The emerging role of HP1 in the DNA damage response

Abstract

Heterochromatin protein 1 (HP1) family members are versatile proteins involved in transcription, chromatin organization, and replication. Recent findings now have implicated HP1 proteins in the DNA damage response as well. Cell-biological approaches showed that reducing the levels of all three HP1 isoforms enhances DNA repair, possibly due to heterochromatin relaxation. Additionally, HP1 is phosphorylated in response to DNA damage, which was suggested to initiate the DNA damage response. These findings have led to the conclusion that heterochromatic proteins are inhibitory to repair and that their dissociation from heterochromatin may facilitate repair. In contrast with an inhibitory role, a more active role for HP1 in DNA repair also was proposed based on the finding that all HP1 isoforms are recruited to UV-induced lesions, oxidative lesions, and DNA breaks. The loss of HP1 renders nematodes highly sensitive to DNA damage, and mice lacking HP1beta suffer from genomic instability, suggesting that the loss of HP1 is not necessarily beneficial for repair. These findings raise the possibility that HP1 facilitates DNA repair by reorganizing chromatin, which may involve interactions between phosphorylated HP1 and other DNA damage response proteins. Taken together, these studies illustrate an emerging role of HP1 proteins in the response to genotoxic stress.

FIG. 1.

Accumulation of HP1 at sites of DNA damage. (A) Immunolocalization of endogenous HP1β (red) in a locally UV-irradiated confluent human fibroblast irradiated at 100 J·m⁻² through 3-μm pores (cells are shown 30 min after irradiation). UV-damaged sites are visualized by the immunolocalization of NER protein XPA (green). (B) Immunolocalization of endogenous HP1β (green) in a U2OS cell irradiated with α-particles from a radioactive Americium (Am-241) source (cells are shown 30 min after IR). Sites containing DSBs are visualized by the immunolocalization of γH2A.X (red).

FIG. 2.

Model of H3K9me-independent association of HP1 with damaged sites. (A) Graphical representation of nucleosomes (histone octamer in purple, DNA in blue) containing H3K9 methylation (yellow circles) to which HP1 is bound (green). DNA damage, such as DSBs, triggers the dissociation of HP1 from H3K9me as a result of Thr51 phosphorylation. Upon the removal of the genomic injury, HP1 rebinds H3K9me restoring the levels of H3K9me-bound HP1. (B) DNA damage triggers the association of HP1 through its CSD independently of H3K9 methylation. Recruited HP1 may be phosphorylated at Thr51 located in the CD (indicated by P). Upon the removal of the genomic injury, the steady-state level of damage-bound HP1 decreases to the predamage situation. (C) A model similar to that shown in panel B with the difference being that, besides the association of HP1 through the CSD, the presence of DNA damage also can trigger the dissociation of HP1 molecules bound to H3K9 methylation. It should be noted that in this scenario, the number of molecules that binds damaged sites (through the CSD) is in excess above the number of molecules that dissociates from H3K9me to explain the net increase of HP1 at the site of DNA damage (see the legend to Fig. 1B and the text for details). For simplicity, HP1 is depicted to bind two H3K9me sites on the same nucleosome simultaneously, resulting in intranucleosomal bridging. However, internucleosomal bridging between adjacent nucleosomes or discontinuous bridging between nonadjacent nucleosomes that are spatially close together is also possible (22).

FIG. 3.

Model of the recruitment of DNA damage response (DDR) proteins to phosphorylated HP1 at damaged sites. Shown is the graphical representation of nucleosomes (histone octamer in purple, DNA in blue) containing H3K9 methylation (yellow circles) to which HP1 is bound (green). DNA damage (indicated by DSB) could trigger the CSD-dependent binding of HP1 to chromatin directly (upper pathway). In this scenario, damage-bound HP1 is phosphorylated (indicated by P), which may allow phosphorylation-dependent interactions with DDR proteins (in yellow) containing, for instance, a BRCT or FHA domain. Alternatively, the binding of HP1 could be preceded by the binding of a damage-binding protein containing a PXVXL motif (indicated in red in the lower pathway). This protein subsequently recruits HP1 through interactions between the PXVXL motif and the CSD of HP1. After that, phosphorylated HP1 may have phosphorylation-dependent interactions with DDR proteins.