PUG DOG ENCEPHALITIS STUDY UPDATE

Report to Grant Sponsors (Pug Dog Club of America) from Dr. Kimberley Greer:

Statistical analyses were conducted on the set of Pugs with complete records (Fall ‘07), and included 2,875 females with 2,831 unaffected and 44 affected. There were 1,823 males with 1,809 unaffected and 14 affected. Of the Pugs with recorded coat color, there were both PDE affected and unaffected dogs included in the “fawn” group and in the “black” group with significantly more fawn dogs participating in the investigation. The median onset age of PDE is 19 months, with a range of 3 to 84 months (7 years). Median survival time is 23 days with an age of death, whether by euthanasia or natural death, ranging from 3 to 85 months.

The pedigrees for the 4986 Pugs have been entered into the dedicated database. Of the total dogs entered in this study, 51.1% were bred with an average inbreeding coefficient of 0.084, a value slightly higher than the predicted inbreeding coefficient for first cousin matings (0.0625), and slightly lower than the inbreeding coefficient for uncle-niece or aunt-nephew mating (0.125). This lack of random assortment in the Pug mating population expectedly yields increased homozygosity across the genome, pushing the proportion of potentially identical alleles beyond that predicted by Hardy-Weinberg equilibrium. Further demonstration of the high heritability of PDE is indicated by their heritability coefficient which is between 0.52 and 0.82, on a scale of 0.1 to 1.0. This analysis also reveals a significant association between the dogs’ gender, coat color, and disease heritability, clearly indicating that within this cohort, the majority of PDE affected dogs are fawn females while the least frequent combination of affected gender and color is black males.

Having shown that PDE is highly heritable, the definitive mode of transmission was the parameter of primary interest. The results revealed the potential for two or three significantly influential genes lending towards inheritance of PDE, or the potential for a primary locus with numerous modifier loci influencing PDE inheritance. The shift in statistical analyses slightly away from a single locus for inheritance may be due to the influence of environmental factors lending towards disease inheritance. To this end, PCR screening for herpes-, adeno-, and parvoviruses was completed. All of these tests proved negative on all included cases, arguing strongly against these viruses as environmental triggers for PDE.

The genome-wide scan was initially conducted on a subset of approximately 100 Pugs. We included an equal number of PDE affected and unaffected, unrelated Pugs. An initial overview scan revealed two markers which were significantly associated with the occurrence of PDE. A secondary analysis, utilizing a more dense population of markers, revealed that there is clearly a single point of significant association on a single canine chromosome. Although there is a genome-wide baseline association, most likely resulting from breed specificity, the region shows association significantly above the baseline. We have successfully identified one primary genotype segregating in the PDE affected dogs. This genotype is currently being explored and investigated in an extended sample of PDE affected and unaffected Pugs for verification and validation. Assuming that the genotype maintains its association with disease presentation, it will be a prime candidate for design of a genetic test.