The SOD1:c.118G > A mutation, associated with CDM, was originally identified in a genome-wide association study and characterised by sequence-based typing [13]. Recent studies have used conventional and real-time PCR techniques to develop more rapid and cost-effective methods of genotyping dogs for this particular mutation [14–16]. The present study employed PCR followed by RFLP analysis, utilising the Eco571 enzyme, demonstrating that this technique allows rapid and simple genotyping dogs for the SOD1:c.118G > A mutation.

There was no significant difference in allele or genotype frequencies comparing young GSDs (n = 50) with those over 8 years of age (n = 100) in the sample population from a large tertiary veterinary hospital. In dogs over 8 years of age, homozygosity for the SOD1:c.118A allele was strongly associated with those affected with pelvic limb ataxia, compared to those referred with non-neurological disease. None of the 50 dogs with non-neurological conditions were of the A/A genotype, compared with 21 of 50 dogs with neurological signs being homozygous for this mutation. This is a lower proportion of homozygous A/A mutant dogs than that identified in the American GSD CDM case population examined by Awano and colleagues [13], where 4 of 5 dogs were homozygous for the A allele, but similar to that found in an Italian study, where 3 of 10 GSD in their case population were homozygous A/A [15]. This might represent genetic variability, due to geographical differences in the gene pool of GSD populations studied, but more likely results from differences in selection criteria used for identifying the case populations. In both the current and the Italian study, selection of the case population was based on a broad phenotype, relating to neurological signs (such lower limb ataxia in this study), compared with the study of dogs in the USA, where a more definite diagnosis of CDM was made following post-mortem histopathology or MRI studies. Thus, although CDM cases would be included in our case population, this is a relatively heterogeneous group, with other causes of pelvic limb ataxia also likely to be represented.

Use of SOD1 genotyping in a clinical setting is now considered to be a useful component of the diagnostic panel, with dogs showing consistent clinical signs and being homozygous for the mutation, raising the index of suspicion of CDM, whereas those with heterozygous or homozygous wild-type genotypes considered more likely to be affected by other disease processes. As management of CDM is problematic, with the disease often rapidly progressing, the results of such a genetic test may influence the treatment the dog receives, particularly in those dog with co-morbidities, whereby it may not be appropriate to consider surgical intervention for concurrent spinal conditions, in a dog that is homozygous for the SOD1:c.118G > A mutation, but to opt instead for physiotherapy which has been shown to prolong life in dogs with CDM [17].

Although CDM appears to be inherited in an autosomal recessive manner, with incomplete penetrance [13], in our study of UK GSDs, all dogs over 8 years of age that expressed the A/A genotype were showing clinical signs of ataxia, with none of the ‘neurologically healthy’ dogs expressing this genotype. This suggests that, in the UK at least, penetrance is relatively high if not complete.

We also assessed the prevalence of the SOD1:c.118G > A mutation in a cross-sectional study of relatively young GSDs, which would be unbiased with respect to CDM susceptibility because they are too young to show clinical signs. The prevalence of the SOD1:c.118A allele in this population was estimated to be 0.35, with 8 of 50 dogs (16%) being homozygous for the A allele. Other studies [13, 15] using limited numbers of dogs have estimated the prevalence of the SOD1:c.118A allele in GSDs to be 0.17-0.21 with 4–6% being AA homozygous. However, these GSD populations were from different geographical locations and included dogs aged 0 to 14 years with and without evidence of neurological disease, and therefore do not necessarily represent the GSD population as a whole. A much larger study [16], where over 6000 GSDs were genotyped for the SOD1:c.118G > A mutation, revealed a frequency of 0.37 for the SOD1:c.118A allele with 22% being AA homozygotes, which is similar to the results from the present study. Follow up on the eight A/A homozygous dogs identified in the present study revealed that one dog had been euthanased due to clinical signs consistent with CDM, four dogs had been euthanased before reaching 7 years of age for other conditions such as neoplasia and liver disease, and three dogs currently remain disease free at 8 years of age, although they might still develop CDM in the future.

It has been suggested that genetic screening of dogs for the SOD1:c.118G > A mutation in breeds known to be susceptible to CDM might be used to influence breeding programs [3]. Genetic testing and identification of heterozygous or homozygous mutant dogs, alongside a strategy for ensuring that they are only bred to homozygous wild-type dogs, would reduce the prevalence of the SOD1 mutation in the population over several generations. A more radical approach, based on selection against all carriers of the mutation for breeding purposes, could pose a problem in terms of restricting genetic diversity, particularly in a breed such as the GSD, where the prevalence of the A allele is relatively high. In breeds such as the Boxer and Pembroke Welsh Corgi, where the prevalence of the A allele is even higher (A allele frequency is estimated to be over 0.7 in some studies [13]), breeding programs to select against dogs carrying the mutant A allele would likely have a major impact on the overall genetic health of these breeds, thus eliminating desirable qualities or unintentionally selecting for other genetic mutations.