Mutation of LMNA, encoding Lamin A/C nuclear proteins, cause dilated cardiomyopathy and conduction disorders. Here, the authors show that patient-specific iPSC-derived CMs carrying the K219T LMNA mutation have downregulated Nav1.5 channels due to dynamic cooperation of Lamin A/C and Polycomb repressor complex 2 at the SCN5A promoter.