Research paperFamilial Hypertrophic Cardiomyopathy - Identification of cause and risk stratification through exome sequencing
Introduction
Hypertrophic cardiomyopathy (HCM, OMIM#192600) is an autosomal dominant inherited disorder with heterogeneous phenotype resulting in thickening of left ventricle or/and intra-ventricular septum leading to heart failure (Maron et al., 2012). Prevalence of 1 in 500 globally and most common reason of sudden deaths in young adults (Maron, 2003; Maron, 2004). In view of paradigm shift from common variants to rare variants in understanding complex disease pathology, next generation sequencing methods has immensely facilitated identification of large numbers of rare variants. However, unlike other complex diseases, Cardiomyopathies reported private mutations, that require sequencing all candidate genes, which is time consuming and cost intensive. Exome sequencing is found to be explorer of disease variants in clinical samples with the success rate of nearly 30–40% in identifying casual variant in patients where disease-identifying variants have not been detected in candidate gene screening (Yang et al., 2013).
Till date, a large number of genetic studies have shown association of 11 genes encoding sarcomere proteins or z-disc in most of the HCM cases (Maron et al., 2012; Maron et al., 2014). Mutations in any of these genes result in structural changes in the sarcomere which results in disarray leading to hypertrophy. Three most common genes mutated in HCM patients encoding β- myosin heavy chain (MYH7), cardiac troponin T (TNNT2) and myosin binding protein C (MYBPC3) explains most of HCM cases where genetic abnormalities are found. From these genetic defects, phenotype outcome may vary greatly from gene to gene, like mutations in TNNT2 gene in human and animal models have shown higher frequency of sudden cardiac death (SCD) in presence of mild or absent cardiac hypertrophy, whereas MYH7 mutants had severe LV hypertrophy and had worse prognosis and symptoms, but not associated with sudden death (Maron et al., 2014).
In this study, we undertook whole exome sequencing in the familial HCM. The deductive inference of implicating the candidate gene variant has support from literature on the functionality of this gene variant reported earlier. However, the importance of this study lies in, how exome of a small number of affected and unaffected family, could negate other novel genes and conclusive in implicating a candidate gene variant.
Section snippets
Clinical evaluation of the family
All subjects from the HCM family were enrolled through, EPOCH-H study conducted by cardiology department of the All India Institute of Medical Sciences (AIIMS), New Delhi, India (Biswas et al., 2015). Two brothers were affected with severe form of HCM and relatively moderate LV thickness and had family history of sudden death of their father where we didn't have sample but reported to be HCM patient. This study was approved by the ethics committee of the AIIMS and Department of Anthropology,
Clinical characteristics of HCM family investigated
Clinical details of the HCM family members are shown in Table 1 and pedigree of the family is represented in Fig. 2. At the age of 30 yrs, after careful clinical evaluation, proband was diagnosed for HCM and immediate family screening was called due to previous family history of sudden cardiac death. ECG and Echocardiographic examination revealed that proband (V) had atrial fibrillation with left ventricular hypertrophy pattern without LVOT obstruction. Proband had pain in chest, shortness of
Discussion
Due to tremendous increase of NGS application in clinical genetics in the form of targeted sequencing and whole exome sequencing, detection of causal variant in cardiomyopathy patients had also increased (Kummeling et al., 2015). This is the first study to validate the TNNT2 (Arg92Trp) as causative variant of familial HCM using WES. One of the proband daughters, who had atrial septal defect, had Arg92Trp in TNNT2 but no HCM. She is at risk of developing HCM at the later age. As a recent study
Conclusion
With sequential filtering and systematic approach, whole exome sequencing facilitated successful identification of one of the putative variant, the TNNT2 R92W in a small HCM family. Mutation is implicated by exclusion of all other variants/genes, and support from literature on its functional validity. Further, this mutation was associated with high penetrance, risk of SCD and moderate hypertrophy. Risk stratification of the disease on the basis of clinical severity may be achieved to holistic
Funding
This work was supported by the Department of Biotechnology (DBT), Government of India [BT/PR 5767/MED/12/563/2012] and Indian Council Medical Research Emeritus Medical Scientist award to VRR (ICMR/74/1/2016-Pers, EMS).
Conflict of interest
None declared.
Acknowledgments
We acknowledge CSIR-Institute of Genomics and Integrative Biology, New Delhi for exome sequencing facility and HCM patients and their family for participating in the study.
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