Vitamin D deficiency rickets (type 1A) in Siamese cats

Type 1A rickets is caused by a deficiency of the enzyme 1-hydroxylase. Its function is to convert vitamin D (cholecalciferol), synthesized in the skin by UV light, into a biologically active form that plays a vital role in calcium homeostasis. Clinical symptoms are consistent with those of insufficient exposure to sunlight or dietary calcium deficiency and include failure of bone calcification with consequent change in shape and delayed growth of the teeth. Affected individuals suffer from increased bone pain and resultant gait abnormalities and lethargy.

The disease is caused by a nonsense mutation c.637G>T in the CYP27B1 gene.

Rickets is an autosomal recessive disorder. The disease affects cats with P/P (positive / positive) genotype only. Cats with P/N (positive /negative) genotype are clinically without any symptoms. They are genetically considered carriers of the disease (heterozygotes). In offspring of two heterozygous animals following genotype distribution can be expected: 25 % N/N (healthy non-carriers), 25 % P/P (affected), and 50 % N/P (healthy carriers). Because of high risk of producing affected offspring, mating of two N/P animals (carriers) can not be recommended.

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References:

Grahn, R.A., Ellis, M.R., Grahn, J.C., Lyons, L.A. : A novel CYP27B1 mutation causes a feline vitamin D-dependent rickets type IA. J Feline Med Surg 14:587-90, 2012. Pubmed reference: 22553308