Coordinated activities of wild-type plus mutant EZH2 drive tumor-associated hypertrimethylation of lysine 27 on histone H3 (H3K27) in human B-cell lymphomas

Abstract

EZH2, the catalytic subunit of the PRC2 complex, catalyzes the mono- through trimethylation of lysine 27 on histone H3 (H3K27). Histone H3K27 trimethylation is a mechanism for suppressing transcription of specific genes that are proximal to the site of histone modification. Point mutations of the EZH2 gene (Tyr641) have been reported to be linked to subsets of human B-cell lymphoma. The mutant allele is always found associated with a wild-type allele (heterozygous) in disease cells, and the mutations were reported to ablate the enzymatic activity of the PRC2 complex for methylating an unmodified peptide substrate. Here we demonstrate that the WT enzyme displays greatest catalytic efficiency ( k cat / K ) for the zero to monomethylation reaction of H3K27 and diminished efficiency for subsequent (mono- to di- and di- to trimethylation) reactions. In stark contrast, the disease-associated Y641 mutations display very limited ability to perform the first methylation reaction, but have enhanced catalytic efficiency for the subsequent reactions, relative to the WT enzyme. These results imply that the malignant phenotype of disease requires the combined activities of a H3K27 monomethylating enzyme (PRC2 containing WT EZH2 or EZH1) together with the mutant PRC2s for augmented conversion of H3K27 to the trimethylated form. To our knowledge, this is the first example of a human disease that is dependent on the coordinated activities of normal and disease-associated mutant enzymatic function.

Epigenetic cancer therapy: rationales, targets and drugs

“…Importantly, while cancer genome sequencing approaches detected inactivating EZH2 mutations in myeloid leukemias (Ernst et al, 2010;Nikoloski et al, 2010), they also uncovered specific heterozygous point mutations of EZH2 in lymphomas (Morin et al, 2010). These point mutations cause an enhanced capacity of EZH2 to di and trimethylate H3K27 (Sneeringer et al, 2010) and could thus drive the establishment of cancerspecific epigenetic programs. Intriguingly, the mutant EZH2 enzyme appears to be druggable, which should allow the development of potent inhibitors with a high degree of cancer specificity for the treatment of patients that carry the corresponding mutation.…”

Chromatin proteins and modifications as drug targets

“…In some cases (e.g. EZH2 below), heterozygous point mutations in the catalytic SET domain lead to a gain of function of the wild type enzyme 15,16 favouring trimethylation and the silencing of tumour suppressor genes and/or differentiation specific genes. Similarly, in other cancers (e.g.…”

Au cœur d’une complexité biologique

“…Des mutations affectant la tyrosine en position 641 du domaine catalytique de la protéine (domaine SET, suppressor of variegation 3-9, enhancer of zeste and trithorax) ont été décrites dans 7,2 % des cas de lymphomes folliculaires et dans 21,7 % des cas de lymphomes diffus à grandes cellules B [32]. Ces mutations étaient considérées initialement comme des mutations perte de fonction mais elles se sont révélées comme des mutations gain de fonction à l'origine d'une accumulation de l'H3K27me3 [33]. Chez des patients atteints de syndromes myéloprolifératifs et myélodysplasiques (MPN/MDS), la surexpression d'EZH2, qui est associée à un mauvais pronostic, entraîne également une hyperméthylation de certains gènes suppresseurs de tumeurs [34].…”