Modeling Somatic Second-Hit Mutations in Novel Mouse Models of Hereditary Hemorrhagic Telangiectasia

Hereditary Hemorrhagic Telangiectasia (HHT) is a genetic vascular disorder characterized by distinct vascular malformations, including deep organ arteriovenous malformations (AVMs) and mucocutaneous telangiectasias. People with HHT inherit monoallelic pathogenic variants in members of the TGFβ signaling cascade (<i>ACVRL1</i>, <i>ENG</i> and <i>SMAD4</i>), resulting in a loss of gene function and dysangiogenesis. While these heterozygous inactivating mutations are present in all cells, malformations develop locally, indicating a focal trigger of onset. Indeed, recent human sequencing studies revealed that second-hit somatic mutations, resulting in complete bi-allelic loss of gene function, are linked to lesion formation in the three major types of HHT (HHT1, HHT2, JP/HHT). To model the loss of heterozygosity (LOH) associated with HHT patients, we generated new Eng and Smad4 HHT mouse models whereby endothelial cell-specific, somatic LOH mutations are induced within a heterozygous loss of function background (<i>HHT-</i>iEC-LOH). The <i>HHT-</i>iEC-LOH models recapitulate the mosaic makeup of patient malformations and indicate that multiple, distinct secondary somatic mutations can contribute to AVM onset. Utilizing immunofluorescent staining, blue latex vasculature casting, weighted tracer perfusions, and lineage tracing studies, <i>HHT-</i>iEC-LOH models were phenotypically assessed and compared to traditional inducible endothelial cell knockout (<i>HHT-iECKO</i>) HHT models. Overall, <i>HHT-</i>iEC-LOH mice exhibit increased malformation frequency and vascular phenotypes that are comparable or exceed the severity of iECKO models. Significantly, <i>HHT-</i>iEC-LOH mice can be induced early in development and live into adulthood, displaying persistent cerebrovascular phenotypes. The heightened patient representation offered by these newly developed models enables the study of long-term disease progression and testing of therapeutic interventions.