Abstract 3665: COL4A1 and COL4A2 Mutations cause Genetically Modifiable Cerebrovascular Diseases

Mutations in the type IV collagen alpha 1 gene (COL4A1) cause Cerebrovascular Diseases (CVDs) in mice and human patients. Patients with COL4A1 mutations suffer from a broad range of CVDs, from infantile porencephaly to debilitating or fatal intracerebral hemorrhage (ICH), to subclinical cerebral microbleeds, suggesting that environmental and other genetic factors may influence their phenotypes. COL4A1 is one of the most abundant proteins in basement membranes and forms heterotrimers with COL4A2. Among possible pathogenic mechanisms are cellular stress due to the toxic intracellular aggregation of the COL4A1 and COL4A2 proteins and/or their absence in the basement membrane. Our first goal is to identify the relative contributions of COL4A1 and COL4A2 mutations to sporadic ICH and to understand the cellular mechanisms and genetic complexity underlying the disease. We identified novel COL4A1 mutations and for the first time, we discovered COL4A2 mutations in a cohort of 96 patients with sporadic ICH. Using a cell-based assay we determined that the mutations impair COL4A1 and COL4A2 secretion. We showed that mutant COL4A1 or COL4A2 proteins accumulate within the cell where they titrate normal COL4A1 and COL4A2 proteins. Interestingly, we found that some of the mutations can ultimately result in endoplasmic reticulum (ER) stress and activation of the Unfolded Protein Response. Our second goal was to test the hypothesis that differences in genetic context could contribute to phenotypic variability in human patients. Thus, we characterized CVD in Col4a1 mutant mice with two different genetic backgrounds. Using cerebral magnetic resonance imaging and histological analysis, we show that one or more genetic modifiers from the CAST/EiJ strain significantly reduce the size and frequency of ICHs detected in Col4a1 mutant mice on a C57BL/6J background. In conclusion, we found that both COL4A1 and COL4A2 mutations cause ICH in human patients, our results support that ER stress could be involved in the pathogenesis and we showed that genetic context is crucial for expressivity and severity of the CVD. We predict that ongoing experiments to better understand the cell biology of COL4A1 and COL4A2 mutations and the mechanisms of genetic modification could lead to targeted therapeutics to reduce the risk of CVD in patients with COL4A1 or COL4A2 mutations.