Atrial arrhythmopathy of the RBM20-Cardiomyopathy – severe alternations in cellular electrophysiology by dysfunction in splicing factor RBM20

L. Weirauch (Heidelberg)¹, F. Wiedmann (Heidelberg)¹, L. Schraft (Heidelberg)², M. van den Hoogenhof (Heidelberg)³, Y. Wang (Mannheim)⁴, G. Dobreva (Mannheim)⁵, L. Steinmetz (Heidelberg)², C. Schmidt (Göttingen)^1
1Universitätsklinikum Heidelberg Klinik für Innere Med. III, Kardiologie, Angiologie u. Pneumologie Heidelberg, Deutschland; ²EMBL Heidelberg, Deutschland; ³Universitätsklinikum Heidelberg Institut für experimentelle Kardiologie Heidelberg, Deutschland; ⁴Medizinische Fakultät Mannheim der Universität Heidelberg Kardiovaskuläre Physiologie Mannheim, Deutschland; ⁵Medizinische Fakultät Mannheim der Universität Heidelberg Anatomie und Entwicklungsbiologie Mannheim, Deutschland

Dilated cardiomyopathy (DCM) is a significant contributor to heart failure, with a substantial proportion of cases having a familial basis. Among these cases, 3-5% are associated with mutations in the RNA-binding motif protein 20 (Rbm20). This aggressive form of DCM is characterized by progressive cardiac dysfunction, an elevated risk of ventricular arrhythmia, and a high prevalence of atrial fibrillation (AF). The treatment options for this kind of arrhythmia in context of the Rbm20-cardiomyopathy are an important but unmet need in therapy options.    To develop more sufficient  treatment approaches we need to understand underlying remodelling patterns to determine specific targets with efficient effect.
The systematic investigation of molecular changes in context of atrial arrhythmopathy provides us more knowledge about the function of Rbm20 as a splicing factor and its role in cellular electrophysiology.
For comprehensive analysis of molecular changes in context of a mutation in Rbm20 we characterised the electrophysiological properties of Rbm20-mutatant mice performing in-vivo ECG measurements and atrial echocardiography as well as cellular characterisation techniques like patch clamp experiments and RNA sequencing. The mice used for our analysis carry a mutation in  Rbm20  (R636Q). For a better subsumption of the Rbm20-CM specific remodelling patterns we also included a Rbm20^-/- and Lmna^+/- model. 
To confirm the DCM phenotype on atrial level, we quantified the area of the left atrium (LA) which was increased in Rbm20-mutant mice. Non-invasive ECG measurements revealed AF in seven out of eight mice in this group. To investigate the correlating cellular remodelling patterns, we performed patch clamp measurements on isolated atrial cardiomyocytes. The provoked action potentials (AP) showed significant changes in morphological properties promoted in the repolarisation phase. The AP duration at 50% of repolarisation (APD50) was significantly prolonged where at the AP duration at 90% of repolarisation (APD90) exhibited a significant shortening. To gain a deeper understanding of the underlying remodelling patterns that led to those alterations we measured single ion channel groups. We found significant alterations in several of those ion channel currents whereas a significantly decreased outward potassium current and increased TASK-1 current explain those AP morphology changes.
To elaborate potential RBM20-CM specific remodelling patterns we compared those findings to other DCM models. We performed the same experiments on a Rbm20-KO and a Lmna-KO model and all those mice presented their self with a DCM phenotype but in sinus rhythm. Correspondingly, both models also exhibited changes in AP morphology but only in form of APD90 shortening and an increased TASK-1 current.
Consulting the revealed electrophysiological changes, we thoroughly investigated the cellular effects the recently discovered group of the SGLTi of treating heart failure which also had beneficial effects for the AF-burden.
Applying dapa-, empa- and sotagliflozin during patch clamp measurements, we observed a class-I-antiarrhythmic effect, further the inhibition of the fast inward sodium current, a reduced AP inducibility and AP amplitude as well as APD prolonging effects which categorize in class-III.

A mutation in the Rbm20 splicing factor causes several and unique alternations a in cellular electrophysiology and the use of SGLTi as antiarrhythmic treatment seems promising.