3273 Inherited Disorders of Amino Acid Metabolism in Adults CHAPTER 420

Urea Cycle

Acetyl-CoA+Glutamate

N-acetyl-Glutamate

H2CO3+NH3+2ATP Carbamylphosphate + Ornithine

Citrulline

Citrulline

Ornithine

NAG Synthase

CPS-1

OTC

Carbamyl

Phosphate

Orotic Acid

Argininosuccinic

Acid

Arginine

Fumarate

Urea

ASA Synthase

ASA Lyase

ASL

Arginase

ARG

Cytosol

Mitochondrion

UTP CTP

Aspartate

Aspartate +

ORNT1

(HHH)

Citrin ORNT1

NAGS

ASS

CO2+H2O

CA5A

FIGURE 420-2 The urea cycle. This cycle, which is fully expressed only in the liver, forms urea starting from ammonia (NH3

) derived from the nitrogen group of all amino acids.

It requires many enzymes and mitochondrial transporters, any of which can be defective and may impair the function of the urea cycle. Ammonia escaping the urea cycle

in periportal hepatocytes is conjugated with glutamate by glutamine synthase in perivenous hepatocytes to generate glutamine. ARG, arginase; ASA, argininosuccinic acid;

ASL, argininosuccinate lyase; ASS, argininosuccinate synthase; CA5A, carbonic anhydrase 5a; citrin (SLC25A13), aspartate/glutamate exchanger; CP, carbamylphosphate;

CPS-1, carbamylphosphate synthase 1; CTP, cytidine triphosphate; HHH, hyperammonemia, hyperornithinemia, homocitrullinuria syndrome; NAG, N-acetylglutamate; NAGS,

N-acetylglutamate synthase; ORNT1 (SLC25A15), ornithine/citrulline mitochondrial transporter; OTC, ornithine transcarbamylase; UTP, uridine triphosphate.

variants can be seen in adults. The accumulation of ammonia and glutamine leads to direct neuronal toxicity and brain edema. Deficiencies

in urea cycle enzymes are individually rare, but as a group, they affect

~1:35,000 individuals. They are all transmitted as autosomal recessive

traits, with the exception of ornithine transcarbamylase deficiency,

which is X-linked and the most frequent urea cycle defect. Hepatocytes

of females with ornithine transcarbamylase deficiency express either

the normal or the mutant allele due to random X-inactivation and may

be unable to remove excess ammonia if mutant cells are predominant.

Infants with classic urea cycle defects present at 1–4 days of life

with refusal to eat and lethargy progressing to coma and death. Milder

enzyme deficiencies present with protein avoidance, recurrent vomiting, migraine, mood swings, chronic fatigue, irritability, and disorientation that can progress to coma. Some cases have presented with acute

or chronic hepatic dysfunction. Females with ornithine transcarbamylase deficiency can present at time of childbirth due to the combination

of involuntary fasting and stress that favors catabolism. Administration

of systemic corticosteroids or chemotherapy can precipitate hyperammonemia and can be fatal in previously asymptomatic individuals of

any age. These patients may be misdiagnosed as having gastrointestinal

disorders, food allergies, behavioral problems, or nonspecific hepatitis.

The diagnosis requires measurement of plasma ammonia, plasma

amino acids, and urine orotic acid, useful for differentiating ornithine

transcarbamylase deficiency from carbamyl phosphate synthase-1 and

N-acetylglutamate synthase deficiency. Increased plasma glutamine is

seen with all urea cycle defects since ammonia not removed by the urea

cycle in periportal hepatocytes is conjugated to glutamate by glutamine

synthase in perivenous hepatocytes. Citrulline is low or undetectable

in proximal defects of the urea cycle (N-acetylglutamate synthase,

carbamylphosphate synthase 1, and ornithine transcarbamylase deficiency), with urine orotic acid being increased only in ornithine

transcarbamylase deficiency. Plasma citrulline is markedly increased

in argininosuccinic acid synthase deficiency (citrullinemia type 1),

with a milder elevation in argininosuccinic acid lyase deficiency in

the presence of argininosuccinic acid (argininosuccinic aciduria).

Arginine levels are usually normal to low in these conditions and

become markedly elevated only in patients with arginase deficiency.

