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Double heterozygous LMNA and TTN mutations revealed by exome sequencing in a severe form of dilated cardiomyopathy

Roncarati and co-workers performed comprehensive clinical and genetic investigations of a large 6- generation Italian family consisting of 83 family members of which 41 were alive with an age ranging between 5-77 years. Fourteen surviving individuals were diagnosed with dilated cardiomyopathy (DCM) with a variable disease expression ranging from mild exertional dyspnea to severe congestive heart failure and a need for urgent heart transplantation (HTx). Analysis of the pedigree suggested an autosomal dominant pattern of inheritance and thus the presence of a heterozygous disease-causing mutation in affected individuals. However the DCM phenotype,  was most severe in four siblings who required HTx at an early age and based on these observations the authors hypothesized that the severely affected patients carried additional disease modifying variants. Subsequent genetic investigations were performed by whole exome sequencing (WES) in the 3 severely affected sibling and one unaffected relative. A total of 42,000 single nucleotide variants were identified which could be reduced to 28 following filtering against the sequencing results of the unaffected proband, 410 Italian controls, the 1000 Genomes and the  Exome Sequencing Project. Further investigations using data from the Human Phenotype Ontology Project, narrowed the variant list to include only two variations in the genes encoding lamin A/C (LMNA) and titin (TTN), respectively.
Sanger sequencing confirmed that the previously reported missense mutation in LMNA (c.656A>C; p.K219T) was highly penetrant and co-segregated with the DCM phenotype since all affected relatives (n=14) were heterozygous carriers  and all unaffected relatives were non-carriers. Furthermore, five affected individuals were double heterozygous and carried a novel missense variant in TTN (c.14563C>T; p.L4855F). Notably, double heterozygotes expressed a more severe phenotype with an early onset of disease requiring HTx before the age of 35. No one carried the TTN mutation in isolation and it was therefore not possible to elucidate if this variation in itself would cause any disease expression.
Endomyocardial biopsies were available from all family members with DCM (n=14). Myocardial fibrosis and changes of nuclear abnormalities were present in all biopsies with significantly more and widespread changes in myocardial tissue of carriers of both sequence variants.

In the present study, Roncarati and co-workers elegantly demonstrates the feasibility of WES to identify and map the genetic determinants of inherited cardiomyopathies. Furthermore this study points out the importance of performing linkage studies in larger families since such studies provide strong evidence of disease association of identified variants compared to candidate gene studies in cohorts of probands.
Lamin A/C is a frequent disease gene in patients with DCM associated with conduction disease and ventricular tachyarrhythmias. This results confirms that LMNA mutations are often associated with a high penetrance although there is a variable disease expression among mutation carriers.
Next generation sequencing technology has made sequencing of very large genes possible and clinically feasible. The discovery of missense variants in TTN gene in patients carrying a highly penetrant LMNA mutation suggests that TTN missense mutations may aggravate the DCM phenotype when the TTN mutation is co-inherited with a disease causing mutation in another gene.
The current study emphasizes that the genetic background in DCM is complex and suggests that the heterogeneous disease expression may be explained by modifying effects of rare missense variants in compound or double heterozygous individuals.  Next generation sequencing has made it feasible to identify such modifiers which is of great clinical value in individualizing risk stratification and thereby improve management and counseling of DCM patients. The study also underscores the need for thorough bioinformatic analyses before sequence variants can be used for genetic counseling and risk stratification.    


European Journal of Human Genetics, advance online publication, 6 March 2013, doi: 10.1038/ejhg.2013.16

Notes to editor

Presented by: Torsten B. Rasmussen1, MD, PhD, and Prof. Jens Mogensen2, MD, PhD
1Dept. of Cardiology, Aarhus University Hospital, DK; 2Dept. of Cardiology, Odense University Hospital, DK
The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology.