Jim Berry: [00:00:00] Welcome to the WSAVA podcast. Today's discussion looks at genetic testing in complex diseases and in feline heart disease, an area that challenges veterinarians to balance science, diagnosis, and patient care. While you listen, consider, how do you weigh up genetic information when deciding on treatments?

Our host, Petra Cerna speaks first with Jerold Bell and later with Joshua Stern. Let's listen in.

Petra Černá: I am so excited to be here today and talking with Dr. Jerry Bell about genetic testing. So interpreting genetic test results can be complicated and often also confusing, but we are here today to break things down for you to be more comfortable and make genetic testings more fun. So first of all, Dr. Jerry Bell, could you please tell us more about the different types of genetic testing? 

Jerold (Jerry) Bell: Yes. Thank you for having me, Petra. The thing about genetic [00:01:00] tests is that it's not a simple yes no answer, and every genetic test has its variables that we have to consider when determining how relevant it is to our patients. So there are different types of genetic tests.

There are direct DNA tests, which actually identifies a mutation in a dog that has been determined to be causative of a genetic disease. And so those tests are more black and white in terms of affected normal or carrier, but they give us more definitive results. There are linked marker based tests, which is testing for a marker on a chromosome that lies close to a causative gene, but maybe the causative gene has not been identified yet or found by the researchers, but the marker is on the chromosome very close to it, so it usually moves with the disease and [00:02:00] is fairly accurate in identifying liability for disease.

But we do have to understand that false results are a possibility with linked marker based tests. So we always want to try to work with direct genetic tests. There are liability genes that have been identified, and we're seeing more and more of those, especially with complexly inherited disease, where multiple genes have to combine together to cause the disease.

And so, these tests tell us that there is increased liability for a disease. And maybe we sometimes have statistics that say 60% 80% chance of developing the disease, but it is not the complete answer. And then the last thing to understand is that there are dominant genes where only a single gene is necessary to cause disease.

And there are recessive genes that if it's a simple recessive, you need two copies to cause that disease. So those are some of the variables that we have and different [00:03:00] types of tests that we have to interpret them. 

Petra Černá: Yeah, no, this is very exciting and I personally find genetic testing sometimes quite complicated as well.

Do you want to give us some examples of those dominant versus recessive traits in cats and dogs, and what are those tests that we are usually testing for? And what are the main diseases with those dominant or recessive traits in both cats and dogs? 

Jerold (Jerry) Bell: So in cats, the test for polycystic kidney disease, which came from the Persian lineage of cats, but has been spread now into oriental breeds as well as to some random bred cat breeds or domestic cat breeds is a dominant gene.

And in actuality, if a cat receives that gene from both parents, it is lethal in the uterus. So that the only cats that are born are those that are either normal or carry the gene for PKD. Almost [00:04:00] all of those cats will develop cysts in their kidneys, sometimes in the liver as well, and the vast majority of them will develop kidney disease due to the destruction of the parenchyma or the kidney tissue that needs to be functioning.

In dogs PRCD is one of the highest frequency Mendelian genes that is recessive and you need two copies of that gene to cause late onset blindness. It occurs in many breeds as well as mixed breed dogs. It is an ancient mutation that occurred a long time ago in an individual that proceeded the formation and separation of breeds.

So it's shared by many breeds, but it is late onset. Some dogs can live to 10, 12 years old. And while it could be identified by a ophthalmologist with a scope, they may not be functionally blind, but many or most dogs, if they reach an elderly age, will show blindness, [00:05:00] especially night blindness and blindness in dim light.

The ABCB1 gene, which is more commonly known as the MDR1 mutation for drug sensitivities occurs both in dogs and in cats. So this is a disease process where the individual patient cannot metabolize certain drugs and they become toxic in the brain. It can cause tremors and seizures and death, and this includes ivermectin sensitivity as well as many chemo drugs and other drugs.

So these are important tests to be testing for and identify in our patients. 

Petra Černá: No, absolutely. I think the MDR1 mutation is definitely quite interesting, so definitely something to keep an eye out for, I think. And Jerry, what would be your main tips and tricks for interpreting genetic test results? It's one thing is to run all those tests, but then how do we actually interpret them and help [00:06:00] our clients make some decisions about their pets.

