A Lineage-Specific Requirement for YY1 Polycomb Group Protein Function in Early T Cell Development

T cell development originates from hematopoietic stem and progenitor cells in the bone marrow, which migrate to the thymus and obtain T cell identification. Transcription factors play critical roles in regulating early T cell development. While Notch signals are critically required at the early stage of T cell development, the completion of T cell lineage commitment is far from the initial response to Notch signaling. Other transcription factors such as PU.1, Ikaros, and RUNX1 are required to enable progenitor cells to committee T cell lineage before Notch signaling. YY1 is a ubiquitous transcription factor and mammalian Polycomb Group Protein (PcG) with important functions to regulate lymphocytes development, stem cell self-renewal, cell proliferation, and survival. Previous study showed that YY1 can interact with the Notch1 receptor intracellular domain and regulate Notch1 transactivation activities in vitro. Thus, YY1 may also belong to the core T cell lineage regulatory factors and is required for progenitor cell commitment to T cell development.

To test how loss-of-function of YY1 impacts early T cell development, we utilized a conditional Yy1 knockout allele Yy1^f/f with loxP sites flanking the Yy1 promoter region and exon 1. Yy1^f/f mice were crossed to the inducible Mx1-Cre. In Yy1^f/f Mx1-Cre mice, YY1 deletion was achieved after treatment with the pI-pC. Yy1^-/- mice had significantly reduced numbers of lymphoid-primed multipotent progenitor, (LMPP), common lymphoid progenitor (CLP), and double-negative (DN) T cells compared to Yy1^+/+ mice. YY1 deficiency resulted in an early T cell developmental blockage at the DN1 stage. In addition, Notch1 mRNA and protein expressions were significantly reduced in Yy1^-/- thymocytes compared to Yy1^+/+ thymocytes. In Yy1^-/- thymocytes, Notch target gene Hes1 was also downregulated. Thus, YY1 is required for early T cell development and Notch1 signaling.

YY1 mediates stable PcG-dependent transcriptional repression via recruitment of PcG proteins that catalyze histone modifications. Our previous results demonstrated that YY1 PcG function is required for Igκ chain rearrangement in early B cell development, however, it is not required for YY1 functions in promoting HSC self-renewal and maintaining HSC quiescence. Many questions remain unanswered regarding how cell- and tissue-specificity is achieved by PcG proteins. Herein, we utilized a YY1 REPO domain mutant (YY1ΔREPO). The small 25 amino acid REPO domain is necessary and sufficient for recruiting other PcG proteins to YY1-bound chromatin sites in Drosophila. While YY1ΔREPO is competent for DNA binding, transcriptional activation, transient transcriptional repression, and interaction with transcriptional coregulators such as HDACs, it is defective in all YY1 PcG functions and unable to recruit other PcG proteins to DNA. This mutant is therefore a powerful tool for dissecting mechanisms governing YY1 PcG-dependent versus -independent functions.

Bone marrow cells from Yy1^f/f Mx1-Cre mice were transduced retrovirally with MigR1-FlagYY1, MigR1-FlagYY1ΔREPO or MigR1 vector and transplanted into lethally irradiated CD45.1⁺ mice. In addition, Mx1-Cre bone marrow cells infected with MigR1 vector were used as the wild-type control and transplanted into CD45.1⁺ mice. While YY1 is required for DN1 to DN2 transition, YY1 PcG function/REPO domain is not required for DN1 transition. Instead, in mice lack of YY1 PcG function/REPO domain, early T cells had increased cell apoptosis and failed to survive. Interestingly, although YY1 PcG function/REPO domain is critical for early T cell survival, it is not required for YY1 regulation of Notch1 expression.

We concluded that YY1 is a critical regulator for early T cell development and Notch signaling. There is a lineage-specific requirement for the YY1 PcG function/REPO domain for early T cell development. While YY1 PcG function is required for early T cell survival, it is not required for YY1 regulation of Notch1 expression. YY1 PcG and non-PcG functions promotes T cell development by unique mechanisms of promoting cell survival and Notch1 expression respectively.