Understanding the mechanistic basis of phenotypic heterogeneity in HTRA1-related vascular disease

HTRA1 is a secreted homotrimeric protease that cleaves a wide range of substrates. Biallelic loss of function mutations cause a monogenic small vessel stroke syndrome which is characterised pathologically by loss of medial vascular smooth muscle cells in the arterial cerebrovasculature. Recent work in UK biobank has identified that rare, heterozygous, missense mutations in HTRA1 increase risk of ischaemic stroke and coronary artery disease (CAD). It is assumed that loss of protease activity results in accumulation of HTRA1 substrates which drive vascular pathology but the specific mechanisms remain unclear. I have found that the risk of CAD due to mutations in HTRA1 is driven by a single missense mutation (p.HTRA1 R227W) carried by ~0.01% of UK biobank participants. This mutation does not affect cerebrovascular disease risk suggesting that distinct mechanisms drive HTRA1-related vascular disease in the brain and heart. We have shown that R227W selectively impairs proteolysis to some but not all HTRA1 substrates providing a plausible explanation for its effects in different vascular beds. I will explore the mechanistic basis of this phenotypic heterogeneity by profiling wildtype and mutant HTRA1 substrates alongside detailed phenotypic characterisation of wildtype and mutant iPSC-derived vascular organoids and cells. This will identify plausible substrates driving vascular risk in each bed and describe the cellular and molecular consequences of cerebrovascular and CAD causing mutations in vascular cells. This data will extend our understanding of the vascular biology of HTRA1 and inform mechanistic models of HTRA1-related vascular disease to be tested in future work.