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Development of a DNA Test

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Some Jack Russell Terriers (JRTs) suffer from a progressive neonatal cerebellar ataxia that differs from a previously reported spinocerebellar ataxia in this breed. Serial neurologic examinations of affected pups revealed normal mentation, intention tremors and severe gait, stance and ocular ataxia beginning at 2 weeks of age. A dancing gait, hypometria in all limbs, decrease in forward ambulation and greater tendency to fall were observed as the disorder progressed. No specific abnormalities were identified in routine laboratory screening of blood and urine. Magnetic resonance imaging showed severe atrophy of the cerebellum. Postmortem examinations were performed on 10 affected dogs between 1.5 and 16 months of age. Histologically, the lesion is described as primary cerebellar granule cell degeneration characterized by depletion of granule cells in the cerebellar cortex and Purkinje cell vacuolation. Preliminary pedigree studies suggest an autosomal recessive trait causing degeneration of the cerebellum.

We recently conducted a genome­mapping experiment using the newly available Illumina canine SNPchip with 18 ataxia­affected JRTs and 78 controls consisting of normal JRTs and JRTs with other diseases. This experiment identified a region of a specific canine chromosome likely to contain the mutation responsible for the ataxia. The mapping of this JRT ataxia locus is a major step towards the identification of the causative mutation and the development of a DNA test that will identify genetic carriers of the disease. Examination of the genes within the mapped chromosomal region revealed a candidate gene that is likely to contain the causative mutation because mutations in the corresponding mouse gene cause an ataxia with symptoms similar to those of affected JRTs.

We are requesting $10,000 from the Jack Russell Terrier Research Foundation so that we can determine if the candidate gene we identified contains the mutation responsible for JRT ataxia. Once we have identified the causative mutation, we will devise a DNA test that uses DNA from blood or cheek swabs and distinguishes normal JRTs from ataxia­carriers. Breeders of JRTs can then use this test to avoid breeding two carriers together and thereby avoid producing future generations of JRTs with progressive neonatal cerebellar ataxia.

Submitted by Gary S. Johnson and Joan R. Coates