In addition to urea cycle defects, hyperammonemia can also be caused

by liver disease from any cause and several organic acidemias and fatty

acid oxidation defects (the latter two excluded by the analysis of urine

organic acids and plasma acylcarnitine profile).

TREATMENT

Urea Cycle Defects

Therapy is aimed at stopping catabolism and ammonia production by providing adequate calories (as IV glucose and lipids in

the comatose patient) and, if needed, insulin. Excess nitrogen

is removed by IV phenylacetate and benzoate (0.25 g/kg for the

priming dose and subsequently as an infusion over 24 h) that conjugate with glutamine and glycine, respectively, to form phenylacetylglutamine and hippuric acid, water-soluble molecules efficiently

excreted in urine. Arginine (200 mg/kg per d) becomes an essential

amino acid (except in arginase deficiency) and should be provided

intravenously to resume protein synthesis. If these measures fail

to reduce ammonia, hemodialysis should be initiated promptly.

Chronic therapy consists of a protein-restricted diet, phenylbutyrate, glycerol phenylbutyrate (a liquid drug better tolerated by

most patients), arginine, or citrulline supplements, depending on

the specific diagnosis. Oral carglumic acid can restore a functional

urea cycle in patients with N-acetylglutamate synthase deficiency

and renders other therapies unnecessary. Liver transplantation

should be considered in patients with severe urea cycle defects that

are difficult to control medically.

Hyperammonemia due to a functional deficiency of glutamine

synthase can occur in patients receiving chemotherapy for different

malignancies or undergoing solid organ transplants. It can also

be seen with hepatic cirrhosis. Several of these patients have been

successfully rescued from hyperammonemia using the protocol

described above for urea cycle defects.

■ FURTHER READING

Morris AA et al: Guidelines for the diagnosis and management of

cystathionine beta-synthase deficiency. J Inherit Metab Dis 40:49,

2017.

Ranganath LR et al: Efficacy and safety of once-daily nitisinone for

patients with alkaptonuria (SONIA 2): An international, multicentre,

3274 PART 12 Endocrinology and Metabolism

Specific membrane transporters mediate the passage of amino acids,

oligopeptides, sugars, cations, anions, vitamins, water, and many other

molecules across cellular membranes. They are encoded by members

of the solute-carrier gene (SLC) superfamily. These transporters are

located on the plasma membrane or intracellular organelles, and their

cellular and tissue distribution in addition to the presence (or absence)

of redundant transporters explains organ involvement and possible metabolic disturbances. The first transport disorders identified

affected the gut or the kidney, but transport processes are essential for

the normal function of every organ, but especially the brain and sensory organs (Table 421-1). Inherited defects impairing the transport

of selected amino acids that can present in adults are discussed here

as examples of the abnormalities encountered; others are considered

elsewhere in this text.

■ CYSTINURIA

Cystinuria (worldwide frequency of 1 in 7000) is an autosomal recessive

disorder caused by defective transporters in the apical brush border of

proximal renal tubule and small intestinal cells. It is characterized by

impaired reabsorption and excessive urinary excretion of the dibasic

amino acids lysine, arginine, ornithine, and cystine. Because cystine

is poorly soluble, its excess excretion predisposes to the formation of

renal, ureteral, and bladder stones. Such stones are responsible for the

signs and symptoms of the disorder.

There are two variants of cystinuria. Homozygotes for both variants

have high urinary excretion of cystine, lysine, arginine, and ornithine.

Type A heterozygotes usually have normal urinary amino acid excretion, whereas most type B heterozygotes have moderately increased

urinary excretion of cystine that under some circumstances can result

in the formation of kidney stones. The gene for type A cystinuria

(SLC3A1, chromosome 2p16.3) encodes a membrane glycoprotein.

Type B cystinuria is caused by mutations in SLC7A9 (chromosome

19q13) that encodes the b0,+ amino acid transporter. The glycoprotein

encoded by SLC3A1 favors the correct processing of the b0,+ membrane

transporter and explains why mutations in two different genes cause a

similar disease.

