Kathryn M. Meurs, DVM, PhD, DACVIM (Cardiology)
College of Veterinary Medicine, North Carolina State University, Raleigh, NC, USA
Congenital heart disease: Present since birth, may be inherited or may not be inherited
Acquired heart disease: Develops after the animal reaches maturity, may be inherited or may not be inherited
Heterozygote: Has 1 copy of the mutated gene and 1 copy of a normal gene
Homozygote: Has 2 copies of the mutated gene
Penetrance: Percentage of population with a mutation that shows the disease
Expression: Severity of disease
Utilization of Molecular Information for Screening and Therapeutic Issues
Increasingly heart disease in dogs and cats is found to be of inherited origin. This seminar will discuss common testing for known genetic mutations for cats and dogs. Genetic tests are generally a PCR test that identifies either a marker for the disease or that identifies the actual genetic mutation. PCR is a method that takes a small amount of DNA provided by the clinician or owner and amplifies a region of interest so it can be carefully inspected. DNA can be usually provided in a blood sample in an EDTA tube, a buccal swab or even a semen sample. The DNA will be inspected for the abnormality by the lab and the presence or absence of the marker or mutation identified. However, breeders and owners should be cautioned and advised how to best use the information. The results should be carefully considered and should be weighed against the severity of the trait, the size of the breed's gene pool, the mode of inheritance of the trait, and the positive traits that this individual animal brings to a breed. In some cases, strict screening and removal programs may be very detrimental to small gene pools in specific breeds; breeding recommendations should be carefully designed.
We will use two examples for illustration - feline hypertrophic cardiomyopathy and Boxer arrhythmogenic right ventricular cardiomyopathy (ARVC).
Feline Hypertrophic Cardiomyopathy
Hypertrophic cardiomyopathy is the most common form of heart disease in the cat. It is adult-onset and known to be inherited in the Maine Coon and Ragdoll breeds and thought to be inherited in Norwegian Forest, Siberian, Sphynx and Bengal cats among others. Causative genetic mutations have now been identified in the Maine Coon and Ragdoll.
In the Maine Coon, a genetic mutation has been identified in the myosin-binding protein C (MYBPC3) gene. The penetrance of the disease is fairly low for heterozygotes (about 30% show disease) but high for homozygotes (about 80%). The Maine Coon mutation appears to be quite breed specific. It is unlikely to be associated with hypertrophic cardiomyopathy in other breeds of cats unless they are closely related to the Maine Coon breed. Additionally, although this mutation has been shown to lead to the development of this disease in this breed, not all cats with the disease have this mutation, so it is clear that there is more than 1 mutation in the Maine Coon cat.
A substitution mutation has also been identified in the myosin-binding protein C gene in the Ragdoll cat. However, the Ragdoll mutation is different from the Maine Coon mutation and is at a different location.
It is extremely unlikely that the Maine Coon and Ragdoll mutations were inherited from a common ancestor since the mutations are different and are located in such different regions of the gene. Additionally, it is very unlikely that other breeds of cats have the identical mutation.
Genetic testing is now available to test a cat for either mutation by submitting a DNA sample to a reputable screening laboratory (www.ncstatevets.org/genetics/). Good-quality DNA samples can be obtained either from a blood sample in an EDTA tube or by brushing the oral gums of the cat with a special buccal swab, although many labs will even accept samples submitted on a cotton swab.
The test results should verify that the cat is negative, heterozygous or homozygous for the mutation. Cats that test negative do not have the mutation. This does not mean that they cannot ever develop hypertrophic cardiomyopathy; it simply means that they will not develop the form of the disease caused by the specific genetic mutation.
Due to an apparently fairly high prevalence of the mutation in both breeds, it would seem to be unwise to recommend that all cats with the mutation be removed from the breeding programs since this could result in dramatically altering the genetic makeup of these breeds. Additionally, it should be emphasized that not all cats that have the mutation, particularly if they are heterozygous, will develop a clinical form of the disease. Our current recommendations for both breeds are to not use cats that are homozygous for the mutation for breeding purposes since they will certainly pass on the mutation and they have the highest risk of developing the disease. Heterozygous cats should be carefully evaluated. Cats that have many strong positive breed attributes and are disease negative at time of breeding could be bred to a mutation negative cat. Their lack of clinical disease may suggest that they have a less penetrant form of the disease or that they just do not show evidence of this adult-onset clinical disease yet. Therefore, these cats should only be used if they are exceptional for the breed, and they should be clinically evaluated for the disease every year. If they develop the clinical disease, they should no longer be kept in the breeding program. The offspring of the mating of a positive heterozygous and a negative should be screened for the mutation, and, if possible, a mutation-negative kitten with desirable traits should be selected to replace the mutation-positive parent in the breeding colony. Over a few generations this will decrease the prevalence of the disease mutation in the population - hopefully without greatly altering the genetic makeup of the breed too significantly. Finally, disease-negative but mutation-positive cats should be evaluated annually for presence of disease.
Arrhythmogenic Right Ventricular Cardiomyopathy in the Boxer
Since the early 1980s, the term Boxer cardiomyopathy has been used to describe adult Boxer dogs that present with ventricular arrhythmias and, sometimes, syncope. Recent studies have demonstrated that the disease has many similarities to a human disease called arrhythmogenic right ventricular cardiomyopathy (ARVC). The similarities between the diseases include clinical presentation, etiology, and a fairly unique histopathology that includes a fibrous fatty infiltrate of the right ventricular free wall. The disease is most commonly characterized by ventricular arrhythmias, syncope, and sudden death. However, systolic dysfunction and ventricular dilatation are seen in a small percentage of cases.
Arrhythmogenic right ventricular cardiomyopathy is a familial disease in the Boxer and appears to be inherited as an autosomal-dominant trait. Unfortunately, the disease also appears to be a disease of variable genetic penetrance, and affected dogs can have many different presentations including asymptomatic, syncope, sudden death, and systolic dysfunction with CHF. A genetic mutation has now been identified for Boxer arrhythmogenic right ventricular cardiomyopathy, although it is not yet known if this will be responsible for all cases of the disease, since in human beings there are several known mutations. Individuals interested in screening for the disease in breeding animals may do so with either a buccal swab or blood sample (www.ncstatevets.org/genetics/).
In human beings with ARVC, there are multiple genetic mutations that can lead to the development of the disease. At this time we do not know if this mutation is the only cause in the Boxer; therefore, clinical screening is still recommended. Since ARVC presents as an electrical abnormality more often than one of myocardial dysfunction, screening efforts should be based on annual Holter monitoring as well as annual echocardiography. Until a greater understanding of disease inheritance and disease progression exists, caution should be used when advising breeders to remove dogs from breeding programs. Overzealous removal of animals based on the results of a single Holter monitor may have a significant negative impact on the breed.
Genetic testing is becoming increasingly available for the pet population. It should be remembered that inherited disease is complex and that there is no perfect test. Results of the genetic test should be carefully considered and should be weighed against the severity of the trait, the size of the breed's gene pool, the mode of inheritance of the trait, and the positive traits that this individual animal brings to a breed. In some cases, strict screening and removal programs may be very detrimental to small gene pools in specific breeds; breeding recommendations should be carefully designed.