Background: We have previously described 19 pedigrees with apparent lamin (LMNA)-related dilated cardiomyopathy (DCM) manifesting in affected family members across multiple generations. In 6 of 19 families, at least one individual with idiopathic DCM did not carry the family’s LMNA variant. We hypothesized that additional genetic cause may underlie DCM in these families. Methods: Affected family members underwent exome sequencing to identify additional genetic cause of DCM in the 6 families with non-segregating LMNA variants. Results: In 5 of 6 pedigrees, we identified at least one additional rare variant in a known DCM gene that could plausibly contribute to disease in the LMNA variant-negative individuals. Bilineal inheritance was clear or presumed to be present in 3 of 5 families and was possible in the remaining two. At least one individual with a LMNA variant also carried a variant in an additional identified DCM gene in each family. Using a multivariate linear mixed model for quantitative traits, we demonstrated that the presence of these additional variants was associated with a more severe phenotype after adjusting for sex, age, and the presence/absence of the family’s non-segregating LMNA variant. Conclusions: Our data support DCM as a genetically heterogeneous disease with, at times, multigene causation. Although the frequency of DCM resulting from multigenic cause is uncertain, our data suggest it may be higher than previously anticipated.

Background: Dilated cardiomyopathy (DCM) is a genetically heterogeneous cardiac disease characterized by progressive ventricular enlargement and reduced systolic function. Here, we report genetic and functional analyses implicating the rat sarcoma signaling protein, SOS1 (Son of sevenless homolog 1), in DCM pathogenesis. Methods: Exome sequencing was performed on 412 probands and family members from our DCM cohort, identifying several SOS1 variants with potential disease involvement. As several lines of evidence have implicated dysregulated rat sarcoma signaling in the pathogenesis of DCM, we assessed functional impact of each variant on the activation of ERK (extracellular signal-regulated kinase), AKT (protein kinase B), and JNK (c-Jun N-terminal kinase) pathways. Relative expression levels were determined by Western blot in HEK293T cells transfected with variant or wild-type human SOS1 expression constructs. Results: A rare SOS1 variant [c.571G>A, p.(Glu191Lys)] was found to segregate alongside an A-band TTN truncating variant in a pedigree with aggressive, early-onset DCM. Reduced disease severity in the absence of the SOS1 variant suggested its potential involvement as a genetic risk factor for DCM in this family. Exome sequencing identified 5 additional SOS1 variants with potential disease involvement in 4 other families [c.1820T>C, p.(Ile607Thr); c.2156G>C, p.(Gly719Ala); c.2230A>G, p.(Arg744Gly); c.2728G>C, p.(Asp910His); c.3601C>T, p.(Arg1201Trp)]. Impacted amino acids occupied a number of functional domains relevant to SOS1 activity, including the N-terminal histone fold, as well as the C-terminal REM (rat sarcoma exchange motif), CDC25 (cell division cycle 25), and PR (proline-rich) tail domains. Increased phosphorylated ERK expression relative to wild-type levels was seen for all 6 SOS1 variants, paralleling known disease-relevant SOS1 signaling profiles. Conclusions: These data support gain-of-function variation in SOS1 as a contributing factor to isolated DCM.

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Diastolic dysfunction contributes to disease in both heart failure with preserved and reduced ejection fraction. There are currently no approved therapies to accelerate impaired relaxation for heart failure patients with diastolic dysfunction. A healthy heart can modify its ability to relax through post-translational modifications (e.g., phosphorylation) of myofilament proteins. The inhibitory subunit of the troponin complex, troponin I (TnI), is a key regulator of cardiac contraction and relaxation and is endogenously phosphorylated at Tyr-26. We previously demonstrated that this novel tyrosine phosphorylation decreases calcium sensitivity and accelerates calcium dissociation from troponin C. We therefore hypothesize that increasing TnI Tyr-26 phosphorylation will accelerate relaxation and be beneficial during pathological diastolic dysfunction. To determine the effects of TnI Tyr-26 phosphorylation in vivo , we generated a phospho-mimetic mouse with TnI Tyr-26 mutated to Glu. Structural and functional echocardiography and hemodynamics measurements demonstrate that TnI Tyr-26 phosphorylation accelerates relaxation and increases diastolic function in vivo . To determine the effect of this TnI Tyr-26 phosphorylation mediated acceleration of diastolic function during disease, we performed unilateral nephrectomy with DOCA-salt (neph/DOCA) to induce diastolic dysfunction. Wild-type mice subjected to neph/DOCA develop left ventricular hypertrophy, enlarged left atria, elevated diastolic pressure, and slowed relaxation, indicative of diastolic dysfunction. In contrast with wild-type, TnI Tyr-26 phosphorylation mice subjected to neph/DOCA display normal diastolic function without any cardiac remodeling. Overall, we demonstrate that TnI Tyr-26 phosphorylation inhibits the development of diastolic dysfunction and is therefore a novel mechanism to accelerate myocardial relaxation in vivo.