Human EHMT2/G9a activates p53 through methylation-independent mechanism

Abstract

ABSTRACTp53 is a critical tumor suppressor in humans. It functions mostly as a transcriptional factor and its activity is regulated by numerous post-translational modifications. Among different covalent modifications found on p53 the most controversial one is lysine methylation. We found that human G9a (hG9a) unlike its mouse orthologue (mG9a) potently stimulated p53 transcriptional activity. Both ectopic and endogenous hG9a augmented p53-dependent transcription of pro-apoptotic genes, including Bax and PUMA, resulting in enhanced apoptosis and reduced colony formation. Significantly, siRNA-mediated knockdown of hG9a attenuated p53-dependent activation of PUMA. On the molecular level, hG9a interacted with histone acetyltransferase, p300/CBP, resulting in increased histone acetylation at the promoter of PUMA. The bioinformatics data substantiated our findings showing that positive correlation between G9a and p53 expression is associated with better survival of lung cancer patients. Collectively, this study demonstrates that depending on the cellular and organismal context, orthologous proteins may exert both overlapping and opposing functions.Furthermore, this finding has important ramifications on the use of G9a inhibitors in combination with genotoxic drugs to treat p53-positive tumors.3

The expanding role of the Ehmt2/G9a complex in neurodevelopment

“…However, it has been recently shown that one splice variant of hEHMT2 is also necessary to attenuate p53's transcriptional activation of its target genes in vitro. 53 This demonstrates an added level of complexity in Ehmt2's regulation of p53, and that its function as a repressor or co-activator is likely context dependent. This context-dependent activity also extends to the p53 target gene p21, where Ehmt2 is required to both epigenetically repress, 16,43 and activate 46 expression.…”

“…Multiple splice variants have been noted both in the human and mouse, denoted by inclusion/exclusion of a 5 0 exon, as well as inclusion/exclusion of exon 10. [53][54][55][56] Although none of these splice variants has been reported to effect the overall HMT function of the Ehmt1/2 dimer, both alternatively spliced 5 0 domain and exon 10 have independently been reported to be required for nuclear localization. 54,56 Furthermore, activation by the human EHMT2 protein has been found to be predominantly dependent of the hEHMT2-s variant which lacks part of the 5 0 TAD, in at least some contexts.…”

“…54,56 Furthermore, activation by the human EHMT2 protein has been found to be predominantly dependent of the hEHMT2-s variant which lacks part of the 5 0 TAD, in at least some contexts. 53 It is therefore possible that alternative splicing the Ehmt2 transcript acts to balance both activator and repressor, as well as nuclear versus cytoplasmic functions.…”

A drive in SUVs: From development to disease

“…Human G9a, unlike its mouse counterpart, is also associated with p300/ CBP leading to acetylation of PUMA promoter that results in apoptosis. 62,63 These studies highlight the methyltransferase dependent and independent function of G9a.…”

“…107 It is, however, important to note that recent studies have identified methyltransferase activity independent functions of KMTs. 27,62,108 In addition, although each SUV39 KMT is known to regulate gene expression and exert independent functions, a subset (G9a, GLP, SETDB1, and SUV39H1) have been shown to co-exist in the same complex, and their stability is interlinked. 23 Thus multiple KMTs may function synergistically in human pathologies.…”