Our Research

XCI maintenance is different in mammalian female lymphocytes

Human Naive T Cells

Human Activated T Cells

We have discovered that the inactive X chromosome lacks the typical heterochromatic modifications in female mature naïve T and B cells. This is the first example of physiologically-relevant female somatic cells that don’t have XIST/Xist RNA, H3K27me3, macroH2A, H2A-ubiquitin, and H4K20me modifications enriched on the inactive X. We also observed that in vitro activation of T and B cells stimulates the return of XIST/Xist RNA and some heterochromatic modifications back to the inactive X.

Using loss of function assays, we identified protein factors responsible for localizing XIST/Xist RNA back to the inactive X during lymphocyte activation.  YY1 and hnRNP U, known XIST RNA binding proteins, return the XIST/Xist transcripts back to the inactive X. Elucidating the molecular mechanism of XIST/Xist RNA relocalization back to the inactive X, and how this transcript initiates the return of heterochromatin modifications, will reveal how the memory of transcriptional silencing is maintained after cell division in lymphocytes.

XIST/Xist RNA localization on the inactive X is perturbed in lymphocytes from female lupus patients and lupus mouse models

Healthy Female Activated T Cells

Lupus patient activated T cells

Remarkably, we have recently found that female lupus patients exhibit differences with XCI compared to healthy controls, which may explain the female bias of autoimmune disorders and why female lupus patients express higher levels of X-linked autoimmunity related genes compared to male patients. SLE-patient B cells have fewer typical XIST RNA clouds and more cells with dispersed patterns of XIST RNA or completely lacking XIST localization. Abnormal Xist RNA localization suggests partial reactivation of the inactive X-chromosome, which could increase gene expression. Using RNA FISH, we demonstrated that female lupus T and B cells frequently exhibit biallelic expression of CD40LG, CXCR3 and TLR7, which correlates with elevated expression of these genes. Moving forward, we are investigating causality of partial X-reactivation for lupus pathogenesis using mouse models. These will be the first experiments that directly connect transcription from the inactive X to lupus pathology.

Diversity of XCI maintenance mechanisms in immune cells

Our lab recently found that other immune cell populations, including dendritic cells, NK cells, and monocytes, have different mechanisms to maintain XCI.  Using single-cell microscopy to image Xist RNA and heterochromatin modifications, we observed that monocytes and NK cells have faint Xist RNA ‘clouds’ and H3K27me3 foci, yet plasmacytoid dentritic cells (p-DCs) completely lack Xist RNA signals and enrichment of this histone mark. Unlike in humans, mouse p-DCs do not exhibit sex differences with interferon alpha production, and interferon signature gene expression in p-DCs is similar between males and females. Thus, immune cells use diverse (and unknown) mechanisms to maintain XCI, and we propose that these mechanisms are susceptible to error, thereby contributing to sex-linked autoimmune diseases. Our lab is actively investigating the molecular mechanisms that maintain XCI in a variety of immune cells.