Cystine stones account for 1–2% of all urinary tract calculi and for

~4–5% of stones in children. Cystinuria homozygotes regularly excrete

2400–7200 μmol (600–1800 mg) of cystine daily. Since the maximum

solubility of cystine in the physiologic urinary pH range of 4.5–7.0 is

~1200 μmol/L (300 mg/L), cystine needs to be diluted to 2.5–7 L of

water to prevent crystalluria. Stone formation usually manifests in the

second or third decade but may occur in the first year of life. Symptoms

and signs are those typical of urolithiasis: hematuria, flank pain, renal

colic, obstructive uropathy, and infection (Chap. 318). Recurrent urolithiasis may lead to progressive renal insufficiency.

Cystinuria is suspected after observing typical hexagonal crystals in

the sediment of acidified, concentrated, chilled urine or after performing a urinary nitroprusside test. Quantitative urine amino acid analysis

421 Inherited Defects of

Membrane Transport

Nicola Longo

open-label, randomised controlled trial. Lancet Diabetes Endocrinol

8:762, 2020.

Smith AD, Refsum H: Homocysteine, B vitamins, and cognitive

impairment. Annu Rev Nutr 36:211, 2016.

Zori R et al: Long-term comparative effectiveness of pegvaliase versus

standard of care comparators in adults with phenylketonuria. Mol

Genet Metab 128:92, 2019.

confirms the diagnosis of cystinuria by showing selective overexcretion

of cystine, lysine, arginine, and ornithine. Quantitative measurements

are important for differentiating heterozygotes from homozygotes and

for following free cystine excretion during therapy.

Management is aimed at preventing cystine crystal formation by

increasing urinary volume and by maintaining an alkaline urine pH.

Fluid ingestion in excess of 4 L/d is essential, and 5–7 L/d is optimal.

Urinary cystine concentration should be <1000 μmol/L (250 mg/L).

The daily fluid ingestion necessary to maintain this dilution of excreted

cystine should be spaced over 24 h, with one-third of the total volume

ingested between bedtime and 3 a.m. Cystine solubility rises sharply

above pH 7.5, and urinary alkalinization (with potassium citrate) can

be therapeutic. Penicillamine (1–3 g/d) and tiopronin (α-mercaptopropionylglycine, 800–1200 mg/d in four divided doses) undergo

sulfhydryl-disulfide exchange with cystine to form mixed disulfides.

Because these disulfides are much more soluble than cystine, pharmacologic therapy can prevent and promote dissolution of calculi.

Penicillamine can have significant side effects and should be reserved

for patients who fail to respond to hydration alone or who are in a highrisk category (e.g., one remaining kidney, renal insufficiency). When

medical management fails, shock wave lithotripsy, ureteroscopy, and

percutaneous nephrolithotomy are effective for most stones. Open urologic surgery is considered only for complex staghorn stones or when

the patient has concomitant renal or ureteral abnormalities. Occasional

patients progress to renal failure and require kidney transplantation.

■ LYSINURIC PROTEIN INTOLERANCE

This disorder is characterized by a defect in renal tubular reabsorption and intestinal transport of the three dibasic amino acids lysine,

arginine, and ornithine but not cystine (lysinuric protein intolerance).

Lysinuric protein intolerance is most common in Finland (1 in 60,000),

southern Italy, and Japan, but is rare elsewhere. The transport defect

affects basolateral rather than luminal membrane transport and causes

secondary impairment of the urea cycle. The defective gene (SLC7A7,

chromosome 14q11.2) encodes the y+LAT membrane transporter,

which associates with the cell-surface glycoprotein 4F2 heavy chain to

form the complete sodium-independent transporter y+L.

Manifestations are related to impairment of the urea cycle and to

immune dysfunction potentially attributable to nitric oxide overproduction secondary to arginine intracellular trapping within

macrophages. Affected patients present in childhood with hepatosplenomegaly, protein intolerance, and episodic ammonia intoxication.

Older patients may present with severe osteoporosis, impaired renal

function, pulmonary alveolar proteinosis, various autoimmune disorders, and an incompletely characterized immune deficiency. Plasma

concentrations of lysine, arginine, and ornithine are reduced, whereas

urinary excretion of lysine and orotic acid is increased. Hyperammonemia may develop after the ingestion of protein loads or with infections,

probably because of insufficient amounts of arginine and ornithine

to maintain proper function of the urea cycle. Therapy consists of

dietary protein restriction and supplementation of citrulline (2–8 g/d),

a neutral amino acid that fuels the urea cycle when metabolized to

arginine and ornithine. Pulmonary disease responds to glucocorticoids

or recombinant human granulocyte-macrophage colony-stimulating

factor in some patients. Women with lysinuric protein intolerance

who become pregnant have an increased risk of anemia, toxemia, and

bleeding complications during delivery. These can be minimized by

aggressive nutritional therapy and control of blood pressure. Their

infants can have intrauterine growth restriction but have normal neurologic function.

