rs146582474 — SLC7A7

Finnish founder splice acceptor mutation abolishing y+LAT1 transport activity at the SLC7A7 intron 6 splice site, causing lysinuric protein intolerance when homozygous — a multisystem recessive disorder of cationic amino acid transport

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SLC7A7 Finnish Founder Mutation — The Genetic Root of Lysinuric Protein Intolerance

Every cell in the small intestine and kidney proximal tubule faces a fundamental challenge: cationic amino acids — lysine, arginine, and ornithine11 lysine, arginine, and ornithine
These positively charged (cationic) amino acids are essential for protein synthesis, urea cycle function, and nitric oxide production. They share a common transporter because of their similar charge and size properties — must be absorbed across the basolateral membrane and returned to the bloodstream. SLC7A7 encodes y+LAT1, the catalytic subunit of the heterodimeric transporter that performs this export step. When both copies of SLC7A7 are disrupted, cationic amino acids become trapped inside intestinal and renal tubular cells, never reaching the bloodstream in meaningful quantities after meals. The result is lysinuric protein intolerance (LPI)22 lysinuric protein intolerance (LPI)
OMIM #222700 — autosomal recessive aminoaciduria; Finnish prevalence 1:60,000; worldwide approximately 200 documented cases.

The c.895-2A>T change (NM_003982.4) obliterates the acceptor splice site at the 5′ end of exon 7 of SLC7A7. Because SLC7A7 is transcribed from the minus strand of chromosome 14, the plus-strand (genome file) change is T>A at position 22,775,938. In Finnish patients, this single mutation accounts for essentially all LPI cases — a founder effect from a single ancestral carrier whose descendants spread through the Finnish population over centuries. In the rest of the world, LPI arises from compound heterozygosity among the ~60 known pathogenic SLC7A7 variants; c.895-2A>T is rare outside Finland.

The Mechanism

The c.895-2A>T transversion converts the invariant AG of the intron 6 splice acceptor consensus to TG. Spliceosomes cannot recognize this disrupted consensus, and normal exon 7 inclusion is abolished. The aberrant splicing creates a 10-bp deletion at the start of exon 7, shifting the reading frame and generating a premature stop codon 26 codons downstream. The truncated y+LAT1 protein retains no residual transport activity and is retained intracellularly rather than trafficking to the plasma membrane — a complete loss of function.

Without functional y+LAT1 at the basolateral membrane, cationic amino acids accumulate in intestinal epithelial cells and are excreted in urine rather than reabsorbed. Plasma lysine, arginine, and ornithine fall chronically low. Arginine deficit impairs urea cycle function, causing postprandial hyperammonemia33 postprandial hyperammonemia
Ammonia normally converted to urea via the ornithine-citrulline-arginine cycle cannot proceed efficiently when ornithine and arginine are deficient; citrulline supplementation bypasses this block by providing a urea-cycle intermediate via a neutral amino acid transporter that is not affected by the SLC7A7 defect. Intracellular arginine trapping also drives excessive nitric oxide (NO) synthesis, which Mannucci et al.44 Mannucci et al.
Mannucci et al., J Inherit Metab Dis, 2005 — elevated plasma nitrate and enhanced nitrite production from LPI fibroblasts propose as a unifying mechanism for the diverse multi-organ complications of LPI.

The Evidence

LPI was recognised as a distinct genetic disease in Finland in the 1960s, but its molecular basis remained unknown until 1999, when two groups simultaneously identified SLC7A7 mutations as the cause. Torrents et al.55 Torrents et al.
Torrents et al., Nature Genetics, 1999 — identified the gene encoding y+LAT1; confirmed transport abolition in Xenopus oocyte functional assay showed that the Finnish founder allele completely abolished transport of cationic amino acids when expressed in Xenopus oocytes with the heavy chain partner 4F2hc.

Sperandeo et al.66 Sperandeo et al.
Sperandeo et al., Human Mutation, 2008 — comprehensive mutation analysis, 130 patients, ≥98 families, 43 distinct variants described 43 distinct pathogenic SLC7A7 variants and confirmed that c.895-2A>T is the sole founder allele in Finland. Despite sharing the same homozygous genotype, Finnish LPI patients show extreme phenotypic variability — ranging from mild growth failure to life-threatening pulmonary alveolar proteinosis — with no genotype-phenotype correlation.

Tringham et al.77 Tringham et al.
Tringham et al., Mol Genet Metab, 2012 — genome-wide microarray in Finnish LPI patients vs controls; 926 differentially expressed genes used genome-wide microarray analysis to show that the Finnish founder mutation triggers widespread secondary transcriptional dysregulation — 926 differentially expressed genes enriched in inflammatory response, immune system processes, and apoptosis pathways. This explains why LPI is far more complex than a simple amino acid transport defect.

Multi-organ complications are well characterised from long-term Finnish cohort data: pulmonary alveolar proteinosis occurs in a significant subset and can be fatal; renal disease including immune complex-mediated glomerulonephritis and proximal tubular dysfunction develops over decades; hemophagocytic lymphohistiocytosis/macrophage activation syndrome is a recognized life-threatening complication; and autoimmune manifestations including lupus-like disease have been reported. Parto et al.88 Parto et al.
Parto et al., Human Pathology, 1994 — autopsy findings in 4 Finnish pediatric LPI fatalities documented pulmonary alveolar proteinosis, immune complex glomerulonephritis, hepatic dysfunction, and amyloid deposits across all four cases.

