A great deal of progress has recently been made in understanding the genetic basis of DCM with mutations identified in more than 20 genes (reviewed in 2-4
). This progress has led to the recent release of clinical guidelines for the genetic cardiomyopathies,1
and to a rapid expansion of clinical molecular genetic testing. However, molecular diagnosis of DCM is challenging. For example, genetic DCM, usually presenting as adult-onset disease, varies greatly in its age of onset. Thus, even if first degree relatives are available for molecular genetic testing, family members carrying the putative disease-causing mutation may have little or no evidence of disease, confounding efforts to assess segregation of the mutation with disease and therefore confirmation of the variant as disease-causing. Yet paradoxically, it is these family members who may most benefit from clinical surveillance and early intervention to prevent disease progression. Further, although not yet reported for TNNT2
, sequencing the many genes implicated in DCM will likely reveal increasing numbers of variants of unknown significance. Ongoing research integrating molecular, clinical and functional data, as shown here, may facilitate clinical management by adding to the interpretation of gene variations thought to represent disease-causing DCM mutations.
In this study the molecular, clinical and pedigree data for troponin T mutations in these families were supplemented with functional studies with mutant troponin T proteins reconstituted into porcine cardiac myocytes. This approach yielded highly informative calcium sensitivity and maximal force response data that was helpful to augment molecular genetic data for the identified novel mutations.
Aside from conduction and rhythm perturbations observed with LMNA
few genotype/phenotype correlations exist for genetic DCM. However, one useful genotype/phenotype correlation may be the fully penetrant, early onset, aggressive disease observed in this and prior reports9, 15
of troponin T DCM. In the 30 affected subjects herein, the median age of onset was 32.5 years, with an average age of onset of 31.1 years; almost one-half (47%) of those affected had disease onset by 30 years of age. This compares with an earlier analysis from our research database of 304 probands and 166 family members affected with FDC/IDC, where the average age at diagnosis was 43 years.24
The mechanism of the early onset and aggressive nature of troponin T cardiomyopathy is likely related to the severity of disruption of the force-generating components of the thick and thin filaments. Functional reports of DCM mutations have shown decreased Ca2+
sensitivity of the myofilament in the tropomyosin/troponin regulatory complex.17-19, 22, 23
Cardiac troponin T is the subunit that connects the troponin complex to the thin filament. The additional subunits are troponin C that binds Ca2+
, and troponin I that binds to actin and is involved in inhibition of muscle contraction.31-33
This decrease in Ca2+
sensitivity of force development is sufficient to alter the contractility dynamics of the heart leading to systolic dysfunction, as previously observed for the Lys210 deletion and the R141W mutants.18
While skinned porcine myocytes present an ideal model to evaluate functional changes in troponin cardiomyopathy, other systems able to evaluate the functional impact of mutations of other DCM genes (e.g., those encoding contractile, channel, calcium handling and other proteins) could also augment clinical and molecular genetic data to help confirm that a specific sequence variant causes DCM.
In conclusion, the functional data obtained for the six TNNT2 mutations reported in this study suggests that these variants are highly likely to be causative of DCM considering the cumulative data from molecular genetic, clinical, pedigree and functional studies. Adding functional data to existing clinical and genetic data will be key to understanding the molecular etiology of genetic DCM while moving the cardiovascular genetic medicine field ahead.