Dilated cardiomyopathy (DCM) is a condition in which the heart muscle becomes weakened and the heart enlarges. The resulting heart failure is the most frequent cause of heart transplantation.
DCM not only affects individuals but also other family members in at least half of the cases. Termed familial dilated cardiomyopathy, it has been shown to be caused by rare variant mutations in more than 30 genes, yet only 35 percent of its genetic cause has been identified.
Ray E. Hershberger, M.D., professor of medicine and associate chief of the Cardiovascular Division at the Miller School, has been leading the Familial Dilated Cardiomyopathy Research Project in a search to discover its genetic causes since 1993. Using two new genomic technologies and a zebrafish model, a team of researchers led by Hershberger has identified rare variants of a gene, BAG3, which causes DCM.
“This is another piece of a very exciting but complicated puzzle,” says Hershberger, who began the FDC Project at Oregon Health & Science University. Collaborating with scientists at the John P. Hussman Institute for Human Genomics and the Division of Nephrology at UM, and the Department of Genome Sciences at the University of Washington, the researchers used exome sequencing, a new targeted method of studying all of the coding regions within the approximately 20,000 genes in a person’s DNA.
Nadine Norton, Ph.D., research assistant professor of medicine in the Cardiovascular Division and first author of the study, led the team in also examining copy number variants, a new technique that finds large portions of genes that are deleted or duplicated. They found that part of the BAG3 gene was deleted in several individuals in one large family with dilated cardiomyopathy. “That led us to suspect that a mutant BAG3 gene is what caused DCM in this family,” says Norton.
The next step was to sequence the coding regions of BAG3 in 312 other DCM families enrolled in the Familial Dilated Cardiomyopathy study. They identified other BAG3 mutations in seven more families, strengthening the genetic evidence that BAG3 rare variants were capable of causing DCM.
“This marked the first discovery of mutations in BAG3 in people with dilated cardiomyopathy,” says Hershberger.
The study team moved to an animal model to confirm that an alteration of BAG3 leads to dilated cardiomyopathy. Steve Mangos, Ph.D., research assistant professor of medicine, works with zebrafish, a fast-growing fish that scientists often use to verify laboratory findings. In this case, the BAG3 gene was knocked down in the fish, which then showed clear evidence of an enlarged heart that pumped less efficiently.
Hershberger says this study is significant for the Familial Dilated Cardiomyopathy Project on many levels. “Sequencing 20,000 genes simultaneously is an enormous leap forward and is revolutionizing our discovery approaches. We also now know that copy number variants contribute to genetic DCM, and that the zebrafish model can be extremely efficient to validate our findings,” he says. “On the human side, we can now help these families understand why their loved ones developed DCM. This discovery is also a key new finding for the field in general because it introduces a previously unknown protein cascade that is highly relevant to understand the cause of dilated cardiomyopathy.”
The study authors also include Duanxiang Li, M.D., M.S., research assistant professor of medicine, Jill D. Siegfried, M.S., C.G.C., genetic counselor, Jorge Gonzalez-Quintana, B.S., research associate, all from the Cardiovascular Division; three members of the John P. Hussman Institute for Human Genomics, Evadnie Rampersaud, Ph.D., research assistant professor of genetics, Stephan Züchner, M.D., Ph.D., associate professor of genetics, and Eden Martin, Ph.D., professor of the Dr. John T. Macdonald Foundation Department of Human Genetics; Libin Wang, M.D., Ph.D. internal medicine resident; Jochen Reiser, M.D., Ph.D., professor of medicine and chief of the Division of Nephrology; and Deborah Nickerson, Ph.D., Mark Rieder, Ph.D., and Sean McGee, Ph.D., from the University of Washington Genome Sciences in Seattle.
Hershberger expects the Familial Dilated Cardiomyopathy Project to discover at least five to ten more genes related to DCM using similar approaches in the next few years.