Hereditary cataract (HC)

Cataract is an eye disorder affecting the lens transparency. Cataracts are very often inherited and occur in more than 70 dog breeds. In connection with the hereditary forms of cataracts in dogs, there have been described several modes of inheritance, the majority being autosomal recessive. There are also autosomal dominant or polygenic modes of inheritance. Cataracts are the leading cause of blindness in dogs.
The hereditary cataracts are often called primary cataracts. Secondary cataracts are forms of a cataract that accompanies other eye disorders, for example, progressive retinal atrophy, glaucoma, retinal dysplasia, metabolic diseases and other disorders. The cataracts may develop after an eye injury (traumatic cataracts) or are related to ageing (senile cataracts).
The cataracts are very specific in individual breeds with regard to the affected parts of the lens, age of disease onset, progression rate and the rate of bilateral development of the cataract. HC in Staffordshire Bull Terriers, French Bulldogs and Boston Terriers (in Boston Terriers it is a juvenile form of HC, this form is supposed to occur in French Bulldogs as well) is an autosomal recessively inherited disease. The clinical signs occur during the first year of life. The cataract always affects both eyes (i.e. it is bilateral). It is a progressive disease that means it becomes worse over years and results in total blindness by approximately three years of age.

HC (in Staffordshire Bull Terriers and Boston Terriers) is caused by mutation of insertion 1 bp in exon 9 of HSF4 gene (Heat shock factor protein 4). This mutation causes shifting of the reading frame and creation of premature stop codon (Mellersh et al. 2006). This mutation was first discovered and described in relation with HC in Staffordshire Bull Terrier breed (Mellersh et al. 2007) and the dogs of this breed are affected only with the juvenile form of the cataract, so-called EHC (early onset hereditary cataract) and not with the late form of the cataract, so-called LHC (late-onset hereditary cataract). The above insertion was also discovered in French Bulldog breed.

HC is autosomal recessive inherited disease. That means the disease affects dogs with P/P (positive / positive) genotype only. The dogs with P/N (positive / negative) genotype are clinically without any symptom. 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. Molecular genetic test is based on direct DNA sequencing and allows detecting of presence or absence of the mutations and determining the carriers.