rs72552272 — SLC7A7 L334R

SLC7A7 L334R — When the Amino Acid Gate Fails

Every meal you eat is a negotiation between the gut wall and your bloodstream. y+LAT1¹¹ y+LAT1
y+LAT1 (y+ large amino acid transporter 1) is encoded by SLC7A7 and forms a heterodimer with 4F2hc (CD98) at the basolateral membrane of intestinal and renal epithelial cells. It exports cationic amino acids — lysine, arginine, and ornithine — from inside epithelial cells into the portal circulation and bloodstream. sits at the basolateral membrane of intestinal and renal tubular cells, its job being to ferry the three dibasic amino acids — lysine, arginine, and ornithine — out of epithelial cells and into the body. The L334R missense variant swaps a small nonpolar leucine at position 334 for a bulky, positively charged arginine. That single substitution does not prevent the transporter from reaching the cell membrane, but it completely silences its transport activity. The result is lysinuric protein intolerance (LPI), a rare autosomal recessive disorder where every protein-containing meal becomes a metabolic emergency.

The Mechanism

After absorbing dietary protein, intestinal epithelial cells trap lysine, arginine, and ornithine inside because the basolateral y+LAT1 gate is broken. Renal tubular cells face the same failure: these amino acids are filtered by the glomerulus but cannot be reabsorbed, producing the hallmark aminoaciduria — massive urinary losses of all three dibasic acids despite near-absent plasma levels.

The depletion of arginine and ornithine breaks the urea cycle²² urea cycle
The urea cycle is the series of enzymatic reactions in the liver that converts toxic ammonia into urea for urinary excretion. Arginine and ornithine are both obligate intermediates; when they are chronically depleted, ammonia accumulates in blood.. Post-meal protein loads flood the liver with amino acids that cannot be cleared to urea, producing acute hyperammonemia — drowsiness, vomiting, and, in severe episodes, coma. Chronically low lysine impairs collagen cross-linking and bone matrix synthesis, contributing to osteoporosis. An emerging mechanism involves trapped intracellular arginine being shunted into excessive nitric oxide (NO) production³³ nitric oxide (NO) production
Arginine is the sole substrate for nitric oxide synthase. When intracellular arginine cannot exit cells via the broken transporter, it is over-converted to NO, leading to systemic NO excess. in macrophages and other cells — a likely driver of immune dysfunction, pulmonary alveolar proteinosis, and glomerulonephritis.

Mykkanen et al. (2000)⁴⁴ Mykkanen et al. (2000)
Mykkanen J et al. Functional analysis of novel mutations in y(+)LAT-1 amino acid transporter gene causing lysinuric protein intolerance. Hum Mol Genet, 2000 demonstrated in Xenopus oocyte expression experiments that L334R protein localises normally to the plasma membrane when co-expressed with 4F2hc, but carries zero transport activity. Leucine 334 is required for the conformational dynamics of transport, not for membrane targeting.

The Evidence

LPI is rare globally — estimated prevalence around 1 in 60,000 in Finland (enriched by founder effect) and 1 in 500,000 elsewhere — so controlled trials are infeasible. Evidence for management comes from case series, registry data, and mouse models.

Sebastio et al. (2011)⁵⁵ Sebastio et al. (2011)
Sebastio G et al. Lysinuric protein intolerance: reviewing concepts on a multisystem disease. Am J Med Genet C, 2011 reviewed the full LPI phenotype in over 200 published cases: protein aversion beginning at weaning, failure to thrive, hepatosplenomegaly, and episodic hyperammonemic crises. Long-term complications in survivors include pulmonary alveolar proteinosis (PAP) — abnormal surfactant accumulation in alveoli — and progressive glomerulonephritis. Genotype-phenotype correlations are absent; the same mutation can produce mild or severe disease in different patients.

Parto et al. (1994)⁶⁶ Parto et al. (1994)
Parto K et al. Pulmonary alveolar proteinosis and glomerulonephritis in lysinuric protein intolerance: case reports and autopsy findings of four pediatric patients. Hum Pathol, 1994 documented histologic glomerulonephritis in all four LPI patients at autopsy, and PAP in three of four — establishing these as near-universal rather than exceptional complications.

Ogier de Baulny et al. (2012)⁷⁷ Ogier de Baulny et al. (2012)
Ogier de Baulny H et al. Lysinuric protein intolerance (LPI): a multi organ disease by far more complex than a classic urea cycle disorder. Mol Genet Metab, 2012 identified a therapeutic paradox: while citrulline supplementation corrects the urea cycle defect, excessive citrulline increases intracellular arginine accumulation (citrulline is converted to arginine intracellularly), which can worsen NO overproduction and macrophage activation. This finding refined citrulline dosing toward lower, carefully titrated doses.

Practical Actions

The standard treatment for biallelic LPI combines three elements: (1) oral citrulline supplementation at low-to-moderate doses (typically 2–8 mmol/kg/day in children, lower in adults) to replenish the urea cycle without flooding cells with arginine; (2) protein restriction to reduce the ammonia load per meal; and (3) periodic whole-lung lavage when PAP causes respiratory compromise.

Carriers (one functional copy) are phenotypically normal — the remaining allele provides sufficient transport activity. Carrier detection matters only for reproductive planning.

Interactions

The severe phenotype of LPI arises exclusively in biallelic (homozygous or compound heterozygous) states. Heterozygous compound mutations involving L334R with other loss-of-function alleles (frameshift, nonsense, splice) produce the same clinical picture as L334R homozygotes, because both alleles must be functional for normal transport. No documented gene-gene modifier effects are known that materially alter LPI severity; phenotypic variability is thought to reflect modifier loci yet to be identified.