Noonan syndrome (NS) is a multisystemic developmental disorder characterized by its clinical variability with common symptoms such as typical facial dysmorphism, short stature, developmental delay and intellectual disability as well as congenital heart disease. The disease is causally linked to gain-of-function mutations in a number of genes leading to an increased signal transduction along the RAS-MAP kinase (MAPK) signaling pathway. However, our understanding of the pathophysiological alterations and mechanisms, especially of the associated cardiomyopathy, remains limited and effective therapeutic options are lacking. In this study, we present a family with two siblings displaying an autosomal recessive form of NS with severe hypertrophic cardiomyopathy caused by biallelic mutations within leucine zipper like transcription regulator 1 (LZTR1). Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) of the affected siblings recapitulated the hypertrophic phenotype and uncovered a causal link between LZTR1 dysfunction, RAS accumulation, RAS-MAPK signaling hyperactivity, hypertrophic gene response and cellular hypertrophy. Intronic CRISPR repair in the patients’ iPSCs normalized RAS-MAPK signaling activity and cellular hypertrophy paving the way for personalized medical treatment.