Chief Scientist, AstraZeneca R&D and adjunct Professor, Karolinska Institute, Sweden
Director, Erich & Hanna Klessmann Institut for Cardiovascular Sciences at the Heart and Diabetes Center, University Hospital of the Ruhr-University, Germany
Director of the Institute of Physiology II, University of Munster, Germany
Heart failure is a life-threatening disease that occurs when the heart cannot pump enough blood around the body. It affects 64 million people worldwide, and is a leading cause of death and hospitalisation.1
The majority of HF cases are due to cardiomyopathies (maladies of the cardiac muscle), heart disease or hypertension. Studies have revealed a significant hereditary component to heart failure, and variants in the genome have been identified to play a role in the development of the disease. The TTN gene provides instructions for making a very large protein called titin, which plays an important role in various muscles in the body, including myocardium (heart muscle). TTN truncating variants (TTNtv) are a significant cause of genetic heart failure, including dilated cardiomyopathy (DCM), but the underlying molecular mechanisms are not well understood and, hence, available therapeutic approaches are limited.
In our recent publication in Science Translational Medicine, we used next-generation sequencing to study myocardial tissues from 14 nonfailing donor hearts and 113 end-stage failing hearts from DCM patients for titin expression and identified a TTNtv in 22 DCM patients (19.5%).
We found that one of the parental gene copies was truncated in the TTNtv-DCM patients and the other was incapable of producing sufficient titin protein (titin haploinsufficiency). This deficiency in the production of titin resulted in the loss of sarcomeres (the contractile units of muscle cells, including cardiomyocytes) in TTNtv patient hearts and, therefore, reduced cardiac pump function, a key characteristic of heart failure.
For the first time, we were also able to demonstrate that adult TTNtv-DCM hearts produce truncated (shortened) titin proteins. The expression levels of these truncated proteins vary considerably and may determine the manifestation of the disease in adult TTNtv-DCM patients. This suggests that truncated titin proteins contribute significantly to disease burden.
Interestingly, truncated titin proteins were not detected in sarcomeres but were present in intracellular clumps. This suggested that the truncated titin proteins are produced and then immediately attacked by the cell’s protein quality control machinery, to reduce their poisonous effect on cardiac cell function.
To obtain deeper insight into the underlying pathophysiology, we used human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and compared non-mutant (wildtype) controls to cells with a patient-derived or induced (by the gene scissors CRISPR-Cas9) TTNtv. TTNtv-hiPSC-CMs showed reduced wildtype-titin expression and contained truncated titin proteins, and these were increased when protein degradation processes were inhibited.
Moreover, we also explored possible innovative therapeutic strategies in engineered heart muscle generated from hiPSC-CMs and were able to reverse the reduced contractility caused by TTNtv by correcting the mutation using CRISPR-Cas9. This eliminated the truncated titin proteins and increased the wildtype-titin content.
This research helps our understanding of the onset and progression of cardiomyopathies by uncovering how a TTNtv causes heart failure. The new insights are being explored for the development of potential new therapeutic targets.
The study was coordinated by Prof. Wolfgang Linke and his team from the University Hospital Münster and also conducted in collaboration with the University Hospital Goettingen, Germany; Heart and Diabetes Centre of Bad Oeynhausen, University Hospital of the Ruhr-University Bochum, Germany; Technical University of Munich, Germany; Karolinska Institute, Stockholm (Sweden), Victor Chang Cardiac Research Institute, Australia; and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Germany