■ CITRULLINEMIA TYPE 2 (CITRIN DEFICIENCY)

Citrullinemia type 2 is a recessive condition caused by deficiency of

the mitochondrial aspartate-glutamate carrier AGC2 (citrin). A defect

in this transporter reduces the availability of cytoplasmic aspartate to

combine with citrulline to form argininosuccinate (see Fig. 420-1),

impairing the urea cycle and decreasing the transfer of reducing equivalents from the cytosol to the mitochondria through the malate-aspartate

NADH shuttle. Mutations in the SLC25A13 gene on chromosome

3275 Inherited Defects of Membrane Transport CHAPTER 421

7q21.3 that encodes for this transporter are rare in Caucasians but

affect ~1:20,000 people with ancestry from Japan, China, and Southeast

Asia with variable penetrance.

The disease can present in children with neonatal intrahepatic

cholestasis, failure to thrive, and dyslipidemia but usually presents

with sudden onset between 20 and 50 years of age with recurring

episodes of hyperammonemia with associated neuropsychiatric symptoms such as altered mental status, irritability, seizures, or comaresembling hepatic encephalopathy. Some patients might come to

medical attention for hypertriglyceridemia, pancreatitis, hepatoma,

or fatty liver histologically similar to nonalcoholic steatohepatitis.

Without therapy, most patients die with cerebral edema within a few

years of onset. Episodes are usually triggered by medications (such as

acetaminophen), surgery, alcohol consumption, or high sugar intake,

with the latter conditions causing NADH production in the cytoplasm.

NADH is not generated by the metabolism of proteins or fats, and

many individuals with citrullinemia type 2 spontaneously prefer foods

such as meat, eggs, and fish and avoid carbohydrates.

TABLE 421-1 Genetic Disorders of Amino Acid Transport

DISORDER SUBSTRATES

TISSUES MANIFESTING

TRANSPORT DEFECT MOLECULAR DEFECT

MAJOR CLINICAL

MANIFESTATIONS INHERITANCE

Cystinuria Cystine, lysine, arginine,

ornithine

Proximal renal tubule,

jejunal mucosa

Shared dibasic-cystine

transporter SLC3A1, SLC7A9

Cystine nephrolithiasis AR

Lysinuric protein

intolerance

Lysine, arginine, ornithine Proximal renal tubule,

jejunal mucosa

Dibasic transporter SLC7A7 Protein intolerance,

hyperammonemia, intellectual

disability

AR

Hartnup disease Neutral amino acids Proximal renal tubule,

jejunal mucosa

Neutral amino acid transporter

SLC6A19

Constant neutral aminoaciduria,

intermittent symptoms of pellagra

AR

Histidinuria Histidine Proximal renal tubule,

jejunal mucosa

Histidine transporter Intellectual disability AR

Iminoglycinuria Glycine, proline,

hydroxyproline

Proximal renal tubule,

jejunal mucosa

Shared glycine–imino acid

transporter SLC6A20, SLC6A18,

SLC36A2

None AR

Dicarboxylic

aminoaciduria

Glutamic acid, aspartic

acid

Proximal renal tubule,

jejunal mucosa

Shared dicarboxylic amino

acid transporter SLC1A1

None AR

Hyperargininemia Arginine, lysine, ornithine Ubiquitous CAT2 cationic amino acid

transporter SLC7A2

Hyperargininemia,

Hyperammonemia (?)

