Inherited Defects of Membrane Transport¶
Chapter 432 | Part 12: Endocrinology and Metabolism
KEY CLINICAL POINTS¶
- Cystinuria is an autosomal recessive disorder caused by defective apical transporters (SLC3A1 and SLC7A9) leading to cystine stone formation (1-2% of urinary tract calculi).
- Lysinuric protein intolerance (AR) involves SLC7A7 mutations causing urea cycle impairment, hyperammonemia, and immune dysfunction with severe complications including renal failure.
- Cystinosis (AR) is caused by CTNS gene mutations leading to lysosomal cystine accumulation, requiring cysteamine therapy and renal transplantation for end-stage disease.
- Hartnup disease (AR) presents with neutral aminoaciduria, pellagra-like skin lesions, and neurologic symptoms due to defective SLC6A19 transporter function.
- Citrullinemia type 2 (AR) involves SLC25A13 mutations causing mitochondrial dysfunction, hyperammonemia, and neurological crises requiring liver transplantation.
1. DEFINITION & OVERVIEW¶
Membrane transporters mediate passage of amino acids, sugars, ions, and water across cellular membranes via the solute-carrier (SLC) superfamily. Inherited defects disrupt transport processes critical for organ function, particularly in the brain and sensory organs. Disorders include cystinuria, lysinuric protein intolerance, citrullinemia, Hartnup disease, and cystinosis.
Table 432-1 Genetic Disorders of Amino Acid Transport¶
| DISORDER | SUBSTRATES | TISSUES MANIFESTING TRANSPORT DEFECT | MOLECULAR DEFECT | MAJOR CLINICAL MANI FESTATIONS | 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, hyp erammonemia, intellectual disability | AR |
| DISORDER | SUBSTRATES | TISSUES MANIFESTING TRANSPORT DEFECT | MOLECULAR DEFECT | MAJOR CLINICAL MANI FESTATIONS | INHERITANCE |
|---|---|---|---|---|---|
| Hartnup disease | Neutral amino acids | Proximal renal tubule, jejunal mucosa | Neutral amino acid transporter SLC6A19 | Constant neutral aminoaciduria, intermittent pellagra-like symptoms | AR |
| Iminoglycinuria | Glycine, proline, hydroxyproline | Prox, jejunal mucosa | Shared glycine–amino acid transporter SLC6A20, SLC6A18, SLC36A2 | None | AR |
| Dicarboxylic aminoaciduria | Glutamic acid, aspartic acid | Prox, jejunal mucosa | Shared dicarboxylic amino acid transporter SLC1A1 | None | AR |
| Hyperargininemi a | Arginine, lysine, ornithine | Ubiquitous | CAT2 cationic amino acid transporter SLC7A2 | Hyperargininemi a, hyperammone mia (?) | AR |
| Citrullinemia type 2 | Aspartate, glutamate, malate | Inner mitochondrial membrane | Mitochondrial as partate/glutamat e carrier 2 (SLC25A13) | Hyperammonem ia, neurological crises | AR |
| Cystinosis | Cystine | Lysosomal membranes | Lysosomal cystine transporter | Renal failure, hypothyroidism, blindness | AR |
1.1 Genetic Basis¶
Transporters are encoded by SLC genes (e.g., SLC3A1, SLC7A7, SLC25A13). Defects in redundant transporters or molecular pathways cause organ-specific involvement. Over 20 amino acid transport disorders are described, with 10% of all inherited metabolic disorders linked to these defects.
1.2 Clinical Spectrum¶
Disorders range from asymptomatic aminoaciduria to life-threatening complications like renal failure, hyperammonemia, and neurological crises. Early diagnosis and targeted therapies (e.g., cysteamine, dietary modifications) are critical for preventing morbidity.
2. EPIDEMIOLOGY¶
Cystinuria: Global prevalence 1:7000; 1:60,000 in Finland. Lysinuric protein intolerance: 1:60,000 in Finland, 1:20,000 in Japan. Hartnup disease: 1:24,000. Cystinosis: 1:100,000–200,000. Most disorders are rare, with regional variations (e.g., Finland, Japan).
