Lysinuric Protein Intolerance

CLINICAL CHARACTERISTICS

Lysinuric protein intolerance (LPI) typically presents after an infant is weaned from breast milk or formula; variable findings include recurrent vomiting and episodes of diarrhea, episodes of stupor and coma after a protein-rich meal, poor feeding, aversion to protein-rich food, failure to thrive, hepatosplenomegaly, and muscular hypotonia. Over time, findings include: poor growth, osteoporosis, involvement of the lungs (progressive interstitial changes, pulmonary alveolar proteinosis) and of the kidneys (progressive glomerular and proximal tubular disease), hematologic abnormalities (normochromic or hypochromic anemia, leukopenia, thrombocytopenia, erythroblastophagocytosis in the bone marrow aspirate), and a clinical presentation resembling the hemophagocytic lymphohistiocytosis/macrophagic activation syndrome. Hypercholesterolemia, hypertriglyceridemia, and acute pancreatitis can also be seen.

DIAGNOSIS/TESTING: The diagnosis is established in an individual with clinical and laboratory features suggestive of LPI including elevated 24-hour urinary excretion of cationic amino acids, especially lysine. Identification of biallelic pathogenic variants confirms the diagnosis.

MANAGEMENT

In acute hyperammonemic crises: intravenous administration of arginine chloride and nitrogen-scavenger drugs (sodium benzoate, sodium phenylacetate) to block ammonia production; reduction of excess nitrogen in the diet; provision of energy as carbohydrates to reduce catabolism. Long-term: dietary protein restriction; oral supplementation with citrulline and nitrogen-scavenger drugs, L-lysine-HCl, and carnitine; whole-lung lavage to improve respiratory function in persons with pulmonary alveolar proteinosis. Long-term protein restriction and administration of citrulline and nitrogen-scavenging drugs. Minimize the risk of respiratory infections; vaccination against influenza is recommended. Varicella immunization in those without previous history of chickenpox or varicella zoster; treatment of those exposed as immune-compromised persons; revaccination may be required if poor response to polysaccharide-containing vaccines. Plasma concentration of amino acids to identify deficiencies of essential amino acids secondary to protein-restricted diet; fasting and postprandial blood ammonia concentrations and attention to signs of hyperammonemia, urinary orotic acid excretion; periodic evaluation of renal function; evaluation of lung involvement; periodic serum LDH and ferritin. Large boluses of protein or amino acids. It is appropriate to evaluate at-risk sibs of a proband by molecular genetic testing or biochemical testing in order to reduce morbidity and mortality through early diagnosis and treatment.

GENETIC COUNSELING

LPI is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk family members and prenatal diagnosis for a pregnancy at increased risk are possible using molecular genetic techniques if both pathogenic variants have been identified in an affected family member.