Jerold (Jerry) Bell: Yes, many of the tests and the test results that you're going to receive as veterinarians is going to be based on a panel testing, a gene chip, that tests for over 200 different types of traits and diseases, conditions, colors, size, and so forth. So what's important is understanding what test results are actually going to be applied to your patient. There are many genetic tests for diseases that only occur in a single breed, and if it isn't your patient's breed, then those test results are of no consequence because it doesn't cause disease in that breed. There are many tests that are liability genes that again, also are for diseases that may not occur in your patient's breed and are not applicable.

A carrier of a [00:07:00] disease causing gene that is specific for your individual patients will never produce an affected individual, so you don't have to worry about the lifetime risk of that. It's only if they're used for breeding that they would pass on that risk to 50% of their offspring and should not be bred to another carrier to prevent the risk of producing affected individuals.

Petra Černá: I think this is extremely important to always be able to interpret those tests. Do you have any recommendations where we can go and look up information about test, or is it different from a test to a test?

Jerold (Jerry) Bell: The International Partnership for Dogs, IPFD, has a website called DogWellNet, no spaces, and they have done a really good job in collating information on the genetic tests that are available.

The applicability of those tests to individual breeds or mixed breed dogs, so that is a good source and probably the [00:08:00] most complete source that's available. ~The iCat website is a feline website and they do a good job in collating a lot of information.~ The commercial laboratory companies do have a responsibility to offer some genetic counseling as to what their test results mean, and they are rapidly grasping that concept and being able to counsel veterinarians and owners as to what those test results mean. 

Petra Černá: I think it's a kind of take home message from this, don't be afraid to ask about the test results if you're not sure what they mean. Could you tell me a little bit more about the importance of heterozygous versus homozygous results and what does it mean for our patients and for us?

Jerold (Jerry) Bell: So heterozygous means that you have one of the disease related genes, all genes come in pairs on paired chromosomes and it's how those two genes, when they pair up, react with each other that determines what the effect is going to be. And especially with liability genes, with genes that aren't just involved with the gene pair [00:09:00] itself, but where other genes and environmental factors can play a part in how that's being expressed.

We can see a dose type of effect where if you have one copy of the gene, you have some liability, maybe low liability for developing that disease, and if you have two copies, you can have a much higher liability or frequency of developing that disease, as well as increased severity of the disease as well as a possibly earlier age of onset of that disease. And many of the cardiac related genes that have been identified in dogs and in cats fall to that category. And I know Dr. Joshua Stern is going to be part of this podcast and joining you next, so I'm going to leave that to him. 

Petra Černá: What do you think, Jerry, are some of the most common pitfalls in interpreting and over interpreting some genetic results?

What do you think would be the most important things to know about the interpreting of [00:10:00] these tests? 

Jerold (Jerry) Bell: The most important thing is that genetic testing is a rapidly evolving field, and the ability of us as veterinarians to utilize those tests continues to evolve as well. So we don't have all the answers and sometimes we still don't understand what answers we don't have.

So, it's very interesting when a researcher finds a liability gene and wants that to be used widespread in all dogs or cats when it may only be specific to a certain limited family. We have valid tests, but we need to understand and really put forth what are the limitations of each test and what are the strengths of each test? What does it actually tell us?

Because misunderstanding genetic tests and misinterpreting them causes our clients to lose faith in genetic [00:11:00] testing and so it's very important for us to understand as best as we can, exactly what that genetic test is telling us. 

Petra Černá: This is so, so, so important. And one important thing in genetic testing is also recognizing the limits of the test relevance across populations.

So could you tell us why genetic test results are usually only clinically meaningful in specific breed or population, and what affects the test validity as well? 

Jerold (Jerry) Bell: So I will use the SOD1 mutation for degenerative myelopathy susceptibility as an example for this one, because as clinicians, greater than 90 to 95% or more of our patients that actually develop degenerative myelopathy are going to be from one of two breeds, either a Boxer or a German Shepherd dog. And those are the breeds where [00:12:00] it is an endemic type of disease. We rarely see the disease in other breeds or mixed breed individuals. We do see them in other breeds and mixed breed individuals, but not as a breed related genetic disease. And the reason is that the other breeds and mixed breed individuals do not carry other mutations or variants that are necessary for that disease to occur. So we have to just watch how we interpret tests and how we pass on these results to our owners. Can it increase liability? Yes. Is it likely to cause disease in most instances? No. And it's not something that we need to emphasize to our clients that this test result is a death sentence for their family member.

Petra Černá: I think this is really important and thank you so much for covering all of those things and giving us great examples [00:13:00] on how we can really over interpret sometimes the results of the genetic testing. Is there anything else you would like to share with us today regarding genetic testing? 

