MLL3 adaptor function, not methyltransferase catalytic activity, is essential for breast tumor suppression

Area of Science:

• Epigenetics
• Cancer Biology
• Molecular Oncology

Background:

• Mixed-Lineage Leukemia 3 (MLL3), also known as KMT2C, is a frequently altered epigenetic regulator in breast cancer.
• MLL3 loss-of-function accelerates tumor onset, growth, and metastasis.
• The precise role of MLL3's catalytic versus non-catalytic functions in tumor suppression is unclear due to truncating mutations causing protein degradation.

Purpose of the Study:

• To dissect the dosage-dependent and domain-specific functions of MLL3 in breast tumor suppression.
• To determine if MLL3's tumor suppressor activity relies on its histone methyltransferase function or its role as a nuclear protein adaptor.
• To elucidate the molecular mechanisms underlying MLL3's role in breast cancer.

Main Methods:

• CRISPR-engineered mouse mammary stem cell organoid-based breast tumor models.
• Analysis of MLL3 heterozygous and homozygous catalytic-dead (Y4792A) or domain (G367V) mutant models.
• Genetic depletion of UTX (an MLL3-interacting protein).
• Integrated transcriptomic (RNA-seq), epigenomic (CUT&TAG), and chromatin accessibility (ATAC-seq) analyses.

Main Results:

• MLL3 is haplo-insufficient for breast tumor suppression.
• A catalytic-dead MLL3 mutation (Y4792A) did not accelerate tumor progression, indicating catalytic activity is dispensable.
• A mutation affecting BAP1 complex binding (G367V) phenocopied MLL3 loss, accelerating tumor onset and growth.
• MLL3 loss impaired UTX chromatin localization and promoter-proximal epigenetic states (H3K27Ac, H3K27me3, chromatin accessibility).

Conclusions:

• MLL3 suppresses breast tumor initiation through a dosage-sensitive, catalytic-independent adaptor function.
• This adaptor function is critical for maintaining normal promoter-proximal epigenetic states.
• MLL3's non-catalytic role in regulating chromatin interactions is key to its tumor suppressor activity in breast cancer.