Abnormal neurodevelopment predisposes to cortical hyperexcitability in Huntington's disease

Accumulating morphological and electrophysiological evidence demonstrates that abnormal brain development is a key element in the progression of Huntington's disease (HD). Mutant huntingtin affects corticogenesis, cell migration, and differentiation. Cortical changes are reminiscent of focal cortical dysplasia, a malformation of cortical development that leads to hyperexcitability and epilepsy. Striatal development also is affected by the mutation. In animal models, recent studies provide additional evidence that neuronal morphology and intrinsic and electrophysiological properties deviate from normal development. Some changes indicate delayed development of cortical pyramidal neurons, while a subtype of striatal projection neuron displays a transient accelerated maturation. However, the brain is able to compensate for early abnormalities and, during a variable latent period, brain function appears normal. Eventually, homeostatic mechanisms begin to fail, resulting in the emergence of HD symptoms. The realization that neurodevelopment in HD is abnormal offers new insights and opens new avenues for early treatment. In this review, we present a brief summary of imaging and morphological studies from human carriers of the HD mutation followed by a more in-depth examination of recent findings in genetic animal models.