[Cystinuria caused by a SLC7A9 missense mutation in Siamese-crossbred littermates in Germany]. / Cystinurie aufgrund einer SLC7A9-Missense-Mutation bei Siammischlingskatzen eines Wurfs in Deutschland.

Cystinuria is caused by defective proximal renal tubular reabsorption of the amino acids cystine, ornithine, lysine, and arginine (COLA). The low solubility of cystine in mildly acidic urine may lead to the formation of urinary cystine crystals and uroliths. Much progress has been made recently in the diagnosis and understanding of cystinuria in companion animals. In cats, cystinuria affects equally both genders independent of neutering status and, despite being rare, already more cystinuria-causing mutations have been detected in cats compared to dogs. In this study a litter of Siamese-crossbred cats in Germany was assessed clinically for cystinuria and screened for mutations known to cause cystinuria in cats. An adult male castrated cat was presented with cystine crystalluria and calculi-related urinary obstruction and treated with perineal urethrostomy, cystotomy, and medical management. This cat and a neutered male littermate without evidence of urinary tract disease were found to be positive for cystine by urinary nitroprusside test, to have increased urinary COLA values and to be homozygous for the p.Val294Glu mutation in the SLC7A9 gene coding for b0,+AT subunit of the b0,+ renal COLA transporter. Another littermate was non-cystinuric and did not carry this mutation. The same SLC7A9 mutation was previously found in a Maine coon, a Sphinx and a medium-haired cat in North America suggesting a common ancestor and likely first widespread SLC7A9 mutation causing cystinuria in cats. Genetic screening for this mutation may offer a simple and precise mean to diagnose other cats for cystinuria and offer specific management.

Cystinuria Associated with Different SLC7A9 Gene Variants in the Cat.

Cystinuria is a classical inborn error of metabolism characterized by a selective proximal renal tubular defect affecting cystine, ornithine, lysine, and arginine (COLA) reabsorption, which can lead to uroliths and urinary obstruction. In humans, dogs and mice, cystinuria is caused by variants in one of two genes, SLC3A1 and SLC7A9, which encode the rBAT and bo,+AT subunits of the bo,+ basic amino acid transporter system, respectively. In this study, exons and flanking regions of the SLC3A1 and SLC7A9 genes were sequenced from genomic DNA of cats (Felis catus) with COLAuria and cystine calculi. Relative to the Felis catus-6.2 reference genome sequence, DNA sequences from these affected cats revealed 3 unique homozygous SLC7A9 missense variants: one in exon 5 (p.Asp236Asn) from a non-purpose-bred medium-haired cat, one in exon 7 (p.Val294Glu) in a Maine Coon and a Sphinx cat, and one in exon 10 (p.Thr392Met) from a non-purpose-bred long-haired cat. A genotyping assay subsequently identified another cystinuric domestic medium-haired cat that was homozygous for the variant originally identified in the purebred cats. These missense variants result in deleterious amino acid substitutions of highly conserved residues in the bo,+AT protein. A limited population survey supported that the variants found were likely causative. The remaining 2 sequenced domestic short-haired cats had a heterozygous variant at a splice donor site in intron 10 and a homozygous single nucleotide variant at a branchpoint in intron 11 of SLC7A9, respectively. This study identifies the first SLC7A9 variants causing feline cystinuria and reveals that, as in humans and dogs, this disease is genetically heterogeneous in cats.

Role of rBAT gene products in cystinuria.

To investigate whether rBAT gene products function as a crystine transporter component or as a transport activator, we microinjected several C-terminal deletion mutants of rBAT cRNA into Xenopus oocytes, and measured transport activity for arginine, leucine and cystine in the presence and absence of sodium. Wild type rBAT significantly stimulated the uptake of all 3 amino acids 10-20 fold compared to control mutants. On the other hand, no mutant, except a Δ511-685 mutant, stimulated the uptake of these amino acids. However, the Δ511-685 mutant significantly increased the uptake of arginine. In the presence of sodium, the Δ511-685 mutant also increased the uptake of leucine. The Δ511-685 mutant did not stimulate crystine uptake in the presence and absence of sodium. Furthermore, inhibition of L-arginine uptake by L-homoserine was seen only in the presence of sodium. These results suggest that mutant rBAT stimulates the endogenous amino acid transport system y+ in oocytes. Finally, rBAT gene products, as the primary cause of cystinuria, may function as activators of the amino acid transport system in renal brush border membrane.