PHF20L1 antagonizes SOX2 proteolysis triggered by the MLL1/WDR5 complexes

Area of Science:

• Molecular Biology
• Cancer Biology
• Epigenetics

Background:

• SOX2 is a crucial transcription factor regulating stem cell pluripotency, differentiation, and tumorigenesis.
• SOX2 expression is tightly controlled at transcriptional and post-translational levels, but the mechanisms governing its protein stability are not fully understood.

Purpose of the Study:

• To elucidate the regulatory mechanisms controlling SOX2 protein stability.
• To investigate the roles of MLL1/WDR5 complexes and PHF20L1 in SOX2 proteolysis and stabilization.

Main Methods:

• Small interfering RNA (siRNA)-mediated gene silencing of MLL1/WDR5 complex components (WDR5, MLL1, RBBP5, ASH2L).
• Forced expression of WDR5 and PHF20L1.
• Analysis of SOX2 ubiquitination and proteolysis.
• Mutation of SOX2 at lysine 42 (K42R).
• Treatment with MBT domain inhibitors (UNC1215, UNC669).
• Tumorigenesis assays in PA-1 cells.

Main Results:

• MLL1/WDR5 complexes interact with SOX2 and promote its proteolysis, possibly via methylation at K42.
• Silencing MLL1/WDR5 components leads to SOX2 accumulation; WDR5 overexpression enhances SOX2 ubiquitination and degradation.
• PHF20L1 antagonizes MLL1/WDR5-induced SOX2 degradation by inhibiting ubiquitination.
• PHF20L1 depletion accelerates SOX2 degradation, while its overexpression stabilizes SOX2.
• SOX2 mutant K42R is resistant to MLL1/WDR5 and PHF20L1 regulation.
• MBT domain inhibitors accelerate SOX2 degradation.
• SOX2 is highly expressed in immature ovarian teratoma; PHF20L1 loss impairs tumor initiation, while MLL1 ablation promotes tumor growth.

Conclusions:

• An antagonistic mechanism involving MLL1/WDR5 complexes and PHF20L1 regulates SOX2 protein stability, potentially through SOX2 methylation.
• This regulatory axis provides a novel therapeutic target for SOX2-positive cancers.