Is this THE gene?

Dog breeders often hold out hope that researchers will find them “the gene”. The gene that plagues their breed and if they could find it, just IF, then all would be good and their breed would thrive. This is a somewhat faulty way to think as many genetic diseases are complex – meaning more than one defective gene is necessary for expression of the disease, and we are unlikely to get a definitive single gene for every issue out there. For instance, many years ago a test was developed for juvenile renal disease (JRD), which has devastating effects on puppies and young dogs. Standard Poodle breeders were very hopeful it would help, and tested a great many dogs. It was said to be a recessive genetic disease and two copies of the mutation were likely to be lethal because homozygotes had not been found. Soon, however, with wide scale testing, breeders discovered the mutation was “carried” by an enormous percentage of the breed, lines that had never produced JRD, and there were even a number of healthy dogs with two copies. Breeders promptly lost faith that the test offered any meaningful information and stopped testing – and they became yet more reluctant to trust science over tradition. Their skepticism is legitimate. Similar stories have happened in many breeds, a gene will be identified only to determine it’s not the whole story of what triggers that disease expression. Nevertheless, we breeders still often hope we will find an answer to those diseases that cause us, our dogs, and our homes such pain. Finding “the gene” when it’s a “single gene of major effect” is usually easy – if it’s there – especially with the rapid advance of technology. So when the research drags on – that simple test is not likely coming anytime soon, if ever.

Recently, while puttering around the new VGL website, I noticed a test for the Nova Scotia Duck Tolling Retriever for Juvenile Addison’s Disease (JADD). Being that Addison’s disease is a prevalent issue in my own breed, I was curious but didn’t click on the test; surely it wouldn’t be relevant to our breed anyway, as we would know if that were the case. A day or so later, I felt a nagging feeling that I should at least check it out, as our study on Addison’s disease had led us to believe that Addison’s disease is complex and we are unlikely to find “the gene”.

So, I clicked on the test and read that it is a simple recessive gene, with incomplete penetrance. If a dog has two copies of this gene, they have a 75% chance of developing the disease. This test, is REQUIRED for Tollers to receive a CHIC number in their breed.  What is this gene?

I was unable to ascertain exactly what mutation the test at UC Davis is checking for, however, the test offered by UC Davis led me to this paper as a reference at the bottom of the test’s information page. In it, the authors found the following:

“We found seven different haplotypes of which the haplotype DLA‐DRB1*01502/DQA*00601/DQB1*02301 was significantly more prevalent in the diseased dogs (P = 0.044). In addition, these affected dogs also were more likely to be homozygous across the DLA class II region than the control dogs (OR = 6.7, CI = 1.5–29.3, P = 0.011). We also found that homozygous dogs, regardless of their haplotype, tended to have earlier disease onset compared with heterozygous dogs. These data indicate a limited MHC diversity in North American NSDTRs and suggest that the MHC may play a role in the development of hypoadrenocorticism in the NSDTR, supporting the autoimmune origin of the disease.”

 Oberbauer et al 2010

Imagine my surprise, when upon investigating, I found that this is the same haplotype identified in the 2015 study on Standard Poodles found to increase risk of AD in Standard Poodles. In this paper, DLA‐DRB1*01502/DQA*00601/DQB1*02301 was identified as “2006”. From that study,  “dogs with haplotype 2006 were 2.4 times more likely to develop AD (p = 0.01).”  There were no dogs in the study that were homozygous for the haplotype, and the haplotype was rare in the population. Therefore, the recommendation for Standard Poodle breeders was to treat it like it added some risk, and to pay close attention to incidences of the disease in the line, and mitigate risk using other methods, like breeding dogs with the haplotype once over age 4.

Is this coincidence that both studies found an association between this DLA haplotype and AD? Or does it indeed mean that this DLA haplotype is causative? Is the test offered by UC Davis testing for the Class 2 DLA haplotype 2006?

It’s important to note the title of the paper written by Oberbauer in 2010. The title itself is called “Association of a dog leukocyte antigen class II haplotype with hypoadrenocorticism in Nova Scotia Duck Tolling Retrievers”. Association is an important distinction in this case – association is a specific term in statistics. Namely: “association is a statistical relationship between two variables. Two variables may be associated without a causal relationship.”

This means that this association is not proven to be causative. But, it surely is interesting that this association was found in two different studies on two (likely) relatively unrelated breeds. 

So, is this “the gene”? 

We don’t know, but it appears that 2006 is associated with juvenile AD in Novia Scotia Duck Tolling retrievers AND is associated with a 2.4x risk increase for Standard Poodles to develop AD.

