Cardiomyopathies are a group of mostly hereditary, rare heart diseases in which the structure of the heart muscle tissue is impaired. This limits the heart’s pumping capacity, leading to shortness of breath, reduced physical capacity and other typical symptoms of heart failure.

In some affected individuals, the RBM20 protein is mutated; they are currently treated in the same way as people with heart failure. However, a more specific treatment could be beneficial for this particular group in future. This is shown by a new study, led primarily by researchers from Würzburg, Heidelberg and Göttingen. The results have been published in the specialist journal Signal Transduction and Targeted Therapy.

A small difference with major consequences

The study team analysed two families, each of which has a specific form of cardiomyopathy associated with a particular mutation in the RBM20 gene. In one family, the condition is dilated cardiomyopathy (DCM), in which the heart chambers become severely enlarged and the heart can no longer pump effectively. In the other family, the condition is left ventricular non-compaction cardiomyopathy (LVNC), in which the heart tissue is riddled with sponge-like holes.

Both families are under the medical care of, and have undergone genetic testing by Professor Benjamin Meder at Heidelberg University Hospital. Analyses showed that there is only a tiny difference between the mutated RBM20 proteins in the two families: in the DCM family, at a specific site on the RBM20 protein, a single amino acid has been replaced by another – specifically, an arginine has been replaced by a tryptophan. In the second family, that very same arginine has been replaced by leucine.

“We wondered how such a small difference could lead to two such different clinical presentations,” says Professor Katrin Streckfuß-Bömeke from the University of Würzburg, the last author of the publication. The pharmacologist and expert in human cardiomyopathy models has been collaborating in this field for years with her Heidelberg colleague Meder.

Serious consequences for calcium homeostasis identified

Using skin and blood samples from the families, the Würzburg team discovered that the replacement of a single protein building block has serious consequences for calcium homeostasis. The vital mineral calcium acts as a pacemaker in heart cells: it ensures that the heart muscle contracts and relaxes at the correct rhythm.

The researchers discovered that, in the condition characterised by spongy changes in heart tissue, the cells are particularly sensitive to the calcium signal: they have a highly activated calcium cycle and consume an extremely large amount of energy. In DCM heart cells, by contrast, there is a storage problem: calcium leaks out of their internal stores as if there were a leak.

Artificial heart tissue and gene scissors used

To understand the processes described, the team recreated the patients’ condition in the laboratory. For the study, pluripotent stem cells (iPS cells) were generated from the blood and skin cells of the affected families. These cells are ‘jack-of-all-trades’ and can be transformed into any type of cell – in this case, beating heart muscle cells.

Three models were used in the investigations: individual heart cells, spherical organoids and small artificial heart muscle tissues. The latter were produced by Dr Malte Tiburcy’s tissue engineering team at Göttingen University Hospital.

The researchers also used the CRISPR/Cas9 gene-editing tool. Using this technique, they were able to repair the genetic defects in the diseased cells with pinpoint accuracy. They then inserted the DCM mutation into these ‘healed’ LVNC cells. In this way, they discovered that just this one small mutation at the same site on the RBM20 protein is indeed responsible for the very different clinical presentations.

Targets identified for patient-specific therapies

During these analyses, the team identified several ‘molecular players’ that play specific roles in the two inherited heart conditions. ‘We have thus identified new molecular targets that drugs could act upon,’ says Dr Sabine Rebs, first author of the study and a member of Katrin Streckfuß-Bömeke’s team.

The researchers are also able to provide initial indications of potential new active substances. One promising candidate, for example, could be the drug verapamil, which blocks the effects of calcium and is approved for the treatment of cardiac arrhythmias: in laboratory experiments on cells and tissues, the drug was able to partially improve the contractile function of the damaged heart cells.

However, many further studies are needed before new therapeutic strategies are ready for use in humans. Even now, however, the research team is identifying ways in which various forms of cardiomyopathy involving the RBM20 mutation can be treated more individually in the future. Genetic analyses will then help to determine the best possible, tailor-made treatment for each individual affected.

Project participants

In this study, Professor Katrin Streckfuß-Bömeke’s team collaborated closely with the groups led by Professor Christoph Maack from the German Centre for Heart Failure in Würzburg and Professor Markus Sauer from the JMU Chair of Biotechnology and Biophysics.

Also involved were groups from the University Hospitals of Freiburg, Göttingen, Hamburg-Eppendorf and Heidelberg, as well as from Dresden University of Technology, the University of Gießen and Stanford University (USA).