Genetic Basis and Clinical Features of Cardiomyopathy

Idiopathic left ventricular hypertrophy (LVH) occurs in about 4 percent of the general population in the United States. Dr. Christine Seidman reviewed insights of this condition gained from studies of single gene mutations by her group at the Brigham and Women’s Hospital.

Clinical identification of hypertrophic phenotypes

Early studies by Seidman and colleagues used routine cardiovascular techniques in families with hypertrophy to define the mode of inheritance and identify the diseased gene. Newer linkage methodologies shortened this process from many years to weeks. Seidman noted that clinical phenotyping remains the critical yet slowest aspect of the process, although it provides necessary information for understanding the genetic basis of hypertrophy and other forms of cardiac remodeling.

Sarcomere gene mutations leading to LVH

Studies by this group have shown that mutations in genes encoding protein constituents of the sarcomere (contractile apparatus of the heart) give rise to hypertrophic cardiomyopathy. Of the 10 different proteins that can be mutated, the most common mutations occur in the beta-myosin heavy chain (BMHC) and in the myosin binding protein C (MBPC) molecule. The former affects the contractile force generated by the sarcomere, and the latter has functions in the sarcomere that are not well understood. Seidman stated that the large diversity of mutations contributes to some of the differences in clinical manifestations seen in this condition.

About 75 percent of persons with clinically-diagnosed hypertrophic cardiomyopathy have mutations when sequencing of the major hypertrophic genes is performed. Not finding a gene mutation may be due to errors in sequencing or not looking at a particular gene region, or the persons have a different cause for their cardiac hypertrophy.

Glycogen storage mutations leading to LVH

In the persons without sarcomere gene mutations, this group performed familial studies and used the clinical manifestations to identify other gene mutations causing cardiac hypertrophy. This led to the identification of mutations of the gene PRKAG2, a gamma subunit of the AMP kinase complex, which functions as a thermostat for energy in the heart. The PRKAG2 mutation results in the kinase complex remaining active, and ultimately glycogen accumulation. Histopathology reveals “moth-eaten” cardiac tissue in persons with PRKAG2 mutations.

Their mice studies of PRKAG2 mutations showed the animals develop considerable hypertrophy, significant glycogen accumulation, and they have a left ventricular pre-excitation pattern and develop conduction disease over time, all of which is seen in humans.

Further studied revealed that the “moth-eaten” tissue allows the cells enlarged from the glycogen accumulation to rupture through the annulus fibrosis—such that the cells in the atria and ventricle are in contact and atrial electrical activity can jump into the ventricle and bypass the AV node. Seidman and colleagues think this accounts for ventricular pre-excitation in a number of glycogen storage disease, including PRKAG2 disease and Pompe disease.

Mutations of the lysosome-associated membrane protein number 2 (LAMP2), another protein involved in the cardiac glycogen metabolism, results in profound hypertrophy and conduction system abnormalities, in studies by this group. The LAMP2 gene is encoded on the X chromosome, hence women have less resulting disease then men. Seidman stated the LAMP2 mutations are particularly impressive for the amount of resulting hypertrophy. An electrocardiogram of a person with LAMP2 mutation revealed prominent LVH, high-voltage, deep inverted T-waves, and ventricular pre-excitation.

Recognition of glycogen storage mutations is necessary because medical management of the electrophysiologic abnormalities is required, and because children with these mutations usually die before they are 30 years of age.

Sarcomere gene mutations, apical remodeling, and arrhythmia

BMHC mutation generally leads to the development of hypertrophy by 20 to 25 years of age, whereas hypertrophy does not develop until the fifth decade or later in the case of MBPC mutations. This contrast suggests a different disease mechanism for these two mutations.

Seidman and colleagues are working to elucidate early signs of hypertrophic cadiomyopathy in children with these gene mutations in whom the disease is not yet manifested.

Doppler tissue imaging has revealed changes that predate the development of LVH. In healthy young persons without sarcomere gene mutations, there is robust relaxation velocity, about 20 centimeters per second. In contrast, perturbed relaxation capacity is seen in persons with a sarcomere gene mutation without demonstrable cardiac hypertrophy, and this velocity may fall further over time. Seidman stated this suggests that diastolic relaxation abnormalities occur prior to hypertrophic remodeling. Further investigation is needed to determine if this is triggering the remodeling.

The location and distribution of cardiac hypertrophy can be quite variable. Apical remodeling is one of the more interesting and unusual forms. Sequencing of sarcomere genes in a cohort with apical remodeling revealed a number of different mutations in each proband. Notably, Seidman and colleagues found that their family members had other forms of remodeling, such as concentric or asymmetric.

