Mono and biallelic inactivation of Huntingtin gene in patient-specific iPS cells reveal HTT roles in striatal development and in neuronal functions impaired in Huntington’s disease

Mutations in the Huntingtin (HTT) gene cause Huntington disease (HD), a neurodegenerative disorder. While HTT is known to be involved, as a scaffold protein, in several cellular functions, its normal and pathogenic functions during the development of the human forebrain remain poorly understood. To investigate the roles of wild-type and mutant HTT alleles during human neural and striatal development we inactivated HTT alleles in a series of isogenic clones of a human induced pluripotent stem cell (iPSC) line derived from a patient with HD. We show that the loss of wild-type, mutant, or both HTT isoforms did not affect the pluripotency of iPSCs or their transition into neural cells. However, we observed that HTT loss caused division impairments in forebrain neuro-epithelial cells. Moreover, HTT-/- derivatives displayed impaired maturation of medium spiny neurons (MSNs) particularly in the acquisition of DARPP32 expression, a key functional marker of MSNs. Finally, we discovered that MSNs and cortical neurons derived from HTT-/- clones exhibited higher levels of baseline DNA damage and lower BDNF axonal transport, two cellular dysfunctions previously described in HD neurons.