Molecular pathways: deregulation of histone h3 lysine 27 methylation in cancer-different paths, same destination.

Methylation of lysine 27 on histone H3 (H3K27me), a modification associated with gene repression, plays a critical role in regulating the expression of genes that determine the balance between cell differentiation and proliferation. Alteration of the level of this histone modification has emerged as a recurrent theme in many types of cancer, demonstrating that either excess or lack of H3K27 methylation can have oncogenic effects. Cancer genome sequencing has revealed the genetic basis of H3K27me deregulation, including mutations of the components of the H3K27 methyltransferase complex PRC2 and accessory proteins, and deletions and inactivating mutations of the H3K27 demethylase UTX in a wide variety of neoplasms. More recently, mutations of lysine 27 on histone H3 itself were shown to prevent H3K27me in pediatric glioblastomas. Aberrant expression or mutations in proteins that recognize H3K27me3 also occur in cancer and may result in misinterpretation of this mark. In addition, due to the cross-talk between different epigenetic modifications, alterations of chromatin modifiers controlling H3K36me, or even mutations of this residue, can ultimately regulate H3K27me levels and distribution across the genome. The significance of mutations altering H3K27me is underscored by the fact that many tumors harboring such lesions often have a poor clinical outcome. New therapeutic approaches targeting aberrant H3K27 methylation include small molecules that block the action of mutant EZH2 in germinal center-derived lymphoma. Understanding the biologic consequences and gene expression pathways affected by aberrant H3K27 methylation may also lead to other new therapeutic strategies.