Reduced Expression of MTSS1 Increases Sarcomere Number and Improves Contractility in Select Forms of Monogenic DCM
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Background
The I-bar protein MTSS1 is a known modifier of heart failure and contractile phenotypes but its role in modulating contractile dysfunction in genetic forms of Mendelian dilated cardiomyopathy (DCM) is not known.
Methods
The potential role of cardiac MTSS1 in TTN DCM was explored using time-to-event models in longitudinal human datasets. Using induced siRNA and mutant forms of pluripotent stem cell cardiomyocytes (iPSC-CMs) the impact of siRNA knockdown of MTSS upon sarcomere and Cardiomyocyte biology was assessed via quantitative high-content microscopy, and the impact and mechanism of MTSS1 knockdown upon contractility was assessed using engineered heart tissues (EHTs).
Results
Amongst individuals affected with TTN DCM, a variant conferring lower cardiac levels of MTSS1 was associated with significantly improved event-free survival from cardiovascular death or heart transplant (HR 0.29, p=0.0016). Knockdown of MTSS1 by siRNA significantly improved the appearance of iPSC-CM models of TTN (p=2.9e-06), CSRP3 (p=3.1e-14), and RBM20 (p=4.4e-04) DCM as assessed by quantitative microscopy. Correspondingly, siRNA knockdown of MTSS1 increased contractility in EHT models of TTN DCM (p=0.003), CSRP3 DCM (p=0.008), and RBM20 DCM (p<2e-16). Across all genetic backgrounds, knockdown of MTSS1 was observed to increase the number of sarcomeres (p<0.0001), and in co-immunoprecipitation experiments MTSS1 physically interacts with MYO18A a key determinant of early sarcomere formation. Knockdown of MTSS1 resulted in increased transcription of MYH7 (0.29 log 2 FC, p=2.9e-06) along with other sarcomere genes.
Conclusions
In iPSC-CMs Knockdown of MTSS1 by siRNA increased number of sarcomeres and was observed to increase twitch force in select in vitro models, and may suggest MTSS1 plays a previously unrecognized role in modulating sarcomere production. Human observational and iPSC-CM experimental data supports the hypothesis that reduced expression of MTSS1 may be beneficial in Mendelian DCM caused by TTN, RBM20, and CSRP3.