2.1 Risk Factors¶
Consanguinity increases risk for recessive disorders. Dietary protein intake and hydration status modulate clinical expression in cystinuria and lysinuric protein intolerance.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Defective transporters disrupt amino acid reabsorption and metabolism. Cystinuria involves SLC3A1/SLC7A9 defects causing cystine stone formation. Lysinuric protein intolerance impairs urea cycle via SLC7A7 mutations. Citrullinemia type 2 disrupts mitochondrial aspartate/glutamate transport (SLC25A13).
3.1 Molecular Mechanisms¶
SLC3A1 encodes a glycoprotein essential for b0,+ transporter function. SLC7A7 forms the y+LAT transporter with 4F2 heavy chain. SLC25A13 defect impairs malate-aspartate shuttle, reducing urea cycle efficiency.
4. CLINICAL FEATURES¶
Cystinuria: Renal/ureteral stones, hematuria, flank pain. Lysinuric protein intolerance: Hyperammonemia, protein intolerance, intellectual disability. Citrullinemia: Neonatal cholestasis, hyperammonemic crises. Hartnup: Pellagra-like rash, neurologic symptoms. Cystinosis: Fanconi syndrome, renal failure, photophobia.
4.1 Complications¶
Renal failure, osteoporosis, pancreatitis, hepatic dysfunction, and neurological sequelae. Cystinosis may progress to end-stage renal disease requiring transplantation.
5. DIFFERENTIAL DIAGNOSIS¶
Distinguish from urea cycle disorders, renal tubular acidosis, and other aminoacidurias. Cystinuria vs. cystinosis: Stone formation vs. lysosomal accumulation. Hartnup vs. pellagra: Neutral aminoaciduria vs. niacin deficiency.
6. INVESTIGATIONS & DIAGNOSIS¶
Urinalysis: Hexagonal cystine crystals, aminoaciduria. Quantitative amino acid analysis. Genetic testing (SLC3A1, SLC7A7, SLC25A13). Urinary cystine concentration <1000 µ mol/L. Liver biopsy for cystinosis. Citrullinemia: Elevated citrulline, low glutamine.
6.1 Diagnostic Criteria¶
Cystinuria: Urinary cystine >1000 µ mol/L with hexagonal crystals. Lysinuric protein intolerance: Reduced plasma lysine/arginine/or-nithine with increased urinary excretion. Citrullinemia: Hyperammonemia with elevated citrulline.
7. MANAGEMENT & TREATMENT¶
Cystinuria: Alkalinize urine (potassium citrate), penicillamine (1-3g/d), hydration >4L/d. Lysinuric protein intolerance: Dietary restriction, citrulline supplementation (2-8g/d), nitrogen scavengers. Citrullinemia: Liver transplantation, ketogenic diet. Cystinosis: Cysteamine (1.3g/m²), renal replacement therapy.
7.1 Therapeutic Algorithms¶
- Hydration and alkalinization for cystinuria. 2. Penicillamine for refractory stones. 3. Dietary restriction and citrulline for lysinuric protein intolerance. 4. Liver transplantation for citrullinemia. 5. Cysteamine and renal replacement for cystinosis.
8. PROGNOSIS & COMPLICATIONS¶
Cystinuria: Stones may resolve with therapy. Lysinuric protein intolerance: Progressive renal failure. Cystinosis: 50% end-stage renal disease by age 10. Citrullinemia: Mortality without liver transplantation. Early intervention improves outcomes.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Cystinuria patients at risk for delivery complications. Cystinosis: Growth hormone, thyroid replacement, testosterone for hypogonadism. Neonatal screening for citrullinemia. Avoid high-protein diets in Hartnup disease.
9.1 Pediatric Considerations¶
Cystinosis: Early cysteamine therapy prevents renal failure. Hartnup disease: Nutritional supplementation for tryptophan and niacin. Neonatal cholestasis in citrullinemia requires urgent liver transplantation.
10. KEY POINTS & CLINICAL PEARLS¶
- Cystinuria: Urinary cystine <1000 µ mol/L with alkalinization. 2. Lysinuric protein intolerance: SLC7A7 defect with urea cycle impairment. 3. Citrullinemia: SLC25A13 mutation causing mitochondrial dysfunction. 4. Cystinosis: Cysteamine is the mainstay therapy. 5. Hartnup disease: Neutral aminoaciduria with pellagra-like symptoms.