rs146582474 — SLC7A7

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.