Widespread organ tolerance to Xist loss and X reactivation except under chronic stress in the gut

Significance There is growing interest in pharmacological reactivation of the inactive X chromosome (Xi) as a method of treating X-linked disorders. Indeed, unsilencing of the normal gene copies on the Xi could potentially restore expression of the missing protein for therapeutic benefit. Because general Xi reactivation would be expected to increase expression of some other X-linked genes, the in vivo consequences of any dosage imbalance must be investigated with care. Here, we use multiple mouse models to investigate systemic effects of Xi reactivation and observe a surprising tolerance to loss of Xist RNA and partial Xi reactivation. However, we also observe that Xist and XCI are protective to females during chronic stress in the gut. Long thought to be dispensable after establishing X chromosome inactivation (XCI), Xist RNA is now known to also maintain the inactive X (Xi). To what extent somatic X reactivation causes physiological abnormalities is an active area of inquiry. Here, we use multiple mouse models to investigate in vivo consequences. First, when Xist is deleted systemically in post-XCI embryonic cells using the Meox2-Cre driver, female pups exhibit no morbidity or mortality despite partial X reactivation. Second, when Xist is conditionally deleted in epithelial cells using Keratin14-Cre or in B cells using CD19-Cre, female mice have a normal life span without obvious illness. Third, when Xist is deleted in gut using Villin-Cre, female mice remain healthy despite significant X–autosome dosage imbalance. Finally, when the gut is acutely stressed by azoxymethane/dextran sulfate (AOM/DSS) exposure, both Xist-deleted and wild-type mice develop gastrointestinal tumors. Intriguingly, however, under prolonged stress, mutant mice develop larger tumors and have a higher tumor burden. The effect is female specific. Altogether, these observations reveal a surprising systemic tolerance to Xist loss but importantly reveal that Xist and XCI are protective to females during chronic stress.