Lysinuric Protein Intolerance

A number sign (#) is used with this entry because lysinuric protein intolerance (LPI) is caused by homozygous or compound heterozygous mutation in the amino acid transporter gene SLC7A7 (603593) on chromosome 14q11.

Description

Lysinuric protein intolerance is caused by defective cationic amino acid (CAA) transport at the basolateral membrane of epithelial cells in kidney and intestine. Metabolic derangement is characterized by increased renal excretion of CAA, reduced CAA absorption from intestine, and orotic aciduria (Borsani et al., 1999).

See also dibasic amino aciduria I (222690).

Clinical Features

Perheentupa and Visakorpi (1965) first described 3 Finnish infants with an inborn error of metabolism characterized by protein intolerance and deficient transport of basic amino acids. Blood urea was low and urinary lysine and arginine were increased.

Kekomaki et al. (1967) described 10 children, including several pairs of sibs, with vomiting, diarrhea, failure to thrive, hepatomegaly, diffuse cirrhosis, low blood urea, hyperammonemia, and leukopenia. Symptoms were aggravated by high protein intake and relieved by protein restriction. An excess of ornithine, arginine, and lysine, but not of cystine, was excreted in the urine. Intestinal absorption of arginine and lysine was normal. A low concentration of arginine relative to lysine in body fluids was thought responsible for the hyperammonemia and reduced urea synthesis. One of the families was consanguineous.

Kekomaki et al. (1968) reported a 23-year-old man with protein intolerance who refused to eat protein-rich food. Institution of cow's milk at age 1 year resulted in prolonged watery diarrhea and retardation of physical development. He grew physically with increased protein intake in his teens, but mental function deteriorated and he had episodic attacks of stupor and asterixis. The liver was enlarged and fatty. His 15-year-old affected sister also had protein intolerance.

Oyanagi et al. (1970) described severe mental retardation, physical retardation, mild intestinal malabsorption syndrome, and increased urinary excretion of lysine, ornithine, and arginine in 2 Japanese sisters with second-cousin parents. Cystine excretion was always within normal limits.

Malmquist et al. (1971) stated that 13 cases of familial protein intolerance had been observed in Finland. They described a Swedish patient of Finnish origin with intellectual impairment, radiographic evidence of brain atrophy, and marked skeletal fragility. Administration of alanine resulted in elevation of blood ammonia and glucose. Urea cycle function appeared to be normal and the defect was thought to concern the mechanisms by which amino nitrogen is transferred to the urea-synthesizing system.

During citrulline infusion, Rajantie et al. (1981) found that LPI patients had increased plasma citrulline levels similar to controls, but excessive excretion compared to controls. Patients had subnormal increases in plasma arginine and ornithine with massive argininuria and moderate ornithinuria. The excretion rates of the third diamino acid lysine and other amino acids remained practically unaltered, thus excluding mutual competition as the cause for the increases. The results suggested that reabsorption in the normal kidney involves partial conversion of citrulline to arginine and ornithine, and that the diamino acid transport defect in LPI is located at the basolateral cell membrane of the renal tubules. This inhibits the efflux of arginine and ornithine, increasing their cellular concentration, which in turn inhibits the metabolic disposal of citrulline, and causes leakage of arginine, ornithine, and citrulline into the tubular lumen.

Carpenter et al. (1985) emphasized that childhood osteopenia and osteoporosis were nearly constant complications of lysinuric protein intolerance. Laboratory studies suggested defective transport of ornithine and arginine across the plasma membrane of liver cells and across the basolateral membrane of renotubular cells. The defect in transport of dibasic amino acids results in lack of sufficient ornithine to support activity of hepatic ornithine transcarbamylase (OTC; 300461). Episodic hyperammonemia occurs, similar to that observed in OTC deficiency (311250).

Shaw et al. (1989) described a 36-year-old man and his 32-year-old brother who presented in adult life with hyperammonemic coma due to lysinuric protein intolerance. They were of normal intellect and had maintained good health, until presentation in their thirties, by unconscious dietary protein avoidance.

Parto et al. (1994) described the clinical courses and autopsy findings of 4 pediatric LPI patients. All had developed acute respiratory insufficiency. In addition to pulmonary hemorrhages, 3 of them had pulmonary alveolar proteinosis and 1 had cholesterol granulomas. Three patients had clinically obvious renal insufficiency, but all 4 showed histologic signs of immune complex-mediated glomerulonephritis. The patients also developed hepatic insufficiency with fatty degeneration or cirrhosis. All patients showed anemia, thrombocytopenia, and a severe bleeding tendency. Bone marrow of 3 patients was hypercellular, but the number of megakaryocytes was decreased in 2 cases. Amyloid was present in the lymph nodes and spleen. Bone specimens showed osteoporosis. Parto et al. (1994) concluded that in addition to being at risk of protein malnutrition in the active growth phase, probably due to higher requirements for total nitrogen and amino acids, pediatric patients with lysinuric protein intolerance are predisposed to develop pulmonary alveolar proteinosis and glomerulonephritis.

