An oncohistone deranges inhibitory chromatin Missense mutations (that change one amino acid for another) in histone H3 can produce a so-called oncohistone and are found in a number of pediatric cancers. For example, the lysine-36–to-methionine (K36M) mutation is seen in almost all chondroblastomas. Lu et al. show that K36M mutant histones are oncogenic, and they inhibit the normal methylation of this same residue in wild-type H3 histones. The mutant histones also interfere with the normal development of bone-related cells and the deposition of inhibitory chromatin marks. Science , this issue p. 844
Significance Recent exome sequencing studies have uncovered high-frequency histone H3 driver mutations in pediatric cancers. Previous studies have shown that lysine to methionine histone mutations are potent inhibitors of their respective lysine methyltransferases. However, an in-depth understanding of this inhibition was limited by the lack of structural and kinetic information. This study investigates the biochemical and biophysical parameters of lysine to methionine histone mutants using the methyltransferase G9a and H3K9M as a model system. Structural and functional experiments conclude that the methyltransferase cofactor S -adenosyl methionine is required for binding of G9a to the mutant histone.
The Drosophila Myb–Muv B (MMB)/dREAM complex regulates gene expression and DNA replication site-specifically, but its activities in vivo have not been thoroughly explored. In ovarian amplification-stage follicle cell nuclei, the largest subunit, Mip130, is a negative regulator of replication, whereas another subunit, Myb, is a positive regulator. Here, we identified a mutation in mip40 and generated a mutation in mip120 , two additional MMB subunits. Both mutants were viable, but mip120 mutants had many complex phenotypes including shortened longevity and severe eye defects. mip40 mutant females had severely reduced fertility, whereas mip120 mutant females were sterile, substantiating ovarian regulatory role(s) for MMB. Myb accumulation and binding to polytene chromosomes was dependent on the core factors of the MMB complex. In contrast to the documented mip130 mutant phenotypes, both mip40 and mip120 mutant males were sterile. We purified Mip40-containing complexes from testis nuclear extracts and identified tMAC, a new testis-specific meiotic arrest complex that contained Mip40, Caf1/p55, the Mip130 family member, Always early (Aly), and a Mip120 family member, Tombola (Tomb). Together, these data demonstrate that MMB serves diverse roles in different developmental pathways, and members of MMB can be found in alternative, noninteracting complexes in different cell types.
Significance A high frequency of missense mutations was recently discovered at histone H3.3 glycine 34 in 92% of giant cell tumors of the bone and 17% of high-grade astrocytomas. The molecular mechanism by which G34 mutations drive these tumors remains unclear. Here, we demonstrate that the G34-mutated “oncohistones” misregulate the negative cross-talk between two histone methyltransferase enzymes, Polycomb Repressive Complex 2 (PRC2) and SETD2. G34 mutations uniquely promote PRC2 activity by blocking SETD2-mediated H3K36 methylation at active enhancers and drive a gene expression program that enhances tumor growth. We propose that G34 oncohistones exploit the regulatory mechanisms that fine tune PRC2 activity in human malignancies.
Abstract Covalent modifications to both the DNA and the histone proteins allow chromatin to act as a dynamic information hub that integrates diverse biochemical stimuli to regulate genomic DNA access to the transcription machinery and ultimately establish and maintain cellular phenotypes. Moreover, there is increasing appreciation that chromatin alterations per se, including DNA and histone modifications, are involved in the pathogenesis of cancer. Nowhere is this better supported than with the groundbreaking discoveries of high-frequency, somatic mutations in histones that are drivers of oncogenesis. These mutations (collectively called "oncohistones") cause amino acid substitutions that localize to conserved residues in the N-terminal tail of histone H3 and all seem to be linked, either directly or indirectly, to disruption of normal levels and distribution of histone H3 methylation and thus genomic regulation. Specifically, oncohistones directly or indirectly promote aberrant genome-wide distribution of lysine 27 methylation on histone H3. H3K27 methylation, catalyzed by the Polycomb Repressive Complex 2 (PRC2), is mechanistically linked to establishment and maintenance of gene repression. We are currently using a combination of biochemical and genomic approaches to investigate how oncohistone-driven changes in histone H3 K27 methylation lead to altered chromatin states that profoundly influence gene expression patterns. I will discuss some of our recent mechanistic and functional work on various oncohistone mutations. Citation Format: Peter W. Lewis. Polycomb dysregulation by oncohistones [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr SY05-02.