Jerold (Jerry) Bell: I think that genetic testing is important to us as veterinarians and something that, that we were not trained in veterinary school and something that we need to learn and to be able to utilize because it's not just genetic tests for disease liability in terms of a DNA test.

We are using biomarkers and other genetic tests now from our reference laboratories to identify different diseases, liver diseases, cancer liability, cancer diagnosis. So we really need to keep ourselves educated on the tests that are out there, the new tests that are coming out and how it can enable us to become better veterinarians, to be able to help with the health of our patients.

Petra Černá: I couldn't agree more and I think there is becoming a push for this individualized patient [00:14:00] medicine in veterinary medicine. Some things that might work for one patient might not work for another one and I think I've learned outside of vet school about this so much more than I did in vet school. So I do agree that we really need to be improving the education because this is going to probably be the standard of care very soon.

Jerold (Jerry) Bell: Agreed. 

Petra Černá: I do hope that people are feeling more comfortable and excited about interpreting these results, and we'll have a lot more episodes throughout this WSAVA podcast discussing different issues with hereditary diseases. I'm so excited about this. 

Jerold (Jerry) Bell: Thank you, and I will mention that on the WSAVA website, the Hereditary Disease Committee website, we do have additional resources that are available for veterinarians as well to be able to understand some of these topics.

Petra Černá: Thank you so much. Providing more resources is always very important, so please check out the WSAVA website and particularly the section for Hereditary Disease Committee that [00:15:00] actually both Jerry and I are members of. This is a wonderful group of people that is trying to improve the genetic testing and the education about genetic testing and do more research in this field, which I think is really important for future veterinary medicine.

So thank you again, Jerry. 

Jerold (Jerry) Bell: Okay. Thank you, Petra, 

Jim Berry: After Jerold Bell, Petra is joined by Joshua Stern to share his perspective. 

Petra Černá: So I'm very excited that Joshua has made some time to be here with us today because hypertrophic cardiomyopathy is the most common inherited cardiac disease in cats that often leads to heart failure and also  arterial thromboembolism or even sudden cardiac death.

We are going to discuss the use of genetic and other tests to diagnose HCM in our cats. So first of all, Joshua, could you please tell us how useful, or actually, and maybe unuseful is the widespread genetic testing for [00:16:00] causative mutations in diagnosing or prognosticating HCM in feline population? 

Joshua Stern: It's a pleasure to be here and discuss cats and cardiology and genetics.

All three of my very favorite things. So when we think about the kind of utility of genetic testing in cats for hypertrophic cardiomyopathy, we're really working in a really limited landscape. And so genetic testing for certain breeds of cats can be quite valuable. Potentially most valuable as a breeding tool, but it is something that, that we put some stock in and recommend.

And so what I'm talking about is the testing for variants that are known to cause hypertrophic cardiomyopathy in either Maine Coon cats, Ragdoll cats, or potentially Sphynx cats. And those three tests are very breed specific. The  Maine Coon test and the Ragdoll test have been independently [00:17:00] validated to be useful in assessing hypertrophic cardiomyopathy in those breeds through a number of different investigations over many years now.

And so when I think about  Maine Coon and Ragdoll HCM, I do think that genetic testing has a role there. That role is as a breeding tool. So if you are a producer of Maine Coon or Ragdoll kittens, you wanna make sure that your breeding cats that are free of those genetic mutations so that you're having the kind of least chance of producing HCM.

But it also has a role in prognostication, cats that have two copies of the variant that cause HCM in either  Maine Coons or Ragdolls have now been shown across a few different studies to have an earlier onset of disease and a more severe variety of disease. So knowing their genetic status can be helpful in that way.

Petra Černá: What are actually your thoughts on currently breeding [00:18:00] recommendations for those  Maine Coons that have potentially one copy of that mutation? Because we currently, of course, forbid breeding of cats with two mutations, but some organizations still allow mating non-carriers, both  Maine Coons, that have one copy of the mutation.

So how do you feel about that? 

Joshua Stern: Yeah. This is a really nuanced discussion, right? So, HCM in both  Maine Coon and Ragdolls is a dominantly inherited trait. We know that there's this kind of gene dose effect where the cats with two copies have a worse form of the disease that comes on earlier, but cats with one copy can still go on to develop HCM.