However, in persons with actin 101 mutation, identified in three families by Seidman’s group, apical hypertrophy was found in each affected person. Seidman stated this is an unusual instance in which the genotype does seem to predict the type remodeling.

Sudden death and hypertrophic cardiomyopathy

Seidman and colleagues have used genotyping to understand the sudden cardiac death seen in hypertrophic remodeling. Good clinical criteria to identify persons at risk are lacking. Mutations of the MBPC are associated with a very good survival, whereas BMHC mutations, particularly of residue 403, are associated with a very poor survival; 50% of persons with residue 403 mutation die before 40 years of age without ICD implantation.

Mice studies of the myosin mutations by this group showed the animals have earlier onset of hypertrophy, more cellular myocyte hypertrophy, and more disarray and left atrial enlargement. Further, the myosins are more activated than normal, hydrolyze ATP better, and generate greater force, leading this group to call them super myosins. Thus, the hypertrophy is not compensatory for decreased function, and there is another driver for the hypertrophy.

This group hypothesizes that some mutations alter phosphorylation and thereby drive their effects. MBPC mutations exhibit milder manifestations, and studies of the effect of mutations in these animals revealed the importance of phosphorylation of MBPC for normal contractile function.

Although the mice with myosin mutations, like humans, are prone to significant arrhythmia, not all do have arrhythmia, showing that the underlying genetic cause drives the presence of arrhythmia. In mice with 403 or 453 mutations, which are associated with increased risk of sudden death, some are at risk for ventricular tachycardia (VT) and some are not, despite identical genetic backgrounds. Further study in these animals revealed that only the amount of hypertrophy correlated with inducibility for arrhythmia. Thus, this group suggests the risk of arrhythmias is dependent on gene mutations that lead to more hypertrophy.

To determine what causes hypertrophy in the setting of myosin mutations, this group studied mice with early and late stage disease, with hypertrophy, disarray, and dysfunction. They found that treating the animals with pharmacologic agents that affect calcium, it is possible to accelerate or attenuate the disease. This is in line with the evidence from biochemical studies showing that calcium regulation is altered early in the disease process. Other work has shown that in persons with hypertrophic cardiomyopathy without sarcomere gene mutation, there are a number of mutations within the ryanodine receptor, which is known to cause polymorphic VT.

In mice with ryanodine receptor mutations, Seidman and colleagues showed that they develop some degree of hypertrophy, but no polymorphic VT on exercise. Yet, upon electrophysiologic testing, arrhythmia is provoked. Thus providing, Seidman stated, genetic evidence of the importance of calcium in modifying hypertrophic signals.

Treatment with a calcium channel blocker, diltiazem, normalized some of the changes, and over time the treated animals developed less hypertrophy, myocyte fibrosis, and disarray.

Thus, gene-based diagnosis can identify children at risk for these mutations and inherited hypertrophic cardiomyopathy. Seidman and colleagues are starting their first clinical trial in children to determine if it possible to prevent hypertrophy with pharmacologic strategies to normalize calcium in the setting of these mutations.

Home box-only protein and hypertrophy

Seidman and colleagues are studying other genes that may contribute to remodeling. The expression of home box-only protein (HOP) is decreased early in hypertrophic remodeling and remains such as hypertrophy and fibrosis develop. In mice with a HOP transgenic and myosin mutation, overexpression of HOP was associated with severe neonatal hypertrophy, massive interstitial fibrosis, and death within two weeks. This suggests the importance of this regulator of gene transcription for survival. Studies of genes regulated by HOP are ongoing in their laboratory.

Sarcomere gene mutations and unexplained LVH

To determine whether or not sarcomere gene mutations contribute to more common forms of LVH, this group sequenced the sarcomere genes in 60 persons with unexplained LVH in the Framingham Heart Study. A number of sarcomere gene variants were found, with the vast majority affecting the MBPC gene. About 14 percent of the persons within this population with unexplained LVH had a sarcomere gene mutation, the majority in the MBPC. However, early on there is a very different distribution of mutations compared to what is seen in persons with a classic diagnosis of hypertrophic cardiomyopathy.

Conclusion

Sarcomere gene variants occur commonly in the general population. A normal sarcomere gene is associated with a low risk of ventricular remodeling. However, a mutation that causes profound biophysical changes leads to hypertrophic cardiomyopathy. In the general population, there are likely many other variants that can lead to hypertrophy, particularly in the setting of hypertension, other cardiovascular risk factors, or age.

Elucidating how sarcomere gene mutations alter calcium signaling and result in hypertrophic remodeling can identify other causes of hypertrophy and ultimately therapeutic targets to prevent this process.