Epigenetic Moonlighting: Catalytic-Independent Functions
Epigenetic Moonlighting: Catalytic-Independent Functions in Development and Disease Pathogenesis
In addition to revealing non-catalytic functions for histone modifying enzymes, our studies with catalytic-dead methyltransferase mutants raise an important question: if the enzymatic activity of a histone lysine methyltransferase (or other histone modifier) is not required for cellular or even organismal viability, then why is the enzymatic activity there at all? What is its functional role? We have found some evidence of gene- or context-specific requirement for histone lysine methyltransferase activity. For example, we found that otherwise viable and self-renewing mESCs with catalytic-dead MLL2 display defects in differentiation, and we demonstrated that both the methyltransferase activity of MLL2 and the chromatin-targeting association of its own CXXC domain with CpG-rich enhancers are both required for the activation of a subset of primordial germ cell (PGC) specification genes. [Hu 2017]
Importantly, multiple histone modifying enzymes can all deposit the same mark, so that balance between these enzymes and other chromatin modifiers may underly the differential gene- and context-specific function of independent histone-modifying activities. When we examined the effects of mutating different COMPASS methyltransferases on H3K4me3 deposition, we were able to demonstrate a functional redundancy between SET1A and SET1B, which appear to be the primary depositors in mESCs, complemented by the activity of MLL2. [Sze 2020] Unlike MLL2, which has its own chromatin-targeting CXXC domain, SET1A and SET1B instead make use of a separate CXXC1 subunit to target promoter regions. Interestingly, we found that knockdown of the SET1A/B COMPASS subunit CXXC1 rescued lost gene expression in MLL2 KO mESCs, despite the failure of this knockdown to restore lost H3K4me3. We further determined that in the absence of MLL2 from its target loci, the presence of SET1A/B excluded Polycomb histone methyltransferase activity, enabling repressive DNA methylation by DNMT1. [Douillet 2020]