The decompaction and re-establishment of chromatin organization immediately after mitosis is essential for genome regulation. Mechanisms underlying chromatin structure control in daughter cells are not fully understood. Here we show that a chromatin compaction threshold in cells exiting mitosis ensures genome integrity by limiting replication licensing in G1 phase. Upon mitotic exit, chromatin relaxation is controlled by SET8-dependent methylation of histone H4 on lysine 20. In the absence of either SET8 or H4K20 residue, substantial genome-wide chromatin decompaction occurs allowing excessive loading of the origin recognition complex (ORC) in the daughter cells. ORC overloading stimulates aberrant recruitment of the MCM2-7 complex that promotes single-stranded DNA formation and DNA damage. Restoring chromatin compaction restrains excess replication licensing and loss of genome integrity. Our findings identify a cell cycle-specific mechanism whereby fine-tuned chromatin relaxation suppresses excessive detrimental replication licensing and maintains genome integrity at the cellular transition from mitosis to G1 phase.

Oral squamous cell carcinoma (OSCC) is considered to be one of the most fatal diseases worldwide, owing to its late diagnosis and lack of availability of established reliable biomarkers. The aim of this study was to highlight the significance of immunosuppressive cytokines as potential biomarkers in OSCC. Whole unstimulated saliva was collected from each individual (30 OSCC patients and 33 age- and gender-matched healthy controls). Immunosuppressive cytokines, including IL-4, IL-10, IL-13, and IL-1RA, were evaluated in each sample using Luminex multianalyte profiling (xMAP) technology on BioPlex instrument. Our results showed that all the studied salivary cytokines were raised in OSCC patients as compared to controls, where IL-10 and IL-13 salivary levels showed statistically significant difference (p = .004 and p = .010, respectively). Mean levels of salivary cytokines in three histologically defined OSCC categories, compared employing one-way ANOVA, showed that salivary levels of IL-1RA were highest in patients having poorly differentiated OSCC tumors as compared to those having moderately and well-differentiated tumors (p = .000 and p = .002, respectively). Among OSCC individuals, duration of smokeless tobacco correlated positively with IL-1RA (p = .036). We conclude that salivary levels of immunosuppressive cytokines, IL-4, IL-10, IL-13, and IL-1RA, could prove to be potential biomarkers of OSCC and can be further investigated as markers of early detection and disease progression.

Genomic DNA is compacted into chromatin through packaging with histone and non-histone proteins. Importantly, DNA accessibility is dynamically regulated to ensure genome stability. This is exemplified in the response to DNA damage where chromatin relaxation near genomic lesions serves to promote access of relevant enzymes to specific DNA regions for signaling and repair. Furthermore, recent data highlight genome maintenance roles of chromatin through the regulation of endogenous DNA-templated processes including transcription and replication. Here, we review research that shows the importance of chromatin structure regulation in maintaining genome integrity by multiple mechanisms including facilitating DNA repair and directly suppressing endogenous DNA damage.

Histone lysine methylations have primarily been linked to selective recruitment of reader or effector proteins that subsequently modify chromatin regions and mediate genome functions. Here, we describe a divergent role for histone H4 lysine 20 mono-methylation (H4K20me1) and demonstrate that it directly facilitates chromatin openness and accessibility by disrupting chromatin folding. Thus, accumulation of H4K20me1 demarcates highly accessible chromatin at genes, and this is maintained throughout the cell cycle. In vitro, H4K20me1-containing nucleosomal arrays with nucleosome repeat lengths (NRL) of 187 and 197 are less compact than unmethylated (H4K20me0) or trimethylated (H4K20me3) arrays. Concordantly, and in contrast to trimethylated and unmethylated tails, solid-state NMR data shows that H4K20 mono-methylation changes the H4 conformational state and leads to more dynamic histone H4-tails. Notably, the increased chromatin accessibility mediated by H4K20me1 facilitates gene expression, particularly of housekeeping genes. Altogether, we show how the methylation state of a single histone H4 residue operates as a focal point in chromatin structure control. While H4K20me1 directly promotes chromatin openness at highly transcribed genes, it also serves as a stepping-stone for H4K20me3-dependent chromatin compaction.