene to be expressed and

regulated in a unique manner that is dependent on developmental

state, cell type, and numerous extracellular stimuli. Regulatory factors

also bind within the gene itself, particularly in the intronic regions.

The transcription factors that bind to DNA actually represent only the

first level of regulatory control. Other proteins—co-activators and corepressors—interact with the DNA-binding transcription factors to generate large regulatory complexes. These complexes are subject to control

by numerous cell-signaling pathways and enzymes, leading to phosphorylation, acetylation, sumoylation, and ubiquitination. Ultimately,

the recruited transcription factors interact with, and stabilize, components of the basal transcription complex that assembles at the site of the

TATA box and initiator region. This basal transcription factor complex

consists of >30 different proteins. Gene transcription occurs when

RNA polymerase begins to synthesize RNA from the DNA template.

A large number of identified genetic diseases involve transcription

factors (Table 466-2).

The field of functional genomics is based on the concept that understanding alterations of gene expression under various physiologic and

pathologic conditions provides insight into the underlying functional

3644 PART 16 Genes, the Environment, and Disease

1

2

Normal

Deleted

Area

Duplicated

Area

0

–1

–2

log2 (ratio)

Chromosome 8

FIGURE 466-5 Copy number variations (CNV) encompass relatively large regions of the genome that

have been duplicated or deleted. Chromosome 8 is shown with a CNV detected by genomic hybridization.

An increase in the signal strength indicates a duplication, whereas a decrease reflects a deletion of the

covered chromosomal regions.

Homologous

chromosomes

A

B

C

D

a

b

c

d

a

b

c

d

a

b

c

d

Chromatids

A

B

C

D

A

B

C

D

a

b

c

d

No crossover

A

B

C

D

a

b

c

d

A

B

C

D

a

b

C

d

Double crossover

A

B

C

D

a

b

c

d

A

B

c

D

a

b

C

D

Crossover

A

B

C

D

a

b

c

d

A

B

c

d

a

b

c

d

No recombination

in gametes

A

B

C

D

a

b

c

d

A

B

C

D

a

b

C

D

Recombination

in gametes

A

B

C

D

a

b

c

d

A

B

c

d

a

b

C

d

Recombination

in gametes

A

B

C

D

a

b

c

d

A

B

c

D

FIGURE 466-6 Crossing-over and genetic recombination. During chiasma

formation, either of the two sister chromatids on one chromosome pairs with

one of the chromatids of the homologous chromosome. Genetic recombination

occurs through crossing-over and results in recombinant and nonrecombinant

chromosome segments in the gametes. Together with the random segregation of

the maternal and paternal chromosomes, recombination contributes to genetic

diversity and forms the basis of the concept of linkage.

role of the gene. The ENCODE (Encyclopedia of DNA Elements) project aims to compile and annotate all functional sequences in the human

genome. By revealing specific gene expression profiles, this knowledge

can be of diagnostic and therapeutic relevance. The large-scale study

of expression profiles is referred to as transcriptomics because the

complement of mRNAs transcribed by the cellular genome is called

the transcriptome.

Most studies of gene expression have focused on the regulatory

DNA elements of genes that control transcription. However, it should

be emphasized that gene expression requires a series of steps, including

mRNA processing, protein translation, and posttranslational modifications, all of which are actively regulated (Fig. 466-2).

Epigenetic Regulation of Gene Expression (see Chap. 483)

Epigenetics describes mechanisms and phenotypic changes that are not

a result of variation in the primary DNA nucleotide sequence but are

caused by secondary modifications of DNA or histones. These modifications include heritable changes such as X-inactivation and imprinting, but they can also result from dynamic posttranslational protein

modifications in response to environmental influences such as diet,

age, or drugs. The epigenetic modifications result in altered expression

of individual genes or chromosomal loci encompassing multiple genes.

The term epigenome describes the constellation of covalent modifications of DNA and histones that impact chromatin structure, as well

as noncoding transcripts that modulate the transcriptional activity of

DNA. Although the primary DNA sequence is usually identical in all

cells of an organism, tissue-specific changes in the epigenome contribute to determining the transcriptional signature of a cell (transcriptome) and hence the protein expression profile (proteome).

Mechanistically, DNA and histone modifications can result in the

activation or silencing of gene expression (Fig. 466-7). DNA methylation involves the addition of a methyl group to cytosine residues.

This is usually restricted to cytosines of CpG dinucleotides, which

are abundant throughout the genome. Methylation of these dinucleotides is thought to represent a defense mechanism that minimizes the

expression of sequences that have been incorporated into the genome

such as retroviral sequences. CpG dinucleotides also exist in so-called

CpG islands, stretches of DNA characterized by a high

CG content, which are found in the majority of human

gene promoters. CpG islands in promoter regions are

typically unmethylated, and the lack of methylation

facilitates transcription.

Histone methylation involves the addition of a

methyl group to lysine residues in histone proteins

(Fig. 466-7). Depending on the specific lysine residue

being methylated, this alters chromatin configuration,

making it either more open or tightly packed. Acetylation of histone proteins is another well-characterized

mechanism that results in an open chromatin configuration, which favors active transcription. Acetylation

is generally more dynamic than methylation, and

many transcriptional activation complexes have histone acetylase activity, whereas repressor complexes

often contain deacetylases and remove acetyl groups

from histones. Other histone modifications include,

among others, phosphorylation and sumoylation.

Furthermore, noncoding RNAs and RNA regulatory networks that bind to DNA have a significant

impact on transcriptional activity.

Physiologically, epigenetic mechanisms play an

important role in several instances. For example,

X-inactivation refers to the relative silencing of one of

the two X chromosome copies present in females. The

inactivation process is a form of dosage compensation

such that females (XX) do not generally express twice

as many X-chromosomal gene products as males

(XY). In a given cell, the choice of which chromosome

is inactivated occurs randomly in humans. But once

the maternal or paternal X chromosome is inactivated,

Principles of Human Genetics

3645CHAPTER 466

it will remain inactive, and this information is transmitted with each

cell division. The X-inactive specific transcript (Xist) gene encodes a

large noncoding RNA that mediates the silencing of the X chromosome

from which it is transcribed by coating it with Xist RNA. The inactive

X chromosome is highly methylated and has low levels of histone acetylation. While the majority of X-chromosomal genes are silenced by

X-inactivation, ~15% escape inactivation and are expressed.

Epigenetic gene inactivation also occurs on selected chromosomal

regions of autosomes, a phenomenon referred to as genomic imprinting.

Through this mechanism, a small subset of genes is only expressed in

a monoallelic fashion. Imprinting is heritable and leads to the preferential expression of one of the parental alleles, which deviates from the

usual biallelic expression seen for the majority of genes. Remarkably,

imprinting can be limited to a subset of tissues. Imprinting is mediated through DNA methylation of one of the alleles. The epigenetic

marks on imprinted genes are maintained throughout life, but during

zygote formation, they are activated or inactivated in a sex-specific

manner (imprint reset) (Fig. 466-8), which allows a differential

expression pattern in the fertilized egg and the subsequent mitotic

divisions. Appropriate expression of imprinted genes is important for

normal development and cellular functions. Imprinting defects and

uniparental disomy, which is the inheritance of two chromosomes or

chromosomal regions from the same parent, are the cause of several

developmental disorders such as Beckwith-Wiedemann syndrome,

Silver-Russell syndrome, Angelman’s syndrome, and Prader-Willi

syndrome (see below). Monoallelic loss-of-function mutations in the

GNAS1 gene lead to Albright’s hereditary osteodystrophy (AHO).

