The Benefits of a SeXist RNA

Equality between the sexes is not just a social and economic imperative. On a biologic level, X-chromosome inactivation (XCI) ensures balanced gene expression between males and females. A major effector of XCI is the long, non-coding RNA, Xist, a 17-kb X-inactive specific transcript that is a component of the X-chromosome inactivation center on the inactive X chromosome. Transcriptional silencing of one X chromosome commences during embryonic development and mechanistically is mediated by Xist RNA coating of the X chromosome and cis-silencing of X-linked genes through binding of polycomb-repressive complexes. Although expression of Xist continues in the post-embryonic state, some in vitro models indicate that Xist may not be required to maintain XCI since its deletion does not always lead to reactivation of the inactive X-chromosome.

After XCI is established, the biologic effect of deleting Xist is unclear. A corollary issue is whether reactivation of X-linked genes occurs, and if so, what are the consequences? It is known that individuals with supernumerary chromosomes (e.g., males with a XXY karyotype) exhibit an increased risk of developing various types of cancer. In addition, certain tumors are characterized by X chromosome aneuploidies, including loss of the inactive X, duplication of the active X, or acquisition of additional X chromosomes. However, the causal relationship between these abnormalities and tumor initiation or progression remains uncertain.

Using murine models where Xist is conditionally deleted in hematopoietic stem cells after establishment of XCI, Eda Yildirim, Jeannie Lee, and colleagues at Harvard Medical School demonstrate a direct link between Xist and development of cancer. Those investigators created two types of mutants by deleting Xist from either the inactive X in 100 percent of cells or from the inactive X in 50 percent of cells and from the active X in 50 percent of cells. Although pups were born normally, mutants started to die at 1.5 months, and only 10 percent were alive after two years of monitoring. Notably, lethality was restricted to females (and was similar in frequency between the two types of mutants), whereas males and controls demonstrated no pathology. Mutant animals frequently exhibited the clinical and laboratory features of a chronic myelomonocytic leukemia or erythroleukemia. Histopathologic analysis of necropsied mice confirmed features of an overlap myeloproliferative/myelodysplastic syndrome. With progression, animals died of a wasting illness characterized by a predominantly myeloid hyperproliferation accompanied by massive splenomegaly. In these cases, histopathology revealed progressive reticulin fibrosis, leukoerythroblastosis, and extramedullary hematopoiesis involving the spleen and liver, consistent with myelofibrosis. Histiocytic sarcoma, sometimes widely metastatic, and multi-organ lymphoplasmacytic vasculitis were also recurrent disease features of the mutant animals.

Transplantation experiments confirmed that the MDS/MPN phenotype is derived from a hematopoietic cell rather than from a stromal cell origin. In addition, HSCs with deleted Xist exhibited qualitative and quantitative defects, including impaired maturation and a reduced capacity to repopulate lethally irradiated hosts. Gene expression profiling of Xistmutants showed a significant upregulation of expression of X-linked genes, confirming that Xist is in fact required not only for initiation of XCI but also for maintenance of XCI. Changes in expression were not confined to X chromosome genes but also affected the autosomal genome, resulting in perturbation of molecular pathways involved in DNA replication, chromosome segregation, cell-cycle regulation, and hematopoiesis.