In Vivo Chromatin Organization of Mouse Rod Photoreceptors Correlates with Histone Modifications

BACKGROUND: The folding of genetic information into chromatin plays important regulatory roles in many nuclear processes and particularly in gene transcription. Post translational histone modifications are associated with specific chromatin condensation states and with distinct transcriptional activities. The peculiar chromatin organization of rod photoreceptor nuclei, with a large central domain of condensed chromatin surrounded by a thin border of extended chromatin was used as a model to correlate in vivo chromatin structure, histone modifications and transcriptional activity. METHODOLOGY: We investigated the functional relationships between chromatin compaction, distribution of histone modifications and location of RNA polymerase II in intact murine rod photoreceptors using cryo-preparation methods, electron tomography and immunogold labeling. Our results show that the characteristic central heterochromatin of rod nuclei is organized into concentric domains characterized by a progressive loosening of the chromatin architecture from inside towards outside and by specific combinations of silencing histone marks. The peripheral heterochromatin is formed by closely packed 30 nm fibers as revealed by a characteristic optical diffraction signal. Unexpectedly, the still highly condensed most external heterochromatin domain contains acetylated histones, which are usually associated with active transcription and decondensed chromatin. Histone acetylation is thus not sufficient in vivo for complete chromatin decondensation. The euchromatin domain contains several degrees of chromatin compaction and the histone tails are hyperacetylated, enriched in H3K4 monomethylation and hypo trimethylated on H3K9, H3K27 and H4K20. The transcriptionally active RNA polymerases II molecules are confined in the euchromatin domain and are preferentially located at the vicinity of the interface with heterochromatin. CONCLUSIONS: Our results show that transcription is located in the most decondensed and highly acetylated chromatin regions, but since acetylation is found associated with compact chromatin it is not sufficient to decondense chromatin in vivo. We also show that a combination of histone marks defines distinct concentric heterochromatin domains

Targeting Histone Lysine Methyltransferases in Cancer

Post-translational modifications of histones are recognised as important determinants of chromatin architecture and are well appreciated as an instrument to exert control over cellular processes that require access to DNA such as transcription, replication and DNA repair. Among the covalent histone modifications, histone lysine methylation has received increasing attention in the field of oncology. Cancer genomic sequencing campaigns have provided evidence that alterations in histone lysine methylation networks occur frequently in cancer, raising interest in the relevant enzymes responsible for adding and removing them (methyltransferases and demethylases) as potential oncology targets. Many successful drug discovery campaigns executed over recent years have led to high-quality histone lysine methyltransferase inhibitors with remarkable potency and selectivity. These compounds elicit selective cancer cell killing in vitro and robust efficacy in vivo, suggesting that targeting histone lysine methylation pathways may be a useful strategy for cancer treatment. This chapter reviews the relevant histone lysine methyltransferase targets and currently available small molecule inhibitors.</jats:p

Targeting BET Bromodomains in Cancer

Cancer is frequently dependent on aberrant gene expression programs that might be vulnerable to targeting with novel therapeutics. Bromodomain and extraterminal domain (BET) proteins are powerful transcriptional coregulators often found as part of oncogenic transcriptional programs. The bromodomain functionality of BET proteins is highly druggable, and several product candidates are in clinical testing. While initial clinical data created doubt about their benefit for cancer patients, more encouraging data recently reported in myelofibrosis patients may promote additional applications of BET inhibitors in oncology as monotherapy and in combination with other therapeutic agents. Moreover, a growing number of approaches to optimize the therapeutic window by tinkering with the property profiles of BET inhibitors may provide additional clinical opportunities. This review provides an update on the status of ongoing activities to exploit BET bromodomain inhibition as a mechanism for cancer therapy.</jats:p

Beyond histone methyl-lysine binding: how malignant brain tumor (MBT) protein L3MBTL1 impacts chromatin structure.

Alterations in gene expression are commonly accompanied by changes in chromatin structure. Histone lysine residues of the so called "histone tails" are subject to various post-translational modifications among which methylation has been extensively studied over the past years. The presence and the extent of methylation on histone lysine residues somehow mediate chromatin structural changes that contribute to activation or repression of gene expression. Chromatin states are functionally linked with cellular processes including the regulation of gene expression during the cell cycle. For nearly a decade, however, it proved difficult to explain mechanistically how methyl moieties on histone lysine residues impact chromatin structure. We recently found that a member of the malignant brain tumor (MBT) protein family, L3MBTL1, directly compacts chromatin in a strictly histone lysine methylation dependent fashion. Below, we briefly discuss our observations and those of others to provide an overview of how L3MBTL1, partially by chromatin condensation, regulates transcription and functions in cell cycle control

Histone Lysine Demethylases and Their Impact on Epigenetics

Methylation marks on the lysine residues of histone proteins are thought to contribute to epigenetic phenomena in part because of their apparent irreversibility. Will this view change with the recent discovery of histone lysine demethylases that reversibly remove methyl marks

Facultative heterochromatin: is there a distinctive molecular signature?

The Latin word "facultas" literally means "opportunity." Facultative heterochromatin (fHC) then designates genomic regions in the nucleus of a eukaryotic cell that have the opportunity to adopt open or compact conformations within temporal and spatial contexts. This review focuses on the molecular and functional aspects of fHC that distinguish it from constitutive heterochromatin (cHC) and euchromatin (EC) and discusses various concepts regarding the regulation of fHC structure. We begin by revisiting the historical developments that gave rise to our current appreciation of fHC

Complement component C8γ is expressed in human fetal and adult kidney independent of C8α

AbstractHuman complement component C8γ is an unusual complement factor since it shows no homology to other complement proteins but is a member of the lipocalin superfamily. So far, it has been found exclusively in plasma, covalently linked to C8α by disulfide bridging. We have used dot blot and Northern blot analyses of a large number of different human tissues to survey systematically the expression pattern of C8γ. Our experiments clearly showed that besides in liver, this gene is also expressed in fetal and adult kidney. Renal expression of C8γ is not dependent on C8α expression, since we could not detect C8α expression in kidney. Thus its physiological function is not restricted to a specific action in association with complement components. As a prerequisite for further characterization of the structure and binding activities of the uncomplexed C8γ, we have expressed the encoding cDNA in Escherichia coli. To increase the probability for proper folding of the characteristic intramolecular disulfide bridge the recombinant protein was produced by secretion to the periplasm