And if you take that risk of breeding a cat that's heterozygous or has one copy of the variant to a cat that's normal and has no copies of the variant, you are still based on odds going to produce kittens that have the variant in some of the litter, but there's also another thing that we [00:19:00] must consider, and that's genetic diversity.

So if you are a breeder of Maine Coon cats, you might be weighing the risk of genetic diversity and preservation of an important line. And so we really have to judge these animals on an independent basis. And so if the cat in question that has one copy of the mutation is free of hypertrophic cardiomyopathy, which is possible, and an amazing representation of the breed and the last of a line that you're desperate to preserve. And there may be unique circumstances where breeding that individual makes sense from a risk assessment perspective. In general, if none of those things are true, I would not recommend breeding those cats.

There are plenty of cats that are wild type out there that have no copies of the variant that could be used to continue to produce Maine Coon cats, but that is a discussion that is largely based in preservation of breeding lines, [00:20:00] genetic diversity and personal decision making in terms of weighing risk versus benefit.

Petra Černá: Thank you so much. So from everything you had said, it seems like the genetic basis of HCM seems to be very complicated, and unfortunately in most breeds, we'll still have no identified mutations and only few commercially available tests for some breeds like Maine Coons and Ragdolls, and potentially Sphinxes. So what do you think is the role of molecular signatures and gene expressions in understanding the disease progression, diagnostics, but also maybe therapeutic strategies for these cats?

Joshua Stern: I think we are not quite there yet, but we're learning so much more about the genetics and genetic architecture in cats that I do believe we will get there. My team just recently published, Dr. Kaplan, who's one of my graduate students, just recently published a really big genetic investigation. The biggest of its kind so far [00:21:00] into hypertrophic cardiomyopathy, looking at more than a hundred cats with the disease and trying to identify new genetic signatures of the disease. What we learned from that study is that it is a very complicated disease, that there isn't just one big unifying mutation that causes it across all these different cats.

But there are themes and there are things that have come out of that work that have given us new avenues to pursue, and so I think we're learning more about the molecular signature of the disease. I think if I were to flash forward 10 years from now, I bet we'll have an HCM panel that gives you a risk assessment based on, I don't know, maybe more than a hundred markers of potential association with HCM.

Unfortunately, we don't have that yet today. But I do think that what we've learned out of our kind of widespread genetic studies is more about genetic pathways [00:22:00] of interest. Those often shape what we know about disease pathogenesis and send us on quests for novel drug discoveries. I think we've learned about possible gene mutations that are enriched. So genes that have lots of different mutations that look like they stack up in cats with HCM versus control cats, I think might be really important as we learn more about this disease moving forward. And so I think ultimately my answer to this question is unfortunately, stay tuned because we're not quite there.

But I think that that, that really is going to be the secret to understanding risk of disease. 

Petra Černá: When we take it a little bit to the simpler level, without looking too much to the future, what would be the recommendations right now for general practitioners for genetic testing for HCM? And just does your recommendation change, as we mentioned, for some of those specific breeds?

Joshua Stern: I think right now [00:23:00] if you're in practice and you are seeing Maine Coon or Ragdoll kittens, or you're seeing a Maine Coon or Ragdoll for the first time, it's a hundred percent appropriate to ask what their genetic test status is, because knowing if they're at higher risk for development of HCM than the average cat will guide your treatment and care plan for that cat moving forward.

So I think from visit one, if they're a kitten, find out if we know their genetic status and get them tested. I think the thing that's really important to know is sometimes kittens are sold with a known genetic test. And sometimes kittens are sold by parentage. What that means is that the breeder has tested the parents and guaranteed the kittens are normal based on that. In my experience, those kittens should still be tested because parentage isn't foolproof. I can't tell you the number of times that somebody expected a [00:24:00] kitten to be normal and then we tested it and it was positive because the mating pair wasn't precisely what we had thought. Cats are pretty tricky in that way, and so I think genetically testing any Maine Coon or Ragdoll is a smart move.

I think that's where I stand on what I tell family practitioners these days. 

Petra Černá: So thank you so much. Very helpful. So what are we gonna do about those breeds where no genetic test is available or our domestic short hair cats, but especially in our pedigree cats where we do not have genetic testing available?

What are your recommendations for breeding and when should we test?

Joshua Stern: Cats and HCM, it's a huge problem. And particular if you're breeding cats, I think we have a responsibility to ensure that we're not breeding cats that have this condition and so unfortunately, in the absence of a straightforward genetic test scenario, we are left to [00:25:00] screen these cats for hypertrophic cardiomyopathy and the gold standard screening test that we have is echocardiography.

