Colorectal cancer hot spot mutations attenuate the ASXL-MLL4 interaction

Area of Science:

• Molecular Biology
• Cancer Research
• Epigenetics

Background:

• Additional sex combs like (ASXL) proteins regulate gene expression by interacting with chromatin modifiers.
• ASXL genes are frequently altered in colorectal cancer (CRC), with ASXL1 identified as a key hub gene.
• ASXL proteins interact with tumor suppressors BAP1 (a deubiquitinase) and MLL4 (a methyltransferase).

Purpose of the Study:

• To investigate the interdependent roles of MLL4 and BAP1 in gene regulation within human colon carcinoma cells.
• To examine the impact of ASXL proteins on the targeting and function of MLL4 and BAP1 at specific genomic regions.
• To identify the role of CRC-associated mutations in ASXL genes on the MLL4-ASXL-BAP1 complex formation and function.

Main Methods:

• Chromatin immunoprecipitation (ChIP) assays to determine MLL4 and BAP1 genomic occupancy.
• Co-immunoprecipitation assays to assess protein-protein interactions.
• Mutational analysis of ASXL genes and binding assays to evaluate the impact of mutations.

Main Results:

• MLL4 and BAP1 colocalize and interdependently regulate a subset of genes in HCT116 colon cancer cells.
• BAP1 exhibits altered genomic distribution in CRC cells compared to embryonic stem cells, with increased binding at enhancers and promoters.
• Functional ASXL proteins are essential for MLL4 occupancy at MLL4+/BAP1+ regions, and CRC-associated ASXL mutations impair MLL4-ASXL complex formation.

Conclusions:

• The MLL4-ASXL-BAP1 axis plays a critical role in maintaining normal transcriptional programs in colon cells.
• Alterations in the genomic distribution of this axis, coupled with ASXL mutations, can perturb gene expression and contribute to colon cancer pathogenesis.
• Targeting the MLL4-ASXL-BAP1 complex may offer new therapeutic strategies for colorectal cancer.