McManus et al. (1996) reported a 21-year-old woman who had presented at 8 months of age with persistent vomiting and failure to thrive. At that time there was a marked increase in urinary lysine excretion and ornithine and arginine to a lesser extent. The urinary orotic acid concentration was also raised and casein protein loading tests increased the concentrations of all plasma amino acids except lysine, ornithine, and arginine. A protein-restricted diet was recommended and supplements of lysine, arginine, and citrulline were prescribed. During the teenage years, compliance with the diet and amino acid supplements was poor, and she developed osteoporosis. She showed gradual deterioration with episodic disturbances of liver function and hyperammonemia 2 years before her death. Immediately before death she became comatose, had persistently raised serum ammonia concentrations, metabolic acidosis, and a coagulopathy. She died despite intensive therapy, including intravenous arginine for the hyperammonemia. Postmortem examination revealed hepatic micronodular cirrhosis with extensive fatty changes. The lungs showed pulmonary alveolar proteinosis. Immunofluorescence and electron microscopy revealed glomerulonephritis with predominant IgA deposition. McManus et al. (1996) suggested that the glomerulopathy may have been related to the failure of the normal role of the liver in clearance of immune complexes from the circulation. Pulmonary hemorrhage and alveolar proteinosis had also been previously described in Finnish cases.

In a 3-year-old boy of Norwegian descent with LPI and immune complex disease consistent with systemic lupus erythematosus (SLE; 152700), Parsons et al. (1996) presented evidence suggesting that the immune complex disease may be the basis of the respiratory problems.

In 4 patients with LPI, Duval et al. (1999) found features that fulfilled the diagnostic criteria for familial hemophagocytic lymphohistiocytosis (HPLH1; 267700). Mature histiocytes and neutrophil precursors participated in hemophagocytosis in the bone marrow. Serum levels of ferritin and lactate dehydrogenase were elevated, hypercytokinemia was present, and soluble interleukin-2 receptor levels were increased up to 18.6-fold. Duval et al. (1999) suggested that the diagnosis of LPI should be considered in any patient presenting with hemophagocytic lymphohistiocytosis.

Biochemical Features

Smith et al. (1988) found that, contrary to their findings in cultured skin fibroblasts, LPI red cells showed normal net uptake and efflux of cationic amino acids.

Inheritance

The studies of Kekomaki et al. (1967) and Norio et al. (1971), who called the condition 'lysinuric protein intolerance,' confirmed autosomal recessive inheritance.

Diagnosis

Sperandeo et al. (2008) noted that the diagnosis of LPI is often difficult because of vague clinical presentation. Classic symptoms of protein intolerance may remain unnoticed during the first and second decades of life due to unconscious avoidance of dietary protein. However, patients usually present with gastrointestinal symptoms soon after weaning.

Prenatal Diagnosis

Sperandeo et al. (1999) demonstrated the feasibility of prenatal diagnosis of LPI by linkage analysis.

Clinical Management

In a 4-year-old girl with LPI, Carpenter et al. (1985) found that oral citrulline therapy resulted in 'substantial increase in protein tolerance..., striking acceleration of linear growth, as well as increase in bone mass...' Whereas impairment of urea production in LPI results from a defect in the uptake of ornithine in liver cells, citrulline, which is metabolized to arginine and ornithine, is absorbed by a mechanism that is unaffected in LPI.

Mapping

By genomewide linkage analysis of 20 Finnish LPI families, Lauteala et al. (1997) found linkage to chromosome 14q (maximum pairwise lod scores of 5.82 at marker D14S742 and 6.91 at D14S283). Haplotype analysis identified a 10-cM candidate interval between D14S72 and MYH7 (160760), which had previously been mapped to 14q12. There was strong evidence for a founder effect in Finland.

By linkage analysis, Lauteala et al. (1998) concluded that LPI in non-Finnish cases is due to mutation in the same gene on 14q. They studied 19 non-Finnish families, of which 13 were Italian, 1 Swedish, 1 Latvian, 2 Moroccan, 1 Saudi Arabian, and 1 Turkish. These families showed no linkage disequilibrium except in an Italian family cluster.

Molecular Genetics

In 31 Finnish patients with lysinuric protein intolerance, Torrents et al. (1999) identified homozygosity for a founder mutation in the SLC7A7 gene (603593.0001). Borsani et al. (1999) defined the Finnish mutation as a splice acceptor change resulting in a frameshift and premature translation termination.

Torrents et al. (1999) identified compound heterozygosity for 2 SLC7A7 mutations (603593.0005; 603593.0006) in a Spanish LPI patient. In affected members of 2 unrelated Italian LPI families, Borsani et al. (1999) identified 2 different homozygous mutations in the SLC7A7 gene (603593.0002 and 603593.0003, respectively).

Noguchi et al. (2000) identified SLC7A7 mutations (603593.0008; 603593.0009) in Japanese LPI patients.

Mykkanen et al. (2000) performed mutation screening of 20 non-Finnish LPI patients and found 10 novel mutations in the SLC7A7 gene.

Sperandeo et al. (2008) identified 9 novel mutations in the SLC7A7 gene, and noted that a total of 43 different mutations had been identified in over 100 patients with LPI. Mutations were spread throughout the gene with no apparent genotype/phenotype correlations.

Font-Llitjos et al. (2009) identified 11 mutations in the SLC7A7, including 7 novel mutations, in 11 patients from 9 unrelated families with LPI. Two of the mutations were large deletions involving exons 4 to 11 and exons 6 through 11 (603593.0011), respectively. These deletions were identified using multiplex ligation probe amplification (MLPA) assays and were found to result from the recombination of Alu repeats at introns 3 and 5, respectively, and the same AluY sequence in the 3-prime region of the SLC7A7 gene. Patients with the large deletions had the most severe phenotypes, likely resulting from dramatic loss of transport function.