AR

Brain branched chain

amino acid deficiency

Leucine, isoleucine, valine Plasma membrane of

blood-brain barrier

Branched chain amino acid

transporter SLC7A5

Microcephaly, intellectual

disability, seizures, autism

AR

Citrullinemia type 2 Aspartate, glutamate,

malate

Inner mitochondrial

membrane

Mitochondrial aspartate/

glutamate carrier 2 SLC25A13

Sudden behavioral changes with

stupor, coma, hyperammonemia

AR

Hyperornithinemia,

hyperammonemia,

homocitrullinuria

Ornithine, citrulline Inner mitochondrial

membrane

Mitochondrial ornithine carrier

SLC25A15

Lethargy, failure to thrive,

intellectual disability, episodic

confusion, hyperammonemia,

protein intolerance

AR

Epileptic

encephalopathy

Aspartate, glutamate,

malate

Inner mitochondrial

membrane

Mitochondrial aspartate/

glutamate carrier 1 SLC25A12

Intellectual disability, epilepsy,

hypotonia, cerebral atrophy, and

hypomyelination

AR

Epileptic

encephalopathy

Glutamate Inner mitochondrial

membrane

Mitochondrial glutamate

carrier SLC25A22

Intellectual disability, epilepsy AR

Epileptic

encephalopathy

Glutamic acid, aspartic

acid

Presynaptic glutamatergic

nerve endings

EEAT2 neuronal dicarboxylic

amino acid transporter SLC1A2

Developmental and epileptic

encephalopathy

AD

Episodic ataxia Glutamic acid, aspartic

acid

Presynaptic glutamatergic

nerve endings

EEAT1 neuronal dicarboxylic

amino acid transporter SLC1A3

Episodic ataxia AD

Brain serine deficiency Alanine, serine, cysteine,

threonine

Neuronal cells ASCT neutral amino acid

transporter SLC1A4

Progressive microcephaly,

intellectual disability, spasticity

AR

Glycine

encephalopathy with

normal serum glycine

Glycine Astrocytes and neuronal

cells

GLYT1 astrocyte glycine

transporter SLC6A9

Arthrogryposis, apnea, axial

hypotonia, spasticity, intellectual

disability

AR

Hyperekplexia-3 Glycine Neuronal cells GLYT2 presynaptic glycine

transporter SLC6A5

Exaggerated startle response,

hypertonia, apnea

AR

Intellectual disability Proline, glycine, leucine,

and alanine, glutamine

Neuronal cells synaptic

vesicles

NTT4 synaptic vesicle neutral

amino acid transporter

SLC6A17

Intellectual disability, tremor AR

Deafness Glutamic acid Neuronal cortical

synaptic vesicles

VGLUT3 vesicular glutamate

transporter SLC17A8

Deafness AD

Foveal hypoplasia Glutamine Retinal photoreceptors SLC38A8 Foveal hypoplasia, optic nerve

decussation defects, anterior

segment dysgenesis

AR

Retinitis pigmentosa Arginine, lysine, ornithine Retinal photoreceptors Cationic amino acid

transporter SLC7A14

Retinitis pigmentosa, blindness AR

Early retinal

degeneration

Taurine Retinal cells TAUT taurine transporter

SLC6A6

Nystagmus, vision loss, retinal

degeneration

AR

Cystinosis Cystine Lysosomal membranes Lysosomal cystine transporter Renal failure, hypothyroidism,

blindness

AR

Abbreviations: AD, autosomal dominant; AR, autosomal recessive.

3276 PART 12 Endocrinology and Metabolism

Laboratory studies during an acute attack include elevated ammonia, citrulline, and arginine with low or normal levels of glutamine

(the latter is usually increased in classic urea cycle defects). Levels of

galactose-1-phosphate in red blood cells are also increased, reflecting

defective transfer of reducing equivalents from the cytosol to mitochondria. The diagnosis is confirmed by demonstrating mutations in

the SLC25A13 gene. Liver transplantation prevents progression of the

disease and normalizes biochemical parameters. A diet high in fats and

proteins and low in carbohydrates with supplements of medium-chain

triglycerides, arginine, and pyruvate is also effective in preventing further episodes, at least in the short term.

■ HARTNUP DISEASE

Hartnup disease (frequency 1 in 24,000) is an autosomal recessive disorder characterized by pellagra-like skin lesions, variable neurologic

manifestations, and neutral and aromatic aminoaciduria. Alanine,

serine, threonine, valine, leucine, isoleucine, phenylalanine, tyrosine,

tryptophan, glutamine, asparagine, and histidine are excreted in urine

in quantities 5–10 times greater than normal, and intestinal transport

of these same amino acids is defective. The defective neutral amino

acid transporter, B°AT1 encoded by the SLC6A19 gene on chromosome

5p15, requires either collectrin or angiotensin-converting enzyme 2 for

surface expression in the kidney and intestine, respectively.

