Of Xist and Dosage Compensation in Mammalian Development and Disease

Abstract

In mammals, equalized gene dosage between the sexes is ensured by a process called X chromosome inactivation (XCI). XCI, mediated by the long noncoding RNA Xist, initiates early in female embryogenesis, both in extraembryonic tissue (“imprinted XCI”) and in all cells of the embryo proper (“random XCI”) to silence one of two X chromosomes. While Xist has been shown to be critical for the establishment of imprinted XCI, the impact of Xist loss on the initiation as well as maintenance of random XCI in vivo has not been fully explored. In this thesis, I describe our work making use of the Cre/loxP system to ablate Xist at different developmental stages and in different tissue types of the mouse. In embryos where Xist was deleted prior to random XCI using a Sox2-Cre driver crossed to the conditional Xist allele, we made the unexpected discovery that a quarter of homozygous mutants developed to term and survived up to three weeks after birth. Dosage compensation was impaired, but the degree of imbalance was significantly lower than the expected two-fold doubling. However, the demise of mutants soon after birth indicate that small deviations from X-autosomal balance have profound effects on overall fitness. In contrast, our mouse model for Xist ablation after establishment of random XCI in the murine kidney (using the Meox2-Cre driver) did not reveal any overt phenotypes even though we detected some loss of dosage parity. We further verified this in a third mouse model where Xist was deleted specifically in the intestinal epithelia post-XCI using the Villin-Cre driver. We again noted partial dosage imbalance. Interestingly, when we combined Xist loss with a colorectal cancer induction model, female mice with intestine-specific Xist loss developed more and larger colon polyps. Dosage imbalance was also more severe for polyps from the Xist mutants, suggesting that Xist continues to be important for dosage compensation in times of cellular stress. Our results underscore the importance of Xist and dosage compensation throughout the lifespan of the female mammal, both during development and in disease.