*3.3. Modifications of Titin Isoforms Represent a Therapeutic Target for Treatment of Heart Failure*

Downregulation of RBM20 favors the longer, softer N2BA isoform which could improve diastolic dysfunction and serve as a therapy for heart failure with preserved ejection fraction (HFpEF). In animal models, reduction of RBM20 improved diastolic dysfunction as measured by left ventricular wall thickness, echocardiographic markers, and hemodynamic markers including end diastolic pressure [71,72]. A novel approach has been developed to complete high throughput screening of small molecules to identify compounds that decrease RBM20 levels. The initial trial identified cardenolides, which include digoxin and digitoxin, as small molecules that can effectively reduce RBM20 levels and preference N2BA isoform expression [73]. Further studies in animal models are needed to determine if cardenolides represent plausible therapies for HFpEF via reduction of RBM20 and preference for the longer softer N2BA isoform. In addition to RBM20, other molecules are likely to be capable of regulating titin isoform expression and may represent alternative therapeutic targets. Another molecule that can modify titin transcriptionally is RNA binding motif protein 24 (RBM24). RBM24 is expressed in mammalian hearts and when knocked out in mice causes DCM. In one study, RBM24 knockout mice had a disorganized sarcomere and altered titin isoform expression: however, N2BA:N2B isoform expression was not determined [74]. Further studies of RBM24 are necessary including determination of how specifically titin isoform expression is altered by RBM24. In addition to proteins involved directly in transcription, micro RNAs are capable of transcriptional regulation by binding to mRNA and targeting it for degradation. In a mouse model carrying a TTNtv that develops DCM, it was demonstrated that the micro mRNA miR-208b is significantly over-expressed. Over-expression of miR-208b was also found in patients with DCM not related to TTNtv. When miR-208b was inhibited, mice did not develop a DCM phenotype suggesting that miR-208b plays a role in development of DCM possibly through transcriptional regulation of titin, although the exact mechanism for miR-208b has not been demonstrated [75].
