Team Leader Murdoch Childrens Research Institute, United States
Abstract: The pumping function of the heart relies on the precise regulation of a highly specialised cytoskeleton, known as the sarcomere. These sarcomeres respond to calcium to produce the force required for systemic blood flow. Variants in the genes encoding sarcomeric proteins are strongly linked to a complex group of heart disease, called cardiomyopathy. While the aetiology of cardiomyopathies is diverse, they are commonly characterised by the abnormal structure and function of the heart's muscular walls. Recently, bi-allelic truncating variants in ALPK3 (ALPK3tv) have been associated with severe neonatal-onset cardiomyopathy. Using human pluripotent stem cell (hPSC) models, we defined the pathogenic mechanisms of bi-allelic ALPK3tv-induced cardiomyopathy. Specifically, we discovered that ALPK3 localises to the sarcomere, where it regulates a protein network required for the turnover of sarcomeric proteins. ALPK3 deficiency resulted in impaired protein turnover at the sarcomere, leading to aggregation of contractile proteins and impaired contractile function.
Further, autosomal dominant ALPK3tvs have been associated with adult-onset cardiomyopathy, although there is no functional evidence to support causation. To assess the functional impact of heterozygous ALPK3tvs on heart cell function, we introduced three heterozygous ALPK3tvs into healthy hPSCs. In vascularised cardiac organoids, contractile force and calcium handling was impaired for two of three heterozygous ALPK3tvs investigated, whilst the third showed slowed relaxation time. Aged mice carrying a human heterozygous ALPK3tv showed an increase in ventricular wall thickness and mass, impaired relaxation and myocyte hypertrophy, consistent with clinical reports. Proteomic profiling of cardiac organoids and mouse heart tissue unveiled novel disease mechanisms that were distinct to that of homozygous ALPK3tv. Together, our data indicates that heterozygous ALPK3tv are sufficient to cause cardiomyopathy, both in vitro and in vivo. This suggests that heterozygous ALPK3tvs play a clinically relevant role in the development of adult-onset cardiomyopathy. Strikingly, the disease mechanisms appear distinct from homozygous ALPK3tvs.
