Cardiomyopathy comprises a deterioration of the heart muscle that affects it's ability to efficiently pump blood through the body. Forms of the disease were tied to the alternative splicing of titin, a giant protein that determines the heart`s structure and biomechanical properties, but the molecular mechanism remained unknown. Professors Michael Gotthardt and Norbert Hübner of the Max Delbrück Center (MDC), Berlin, Germany, have found that a gene previously tied to hereditary cardiomyopathy, regulates titin splicing. Understanding this mechanism behind heart function and failure, could lead to more efficient diagnosis and therapies for this disease (Nature Medicine, doi: 10.1038/nm.2693)*.
The ventricular filling of the heart is regulated by the different protein isoforms of titin which are produced through alternative splicing, a process in which the protein-coding regions of RNA (the exons) are connected in different ways, resulting in multiple mRNAs (messenger RNAs) that give rise to many proteins.
Professor Marion Greaser of the University of Wisconsin-Madison, USA, had recently identified a naturally occurring rat strain deficient in titin splicing, which resulted in an elongated titin protein. "Titin naturally shortens around birth as the blood flow is redirected through the heart," Professor Gotthardt explained, "but these rats maintained the excessively long embryonic titin isoforms, which suggests a cause for their cardiomyopathy."
Using genome-wide mapping techniques, the researchers found a loss-of-function mutation in RBM20 (RNA binding motif protein 20)in all the rats that expressed the pathological titin isoform. The rats with this mutation also shared many phenotypic similarities with human patients suffering from RBM20 related cardiomyopathy; specifically, ventricular enlargement, arrhythmia, increased rate of sudden death, and extensive fibrosis.
The researchers also identified a set of 31 genes shared by humans and rats that regulate splicing with RBM20. Included in this group was titin, thus validating the group's previous findings. Many of these genes have previously been tied to cardiomyopathy, ion-homeostasis, and sarcomere biology and future analysis will help resolve their individual contribution to the progression of the disease.
Towards utilizing these findings in a clinical setting, Professor Gotthardt has developed a technique to characterize the functional consequences of individual RBM20 mutations. "We can help patients learn if their RBM20 mutation will likely result in the severe form of the disease so that their physician can devise an appropriate therapy," added Professor Gotthardt. "We are currently utilizing this information to develop novel therapeutic strategies for patients suffering from severe forms of cardiomyopathy."
*RBM20, a gene for hereditary cardiomyopathy, regulates titin splicing
Wei Guo1,10, Sebastian Schafer2,10, Marion L. Greaser1, Michael H. Radke3, Martin Liss3, Thirupugal Govindarajan3, Henrike Maatz2, Herbert Schulz2, Shijun Li1, Amanda M. Parrish1, Vita Dauksaite3, Padmanabhan Vakeel3, Sabine Klaassen4, Brenda Gerull4, Ludwig Thierfelder4, Vera Regitz-Zagrosek5, Timothy A. Hacker6, Kurt W. Saupe6, G. William Dec7, Patrick T. Ellinor7, Calum A. MacRae7, Bastian Spallek8, Robert Fischer8, Andreas Perrot9, Cemil Özcelik9, Kathrin Saar2, Norbert Hübner2, Michael Gotthardt3
1Muscle Biology Laboratory, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
2Max-Delbrück-Center for Molecular Medicine, 13125 Berlin, Germany
3Neuromuscular and Cardiovascular Cell Biology, Max Delbrück Center for Molecular Medicine, 13125 Berlin, Germany
4Cardiovascular Molecular Genetics, Max-Delbrück Center for Molecular Medicine, 13125 Berlin, Germany
5Institute of Gender in Medicine and Center for Cardiovascular Research, Charité-University Medicine Berlin, 13353 Berlin, Germany
6Department of Medicine, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
7Cardiology Division, Massachusetts General Hospital, Charlestown, Massachusetts 02129, USA
8Universitätsklinikum Benjamin Franklin, Charité- University Medicine Berlin, 12203 Berlin, Germany
9Department of Cardiology (Campus Virchow-Klinikum), Charité- University Medicine Berlin, 13353 Berlin, Germany.
10These authors contributed equally to this work