The clinical manifestations result from nutritional deficiency of

the essential amino acid tryptophan, caused by its intestinal and renal

malabsorption, and of niacin, which derives in part from tryptophan

metabolism. Only a small fraction of patients with the chemical findings of this disorder develop a pellagra-like syndrome, implying that

manifestations depend on other factors in addition to the transport

defect. The diagnosis of Hartnup disease should be suspected in any

patient with clinical features of pellagra who does not have a history of

dietary niacin deficiency (Chap. 333). The neurologic and psychiatric

manifestations range from attacks of cerebellar ataxia to mild emotional lability to frank delirium, and they are usually accompanied by

exacerbations of the erythematous, eczematoid skin rash. Fever, sunlight, stress, and sulfonamide therapy provoke clinical relapses. Diagnosis is made by detection of the neutral aminoaciduria, which does

not occur in dietary niacin deficiency. Treatment is directed at niacin

repletion and includes a high-protein diet and daily nicotinamide supplementation (50–250 mg).

■ CYSTINOSIS

Cystinosis (frequency 1 in 100,000–200,000) is an autosomal recessive

disorder caused by mutations in the CTNS gene encoding the lysosomal cystine/proton transporter (cystinosin). In this condition, cystine

derived from protein degradation accumulates inside lysosomes and

forms crystals due to its poor solubility. Depending on the degree of

impairment of transporter function, three clinical forms are recognized.

The most severe form, classic nephropathic cystinosis, causes renal

Fanconi syndrome during the first year of life and, without treatment,

evolves to renal failure usually by 10 years of age. Intermediate nephropathic cystinosis leads to kidney failure between 15 and 25 years of age,

whereas photophobia, caused by deposition of cystine crystals in the

cornea, is the only manifestation of ocular nonnephropathic cystinosis.

Cystinosis is suspected by the identification of cystine crystals in the cornea by slit lamp examination and diagnosed by measuring cystine content in white blood cells. DNA testing (including deletion analysis) of the

CTNS gene can further confirm the diagnosis. Therapy consists in the

administration of cysteamine that enters lysosomes, forms a mixed disulfide with cysteine, and is exported from the lysosome using a cationic

amino acid transporter. Oral cysteamine therapy (60–90 mg/kg per d

up to 2 g/d in adults, 0.2–0.3 g/m2

 per d divided into two doses given

every 12 h for the extended-release formulation) can delay renal failure

and is more effective if started early in the course of the disease. Therapy

with cysteamine reduces intracellular cystine accumulation in white

blood cells, but compliance with therapy is difficult due to the unpleasant

odor of the drug and the need for frequent administration. Cysteamine

eye drops can relieve photophobia. Renal replacement therapy with

salts, alkali, and activated vitamin D is necessary for renal Fanconi syndrome. Cystine accumulation occurs in virtually all organs and tissues,

causing additional complications such as hypothyroidism, hypohydrosis,

diabetes, and delayed puberty in both males and females with primary

hypogonadism in males. Growth hormone replacement, l-thyroxine

for hypothyroidism, insulin for diabetes mellitus, and testosterone

for hypogonadism in males may be necessary. Despite therapy, many

patients with cystinosis progress to end-stage renal failure and require

kidney transplantation. Late-onset complications include hepatomegaly

and splenomegaly that occur in approximately one-third of subjects and

a vacuolar myopathy causing weakness (initially involving the distal

extremities), swallowing difficulties, gastrointestinal dysmotility, and

pulmonary insufficiency. Before the availability of cystine-depleting

therapy and renal transplantation, the life span in nephropathic cystinosis was <10 years. With current therapies, affected individuals can

survive into the late forties with satisfactory quality of life.

■ FURTHER READING

Servais A et al: Cystinuria: Clinical practice recommendation. Kidney

Int 99:48, 2021.

Tanner LM et al: Inhaled sargramostim induces resolution of pulmonary alveolar proteinosis in lysinuric protein intolerance. JIMD Rep

34:97, 2017.

Taˇ rlungeanu DC et al: Impaired amino acid transport at the blood

brain barrier is a cause of autism spectrum disorder. Cell 167:1481,

2016.

Yahyaoui R, Pérez-Frías J: Amino acid transport defects in human

inherited metabolic disorders. Int J Mol Sci 21:119, 2019.