rs121908677 — SLC7A7 SLC7A7 p.Gly54Val

SLC7A7 and the Broken Gateway for Cationic Amino Acids

Three amino acids — lysine, arginine, and ornithine — share a specialized transport system for crossing the intestinal wall and for reabsorption in the kidney tubule. That system is y+LAT1, encoded by the SLC7A7 gene. When both copies of SLC7A7 carry loss-of-function variants, cationic amino acids cannot exit the intestinal epithelium into the bloodstream, and cannot be retrieved from urine by the kidney. The result is lysinuric protein intolerance (LPI)¹¹ lysinuric protein intolerance (LPI)
A rare autosomal recessive disorder of cationic amino acid transport; OMIM #222700. Approximately 200 cases reported globally, with the highest prevalence in Finland (1:60,000) and Japan (1:57,000) — a multisystem disorder that is manageable but not curable.

This SNP, rs121908677, captures the c.161G>T missense change (coding strand notation) that converts glycine to valine at position 54 of y+LAT1 (p.Gly54Val). On the genomic plus strand, this appears as a C>A transversion at chr14:22,813,238 (GRCh38). The variant was identified by Mykkänen et al. in 2000²² Mykkänen et al. in 2000
Mykkänen J et al. Functional analysis of novel mutations in y(+)LAT-1 amino acid transporter gene causing lysinuric protein intolerance. Hum Mol Genet, 2000 in a homozygous Latvian patient and a homozygous Estonian patient — a pattern consistent with its rarity and the geographic clustering of LPI cases around the Baltic region and Scandinavia.

The Mechanism

y+LAT1 does not work alone. It forms a heterodimer³³ heterodimer
A protein complex composed of two different subunits. y+LAT1 is the light catalytic subunit; 4F2hc (encoded by SLC3A2) is the heavy structural subunit. Together they form a functional transporter at the basolateral membrane of intestinal and renal epithelial cells with 4F2hc (encoded by SLC3A2), and this complex mediates efflux of cationic amino acids (lysine, arginine, ornithine) from the epithelial cell into the portal circulation, exchanging them for neutral amino acids plus sodium.

Glycine 54 sits in a highly conserved transmembrane region of y+LAT1. The Gly54Val substitution replaces the smallest amino acid (glycine, no side chain) with a branched-chain valine, disrupting the precise geometry of the transmembrane helix packing. Critically, the G54V mutant protein is not degraded intracellularly — it reaches the plasma membrane correctly when coexpressed with 4F2hc in the Xenopus oocyte expression system used by Mykkänen et al. — but once there, it has zero amino acid transport activity⁴⁴ zero amino acid transport activity
In contrast, frameshift mutants in the same study were trapped intracellularly and never reached the membrane. G54V thus represents a surface-displayed "dead" transporter — folded but non-functional, similar to the delta-F508 CFTR mutation in cystic fibrosis. The structural integrity is preserved; the catalytic function is destroyed.

When both SLC7A7 alleles are non-functional, cationic amino acids accumulate inside intestinal and renal epithelial cells while failing to reach the bloodstream. Plasma lysine, arginine, and ornithine are chronically depressed. Arginine and ornithine deficiency undermines the urea cycle, leading to post-prandial hyperammonemia⁵⁵ hyperammonemia
Elevated blood ammonia. The urea cycle requires ornithine and arginine as substrates; when both are scarce, ammonia cannot be adequately cleared after protein meals. Ammonia is neurotoxic even at mildly elevated levels, causing encephalopathy, coma, and permanent neurological damage if untreated after protein-rich meals. Lysine deficiency impairs collagen cross-linking, bone matrix synthesis, and immune function.

The Evidence

SLC7A7 was identified as the LPI gene simultaneously by two groups in 1999: Borsani et al. (Nature Genetics)⁶⁶ Borsani et al. (Nature Genetics)
Borsani G et al. SLC7A7, encoding a putative permease-related protein, is mutated in patients with lysinuric protein intolerance. Nat Genet, 1999 in Italian patients and Torrents et al. in Finnish and Spanish patients. Since then, more than 43 distinct pathogenic SLC7A7 mutations have been catalogued across 130 patients from 98 independent families Sperandeo et al. 2008⁷⁷ Sperandeo et al. 2008
Sperandeo MP et al. Lysinuric protein intolerance: update and extended mutation analysis of the SLC7A7 gene. Hum Mutat, 2008, with no genotype-phenotype correlations established — severity varies enormously even within families carrying identical mutations.

The functional null character of Gly54Val was established by the Mykkänen 2000 oocyte study, which tested five SLC7A7 missense mutations functionally and found that all, including G54V, abolished transport despite variable effects on membrane localization.

Systemic complications beyond hyperammonemia are well-documented. Pulmonary alveolar proteinosis (PAP)⁸⁸ Pulmonary alveolar proteinosis (PAP)
Accumulation of proteinaceous material in the alveoli, impairing gas exchange. In LPI, this is thought to result from macrophage dysfunction secondary to arginine/ornithine deficiency, with abnormal lysosomal processing of surfactant proteins. PAP can be life-threatening and does not reliably respond to citrulline supplementation occurs in a significant minority of LPI patients, sometimes from childhood, and represents the leading cause of LPI-related mortality Parto et al. 1993⁹⁹ Parto et al. 1993
Parto K et al. Pulmonary alveolar proteinosis and glomerulonephritis in lysinuric protein intolerance: case reports and autopsy findings of four pediatric patients. J Pediatr, 1993. Osteoporosis, glomerulonephritis, and hemophagocytic syndrome-like presentations are additional recognized complications.

Practical Actions

Citrulline supplementation is the cornerstone of LPI management because citrulline is a neutral amino acid — it is absorbed normally via a different transporter system — and is converted to arginine and ornithine inside hepatocytes, replenishing the urea cycle substrates that cannot enter via the defective y+LAT1 route. The landmark two-year citrulline trial¹⁰¹⁰ two-year citrulline trial
Rajantie J et al. Lysinuric protein intolerance: a two-year trial of dietary supplementation therapy with citrulline and lysine. J Pediatr, 1980 by Rajantie et al. demonstrated catch-up growth in seven of nine previously stunted children and normalization of urea cycle function. Dosing is now standardized at up to 100 mg/kg/day citrulline in 4 divided doses with meals, combined with protein restriction to 0.8-1.5 g/kg/day to limit ammonia load. Supplemental L-lysine (20-30 mg/kg/day) is added because lysine cannot be recovered adequately from urine. High-dose citrulline may paradoxically worsen nitric oxide overproduction in some patients; dosing should be supervised and individualized.

Carriers (AC genotype) are uniformly asymptomatic and require no treatment. Their significance is reproductive: if both parents carry any SLC7A7 loss-of-function variant, each child has a 25% risk of LPI.

Interactions

Because LPI is autosomal recessive, full disease expression requires biallelic SLC7A7 loss. Many LPI patients are compound heterozygous (two different SLC7A7 mutations rather than homozygous for a single one); a carrier of this G54V allele who also carries a different pathogenic SLC7A7 variant in trans would be clinically affected. Comprehensive SLC7A7 gene sequencing — not single-SNP genotyping — is required for complete clinical evaluation. The compound heterozygous scenario is relevant to the compound action system: rs121908677 AC status combined with any second SLC7A7 pathogenic allele would produce the full LPI phenotype.