Why do some hearts get too thick?

By Megan Mayerle, PhD

January 24, 2019

Worldwide, more people die of cardiovascular disease than any other cause. Hypertrophic cardiomyopathy (HCM) is the most common form of inherited heart disease, affecting one out of every 500 people, and can lead to progressive heart failure and sudden cardiac death. The left ventricular walls of HCM patients’ hearts are unusually thick and have trouble pumping blood. Specific variations of two genes, myosin heavy chain 7 and myosin-binding protein C3 (MYBPC3), have been linked to HCM. Most disease-associated variants of MYBPC3 encode directions to make proteins that are too short, which are broken down by specific cellular quality control mechanisms used by cells to avoid wasting resources on making nonfunctional proteins. Diseases like HCM are thought to arise because cells are unable to make enough normal, functional proteins to work correctly.

In recent years, scientists have developed techniques to create pluripotent stem cells from individual patients and manipulate them in a laboratory dish. Scientists can use various tricks to induce these stem cells to become specific cell types such as heart muscle cells, tweak their genomes, test potential therapeutics, and use these cells to understand how diseases like HCM arise.

In a study recently published in Circulation, lead author Timon Seeger, MD and colleagues generated heart muscle cells from stem cells isolated from HCM patients carrying disease-associated, truncated variants of MYBPC3. Using latest tools to edit the genome of these stem cells (CRISPR), the scientists corrected the underlying mutations generating perfect controls.

"Combining advances in iPSC technology with genome editing is a highly powerful tool to study the impact of genetic variants in cardiac diseases," senior author Joseph C. Wu, MD, PhD stated.

From comparative analyses, the team learned that although the heart muscle cells generated from HCM patients appear similar to their corrected, healthy control heart muscle cells, they have trouble properly handling calcium, which can lead to difficulties in heart muscle cell contraction. Determined to understand how truncated forms of MYBPC3 could cause calcium handling issues, the researchers looked deeper, examining the cellular levels of MYBPC3 mRNA and protein, which are usually highly correlated. As expected, HCM-patients had low levels of MYBPC3 mRNA, indicating that their cellular quality control pathways were functioning properly and degrading the truncated MYBPC3 mRNA. However, despite this decrease in mRNA levels, MYBPC3 protein levels were normal in HCM patients, a totally unexpected finding.  Seeger and colleagues further determined specific genes that were activated in HCM patient cardiac cells but not activated in cells made from healthy individuals. They found that pathways associated with RNA metabolism and NMD, the cellular quality control machinery that degrades truncated mRNAs, were unusually active in HCM patients’ cells.

“These findings indicate very early, even pre-disease molecular mechanisms in the onset of HCM, that can be investigated using iPSC technology aiming at developing specific therapeutic approaches, still absent in HCM to date.” Timon Seeger, MD, lead author of the study explains.

The group confirmed that molecular inhibition of the NMD pathway in HCM patients’ cells restored MYBPC3 mRNA levels to normal and fixed how their cells handled calcium, suggesting a direct connection between the unusually activated NMD pathway and pathogenesis of HCM, and a potential new target for HCM therapeutics.

Stanford Cardiovascular Institute co-authors include Rajani Shrestha, MS, Chi Keung Lam, PhD, Caressa Chen, MD, Wesley L. McKeithan, PhD, Edward Lau, PhD, Alexa Wnorowski, MS, George McMullen, BS, Matthew Greenhaw, BS, Jaecheol Lee, PhD, Angelos Oikonomopoulos, PhD, Soah Lee, PhD, Huaxiao Yang, PhD, Mark Mercola, PhD, Matthew Wheeler, MD, PhD, Euan A. Ashley, PhD, Fan Yang, PhD, Ioannis Karakikes, PhD, and Joseph C. Wu, MD, PhD. The work was supported by grants from the German Funding Foundation, the California Institute of Regenerative Medicine; the NIH (R01 HL139679, R00 HL104002, R01 HL141851, R01 HL128170, R01 HL130020, R01 HL126527), the American Heart Association (17IRG33410532, 33610009), and Burroughs Wellcome Fund (1015009).

Timon Seeger, MD