Practical Actions

For confirmed homozygous individuals, management is lifelong and requires a metabolic specialist. The two pillars are:

  1. Dietary protein restriction (0.8–1.5 g/kg/day in children; 0.5–0.8 g/kg/day in adults) to limit the postprandial cationic amino acid load that cannot be handled by deficient y+LAT1.

  2. Oral citrulline supplementation (≤100 mg/kg/day in 4 divided doses) — citrulline is a neutral amino acid transported by intact systems, bypassing the y+LAT1 defect. It replenishes the urea cycle's ornithine supply, normalising ammonia clearance after meals and permitting modestly higher protein intake.

L-lysine-HCl (20–30 mg/kg/day) can partially correct lysine deficiency but does not address arginine or ornithine. Carnitine (25–50 mg/kg/day) is added when hypocarnitinemia is documented. Nitrogen-scavenger drugs (sodium benzoate, sodium phenylacetate) are used for acute hyperammonemic crises alongside IV arginine and dextrose.

Carriers (one copy) are clinically and biochemically normal; no dietary restrictions or treatment is needed.

Interactions

Compound heterozygotes carrying c.895-2A>T on one chromosome and a different pathogenic SLC7A7 variant on the other also develop full LPI. Because the Finnish founder allele is rare outside Finland, non-Finnish LPI patients are typically compound heterozygous for two different SLC7A7 variants.

The LPI urea cycle defect interacts with arginine-dependent nitric oxide synthesis. Intracellular arginine trapping elevates NO production, which may mediate pulmonary, renal, and immune manifestations independently of the systemic amino acid deficiency — suggesting that treatment targeting only hyperammonemia may be insufficient to prevent end-organ damage.

Nutrient Interactions

lysine impaired_conversion
arginine impaired_conversion
ornithine impaired_conversion
citrulline increased_need

Genotype Interpretations

What each possible genotype means for this variant.

TT Non-carrier Normal

Normal SLC7A7 function — no LPI founder mutation

You carry two copies of the normal SLC7A7 allele at this position. Your y+LAT1 transporter functions normally, exporting lysine, arginine, and ornithine across the basolateral membrane of intestinal and renal tubular cells. The vast majority of people worldwide share this result — the risk allele (A) has a global frequency below 0.1%.

AT Carrier Carrier

Carrier of one SLC7A7 c.895-2A>T allele — no disease risk for you

Lysinuric protein intolerance follows strict autosomal recessive inheritance — both SLC7A7 copies must be non-functional for disease to develop. Heterozygous carriers in all published series are clinically and biochemically indistinguishable from non-carriers. The plasma amino acid profile, urinary amino acid excretion, and ammonia levels are normal in carriers; no biochemical test can distinguish a carrier from a non-carrier without molecular testing.

The primary significance of carrier status is reproductive. If both reproductive partners carry pathogenic SLC7A7 variants (not necessarily the same variant — one could carry c.895-2A>T and the other a different pathogenic allele), each pregnancy has a 25% chance of an affected child, 50% chance of a carrier, and 25% chance of a non-carrier. Given that LPI is a serious multisystem disease with unpredictable severity, prenatal or preimplantation diagnosis is available and worth discussing with a genetic counsellor before conception.

AA Homozygous — LPI Homozygous

Homozygous for SLC7A7 c.895-2A>T — lysinuric protein intolerance requires specialist management

LPI is a multisystem disease substantially more complex than a classic urea cycle disorder. All organ complications are thought to stem from two interconnected mechanisms: (1) chronic deficiency of plasma lysine, arginine, and ornithine causing growth failure, impaired immunity, and urea cycle dysfunction; (2) intracellular accumulation of arginine driving excessive nitric oxide synthesis, which may mediate inflammatory and autoimmune manifestations independently.

Gastrointestinal and metabolic: Protein intolerance is the hallmark — protein-rich meals cause nausea, vomiting, abdominal pain, and acute hyperammonemia. Untreated hyperammonemic episodes can cause stupor, coma, and brain damage. Fasting ammonia is typically normal.

Pulmonary: Progressive interstitial lung disease occurs in many patients; pulmonary alveolar proteinosis (PAP) — surfactant accumulation blocking gas exchange — is a well-recognised life-threatening complication requiring whole-lung lavage in severe cases.

Renal: Proteinuria and haematuria occur in the majority of patients over time. Immune complex-mediated glomerulonephritis and proximal tubular dysfunction (Fanconi syndrome) are documented in long-term follow-up. Beta2-microglobulinuria is an early marker of renal tubular involvement.

Haematologic and immune: Anaemia, leukopenia, and thrombocytopenia occur. Hemophagocytic lymphohistiocytosis (HLH) / macrophage activation syndrome is a recognised life-threatening complication. Autoimmune manifestations including systemic lupus erythematosus-like disease have been reported. During acute HLH crises, intravenous lipid emulsions should be avoided.

Phenotypic variability: Despite sharing the same homozygous genotype, Finnish LPI patients show extreme variability in disease severity — from mild protein aversion and growth delay managed with diet alone, to progressive pulmonary or renal disease requiring organ support. No reliable predictors of outcome currently exist.

Monitoring protocol (per GeneReviews LPI guidelines): - Plasma amino acid concentrations (lysine, arginine, ornithine, citrulline) - Fasting and postprandial blood ammonia - Urinary orotic acid excretion - Renal function: creatinine, urine protein, urine beta2-microglobulin - Pulmonary: chest imaging, lung function - Serum LDH, ferritin (HLH surveillance) - Bone density (osteoporosis risk)