By Arnold Plotnick, MS, DVM, ACVIM
The Heart of the Matter
It seems you can’t turn on the news without hearing about heart disease. Everyone is aware that humans are at risk for cardiovascular disease, but most cat owners probably don’t realize that cardiovascular diseases are common disorders in cats. In fact, disorders of the myocardium (heart muscle) are the major cause of heart failure in cats. The most commonly seen cardiovascular disease in cats is hypertrophic cardiomyopathy (HCM).
The heart has four chambers that pump blood. The normal thickness values of the walls of the heart have been measured and charted, and are very familiar to veterinary cardiologists. In cats suffering from HCM, the walls of the heart become progressively thicker, with one particular chamber, the left ventricle, usually becoming the most affected. The left ventricle pumps blood out through the aorta to the rest of the body. When the left ventricle becomes thickened, it cannot hold as much blood. Imagine the walls of a coffee mug becoming thicker and thicker, growing inward. It would hold less and less coffee. Such is the case with HCM.
As the left ventricle becomes thicker over time, other complications can arise. The left atrium is the chamber directly above the left ventricle, and it supplies blood to the ventricle, which then sends it to the rest of the body. As the left ventricle becomes thicker and stiffer, it becomes more difficult for the atrium to push blood into the ventricle. As the atrium strains to do this, it becomes larger and larger. Once the atrium stretches out, it causes the valve between the atrium and ventricle to malfunction. Pressure eventually builds up in the left atrium, and this pressure can be transmitted to the lungs, resulting in fluid retention in the lungs and congestive heart failure.
HCM can strike any breed of cat, but Maine Coons are predisposed to the disorder. For years, the genetic cause of HCM in this breed was a mystery. In 2005, however, the reason was discovered: a mutation in the gene that codes for a protein, called “myosin-binding protein C” (abbreviated MYBPC3). Mutations in MYBPC3 are the most common cause of hereditary HCM in people, but a genetic mutation causing any type of heart disease in a cat had never been reported until that study was published.
The prevalence of HCM in cats has been reported to be anywhere from 1.6% to 8.3%, depending on the population evaluated and the method of diagnosis. The prevalence of HCM in the Maine Coon cat population is not known, but recently, a study was performed to determine the percentage of cats worldwide with this worrisome mutation.
DNA (either blood or a cheek swab) samples were submitted to the Veterinary Cardiac Genetics Laboratory at Washington State University. Samples came from both clinically normal cats as well as cats known to have HCM. A total of 3,310 samples, from 21 different countries, were submitted. At least 17 different breeds were represented. Maine Coons accounted for 100% of all of the positive samples; no other breed tested positive for the MYBPC3 mutation. Of the 3,238 samples from Maine Coons, 1089 tested positive for the mutation. In other words, the overall worldwide percentage of Maine Coon cats with the mutation was 34%.
Genes come in pairs. Only one gene in a pair may be affected by a mutation, or both genes may be affected. When one gene is affected, we say that the cat is “heterozygous” for the mutation. When both genes in the pair are affected, the cat is termed “homozygous” for the mutation. Of the 1,089 Maine Coon cats that tested positive, 91% were heterozygous for the mutation. Only 9% had the mutation in both copies of the gene.
So what does this mean for Maine Coon owners and breeders? Clearly, the MYBPC3 gene mutation appears to be specific to Maine Coons only, and at 34%, is fairly prevalent within the Maine Coon cat population. Does testing positive for the gene mean that the cat will develop HCM? In this prevalence study, the authors did not attempt to correlate the presence of the gene with the development of HCM, however, it seems clear that not all cats that test positive for the MYBPC3 mutation will go on to develop HCM. Also, in the original study that identified the mutation that causes HCM in Maine Coons, cats that were homozygous for the mutation (both genes in the pair affected) developed severe disease, with most dying before the age of 4, whereas 3 out of 10 Maine Coons that were heterozygous for the mutation (one gene in the pair affected) had moderate disease and were still alive at 8 to 12 years of age.
For Maine Coon breeders, the development of the test is a major breakthrough. The mean age at diagnosis of cats with HCM is 5.9 years. The data we have so far do not yet indicate the percentage of cats with the mutation that do go on to develop clinical HCM, nor do they tell us what percentage of cats without the mutation go on to develop HCM. Because cats with the MYBPC3 gene are believed to be more likely to develop clinical HCM than negative cats, these cats should be evaluated periodically by a veterinary cardiologist to see if HCM is developing. From a breeding perspective, having this genetic test affords an opportunity to detect cats with the mutation years before they develop HCM, and would allow selective breeding programs aimed at eliminating the gene from the population.
Maine Coons with the MYBPC3 mutation are believed to be more likely to develop HCM than if they did not have the mutation.
Maine Coons with the mutation affecting both copies of the gene tend to have more severe disease compared with Maine Coons having the mutation affecting only one gene in the pair
Not all Maine Coons with the mutation will develop HCM
Non-Coons that develop HCM do not have the MYBPC3 mutation; this mutation is only present in Maine Coons.
Whether Maine Coons that test negative for the mutation can go on to develop HCM is not known at this time.