MLL3 and MLL4 sustain hematopoietic stem cell multipotency by opposing a B-cell default state

Area of Science:

• Hematology
• Epigenetics
• Stem Cell Biology

Background:

• Hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs) maintain a balance between lineage priming and multipotency.
• Epigenetic regulators MLL3 and MLL4 have distinct roles in HSC differentiation and homeostasis.
• The precise mechanisms by which MLL3 and MLL4 control HSC/MPP gene expression and fate decisions remain unclear.

Purpose of the Study:

• To elucidate the distinct and combined roles of MLL3 and MLL4 in regulating HSC and MPP gene expression and cell fate.
• To understand how MLL3 and MLL4 maintain the delicate balance required for multilineage hematopoiesis.

Main Methods:

• Single-cell genomic studies were performed on HSCs and MPPs following conditional deletion of Mll3, Mll4, or both.
• Enhancer networks and superenhancers were analyzed to assess gene expression changes and cell identity.
• Histone methyltransferase activity of MLL3/4 was investigated to differentiate enzymatic from non-enzymatic roles.

Main Results:

• MLL3 deletion had minor effects on HSC/MPP enhancers, while MLL4 deletion led to premature myeloid enhancer activation.
• Compound MLL3/4 deletion abolished myeloid, erythroid, and megakaryocytic potential, causing progenitors to adopt a B-cell identity.
• These identity changes correlated with widespread inactivation of HSC/MPP enhancers and ectopic activation of B-cell superenhancers.
• MLL3/4's role in activating HSC/MPP enhancers is independent of their enzymatic histone methyltransferase activity.

Conclusions:

• HSC/MPP multipotency is maintained by a dynamic interplay between MLL3/4-dependent enhancers (preserving myeloid, erythroid, megakaryocyte potential) and MLL3/4-independent enhancers (priming B-cell identity).
• MLL3 and MLL4 are essential regulators, acting as critical linchpins for multilineage hematopoiesis.
• The findings highlight a novel mechanism where MLL3/4 regulate cell fate decisions through enhancer activity, independent of their enzymatic function.