To quote the final lines in the 2010 Oberbauer paper:

“the risk of hypoadrenocorticism is nearly double when an NSDTR is homozygous across its DLA class II region and the risk is potentially greater if the dog is homozygous for the DLA-DRB101502/DQA00601/DQB1*02301 haplotype. Hypoadrenocorticism is likely to be a complex genetic disease and these results suggest that the DLA genes play a role in its development; determining the extent of this involvement will require identification of other genetic components in NSDTRs. Comparison of these results with other breeds predisposed to this disorder may also identify the DLA genes that are specifically involved in the manifestation of hypoadrenocorticism.“

 Oberbauer et al 2010

After reading the 2010 paper referenced at the bottom of the Juvenile Addison’s disease page on VGL’s website, we felt it was not likely that 2006 was what was being tested for in Tollers. We emailed our friends at VGL to inquire what the JADD test was actually testing. It turns out that the mutation being tested has yet to be published, so we were unable to get more specifics regarding it. From the page: “The juvenile form of Addison’s disease is genetically distinguishable from the adult onset form in that all dogs that develop the juvenile form have two identical copies of a specific region within their genome and a specific mutation within a novel gene. The mutation responsible for JADD causes a change in the amino acid sequence in a highly conserved region of this protein. This mutation is not present in any other breeds of dogs based on testing of over 250 individual animals in 80 different breeds.” We are waiting to hear whether Standard Poodles were included in this portion of the study. For Tollers, at least, it looks like they may have found “the gene”.

This brings to mind many questions. Is it possible that Tollers carrying 2006 also tend to have this identified mutation- possibly due to linebreeding or reduction in biodiversity in the breed? The 2010 paper by Oberbauer et al noted that genetic diversity in the breed was suspected to be low (maybe they would be a good candidate for enrollment at UC Davis for canine diversity). The Class 2 DLA haplotype 2006 is relatively rare in the breeds thus far tested at UC Davis VGL for canine genetic diversity, only a handful of breeds have this haplotype and of those, very few dogs are homozygous for it. Is there a reason for this? Or is it coincidence? We know from previous studies to not take DLA associations too seriously, and true deleterious DLA haplotypes are rare. Or perhaps 2006 is completely unrelated from JADD in Tollers and the link was provided because it was previous research on the subject.

Time, and publication of the new study, may tell for us more.

In the meantime, Standard Poodle breeders should pay very careful attention to this DLA haplotype when making breeding decisions. There is no definitive answer here; we should continue to keep an eye towards improving breed health via biodiversity and redistributing the genetic diversity we have as a breed, while still considering this information on DLA. Remember, preserving existing biodiversity of a breed can prevent the rise of breed specific diseases. While a gene was found for JADD in Tollers, many diseases that plague breeds are complex and it’s difficult to find genetic contributors, so we likely cannot hold out hope for a single gene to be identified in many breeds for these breed specific diseases.

All dog breeders feel the same. We want that loyal face looking back at us to be as healthy and happy for as long as possible. We cultivate generations of our canine companions and would never wish a health issue to become prevalent in our breeds. We must not only seek “the gene”, but also prevent the rise in these deleterious genes by ensuring that no single dog becomes so closely related to the others of its breed that that gene becomes fixed in the population. We do this by maximizing retention of the existing biodiversity in each breed. 

References

Hughes, A.M., Jokinen, P., Bannasch, D.L., Lohi, H., & Oberbauer, A.M. (2010). Association of a dog leukocyte antigen class II haplotype with hypoadrenocorticism in Nova Scotia Duck Tolling Retrievers. Tissue Antigens, 75(6), 684-690. doi: 10.1111/j.1399-0039.2010.01440.x

Pedersen NC, Brucker L, Tessier NG, Liu H, Penedo MC, Hughes S, Oberbauer A, Sacks B. The effect of genetic bottlenecks and inbreeding on the incidence of two major autoimmune diseases in Standard Poodles, sebaceous adenitis and Addison’s disease. Canine Gen Epidem. 2015;2:14. https://doi.org/10.1186/s40575-015-0026-5.

Safra N, Pedersen NC, Wolf Z, Johnson EG, Liu HW, Hughes AM, Young A, Bannasch DL (2011) Expanded dog leukocyte antigen (DLA) single nucleotide polymorphism (SNP) genotyping reveals spurious class II associations. Vet J 189(2):220–226. https://doi.org/10.1016/j.tvjl.2011.06.023