H3K4 Methyltransferase Activity Is Required for MLL4 Protein Stability

Area of Science:

• Epigenetics and Gene Regulation
• Molecular Biology
• Developmental Biology

Background:

• Transcriptional enhancers regulate cell-specific gene expression and cell fate.
• Enhancers are marked by histone H3K4 mono- and di-methylation (H3K4me1/2).
• MLL4 (KMT2D) is a key H3K4 methyltransferase for enhancers, with partial redundancy with MLL3 (KMT2C).

Purpose of the Study:

• To investigate the functional importance of MLL4's enzymatic activity.
• To elucidate the role of MLL4's H3K4 methyltransferase function in development and cellular processes.

Main Methods:

• Generation of MLL4 enzyme-dead knock-in (KI) mice and embryonic stem (ES) cells with mutations in the SET domain (Y5477A/Y5523A/Y5563A).
• Phenotypic analysis of MLL4 enzyme-dead KI mice and ES cells.
• Assessment of H3K4me1/2 levels and MLL4 protein stability.

Main Results:

• Homozygous MLL4 enzyme-dead KI mice (Mll4^KI/KI) are embryonic lethal around E10.5, mirroring Mll4 knockout phenotypes.
• Enzyme-dead MLL4 protein is unstable in ES cells.
• Mll4^KI/KI ES cells exhibit reduced H3K4me1/2 levels.
• Histone H3.3 K4M mutant expression destabilizes MLL3 and MLL4, but not SET1A/SET1B, suggesting H3K4 methylation is crucial for MLL4/MLL3 stability.

Conclusions:

• MLL4 protein stability is intrinsically linked to its H3K4 methyltransferase activity.
• The enzymatic function of MLL4 is critical for its stability and likely its biological roles.
• These findings provide new insights into the regulation of enhancer activity and epigenetic maintenance by MLL4.