Now, echo is not always widely available. It is expensive and it is a test that only tells us about disease presence on the day that we do the test and so this is one of those diseases that's really frustrating for breeders because it often leads them doing screening tests for their breeding population year on year until they get to a ripe old age where we think it's unlikely that they'll develop HCM.

What I'm recommending as a kind of best practice and slightly more conservative approach is that we start screening breeding age animals before we produce a first litter, so they have a negative echo or normal echo before they are bred and then we're looking into that presence or absence of disease each year after that.

I do [00:26:00] think it's reasonable for breeders if it's hard to get access to an echo, or if it's expensive to get echos done, to think about using other supportive diagnostic tests and maybe alternating those. And so the best one that we have right now is the NT-proBNP test. And so I think it would be very reasonable to alternate echo and NT-proBNP in a breeding population and say, as long as my NT-proBNP is low, then I can think about skipping that echo and doing it next year and doing kind of an alternating pathway. I think that puts us in a space where it might be more achievable to, to perform best practice. 

Petra Černá: This is definitely very interesting and I do agree. I think we have a huge responsibility if you are a breeder and producing kittens, you really are responsible.

And last but not least, you have been involved recently with a lot of [00:27:00] studies for new treatment for HCM in cats, rapamycin. Could you please tell us a little bit more about the results of your recent study and in which cats we might consider starting rapamycin? 

Joshua Stern: Absolutely. Yeah. I've been really fortunate to be part of an incredible journey, looking at a novel therapy for hypertrophic cardiomyopathy that is a delayed release rapamycin or sirolimus product, now known as Felycin-CA1.

CA1 stands for conditional approval. That is a drug that we've taken all the way through safety studies, target animal safety studies, field studies, known as the RAPACAT study and we're now in the middle of a pivotal clinical trial of many cats looking at the outcome of using this compound in hypertrophic cardiomyopathy.

What we found when we gave this drug to cats with subclinical hypertrophic cardiomyopathy, either what we know as stage [00:28:00] B1 or B2 HCM, is that rapamycin can delay or regress the thickening of the left ventricular walls. That's the primary feature by which we diagnose hypertrophic cardiomyopathy, and it did so with acceptable safety to move on to conditional approval from the FDA.

And so what we know about that drug is that basically the target population is those cats that are not yet showing any symptoms of disease that have a thickened ventricle wall, either with or without left atrial enlargement. And that's the group that we are offering to prescribe this drug to now.

That's the same group that we're now enrolling in what's known the HALT study, which is that 300 cat, more than 25 recruiting centers across the nation. Really exciting for us to have something to finally offer cats with hypertrophic cardiomyopathy as a possible treatment. [00:29:00] And I am so very excited to see the results of the HALT study when that comes out in the next year or two.

We are almost there for concluding recruitment in that study, and then we'll be following those cats for an additional year after that. 

Petra Černá: It is amazing. I think HCM in cats is probably one of the most devastating and heartbreaking diseases because a lot of those are very young cats that are otherwise completely healthy, and I think it is amazing that we now have options to offer some treatment for these cats. And this is probably even more pushing us to start doing those echo screenings early in those cats that potentially might be affected by one of those genes that could be predisposing cats to HCM. So thank you again and is there anything else you would like to share with us today before we wrap this up?

Joshua Stern: Well, I'll just wrap up my part by saying I really appreciate the opportunity to be on the podcast. I think this is probably a podcast that if we're being [00:30:00] honest with ourselves, we'll need rerecorded every 12 to 18 months because the state of genetics and in particular feline genetics is growing so incredibly fast.

I think watching this space is really important for feline enthusiasts. I'm gonna make a prediction at the end, which will be fun. I think the thing we'll probably learn the most about relative to cat genetics in the near term is about modifying variants. I think there's a real role for learning about genetic variants that modify the course of hypertrophic cardiomyopathy and might help us predict which cats are more likely to have congestive heart failure, thromboembolic disease, or sudden death. And so that's the space I'm most excited to continue watching. 

Petra Černá: Amazing. Thank you again for being here with us today. 

Joshua Stern: Thank you. 

Jim Berry: Thanks for joining us on the WSAVA Podcast where we are transforming care, one episode at a time. We hope today's [00:31:00] discussion was helpful, wherever you are in the world. You'll find more information and further resources on the topics discussed in the show notes. And we look forward to sharing our next conversation with you very soon.