Notch Signaling Controls Transcription via the Recruitment of RUNX1 and MYB to Enhancers during T Cell Development

Tcrd and Tcrg display identical developmental programs that depend on the activity of the enhancers E delta and E gamma being "on" in pre-beta-selection thymocytes to activate transcription and V(D) J recombination of the unrearranged genes and "off" in post-beta-selection CD4(+)CD8(+) double-positive thymocytes to inhibit transcription of the rearranged genes and avoid the expression of TCR delta- and TCR gamma-chains in alpha beta T lymphocytes. E delta and E gamma activity depends on transcription factor binding to essential Runx and Myb sites and parallels that of Notch signaling. We performed Notch gain- and loss-of-function experiments and found that Notch signaling activates Tcrd and Tcrg transcription by favoring the recruitment of RUNX1 and MYB to the enhancers. Our results suggest that the dissociation of RUNX1 and MYB from E delta and E gamma chromatin in double-positive thymocytes, which results in enhancer inactivation, is caused by decreased Notch signaling triggered by pre-TCR signaling, thereby deciphering the molecular mechanism of Tcrd and Tcrg silencing during beta-selection. These findings reveal a novel molecular mechanism for gene regulation via Notch signaling through the recruitment of RUNX1 and MYB to enhancer chromatin during thymocyte development.