Huntington's as a developmental disorder: Molecular and neuropathologic considerations.

Huntington's disease (HD) is classically characterized as a late-onset neurodegenerative disorder of adulthood caused by CAG expansion in the HTT gene. However, mounting evidence from both human and experimental studies suggests that both wild-type and mutant huntingtin play important roles during brain development. In this review, we examine the developmental functions of huntingtin, including its role in neuronal migration, synaptogenesis, suppression of apoptosis, mitotic spindle orientation, and transcriptional regulation. We also discuss how mutant HTT may act through both loss- and gain-of-function mechanisms during early brain development. Comparative evolutionary analysis suggests that HTT is highly conserved and that the emergence of the N-terminal polyglutamine tract may have conferred developmental advantages in organisms with more complex nervous systems. Interestingly, studies in pre-symptomatic human carriers and mouse models have identified potential early-life cognitive benefits associated with moderate CAG expansion, raising the possibility of antagonistic pleiotropy. Understanding huntingtin's dual function in neurodevelopment and degeneration is essential in gaining insights into the earliest stages of HD pathogenesis, long before clinical onset.