Huntingtin CAG expansion impairs germ layer patterning in synthetic human gastruloids through polarity defects

Huntingtons disease (HD) is a fatal neurodegenerative disorder caused by an expansion of the CAG repeats in the Huntingtin gene (HTT). While HD has been shown to have a developmental component, how early during human embryogenesis the HTT-CAG expansion can cause embryonic defects remains unknown. Here, we demonstrate a specific and highly reproducible CAG length-dependent phenotypic signature in a synthetic model for human gastrulation derived from human embryonic stem cells (hESCs). Specifically, we observed a reduction in the extension of the ectodermal compartment that is associated with enhanced ACTIVIN signaling. Surprisingly, rather than a cell-autonomous effect, tracking the dynamics of TGF{beta} signaling demonstrated that HTT-CAG expansion perturbs the spatial restriction of ACTIVIN response. This is due to defects in the apicobasal polarization in the context of the polarized epithelium of the gastruloid, leading to ectopic subcellular localization of TGF{beta} receptors. This work refines the earliest developmental window for the prodromal phase of HD to the first two weeks of human development as modeled by our gastruloids.