Etiology of Feline hypertrophic cardiomyopathy just b. cause (Proceedings)

Sarcomeric Gene Mutations in Humans with HCM

In humans, the familial nature of HCM was first reported in 1958. It has been demonstrated that over 70% of human cases of HCM are inherited in an autosomal dominant pattern, with most other cases being sporadic (although often still genetic in origin). Since 1989, over 450 mutations in 11 genes that encode for sarcomeric proteins have been identified in human families with HCM. include the β-myosin heavy chain, α-tropomyosin, cardiac troponins I, C, and T, myosin binding protein C, essential and regulatory light chains, titin, actin, and α-myosin heavy chain genes. The genes with the most mutations described and the ones that most commonly produce disease are the β-myosin heavy chain and cardiac myosin binding protein C genes. It is now known that sarcomeric gene mutations actually cause HCM since several mutations that have been identified in human families with HCM have been placed in transgenic mice and the disease reproduced, thus fulfilling Koch's postulates.

Several families with HCM have also been shown to have mutations in the gene that codes for LIM protein. Another set of families has been identified with mutations in the gene coding for AMP-activated kinase that produces glycogen accumulation in the myocardium, mimicking HCM. Interestingly, mutations in β-myosin heavy chain, α-tropomyosin, titin, and troponin T genes also cause  dilated and restrictive cardiomyopathies along with left ventricular noncompaction, a relatively new form of cardiomyopathy in humans (although the mutations are at different sites than those that cause HCM) and mutations in the LIM protein gene cause DCM in mice.

The pathophysiology of HCM production by sarcomeric mutations is relatively unknown and controversial. One theory is that the abnormal protein produced by a mutated gene results in dysfunctional sarcomeres that contract poorly and this forces the functional sarcomeres to bear a larger load. The myocardium compensates by replacing the dysfunctional sarcomeres with additional functional as well as dysfunctional sarcomeres. The net result is that the heart may end up with almost twice as many sarcomeres as it should have. Because sarcomeres make up a large part of the heart muscle, the heart muscle ultimately almost doubles in thickness in severe cases.

HCM severity and penetrance of mutations appears to be determined by the type of mutation, although this is still somewhat controversial. Cardiac myosin binding protein C (MYBPC3) mutations, however, are well known to produce a less aggressive form of HCM with reduced penetrance and age related penetrance in heterozygous individuals. This means that humans with a MYBPC3 mutation may never have echocardiographic evidence of the disease and if they do it's not uncommon for it to first show up when the individual is 50 or 60 years of age.

Familial feline HCM

The first “family” of cats with an inherited form of HCM was identified in Maine coon cats in 1992 and reported in 1999. The disease is inherited as a simple autosomal dominant trait in this breed and is 100% penetrant in experimental cats that are beyond 5 years of age. However, in Maine coon cats in the real world it appears that the disease is not 100% penetrant, since both parents of an affected cat may have no echocardiographic evidence of the disease. The disease has been reproduced in Maine coon cats by mating affected to unaffected and affected to affected cats as well as by breeding affected Maine coon male cats to domestic shorthair female cats. The course of the disease is accelerated in affected cats produced by mating affected to affected cats where homozygous cats are probably more common. The disease is progressive. In most affected Maine coon cats HCM is not apparent during the first year of life but becomes apparent by 2 years of age in males. Females tend to get the disease later, with many manifesting the disease by 3-4 years of age but some not showing evidence of the disease until 6 or 7 years of age. When both parents have HCM, an affected Maine coon kitten may have echocardiographic evidence of the disease as early as 6 months of age and have severe disease by one year of age, whether male or female. Again these are assumed to be cats that are homozygous for the mutation subsequently identified.

In 2005, the first gene mutation responsible for HCM was identified in Maine Coon cats.This mutation is in exon 3 of MYBPC3. The exact location is codon 31 of the gene where a single point base pair mutation changes alanine to proline in the encoded protein (A31P). This region of the gene is highly conserved across species and the resultant amino acid change results in a change in protein structure and function.

In 2007 a mutation in the same gene (MYBPC3) was identified in Ragdoll cats with HCM (C820T).This mutation is at a completely different location on the gene but it again occurs in a highly conserved region. Ragdoll cats have long been known to have a particularly malignant form of the disease where they often die before reaching one year of age. This now appears to be due to the presence of a larger number of homozygous cats in this highly inbred population.

A family of American shorthair cats, primarily with SAM of the mitral valve, but with other evidence of HCM as well, has also been identified. The disease in this breed also appears to be inherited as an autosomal dominant trait. In addition to these breeds, there is anecdotal evidence of HCM being inherited in numerous other breeds, including Persian, British shorthair, Norwegian forest, Ragdoll, Turkish van, and Scottish fold cats, along with others. HCM is most likely inherited when it is identified in a specific breed. However, HCM is most commonly identified in domestic (mixed-breed) cats. Whether the disease is inherited in these cats, is due to a de novo mutation, or is associated with a different disease process is unknown although suspicion of inheritance has been reported in mixed breed cats. Since the disease is inherited as an autosomal dominant trait in some purebred cats, it is not hard to imagine how these mutations could make their way into the mixed breed population and disseminate.

