Population and molecular genetic analysis of primary cataracts in English Cocker Spaniels and wire-haired Kromfohrlanders

The first purpose of the present study was to analyze systematic and genetic influences on the prevalences of frequently occurring eye diseases in single- and multi-colored English Cocker Spaniels (ECS), to characterize similarities and differences in the prevalence and formation of primary cataract (CAT) in these separately bred color variants, and to use the obtained results for developing a selection scheme against primary cataract. The second purpose of the work was to analyze the molecular genetic background of primary cataracts in ECS. For comparison purposes another dog breed, the wire-haired Kromfohrlander (KFL), known to be commonly affected by primary cataract was considered for the molecular genetic analyses. Ophthalmological records of 615 single-colored and 617 multi-colored English Cocker Spaniels bred in the German kennel club for Spaniels (Jagdspaniel-Klub e.V.) were analyzed for systematic and genetic influences on the prevalence of primary cataract (CAT), persistent pupillary membrane (PPM) and distichiasis (DIST). In refined analyses, age at first diagnosis of CAT was used to define two different age-related forms of CAT, primary cataract diagnosed in dogs of up to three and a half years of age (early onset cataract, CAT-early) and primary cataract diagnosed in dogs of more than three and a half years of age (late onset cataract, CAT-late). Almost 50 % of the affected single- and multi-colored ECS were diagnosed as affected by CAT by the age of three and a half years. A high proportion of animals were diagnosed as affected by CAT with less than four years of age. We therefore analyzed the prevalences of CAT, CAT-early, CAT-late, PPM and DIST as separate binary traits. Because the average coefficient of coancestry in all animals born after 1999 was 2.1% within the single-colored and 1.8% within the multi-colored ECS, but only 0.09% between single- and multi-colored ECS, all analyses were performed separately for single-colored and multi-colored ECS. Models for the genetic analyses were developed on the basis of the results of single and multiple analyses of variance using the GENMOD and GLIMMIX procedures of SAS (Statistical Analysis System), version 9.1.3 (SAS Institute, Cary, NC, USA, 2006). Heritabilities on the underlying liability scale were 0.15 for CAT, 0.34 for CAT-early, 0.13 for CAT-late, 0.46 for PPM, and 0.62 for DIST in single-colored ECS, and 0.06 for CAT, 0.13 for CAT-early, 0.14 for CAT-late, 0.10 for PPM, and 0.61 for DIST in multi-colored ECS. We found indications for the existence of two genetically different forms of CAT with different ages of onset in single-colored ECS and a late-onset form of CAT in multi-colored ECS. Inspection of sample pedigrees of single- and multi-colored ECS affected by CAT was performed to characterize similarities and differences in the prevalence and formation of CAT in these separately bred color variants. The pedigree of the single-colored ECS included 40 ophthalmologically examined dogs with 18 unaffected and 22 affected dogs and the pedigree of the multi-colored ECS 16 ophthalmologically examined dogs with eleven unaffected and five affected dogs. In both color variants of the ECS different forms of CAT with respect to location within the lens occurred among close relatives. Appearance of cataract was very heterogeneous without obvious sex differences. The sample pedigrees did not support the assumption of familial segregation of specific forms of CAT in ECS. The results of ophthalmological examination were used for prediction of breeding values for CAT and the two different age-related forms of primary cataract, i.e. CAT-early and CAT-late, for the German population of single-colored ECS using an animal threshold model and Gibbs sampling. Breeding values were then standardized to a mean of 100 and a standard deviation of 20. Relative efficiencies in reducing prevalence of CAT and CAT-early were compared and we could show that selection based on RBV for CAT-early of both parents is expected to result in the largest relative reduction of the prevalence of all forms of CATs considered here. A selection scheme based on sires or dams with RBV for CAT-early of ≤90 would have decreased the population mean of 6.67% in the next generation by about 50%, while a selection scheme based on sires or dams with RBV for CAT of ≤90 would have decreased the population mean of 14.96% in the next generation by 2.2 to 3.6%. In contrast to these results, selection for CAT-late is difficult due to insufficient numbers of ophthalmologically examined dogs at older ages, which may be due to the management of ophthalmologically examination in the JSK. However, in the present situation the most advantageous selection strategy to reduce the prevalence of primary cataract in the German population of single-colored ECS would be to select using RBVs for CAT. For molecular genetic analyses of primary cataracts in ECS and KFL a candidate gene approach was used to investigate 32 genes, known to be associated with primary cataract in humans, for their potential linkage and association with primary cataracts in these two breeds. None of the 45 microsatellites associated with the analyzed 32 candidate genes displayed linkage with primary cataract in ECS or KFL. Tests for association revealed similar results for CRYAA in the two breeds, and we found additional significant association for MAF and SORD in the ECS and for CRYBA1 in the KFL. In addition, the causative mutation in exon 9 of canine HSF4 described by Mellersh et al (2006) in Staffordshire Bull Terriers and Boston Terriers was neither found in ECS nor in KFL. Indications of association of candidate genes should be approved by further molecular genetic analyses considering larger samples of closely related dogs ophthalmologically examined at a young age. In order to achieve these larger highly informative samples, blood samples should be collected in the course of ophthalmological examination schemes. In addition, lens tissue of dogs which underwent cataract surgery should be collected and included in the molecular genetic analyses.