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Background: Non-ischemic dilated cardiomyopathy (NIDCM) responds variably to intramyocardial injection of mesenchymal stem cells (MSCs). We hypothesized that NIDCM genotype may influence responsiveness to MSC therapy and performed genotyping on all patients in the POSEIDON-DCM trial. Methods: POSEIDON-DCM patients (n = 34) underwent genetic sequence analysis and deletion/duplication testing. The results were classified as positive for pathological variants (PV+; n = 8), negative for any variants (V−; n = 6), or as variants of uncertain significance (VUS; n = 20). All outcomes of therapy were analysed for each category of genetic results. Findings: The 3 groups were indistinguishable at baseline with regard to ejection fraction (EF), demographics, medication use, or functional parameters. V− patients had an increase in EF at 12 months: +13.6% (IQR = +7.8%; +20.5%; p = 0.002), compared with VUS (+6.5%; IQR = +0.9%, +11.1%; p = 0.005) and PV+(−5.9%; IQR = −12.7%, +1.0; p = 0.2; p = 0.01 between groups). Six-minute walk distance improved in V- patients, but not in VUS and PV+. V− patients improved MLHFQ, compared to the other 2 groups, which did not improve over time. EPCCFUs increased by 9.7 ± 1.9 in V− (p = 0.009) compared to VUS and PV+ patients. V− patients had one-year survival (100%) compared with VUS (85%) and PV+ (40%; p = 0.015 log-rank). Similarly, MACE rates were lower in V− (0%) than PV+ (61.9%) or VUS (42.2%; p = 0.021 log-rank). Interpretation: Our findings support the concept that the genetic profile of NIDCM patients plays a role in responsiveness to MSC therapy, with V− patients more likely to benefit and the converse for PV+. This observation emphasizes the need for further genetic studies, because of important implications for the management of NIDCM syndromes. Keywords: Precision medicine, Variants of uncertain significance, Positive for pathologic/likely pathologic variant, Dilated cardiomyopathy

Background Ischemic cardiomyopathy (ICM) and dilated cardiomyopathy (DCM) differ in histopathology and prognosis. Although transendocardial delivery of mesenchymal stem cells is safe and provides cardiovascular benefits in both, a comparison of mesenchymal stem cell efficacy in ICM versus DCM has not been done. Methods and Results We conducted a subanalysis of 3 single‐center, randomized, and blinded clinical trials: (1) TAC‐HFT (Transendocardial Autologous Mesenchymal Stem Cells and Mononuclear Bone Marrow Cells in Ischemic Heart Failure Trial); (2) POSEIDON (A Phase I/II, Randomized Pilot Study of the Comparative Safety and Efficacy of Transendocardial Injection of Autologous Mesenchymal Stem Cells Versus Allogeneic Mesenchymal Stem Cells in Patients With Chronic Ischemic Left Ventricular Dysfunction Secondary to Myocardial Infarction); and (3) POSEIDON‐DCM (Percutaneous Stem Cell Injection Delivery Effects on Neomyogenesis in Dilated Cardiomyopathy). Baseline and 1‐year cardiac structure and function and quality‐of‐life data were compared in a post hoc pooled analysis including ICM (n=46) and DCM (n=33) patients who received autologous or allogeneic mesenchymal stem cells. Ejection fraction improved in DCM by 7% (within‐group, P=0.002) compared to ICM (1.5%; within‐group, P=0.14; between‐group, P=0.003). Similarly, stroke volume increased in DCM by 10.59 mL (P=0.046) versus ICM (−0.2 mL; P=0.73; between‐group, P=0.02). End‐diastolic volume improved only in ICM (10.6 mL; P=0.04) and end‐systolic volume improved only in DCM (17.8 mL; P=0.049). The sphericity index decreased only in ICM (−0.04; P=0.0002). End‐diastolic mass increased in ICM (23.1 g; P