Abstract
Bromodomain and extraterminal domain proteins are epigenetic reader proteins that recognize and bind acetylated histones to facilitate gene transcription. Because of their influence on transcription of genes involved in proliferation and differentiation, small molecule bromodomain inhibitors have entered clinical trials for various indications including several types of cancer. However, our understanding of bromodomain protein function is incomplete, as we do not completely understand their role in vertebrate development. We are investigating the role of the BET protein, Brd4, in cerebellar granule cell progenitor proliferation and cell cycle exit. Dysregulation of cerebellar granule cell progenitor proliferation leads to medulloblastoma, the most common pediatric brain tumor; therefore, understanding the role of Brd4 in granule cell progenitor proliferation is critical. Granule cell progenitor proliferation is dependent on the sonic hedgehog signaling pathway. Our laboratory has used time series expression modeling to identify differential expression of sonic hedgehog pathway genes during granule cell progenitor cell cycle exit and found that expression of the sonic hedgehog activator, Gli1, was dependent on Brd4 phosphorylation. As Brd4 phosphorylation is required for its activity and Gli1 expression, we hypothesized that Brd4 plays a fundamental role in granule cell progenitor proliferation during cerebellar development. Therefore, to specifically examine the role of Brd4 in granule cell progenitor proliferation, we generated a conditional Brd4 knockout mouse line using the Tg (Atoh1-Cre) driver, to limit Brd4 deletion in the cerebellum to granule cell progenitor. Upon verifying Brd4 loss in the granule cell lineage, we determined that granule cell progenitors lacking Brd4 proliferated less, had reduced expression of sonic hedgehog activator genes, and were no longer responsive to sonic hedgehog stimulation. Brd4 loss in in the granule cell lineage diminished overall cerebellum size and led to persistent morphological deficits in the Purkinje cell layer. Adult mice lacking Brd4 in the granule cell lineage displayed behavioral problems including ataxia symptoms and motor and coordination difficulties. Brd4 appears to play a critical role in granule cell progenitor proliferation and leads to lasting problems when depleted in early postnatal development.