Histone H4 lysine 20 methylation: key player in epigenetic regulation of genomic integrity

Abstract

Maintenance of genomic integrity is essential to ensure normal organismal development and to prevent diseases such as cancer. Nuclear DNA is packaged into chromatin, and thus genome maintenance can be influenced by distinct chromatin environments. In particular, post-translational modifications of histones have emerged as key regulators of genomic integrity. Intense research during the past few years has revealed histone H4 lysine 20 methylation (H4K20me) as critically important for the biological processes that ensure genome integrity, such as DNA damage repair, DNA replication and chromatin compaction. The distinct H4K20 methylation states are mediated by SET8/PR-Set7 that catalyses monomethylation of H4K20, whereas SUV4-20H1 and SUV4-20H2 enzymes mediate further H4K20 methylation to H4K20me2 and H4K20me3. Disruption of these H4K20-specific histone methyltransferases leads to genomic instability, demonstrating the important functions of H4K20 methylation in genome maintenance. In this review, we explain molecular mechanisms underlying these defects and discuss novel ideas for furthering our understanding of genome maintenance in higher eukaryotes.

Figure 1.

Cell cycle regulated establishment of the different H4K20 methylation states. Resting cells in G1 or G0 phase have high levels of H4K20me3 at heterochromatic regions and carry H4K20me2 throughout the genome. H4K20me1 is restricted to specific genes. When cells enter S phase, new histone H4 molecules are incorporated, which lack H4K20 methylation (marked in white). As SET8 is kept at low levels, very little H4K20me1 is added during S phase. Towards the end of S phase and in G2, SET8 is stabilized and establishes H4K20me1 at nearly all new histone H4 molecules. This high level of H4K20me1 is preserved during mitosis and is probably protected (shielded) from conversion into H4K20me2 or me3 via currently unknown mechanisms. Directly after mitosis, in early G1, most of the H4K20me1 is then converted to H4K20me2 and me3 by SUV4-20H enzymes.

Figure 2.

Role of histone H4 HMTs during replication. (A) H4K20me1 is added during G2 and M phase and shielded from conversion until G1 phase. In preparation for the following S phase, members of the Orc complex are recruited to ORI. Part of this involves binding between Orc1 and H4K20me2. We speculate that H4K20me1 contributes to a favourable chromatin structure around the ORI that support fork progression. (B) We hypothesise that SET8 directly supports the replication fork structure, either by itself or by methylation of one or more proteins that provide structural or functional support at the fork. Loss of SET8 results in dysfunctional replication and potentially exposure of DNA structures that are targeted and cut by endonucleases, thereby generating DSBs.

Figure 3.

Recruitment of 53BP1 to DNA damage sites through chromatin. DSBs are recognized by the MRE11/RAD50/NBS1 (MRN) complex, which binds open DNA ends. Stable binding of the MRN complex leads to autophosphorylation of the ATM kinase, which then induces phosphorylation of the histone variant H2A.X in the vicinity of the DNA break. Phospho-H2A.X (γH2A.X) allows for accumulation of MDC1 and its partner protein RNF8, which in turn, establishes poly-ubiquitinylation of histones at the break site. 53BP1 is then stably recruited through multiple interactions, including binding to MDC1 and H4K20me2.

Figure 4.

Role of SUV4-20H1/2 in recruitment of 53BP1. In wild-type cells, 53BP1 is stably recruited to DNA break sites through multiple interactions with e.g. MDC1 and H4K20me2. SET8-mediated H4K20me1 at DNA break sites may also contribute to 53BP1 recruitment; however, this link is not yet understood at the molecular level. Tight binding of 53BP1 suppresses long range resection of the DNA ends and therefore inhibits HR repair. High levels of H4K20me1, which is characteristic of Suv4-20h DKO cells, leads to less stable 53BP1 recruitment at break sites. 53BP1 can therefore not efficiently prevent end resection or inhibit HR repair, leading to elevated HR in SUV4-20H mutant cells.