Paternal transmission of GNAS1 mutations leads to an isolated AHO

phenotype (pseudopseudohypoparathyroidism), whereas maternal

transmission leads to AHO in combination with hormone resistance

to parathyroid hormone, thyrotropin, and gonadotropins (pseudohypoparathyroidism type IA). These phenotypic differences are explained

by tissue-specific imprinting of the GNAS1 gene, which is expressed

primarily from the maternal allele in the thyroid, gonadotropes, and

the proximal renal tubule. In most other tissues, the GNAS1 gene is

expressed biallelically. In patients with isolated renal resistance to

parathyroid hormone (pseudohypoparathyroidism type IB), defective

imprinting of the GNAS1 gene results in decreased Gs

α expression in

the proximal renal tubules. Rett’s syndrome is an X-linked dominant

disorder resulting in developmental regression and stereotypic hand

movements in affected girls. It is caused by mutations in the MECP2

gene, which encodes a methyl-binding protein. The ensuing aberrant

methylation results in abnormal gene expression in neurons, which are

otherwise normally developed.

Remarkably, epigenetic differences also occur among monozygotic

twins. Although twins are epigenetically indistinguishable during the

early years of life, older monozygotic twins exhibit differences in the

overall content and genomic distribution of DNA methylation and

histone acetylation, which would be expected to alter gene expression

in various tissues.

In cancer, the epigenome is characterized by simultaneous losses

and gains of DNA methylation in different genomic regions, as well

as repressive histone modifications. Hyper- and hypomethylation are

associated with mutations in genes that control DNA methylation.

Hypomethylation is thought to remove normal control mechanisms

that prevent expression of repressed DNA regions. It is also associated

with genomic instability. Hypermethylation, in contrast, results in

the silencing of CpG islands in promoter regions of genes, including

tumor-suppressor genes. Epigenetic alterations are considered to be

more easily reversible compared to genetic changes; modification of

the epigenome with demethylating agents and histone deacetylases is

being used in the treatment of various malignancies.

■ TRANSMISSION OF GENETIC DISEASE

Origins and Types of Mutations The term mutation or variant

is used to designate the process of generating genetic variations as well

as the outcome of these alterations. A mutation can be defined as any

change in the primary nucleotide sequence of DNA regardless of its

functional consequences, although it often has a negative connotation.

The more neutral term variation is now increasingly used to describe

sequence changes and is recommended by several professional organizations and guidelines instead of mutation. Some variations may be

lethal, others are less deleterious, and some may confer an evolutionary

advantage. Variations can occur in the germline (sperm or oocytes);

these can be transmitted to progeny. Alternatively, variations can occur

during embryogenesis or in somatic tissues. Variations that occur

during development lead to mosaicism, a situation in which tissues are

composed of cells with different genetic constitutions. If the germline

is mosaic, a mutation can be transmitted to some progeny but not

others, which sometimes leads to confusion in assessing the pattern

of inheritance. Somatic mutations that do not affect cell survival can

sometimes be detected because of variable phenotypic effects in tissues (e.g., pigmented lesions in McCune-Albright syndrome). Other

somatic mutations are associated with neoplasia because they confer

a growth advantage to cells. Epigenetic events may also influence gene

expression or facilitate genetic damage. With the exception of triplet

TABLE 466-2 Selected Examples of Diseases Caused by Mutations and

Rearrangements in Transcription Factors

TRANSCRIPTION

FACTOR CLASS EXAMPLE ASSOCIATED DISORDER

Nuclear receptors Androgen

receptor

Complete or partial androgen

insensitivity (recessive missense

mutations)

Spinobulbar muscular atrophy (CAG

repeat expansion)

Zinc finger proteins WT1 WAGR syndrome: Wilms’

tumor, aniridia, genitourinary

malformations, mental retardation

Basic helix-loop-helix MITF Waardenburg’s syndrome type 2A

Homeobox IPF1 Maturity onset of diabetes mellitus

type 4 (heterozygous mutation/

haploinsufficiency)

Pancreatic agenesis (homozygous

mutation)

Leucine zipper Retina leucine

zipper (NRL)

Autosomal dominant retinitis

pigmentosa

High mobility group

(HMG) proteins

SRY Sex reversal

Forkhead HNF4α, HNF1α,

HNF1β

Maturity onset of diabetes mellitus

types 1, 3, 5

Paired box PAX3 Waardenburg’s syndrome types 1

and 3

T-box TBX5 Holt-Oram syndrome (thumb

anomalies, atrial or ventricular

septum defects, phocomelia)

