MED12 Regulates HSC-Specific Enhancers Independently of Mediator Kinase Activity to Control Hematopoiesis

Beatriz Aranda-Orgillés ,Ramya Raviram,Heinrich Schrewe,María Sánchez-Díaz,Stephanie Lau,Ricardo Saldaña-Meyer,Leonard I Zon,Pedro P Rocha,Nan Zhang,Anne Fassl,Eirini Trompouki,Iannis Aifantis,Leonela Amoasii,Jasper Mullenders,Aristotelis Tsirigos,María Guillamot,Avik Choudhuri,Kun Wang,Danny Reinberg,Jane A Skok,Eric Wang,Clarisse Kayembe,Markus Schöber ,Piotr Siciński

doi:10.1016/j.stem.2016.08.004

Abstract

Hematopoietic-specific transcription factors require coactivators to communicate with the general transcription machinery and establish transcriptional programs that maintain hematopoietic stem cell (HSC) self-renewal, promote differentiation, and prevent malignant transformation. Mediator is a large coactivator complex that bridges enhancer-localized transcription factors with promoters, but little is known about Mediator function in adult stem cell self-renewal and differentiation. We show that MED12, a member of the Mediator kinase module, is an essential regulator of HSC homeostasis, as invivo deletion of Med12 causes rapid bone marrow aplasia leading to acute lethality. Deleting other members of the Mediator kinase module does not affect HSC function, suggesting kinase-independent roles of MED12. MED12 deletion destabilizes P300 binding at lineage-specific enhancers, resulting in H3K27Ac depletion, enhancer de-activation, and consequent loss of HSC stemness signatures. As MED12 mutations have been described recently in blood malignancies, alterations in MED12-dependent enhancer regulation may control both physiological and malignant hematopoiesis.

Highlights

Stem cells rely on the finely coordinated activity of cell-specific transcription factors to selfrenew and differentiate (De Los Angeles et al, 2015; Spitz and Furlong, 2012)
We show that MED12 is essential for hematopoietic stem cell (HSC) homeostasis as Med12 deletion led to hematopoietic stem and progenitor cells (HSPCs) loss, bone marrow failure, and rapid lethality
A significant decrease in HSPC numbers was readily observed on fetal livers at E15.5 (Figures S1C and S1D), suggesting that MED12 is required for adequate fetal hematopoiesis

Summary

Introduction

Stem cells rely on the finely coordinated activity of cell-specific transcription factors to selfrenew and differentiate (De Los Angeles et al, 2015; Spitz and Furlong, 2012). Key transcription factors, such as GATA2, RUNX1, GFI1, or TAL1, are required to preserve hematopoietic stem cell (HSC) function (Orkin and Zon, 2008; Wilson et al, 2010). The Mediator complex is a pivotal coactivator of transcription factor activity that acts as a molecular bridge between transcription factors at enhancers and RNA polymerase II at promoters (Allen and Taatjes, 2015). Mediator is a large macromolecular complex arranged in four modules, the head, the middle, the tail, and the kinase module, the latter being comprised by MED12, MED13, CDK8, and CYCLIN C (Tsai et al, 2014)

Results

Discussion

Conclusion