Development. 2018 Jan 29;145(2). pii: dev156844. doi: 10.1242/dev.156844.

Chromosomal instability during neurogenesis in Huntington's disease.

Ruzo A¹, Croft GF¹, Metzger JJ^1,2, Galgoczi S¹, Gerber LJ¹, Pellegrini C¹, Wang H Jr¹, Fenner M¹, Tse S¹, Marks A¹, Nchako C¹, Brivanlou AH³.

Abstract

Huntington's disease (HD) is a fatal neurodegenerative disease caused by expansion of CAG repeats in the Huntingtin gene (HTT). Neither its pathogenic mechanisms nor the normal functions of HTT are well understood. To model HD in humans, we engineered a genetic allelic series of isogenic human embryonic stem cell (hESC) lines with graded increases in CAG repeat length. Neural differentiation of these lines unveiled a novel developmental HD phenotype: the appearance of giant multinucleated telencephalic neurons at an abundance directly proportional to CAG repeat length, generated by a chromosomal instability and failed cytokinesis over multiple rounds of DNA replication. We conclude that disrupted neurogenesis during development is an important, unrecognized aspect of HD pathogenesis. To address the function of normal HTT protein we generated HTT^+/- and HTT^-/- lines. Surprisingly, the same phenotype emerged in HTT^-/- but not HTT^+/- lines. We conclude that HD is a developmental disorder characterized by chromosomal instability that impairs neurogenesis, and that HD represents a genetic dominant-negative loss of function, contrary to the prevalent gain-of-toxic-function hypothesis. The consequences of developmental alterations should be considered as a new target for HD therapies.