*3.2. RBM20 Is a Key Regulator of Titin Isoform Preference*

Selecting isoform expression of titin in cardiac tissue represents a potential therapeutic target in treating both systolic and diastolic dysfunction. In systolic dysfunction including DCM, transcriptionally favoring the shorter, stiffer N2B isoform may increase resting tension, decrease end diastolic volume and improve systolic function. Conversely for diastolic dysfunction where there is impaired cardiac relaxation, favoring the longer, softer N2BA isoform may decrease passive tension and improve diastolic filling. One key regulator of transcriptional selection of titin isoforms is RNA binding motif protein 20 (RBM20) [60]. RBM20 is an RNA splicing factor that participates in the spliceosome during maturation of mRNA [57,61]. RBM20 binds introns near splice sites and adjacent to U1 and U2 small nuclear ribonucleoprotein (snRNP) binding sites to regulate transcription [62]. The exact mechanism by which RBM20 regulates titin isoform selection has not been demonstrated; however, RBM20 clearly plays an essential role. Patients with RBM20 mutations develop severe DCM and are at risk of sudden cardiac death [61,63–65]. Sudden cardiac death associated with RBM20 mutations are related to increased arrhythmias with this mutation that are likely caused by abnormal calcium handling and increased

calcium release from the sarcoplasmic reticulum [66]. In addition, RBM20 mutations have been associated with left ventricular non-compaction [67]. Humans with RBM20 mutations have a marked increase in the N2BA isoform [60]. This is also demonstrated in animal models including a rat knockout of RBM20 [60]. Further animal models for RBM20 and its targets have been described but are not as well characterized [43]. The preference for the longer, softer N2BA isoform in RBM20 mutants results in decreased active and passive tension of cardiomyocytes contributing to dilation of the ventricle [68]. Upregulation of RBM20 to preference the shorter, stiffer N2B isoform may be appealing as a treatment for patients with DCM or systolic dysfunction. To date there has been no clear mechanism or small molecule that will increase RBM20 activity. Recent studies in rats have shown that RBM20 can be upregulated by thyroid hormone-triiodothyronine (T3) resulting in preference of the N2B isoform suggesting that modification of the thyroid pathway may be beneficial in treatment of systolic heart failure [69]. Additionally, in cultured rat cells, administration of insulin activates the mTOR kinase axis and RBM20 to preference the N2B isoform [70]. Further understanding of the thyroid and insulin pathways and how they specifically affect RBM20 and titin isoform selection is needed to identify novel therapeutics for systolic dysfunction and DCM.
