Cystinuria Type B (Variant 3)

Cystinuria type B is a congenital metabolic defect that causes abnormal amino acid transport in the intestine and kidneys, leading to urolithiasis and cystine crystal formation. This disorder has been linked to different variants in the SLC7A9 gene, which encodes a subunit of an amino acid transporter.

Symptoms

Clinical signs of cystinuria usually appear around 3 to 4 years of age, although in some cases they are documented as early as two months of age. One of the main clinical features is the presence of cystine crystals in the urine, although elevated levels of other amino acids such as ornithine, lysine and arginine are also observed. In addition, affected cats show signs of lower urinary tract disease, including hematuria, oliguria and straining to urinate with frequent attempts.

Disease Management

The management of cystinuria focuses on the elimination or dissolution of the uroliths through dietary changes and medical treatment. A low-protein, low-sodium diet with a high water content is recommended.

Genetic basis

This disease follows an autosomal recessive mode of inheritance. Autosomal recessive inheritance means that the cat, regardless of sex, must receive two copies of the mutation or pathogenic variant to be at risk of developing the disease. Both parents of an affected cat must carry at least one copy of the mutation. Animals with only one copy of the mutation are not at increased risk of developing the disease, but may pass the mutation on to future generations. Breeding between cats carrying genetic variants that may cause disease, even if they do not show symptoms, is not recommended.

Technical report

Cystinuria is a congenital metabolic defect that affects the renal proximal tubule, hindering the reabsorption of amino acids such as cystine, ornithine, lysine and arginine. This condition causes the formation of uroliths and urinary obstructions. This condition was first identified in cats in 1991 and is associated with mutations in the SLC7A9 gene, which encodes a protein integral to a plasma membrane transporter essential for the reabsorption of basic amino acids and cystine in the kidneys and intestine. One of the known nonsense variants, c.1175C>T, involves the substitution of a highly conserved threonine for a methionine in a transmembrane domain of the protein. Other mutations such as c.881T>A in breeds such as Maine Coon and Sphynx, and c.706G>A, have also been implicated in this condition.

Most affected breeds

Domestic Longhair
Maine Coon
Siamese
Siberian
Sphynx

Bibliography

DiBartola SP, Chew DJ, Horton ML. Cystinuria in a cat. J Am Vet Med Assoc. 1991 Jan 1.