A recent study (J Vet Intern Med 2010;24(3): 527-532 ) appears to try to cast some doubt on the validity that the A31P mutation in Maine Coon cats causes HCM. This study looked at a group of Maine Coon cats during a screening clinic and tried to correlate the genetic findings with echocardiographic findings. They found some Maine Coon cats with the A31P mutation did not have echocardiographic evidence of HCM and some cats without the A31P mutation that had HCM. The veiled conclusion by the authors was that the A31P mutation might not be causal and that genetic testing for the mutation in this breed may not be warranted. However, the study design was too flawed to make any such conclusion. What was shown instead is that the penetrance of this mutation may be relatively low, at least in young cats, which is not surprising since most MYBPC3mutations in humans have a low and an age-related penetrance. And that there appears to be at least one more cause of HCM in this breed. So the correct interpretation of this study is that it appears (if the genetic analysis was correct) that the A31P mutation is not 100% penetrant (not all cats with the mutation have echocardiographic evidence of HCM) and that there is at least one more cause for HCM in Maine Coon cats that is responsible for HCM. That there is at least one more cause for HCM in Maine coon cats has been apparent in the colony at UCDavis for a number of years.

Another study was published that also made preliminary observations on the echocardiographic appearance of hearts from Maine coon cats with the A31P MYBPC3 mutation.29 In this study, echocardiography was performed on 96 Maine coon cats presented for screening for HCM (J Vet Intern Med 2009;23(1): 91-99 ). Both two-dimensional and tissue Doppler imaging echocardiography were performed. Cats had to have an LV wall thickness greater than 6 mm to make the diagnosis of HCM. Of the 96 cats, 44 of the cats had the A31P mutation (38 were heterozygous and 6 were homozygous). Unfortunately, 45 of the 96 cats were less than 2 years of age and so too young to have evidence of HCM if they were heterozygous. These young cats probably should have been excluded from analysis if they were heterozygous. Of the 38 heterozygous cats, four had clear evidence of at least moderate HCM. However, only 10 of the 34 heterozygous cats that did not have HCM were over 4 years of age. Of the 44 cats that had the mutation, 13 were male and 31were female. This produced an additional bias since female Maine coon cats get HCM at a later age and get less severe disease. Still, this study clearly suggests that the A31P MYBPC3 mutation is not 100% penetrant in Maine coon cats when a cat is heterozygous for the mutation. As expected, homozygous cats appear to be a different story. Of the 6 cats that were homozygous for the A31P MYBPC3 mutation in this same study, 4 had clear echocardiographic evidence of HCM and the other two had abnormal diastolic function as assessed by TDI, an abnormality previously shown to be present in cats that go on to develop HCM. Consequently, it appears that all 6 of these cats had HCM, which once again proves the mutation to be causal. Two cats in this study without the known mutation also had HCM. This once again documents that there is at least one more cause of HCM in Maine coon cats.

And most recently another study confirmed that the penetrance of the A31P mutation in Maine Coon cats heterozygous for the A31P mutation is low in young cats (Acta Vet Scand 2011;53: 7-17). This study also concluded that most likely the majority of young Maine Coon cats that get HCM are homozygous for the mutation. 

Both of the previously referenced studies make only a preliminary attempt to look at penetrance of the A31P mutation in Maine coon cats that are heterozygous for the A31P mutation. What really needs to be done is a longitudinal study looking at the same mutated cats until they are at least 7-8 years of age to tell what the true penetrance is for heterozygous cats rather than looking at a group of cats once at one point in time.

We are currently examining the cellular effects of this mutation and have preliminary evidence to show clear abnormalities produced by this mutation (e.g., myocardium from cats homozygous for the A31P mutation has no myosin binding protein C). Our preliminary findings clearly show that the A31P mutation causes intracellular derangement.

There are currently several labs that test for the A31P MYBPC3 mutation in Maine Coon cats (one in the USA and at least two in Europe). This service has been set up so that breeders can try to rid their breed of this mutation and the HCM caused by this mutation. Many breeders have been reluctant to do test and even more reluctant not to breed mutated cats. To be fair, the method of dealing with this problem is controversial. In the lab at Washington State University, approximately 35% of the DNA samples submitted have had the mutation. This suggests that the mutation is very prevalent in this breed. Concern has been expressed about what would happen if all of these cats were removed from the breeding pool. The concern has to do with decreasing the size of the gene pool and so producing more recessive traits in this breed. In other words, if 35% of the cats could no longer be bred the remaining cats would have to make up the breeding pool and by shrinking that pool the breed may be worse off because of increased inbreeding.  The author's counter to that argument is as follows. Breeders commonly only use around 10% of cats in a purebred population for breeding. That means they already exclude 90% of cats from being bred. So if we recommend that they don't breed any cat with a mutation that means we're recommending that they not use 35% of 10% or 3.5% of the entire population. Yes, that does mean that 35% of the cats they would normally breed (the good-looking 10%) would be no longer eligible for breeding. But it also means that they could still use the 65% of the cats they normally don't breed (the less than good-looking 90%) to breed. What would they sacrifice? Maybe their breed wouldn't be quite as visually attractive as before – but it also might be healthier without the mutation. Of course, getting breeders to breed cats they don't think look quite as good as some others is probably an impossible task. So one recommendation has been made that if a cat is heterozygous for the A31P mutation and it is very good looking, it can be bred once. Its kittens then should be tested for the mutation and only those without should be bred. The question then comes, what do you do with the mutant kittens that have been produced in this process? And this recommendation was first made 3 years ago meaning breeders have had plenty of time to do this and should no longer be relying on this method. Consequently, the author's current recommendation is that no cat with the A31P mutation should be bred.