A theory for polymicrogyria and brain arteriovenous malformations in HHT

Hereditary hemorrhagic telangiectasia (HHT) is generally considered a disorder of endothelial dysfunction, characterized by the development of multiple systemic arteriovenous malformations (AVMs), including within the brain. However, there have recently been a number of reports correlating HHT with malformations of cortical development, of which polymicrogyria is the most common type. Here we present 7 new cases demonstrating polymicrogyria in HHT, 6 of which demonstrate a brain AVM (bAVM) in close spatial proximity, with the aim of providing a common origin for the association. Upon reviewing patient genetics and imaging data and comparing with previously reported findings, we form 2 new conclusions: (1) polymicrogyria in HHT appears exclusively associated with a subset of mutations in the transmembrane protein endoglin that is involved with blood flow-related mechanotransduction signaling during angiogenesis and (2) the polymicrogyria is characteristically unilateral, typically focal, and correlates with vascular regions experiencing low fluid shear stress during corticogenesis in utero. Integrating these with findings in the literature from genetics and molecular biology experiments, we propose a theory suggesting haploinsufficient endoglin mutations, especially those that are dominant-negative, may predispose focal, aberrant hypersprouting angiogenesis during corticogenesis that leads to the production of polymicrogyria. This hypoxic insult may further serve as the revealing trigger for later development of a spatially coincident bAVM. This hypothesis suggests an essential role for endoglin-mediated hemodynamic mechanotransduction in normal corticogenesis.