Sample Collection

To date we have collected blood samples from 284 Tibetan Spaniels, of which 16 are affected with PRA. DNA has been extracted each blood sample and is being stored at -20.

Computer Simulations

Computer simulations have revealed that we still do not have samples from enough affected dogs to pursue Linkage Analysis experiments at the present time. We have therefore decided to persist with the Homozygosity Mapping experiments in the mean time, until sufficient samples have been collected to confirm any putative linkage results by conventional linkage analysis. It still remains important however for samples from affected dogs and their close relatives to be collected. We require samples from affected dogs, their siblings (even if they are unaffected they add a very high level of power to the samples collection) and their parents if available.

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.

Progress

We have analysed 163 markers with 6 affected dogs and 10 carriers. Several markers showed a pattern of alleles similar to that we would expect for a linked marker (i.e. two different alleles in the carriers and identical alleles in the affected dogs). These markers were genotyped on all the affected dogs and their close relatives but unfortunately the results of the broader experiment failed to corroborate the initial 'suggestive' results and it seems unlikely that any of these markers are in fact to PRA in the Tibetan Spaniel.

Current and Future work

The work we are undertaking to arrange genetic markers into 'multiplex' groups that can be analysed together in single experiments (as detailed in the Sept '02 progress report) continues to proceed very well. This labour and time intensive ground work has already paid dividends, as in enabled us to increase the number of markers examined on the homozygosity dogs from 100 to the current 163 in a matter of days. We are currently in the process of optimising a large batch of 54 multiplex groups (which contain several hundred markers); once they are ready we will genotype them on the 16 homozygosity dogs.