Cell cycle control

proteins

P53 Li-Fraumeni syndrome, other

cancers

Co-activators CREB binding

protein (CREBBP)

Rubinstein-Taybi syndrome

General transcription

factors

TATA-binding

protein (TBP)

Spinocerebellar ataxia 17 (CAG

expansion)

Transcription

elongation factor

VHL von Hippel–Lindau syndrome

(renal cell carcinoma,

pheochromocytoma, pancreatic

tumors, hemangioblastomas)

Autosomal dominant inheritance,

somatic inactivation of second allele

(Knudson two-hit model)

Runt RUNX1 Familial thrombocytopenia with

propensity to acute myelogenous

leukemia

Chimeric proteins due

to translocations

PML-RAR Acute promyelocytic leukemia

t(15;17)(q22;q11.2-q12) translocation

Abbreviations: CREB, cAMP responsive element–binding protein; HNF, hepatocyte

nuclear factor; PML, promyelocytic leukemia; RAR, retinoic acid receptor; SRY,

sex-determining region Y; VHL, von Hippel–Lindau.

3646 PART 16 Genes, the Environment, and Disease

Methylated DNA

Methylation

Histone Acetylation

Unmethylated DNA

Transcription

NH2

O

N

N

NH2

O

N

N

CH3

Histone Modifications

Cytosine Methylation

Acetylation

Phosphorylation

Methylation

NH2

FIGURE 466-7 Epigenetic modifications of DNA and histones. Methylation of cytosine residues is associated with

gene silencing. Methylation of certain genomic regions is inherited (imprinting), and it is involved in the silencing of

one of the two X chromosomes in females (X-inactivation). Alterations in methylation can also be acquired, e.g., in

cancer cells. Covalent posttranslational modifications of histones play an important role in altering DNA accessibility

and chromatin structure and hence in regulating transcription. Histones can be reversibly modified in their aminoterminal tails, which protrude from the nucleosome core particle, by acetylation of lysine, phosphorylation of serine,

methylation of lysine and arginine residues, and sumoylation. Acetylation of histones by histone acetylases (HATs),

e.g., leads to unwinding of chromatin and accessibility to transcription factors. Conversely, deacetylation by histone

deacetylases (HDACs) results in a compact chromatin structure and silencing of transcription.

nucleotide repeats, which can expand (see below), variations are usually stable.

Mutations are structurally diverse—they can involve the entire

genome, as in triploidy (one extra set of chromosomes), or gross

numerical or structural alterations in chromosomes or individual

genes. Large deletions may affect a portion of a gene or an entire gene,

or, if several genes are involved, they may lead to a contiguous gene syndrome. Unequal crossing-over between homologous genes can result

in fusion gene mutations, as illustrated by color blindness. Variations

involving single nucleotides are referred to as point mutations. Substitutions are called transitions if a purine is replaced by another purine base

(A ↔ G) or if a pyrimidine is replaced by another pyrimidine (C ↔ T).

Changes from a purine to a pyrimidine, or vice versa, are referred to as

transversions. If the DNA sequence change occurs in a coding region

and alters an amino acid, it is called a missense mutation. Depending

on the functional consequences of such a missense mutation, amino

acid substitutions in different regions of the protein can lead to distinct

phenotypes.

Variations can occur in all domains of a gene (Fig. 466-9). A point

mutation occurring within the coding region leads to an amino acid

substitution if the codon is altered (Fig. 466-10). Point mutations that

introduce a premature stop codon result in a truncated or missing

protein. Large deletions may affect a portion of a gene or an entire

gene, whereas small deletions and insertions alter the reading frame

if they do not represent a multiple of three bases. These “frameshift”

mutations, also designated as amphigoric amino acid changes, lead to

an entirely altered carboxy terminus. Mutations in intronic sequences

or in exon junctions may destroy or create splice donor or splice acceptor sites. Variations may also be found in the regulatory sequences of

genes, resulting in reduced or enhanced gene transcription.

