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Progress Report of Study to Find Genetic Markers Linked to

PRA in Tibetan Spaniels by the Animal Health Trust October 2003


Sample Collection

To date we have collected blood samples from 294 Tibetan Spaniels, of which 19 are affected with PRA. DNA has been extracted from each blood sample and is being stored at -20 .


Computer Simulations

Computer simulations had previously revealed that we have insufficient samples from affected dogs and close relatives to pursue Linkage Analysis experiments at the present time. Our efforts over the last twelve months have therefore focused on a Homozygosity Mapping approach to find markers linked to the PRA gene.

Homozygosity Mapping

Whereas linkage mapping involves the analysis of genetic markers in pedigrees of individuals, homozygosity mapping involves examining small numbers of affected and carrier dogs for genetic markers that are homozygous (two identical alleles) in the affected dogs and heterozygous (two different alleles) in the carriers. The technique is not as powerful as linkage mapping, but can provide a good early indication of markers that are located close on the same chromosome as the disease-causing gene


We have analysed 340 markers with 6 affected dogs and 10 carriers and identified markers, which are homozygous for the same allele in at least 4 affected dogs and heterozygous in at least two carriers. Twenty markers meet the above criteria, these are located on 16 different chromosomes. Four canine chromosomes have two markers each that meet our criteria so we have focused our efforts on those chromosomes / markers.

We identified 8 additional markers from these four promising chromosomes and have typed these markers on all the Tibetan Spaniels we have samples for. Unfortunately the results are inconclusive, meaning we are not able to either confirm or exclude any of these four chromosomes. The results are inconclusive due to the lack of power of the sample collection, as discussed above. The fact that markers which are known to be located on the same chromosome as one another show very weak evidence of linkage to one another when analysed with the Tibetan Spaniel collection supports this theory.

Future Work

We will use the newly available canine whole genome sequence from the United States to isolate genetic markers located in genes that are known to cause retinal disease in humans. We will examine them in the Tibetan Spaniel pedigree for evidence of linkage, in the hope that if one of them is the gene responsible for PRA it will be so tightly linked to the disease within the pedigree we will be able to detect the linkage.

We will also make efforts to contact other researchers working on PRA in Lhasa Apsos, in an attempt to pool our results. The conditions are very similar between the two breeds so it is possible the same gene is responsible for the two conditions.

We will also continue to collect and store blood samples from affected dogs and their close relatives until the point at which the collection is sufficiently powerful to continue with a linkage analysis approach. is home of the PRA Fund and the South East & East Anglian Tibetan Spaniel Society