Summary Aberrations in chromatin dynamics play a fundamental role in tumorigenesis, yet relatively little is known of the molecular mechanisms linking histone lysine methylation to neoplastic disease. ING4 (Inhibitor of Growth 4) is a native subunit of an HBO1 histone acetyltransferase (HAT) complex and a tumor suppressor protein. Here we show a critical role for the specific read-out of histone H3 trimethylated at lysine 4 (H3K4me3) by the ING4 PHD finger in mediating ING4 gene expression and tumor suppressor functions. The interaction between ING4 and H3K4me3 augments the acetylation activity of HBO1 on H3 tails, and drives H3 acetylation at ING4 target promoters to effect a DNA damage-dependent gene expression program. Further, ING4 facilitates apoptosis in response to genotoxic stress and inhibits anchorage-independent cell growth, and these functions are dependent on ING4 interactions with H3K4me3. Together, our results demonstrate a mechanism for brokering crosstalk between H3K4 methylation and histone H3 acetylation, and reveal a new molecular link between chromatin modulation and tumor suppressor mechanisms.

SUMMARY Lysine methylation within histones is crucial for transcriptional regulation and thus links chromatin states to biological outcomes. Although recent studies have extended lysine methylation to nonhistone proteins, underlying molecular mechanisms such as the upstream signaling cascade that induces lysine methylation and downstream target genes modulated by this modification have not been elucidated. Here, we show that Reptin, a chromatin-remodeling factor, is methylated at lysine 67 in hypoxic conditions by the methyltransferase G9a. Methylated Reptin binds to the promoters of a subset of hypoxia-responsive genes and negatively regulates transcription of these genes to modulate cellular responses to hypoxia.

Herpes simplex virus 1 (HSV-1) latency establishment is tightly controlled by promyelocytic leukemia (PML) nuclear bodies (NBs) (or ND10), although their exact contribution is still elusive. A hallmark of HSV-1 latency is the interaction between latent viral genomes and PML NBs, leading to the formation of viral DNA-containing PML NBs (vDCP NBs), and the complete silencing of HSV-1. Using a replication-defective HSV-1-infected human primary fibroblast model reproducing the formation of vDCP NBs, combined with an immuno-FISH approach developed to detect latent/quiescent HSV-1, we show that vDCP NBs contain both histone H3.3 and its chaperone complexes, i.e., DAXX/ATRX and HIRA complex (HIRA, UBN1, CABIN1, and ASF1a). HIRA also co-localizes with vDCP NBs present in trigeminal ganglia (TG) neurons from HSV-1-infected wild type mice. ChIP and Re-ChIP show that vDCP NBs-associated latent/quiescent viral genomes are chromatinized almost exclusively with H3.3 modified on its lysine (K) 9 by trimethylation, consistent with an interaction of the H3.3 chaperones with multiple viral loci and with the transcriptional silencing of HSV-1. Only simultaneous inactivation of both H3.3 chaperone complexes has a significant impact on the deposition of H3.3 on viral genomes, suggesting a compensation mechanism. In contrast, the sole depletion of PML significantly impacts the chromatinization of the latent/quiescent viral genomes with H3.3 without any overall replacement with H3.1. vDCP NBs-associated HSV-1 genomes are not definitively silenced since the destabilization of vDCP NBs by ICP0, which is essential for HSV-1 reactivation in vivo, allows the recovery of a transcriptional lytic program and the replication of viral genomes. Consequently, the present study demonstrates a specific chromatin regulation of vDCP NBs-associated latent/quiescent HSV-1 through an H3.3-dependent HSV-1 chromatinization involving the two H3.3 chaperones DAXX/ATRX and HIRA complexes. Additionally, the study reveals that PML NBs are major actors in latent/quiescent HSV-1 H3.3 chromatinization through a PML NB/histone H3.3/H3.3 chaperone axis.