Certain DNA sequences are particularly susceptible to mutagenesis.

Successive pyrimidine residues (e.g., T-T or C-C) are subject to the

formation of ultraviolet light–induced photoadducts. If these pyrimidine dimers are not repaired by the nucleotide excision repair pathway,

mutations will be introduced after DNA synthesis. The dinucleotide

C-G, or CpG, is also a hot spot for a specific type of mutation. In this

case, methylation of the cytosine is associated with an enhanced rate

of deamination to uracil, which is then replaced with thymine. This

C

■ PHARMACOLOGY AND NUTRITIONAL

IMPACT OF ETHANOL

Ethanol blood levels are expressed as milligrams or grams of ethanol

per deciliter (e.g., 100 mg/dL = 0.10 g/dL), with values of ~0.02 g/dL

resulting from the ingestion of one typical drink. In round figures,

a standard drink is 10–12 g of ethanol, as seen in 340 mL (12 oz) of

beer, 115 mL (4 oz) of nonfortified wine, and 43 mL (1.5 oz) (a shot)

of 80-proof (40% ethanol by volume) beverage (e.g., whisky); 0.5 L

(1 pint) of 80-proof beverage contains ~160 g of ethanol (~16 standard

drinks), and 750 mL of wine contains ~60 g of ethanol. These beverages

also have additional components (congeners) that affect the drink’s taste

and might contribute to adverse effects on the body. Congeners include

methanol, butanol, acetaldehyde, histamine, tannins, iron, and lead. As

a depressant drug, alcohol acutely decreases neuronal activity and has

similar behavioral effects and cross-tolerance with other depressants,

including benzodiazepines, barbiturates, and some anticonvulsants.

Alcohol is absorbed from mucous membranes of the mouth and

esophagus (in small amounts), from the stomach and large bowel

(in modest amounts), and from the proximal portion of the small

intestine (the major site). The rate of absorption is increased by rapid

gastric emptying (as seen with carbonation); by the absence of proteins, fats, or carbohydrates (which interfere with absorption); and

by dilution to a modest percentage of ethanol (maximum at ~20% by

volume).

Between 2% (at low blood alcohol concentrations) and 10% (at high

blood alcohol concentrations) of ethanol is excreted directly through

the lungs, urine, or sweat, but most is metabolized to acetaldehyde,

primarily in the liver. The most important pathway occurs in the cell

cytosol where alcohol dehydrogenase (ADH) produces acetaldehyde,

which is then rapidly destroyed by aldehyde dehydrogenase (ALDH)

in the cytosol and mitochondria (Fig. 453-1). A second pathway

occurs in the microsomes of the smooth endoplasmic reticulum (the

microsomal ethanol-oxidizing system [MEOS]) that is responsible for

≥10% of ethanol oxidation at high blood alcohol concentrations.

Although a standard drink contains ~300 kJ, or 70–100 kcal, these

are devoid of minerals, proteins, and vitamins. In addition, alcohol

interferes with absorption of vitamins in the small intestine and

decreases their storage in the liver with modest effects on folate (folacin

or folic acid), pyridoxine (B6

), thiamine (B1

), nicotinic acid (niacin, B3

),

and vitamin A.

Heavy drinking in a fasting, healthy individual can produce transient hypoglycemia within 6–36 h, secondary to the acute actions of

ethanol that decrease gluconeogenesis. This can result in temporary

abnormal glucose tolerance tests (with a resulting erroneous diagnosis of diabetes mellitus) until the heavy drinker has abstained for

Ethanol

MEOS

20% Acetaldehyde

Alcohol

dehydrogenase

Aldehyde

dehydrogenase

80% Acetaldehyde

Acetate

Citric acid

cycle

CO2 + Water

Acetyl CoA

Fatty acids

FIGURE 453-1 The metabolism of alcohol. CoA, coenzyme A; MEOS, microsomal

ethanol oxidizing system.

