The Competitive Loss of Cerebellar Granule and Purkinje Cells Driven by X-Linked Mosaicism in a Female Mouse Model of CASK-Related Disorders

CASK-related disorders are a form of female-restricted intellectual disabilities associated with cerebellar and pontine hypoplasia. The <i>CASK</i> gene is regulated by X-chromosome inactivation, which results in a mosaic distribution of CASK-expressing and CASK-deficient neurons in the female brain. This mosaic distribution is believed to play a key role in the pathophysiology of X-linked neurological disorders; however, the detailed brain structure has not been extensively characterized. In this study, we used CASK heterozygous knockout (CASK-hKO) mice combined with X-linked GFP reporter mice to investigate motor abilities and the distribution of CASK-expressing cells in the brains of female CASK-hKO mice. The CASK-hKO mice exhibited motor deficits and cerebellar hypoplasia similar to those observed in patients with CASK-related disorders. Interestingly, although half of the cerebellar granule cells were CASK-negative during early postnatal development, almost all Purkinje cells and cerebellar granule cells were CASK-positive in adulthood, suggesting that CASK expression may determine the survival of cerebellar granule cells during postnatal development. We also analyzed CASK-hypomorphic mice, which express 50% less CASK than wild-type mice, and compared hemizygous males and heterozygous females. The CASK-hypomorphic heterozygous females displayed a thinner cerebellar cortex and a higher probability of CASK-positive granule cells in CASK-hKO females, suggesting that the survival of cerebellar granule cells is regulated by a combination of cell-autonomous and cell-competitive mechanisms between CASK-expressing and CASK-deficient cells, which are generated by X-chromosome inactivation. These findings provide new insights into the relationship between the mosaic distribution of cells established by X-chromosome inactivation and the pathophysiology of CASK-related disorders.