Sebaceous adenitis with hyperkeratosis (SA) is a dermatological condition that has been described in several breeds of dog, most commonly in the Standard Poodle (SP). It is a condition in which the sebaceous glands in the skin become inflamed and are eventually destroyed, leading to hair loss and secondary skin infections, and is a significant health and welfare problem in affected breeds.
In 2008 the Canine Genetics team at the AHT, funded by a grant from the American Kennel Club Canine Health Foundation, and with invaluable support from the Standard Poodle Club UK, conducted a study into Sebaceous Adenitis in the Standard Poodle. A whole genome association scan was carried out on DNA samples from 48 Standard Poodles (20 cases and 28 controls). This procedure allows us to compare the genomes of dogs affected with SA (cases) to the genomes of healthy dogs (controls) and to pinpoint any regions where a clear difference can be seen. Although the genotyping was carried out successfully, the study failed to identify any regions of the genome which were significantly associated with the disease.
We are confident that the genotyping data we generated was of high quality, so the likely explanation of our failure to identify a region of the genome associated with SA is because the disease is more complex than was originally thought, and is either caused by more than one gene, or the interaction between genes and the environment. Alternatively, our controls might have included a number of subclinically affected dogs whose skin biopsy results led us to mis-categorise them. In either case, the solution is to collect and genotype more samples, and any new data can be added to what we already have, thus increasing the chances of success.
This is a disappointing result, but as a result of this investigation we now can say fairly confidently that sebaceous adenitis in the Standard Poodle is not inherited as a simple autosomal recessive disease with a high degree of penetrance, and that more samples need to be analysed to identify a genomic region associated with the disease.
We are committed to continuing our study of SA in the Standard Poodle and are continuing to collect and store samples so that we can successfully analyse SA as a complex disease. Data generated during the previous study will be saved, and added to additional data generated in this subsequent phase of the study.
Second study (Collaboration with University of California Davis)
Since the completion of the first study in September 2008 the AHT has received an additional 32 samples from Standard Poodles, 10 of which were from dogs affected with Sebaceous Adenitis. We would like to thank Molly Windebank for her hard work in encouraging owners to send in samples. However these numbers were not sufficient by themselves for a second whole genome scan. Fortunately in 2010 we were contacted by Dr Niels Pedersen from the University of California in Davis, who suggested a collaboration involving pooling of our samples.
As a result of this contact Dr. Niels Pedersen at the Veterinary Genetics Laboratory at the University of California Davis (UC Davis) and Dr. Mike Boursnell in Dr Cathryn Mellersh’s Canine Genetics Group at the Animal Health Trust in the United Kingdom (AHT) have entered into a collaboration centred round the genetics of Sebaceous Adenitis in Standard Poodles (SPs). Dr Pedersen had selected a batch of US SP samples for genotyping, and kindly offered to genotype some SP samples from the AHT. It was hoped that this larger sample set would provide more useful information than either set alone. The data would be analysed both by UC Davis and the AHT, each providing their own experience to the analysis.
The US set included 35 samples from affected dogs (“cases”) and 22 samples from unaffected dogs (“controls”). The UK samples included 23 cases and 16 controls. Some of the UK samples had been previously genotyped using the old Canine array (22,362 SNP markers), and were re-genotyped on the new Canine HD array (173,662 SNP markers). All samples were successfully genotyped, with high call rates, indicating that the DNA was of good quality. As for the previous study, this type of “Whole Genome Scan” is used to search for regions of the genome that are consistently shared among cases but different in controls. Such regions are likely to be associated with the disease and may contain mutations in genes that are involved with Sebaceous Adenitis.
The data from the genotyping is currently being analysed at UC Davis and at the AHT. Initial analysis has been carried out, but there is still a great deal to do, and possible follow up experiments to be conducted. The first stage of the analysis has confirmed the results of the previous smaller whole genome scan with the old Canine SNP array, namely that there is no evidence for a single mutation causing the disease. In fact the data further strengthen our belief that SA is a more complex condition with perhaps several genes involved.
Some other interesting facts emerge from the first stage of the analysis. Firstly that the US and the UK sample sets are clearly distinct genetically. In addition the regions of the genome that are most associated with the disease are also different in the two data sets.
This latter observation raises the possibility that some of the genetic factors causing the disease are different in the US population than in the UK population. If true, this would be interesting scientifically but obviously makes identifying a clear cause a bit more problematical.
On the positive side, the association of some regions of the genome with the disease is more clear cut than with the previous smaller sample set. This is probably due in part to the new combined data set arising from the collaboration, and also from the use of the new high density SNP marker array. This gives us encouragement to follow up these results with further analysis and experimentation.
The enormous amounts of data that we have obtained in this new Whole Genome Scan means that there is still some way to go in completing the analysis. We will try to make sense of the differences that we see between the US and the UK populations, and also to look to see if there are any connections or interactions between the separate associated genome regions. To try to test whether each of these regions are genuinely significant we will need to analyse a limited number of SNP markers in as many more samples as we can find. If we can validate these regions in a large number of extra dogs, then we can be more confident that they are associated with the disease. So, as before, we are still interested in more samples (see the next section for more details).
Definition of cases and controls
To study any disease we require DNA samples from dogs that are affected with the disease (‘cases’) and also dogs of the same breed that are unaffected (‘controls’). The definitions for Sebaceous Adenitis are as follows:
A case of Sebaceous Adenitis is defined as a dog that has been examined by a veterinary dermatopathologist and found to have the disease. This must involve the taking of a skin biopsy.
A control for our Sebaceous Adenitis studies is defined as a dog that has again been examined by a veterinary dermatopathologist with a skin biopsy and pronounced clear of the disease. In addition we would ask that unaffected dogs should be at least 6 years of age.
For all our SA samples it is preferable to have copies of the veterinary skin biopsy examination report.