2–4 weeks. Alcohol ketoacidosis, probably reflecting a decrease in fatty

acid oxidation coupled with poor diet or persistent vomiting, can be

misdiagnosed as diabetic ketosis. With alcohol-related ketoacidosis,

patients show an increase in serum ketones along with a mild increase

in glucose but a large anion gap, a mild to moderate increase in serum

lactate, and a β-hydroxybutyrate/lactate ratio of between 2:1 and 9:1

(with normal being 1:1).

In the brain, alcohol affects almost all neurotransmitter systems,

with acute effects that are often the opposite of those seen following

desistance after a period of heavy drinking. The most prominent

acute actions relate to boosting γ-aminobutyric acid (GABA) activity,

especially at GABAA receptors. Enhancement of this complex chloride

channel system contributes to anticonvulsant, sleep-inducing, antianxiety, and muscle relaxation effects of all GABA-boosting drugs.

Acutely administered alcohol produces a release of GABA, and continued use increases density of GABAA receptors, whereas alcohol

withdrawal states are characterized by decreases in GABA-related

activity. Equally important is the ability of acute alcohol to inhibit

postsynaptic N-methyl-d-aspartate (NMDA) excitatory glutamate

receptors, whereas chronic drinking and desistance are associated with

an upregulation of these excitatory receptor subunits. The relationships

between greater GABA and diminished NMDA receptor activity during acute intoxication and diminished GABA with enhanced NMDA

actions during alcohol withdrawal explain much of intoxication and

withdrawal phenomena.

As with all pleasurable activities, alcohol acutely increases dopamine

levels in the ventral tegmentum and related brain regions, and this

effect plays an important role in continued alcohol use, craving, and

relapse. The changes in dopamine pathways are also linked to increases

in “stress hormones,” including cortisol and adrenocorticotropic hormone (ACTH), during intoxication and in the context of the stresses

of withdrawal. Such alterations are likely to contribute to both feelings

of reward during intoxication and depression during falling blood

alcohol concentrations. Also closely linked to alterations in dopamine

(especially in the nucleus accumbens) are alcohol-induced changes in

opioid receptors, with acute alcohol causing release of β-endorphins.

Additional neurochemical changes include increases in synaptic

levels of serotonin during acute intoxication and subsequent upregulation of serotonin receptors. Acute increases in nicotinic acetylcholine

systems contribute to the impact of alcohol in the ventral tegmental

region, which occurs in concert with enhanced dopamine activity. In

the same regions, alcohol impacts on cannabinol receptors, with resulting release of dopamine, GABA, and glutamate as well as subsequent

effects on brain reward circuits.

■ BEHAVIORAL EFFECTS, TOLERANCE,

 AND WITHDRAWAL

The acute effects of a drug depend on the dose, the rate of increase

in plasma, the concomitant presence of other drugs, and past experience with the agent. “Legal intoxication” with alcohol in most states

is based on a blood alcohol concentration of 0.08 g/dL, some states

are considering lowering acceptable levels to <0.05 g/dL, and levels

of 0.04 g/dL are cited for pilots in the United States and automobile

drivers in some other countries. However, behavioral, psychomotor,

and cognitive changes are seen at 0.02–0.04 g/dL (i.e., after one to

two drinks) (Table 453-1). Deep but disturbed sleep can be seen at

0.15 g/dL in individuals who have not developed tolerance, and death

can occur with levels between 0.30 and 0.40 g/dL. Beverage alcohol is

probably responsible for more overdose deaths than any other drug.

Repeated use of alcohol contributes to the need for a greater number

of standard drinks to produce effects originally observed with fewer

drinks (acquired tolerance), a phenomenon involving at least three

compensatory mechanisms. (1) After 1–2 weeks of daily drinking,

metabolic or pharmacokinetic tolerance can be seen, with up to 30%

increases in the rate of hepatic ethanol metabolism. This alteration

disappears almost as rapidly as it develops. (2) Cellular or pharmacodynamic tolerance develops through neurochemical changes that maintain relatively normal physiologic functioning despite the presence of

alcohol. Subsequent decreases in blood levels contribute to symptoms