Objectives of the Presentation

 To discuss the approach to identifying the genetic mutation of inheritable cardiac diseases in dogs and cats.

 To discuss recent findings of mutations for feline cardiomyopathy.

Overview of the Issue

Evaluation of the causative mutation for a familial disease is a difficult, lengthy (often years!) and expensive endeavor, however it is also very rewarding! The best, most successful studies are those in which the study has been well thought out, well designed and well supported by pet owners who are willing to participate by providing pedigrees, medical information and DNA samples (when possible) to the study investigator. We generally think of the study as having three possible phases.

The first phase is careful recruitment of pets that have the disease and those that do not ("controls"). For some studies it is very helpful to try to gather as many members of a family as possible. A family is most powerful when 3-4 full generations are available. The grandparents, parents and offspring are very important. Numerous siblings and dogs distantly related may be less useful if the parents are not available. In other studies, the families are actually not desired, but a large number of affected and unaffected animals are needed. It is extremely important that the disease status be accurately determined. In animals with inheritable issues that are present at birth, the situation is quite clear. Affected puppies or kittens will have the trait, unaffected do not. However, it is much more complicated with inheritable, adult onset disease, such as hypertrophic cardiomyopathy in the cat and dilated cardiomyopathy in many dogs. In these cases, the disease may not show up until 5 or 6 years of age. Some animals may be affected but will not show the disease until even later, so the pet may need to be evaluated for many years in a row before they can be characterized as affected or unaffected. Ideally DNA samples are collected and stored for future studies as the animals are evaluated. DNA samples can be collected easily using a buccal swab and wiping the inside lip of the mouth. However, sometimes a large amount of DNA may be needed for a genetic study and a small blood sample may provide a larger amount of high quality DNA for years to come!

The next phase could include a "candidate gene" approach. The candidate gene approach is one where known proteins and genes that are causative for the development of the disease in human beings are evaluated. This seems a logical approach in many cases since it seems reasonable to assume an abnormality in a gene that causes the disease in human beings may cause the disease in animals. The genetic sequence for the animal gene can be determined (if it is not already) by using the known human gene. DNA samples from affected and unaffected animals are then compared for changes that are only present in the affected animals.

We have used this approach successfully in the cat with hypertrophic cardiomyopathy and have identified mutations in Maine Coon and Ragdoll cats within a gene (MYBPC3) that causes the human disease. In the Maine Coon we demonstrated that a mutation in the MYBP3 gene is causative for the development of hypertrophic cardiomyopathy (HCM) in some Maine Coon cats. In the past 11 months, we have evaluated 2,337 DNA samples submitted from Maine Coon cats (unaffected, affected) and have identified the mutation in 33% of the samples (96% heterozygous). Although we have not performed extended longitudinal studies on cats other than the original family, we do not believe that all cats with the mutation will develop a severe form of HCM. It does appear likely that cats that are homozygous (have 2 copies of the gene mutation) for the disease are highly likely to develop a severe form of the disease. Due to the large number of cats with the mutation in the population, we can not recommend removal of all mutation positive cats. Our current recommendations are to remove cats that are homozygous for the mutation from the breeding pool. Heterozygous (have 1 copy of the mutation) cats should be carefully evaluated for the ability to pass on other important Maine Coon traits to the population. Cats that have many strong attributes and are heterozygous but disease negative at time of breeding (suggesting that they may have a more mild form of the disease) could be bred to a mutation negative cat. The offspring of that mating should be screened, and if possible, a mutation negative kitten selected to replace the mutation positive parent in the breeding pool. Over a few generations this will decrease the prevalence of the disease mutation in the population, hopefully without greatly altering the gene pool. Disease negative but mutation positive cats should be evaluated annually by a cardiologist. We have also recently identified a different mutation in this same gene in the Ragdoll cat. We do not yet know the prevalence of the mutation in the breed, but at this point our breeding recommendations for the Ragdoll are the same as for the Maine Coon.

However, the candidate gene approach has been less successful in the Doberman pinscher with dilated cardiomyopathy. In human beings there are over 20 genes responsible for dilated cardiomyopathy and it would take several years and thousands of dollars to evaluate all of those genes in this breed of dog. The approach is also difficult in a disease that is more common in our animal patients than in human beings. For instance, subvalvular aortic stenosis is an important, inherited disease in the dog. However, it is uncommon in human beings and there are no known genes that could be studied.

The third phase is either a genome wide scan or association study using nonspecific canine markers that are distributed throughout the canine genome. Markers are evaluated with respect to presence or absence of disease and a statistical pattern is sought that associates a marker and the disease to a specific chromosomal region. Genes that are in that region that could be involved in disease development are then studied. We are currently pursuing this approach in the Doberman pinscher with dilated cardiomyopathy.

Additional Detail

Definitions:

Heterozygote: has 1 copy of the mutated gene and 1 copy of a normal gene

Homozygote: has 2 copies of the mutated gene

Summary

Evaluation of the causative mutation for a familial disease is a difficult endeavor. However, with careful planning and perseverance, it can be quite rewarding!

References/Suggested Reading

1.  Meurs KM, Munro MJ, Dryburgh K, Boyer M, Mathur D, MacDonald KA, Kittleson MD. Identification of a Missense Mutation in the Cardiac Myosin Binding Protein C Gene in a Family of Maine Coon Cats with Hypertrophic Cardiomyopathy. Human Molecular Genetics 2005;14:3587-3593

2.  Parker HG, Meurs KM, Ostrander EA. Finding cardiovascular disease genes in the domestic dog. J Vet Cardiol 2006;8:115-127.

3.  Meurs KM, Fox PR, Spier AW, Koplitz S, Baumwart RD. A prospective pedigree and linkage analysis of familial dilated cardiomyopathy in the Doberman pinscher. Accepted JVIM

4.  Meurs KM, Norgard MM, Ederer MM, Hendrix KP, Kittleson MD. A substitution mutation in the myosin binding protein C gene in Ragdoll cats. Genomics 2007;90:261.