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
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
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
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.