Hypertrophic cardiomyopathy (HCM) is genetic in the majority of cases. It follows an autosomal dominant inheritance pattern, meaning a single copy of an altered gene from one parent is enough to cause the condition. Current U.S. data estimates HCM affects about 1 in 327 people, a higher figure than the commonly cited 1 in 500 from older studies.
Which Genes Are Involved
Most cases of genetic HCM trace back to mutations in genes that build the sarcomere, the tiny contractile unit inside each heart muscle cell. The two most commonly affected genes are MYH7 and MYBPC3. MYH7 produces a protein that forms the main structural component of the thick filaments responsible for muscle contraction. MYBPC3 produces a protein that helps regulate how those filaments contract and relax. Less frequently, mutations in TNNT2 and TNNI3, which affect the thin filaments that work alongside the thick ones, also cause HCM.
Despite involving different proteins, these mutations converge on the same problem: they make the heart muscle hypercontractile. MYH7 mutations can destabilize a resting state that normally keeps muscle fibers from firing unnecessarily, putting more fibers on a hair trigger. MYBPC3 mutations often result in the body producing too little of its regulatory protein, removing a brake on contraction. Thin filament mutations make the muscle fibers respond to lower levels of calcium than they normally would, again producing excessive contraction. Over time, this chronic overwork drives the heart wall to thicken abnormally.
Carrying the Gene Doesn’t Guarantee the Disease
One of the most important things to understand about HCM genetics is that penetrance, the proportion of gene carriers who actually develop the disease, is far from 100%. A large meta-analysis in Circulation found that among family members identified through cascade screening, overall penetrance was about 57%, with the average age at diagnosis being 38 years. Penetrance varied by gene: roughly 65% for MYH7 carriers, 55% for MYBPC3, around 60% for TNNT2 and TNNI3, and as low as 32% for the rarer MYL3 gene.
These numbers shift dramatically depending on how the variant was discovered. In people who had their genes sequenced incidentally (through biobank projects or population studies, not because of symptoms), penetrance dropped to around 11%. This gap highlights how much context matters. Family members of someone with known HCM carry a higher baseline risk than a random person who happens to carry the same variant, likely because of shared genetic background and environmental factors that aren’t yet fully mapped.
Age is a major driver. For every additional year of life, the chance of developing the disease increases by roughly 1%. In longitudinal studies following young gene carriers (starting around age 16) for about eight years, only 15% converted to a clinical diagnosis during that window. Conversion was faster with MYH7 variants (about 23%) than MYBPC3 (about 12%). Other factors linked to higher penetrance include male sex, obesity, hypertension, carrying more than one pathogenic variant, and having a strong family history of HCM. One study found penetrance jumped from 36% to 89% in carriers of a particular variant when they also had hypertension or obesity.
Conditions That Mimic Genetic HCM
Not every case of a thickened heart wall is caused by sarcomere gene mutations. Several other conditions produce a similar appearance on imaging but have entirely different causes and treatments. These “phenocopies” include Fabry disease (a metabolic storage disorder), cardiac amyloidosis (caused by abnormal protein deposits), Danon disease, PRKAG2 syndrome, and RASopathies like Noonan syndrome. Clues that point toward a phenocopy rather than true HCM can include the age when symptoms start, the presence of symptoms outside the heart, and the specific pattern of thickening. Genetic testing is the definitive way to tell them apart, and getting the right diagnosis matters because some phenocopies have targeted treatments that sarcomeric HCM does not.
How Genetic Testing Works for HCM
Genetic testing typically starts with the person who has been diagnosed, called the proband. A blood or saliva sample is analyzed for variants in the known HCM-related genes. The result falls into one of three categories: pathogenic or likely pathogenic (clearly or very likely disease-causing), benign or likely benign (not disease-causing), or a variant of uncertain significance (VUS). A VUS means a change was found in the DNA, but there isn’t enough data yet to say whether it actually contributes to disease. VUS results are common because many genetic variants are simply too rare for researchers to have studied them thoroughly. A VUS should not be used to make clinical decisions or to test family members, though its classification may be updated as more data accumulates over time.
When a clearly pathogenic variant is found, cascade testing becomes available for first-degree relatives (parents, siblings, children). This is a targeted test that looks only for the specific variant already identified in the family. Relatives who don’t carry the variant can be released from ongoing heart surveillance. Those who do carry it enter a regular screening schedule with imaging and electrocardiograms, even if they feel completely fine, because the disease can emerge at any point from childhood through middle age.
Family Screening Even Without a Genetic Answer
About 40% of people clinically diagnosed with HCM will not have a detectable pathogenic variant on current genetic testing panels. This doesn’t mean their condition isn’t genetic. It may involve genes or mechanisms that haven’t been identified yet, or variants too subtle for current tests to classify. The 2024 AHA/ACC guidelines are clear: family members still need periodic clinical screening with imaging and electrocardiograms even when genetic testing comes back negative or inconclusive. The age of onset and severity can vary enormously within the same family, so a normal screening result at age 20 does not guarantee a normal result at 35.
The guidelines also recommend that any reported pathogenicity classification be reconfirmed every two to three years. As genetic databases grow, a variant once labeled uncertain may be reclassified as pathogenic or benign, potentially changing the screening plan for an entire family.
The Link Between Genetics and Sudden Cardiac Arrest
In pediatric HCM, carrying a sarcomere gene pathogenic variant is strongly associated with sudden cardiac arrest, with one study finding an odds ratio of 10.2 compared to children without sarcomere variants. The most common variants in these cases were in MYH7 (40%) and MYBPC3 (24%). Risk scoring systems that incorporate genetic information have shown strong predictive power, though a persistent challenge is that nearly half of sudden cardiac arrest events in children with HCM occur as the very first sign of the disease, before any diagnosis has been made.
In adults, the relationship between specific gene variants and sudden cardiac death risk is less straightforward. Clinical features like the degree of wall thickening, abnormal blood pressure response during exercise, family history of sudden death, and episodes of abnormal heart rhythms remain the primary tools for risk stratification. Genetic information adds a layer of context but has not replaced these clinical markers for guiding decisions about preventive interventions like implantable defibrillators.