Editors' ChoiceDNA Repair

SUMO Wrestling with DNA Damage

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Science Signaling  22 Dec 2009:
Vol. 2, Issue 102, pp. ec404
DOI: 10.1126/scisignal.2102ec404

Double-stranded DNA breaks, such as those caused by ionizing irradiation or DNA-damaging chemicals (such as cisplatin or hydroxyurea), activate signaling pathways that lead to the association of DNA repair proteins, such as breast cancer early onset 1 (BRCA1), p53 binding protein 1 (53BP1), and ring finger protein 168 (RNF168), with the damaged regions. Two groups have independently begun to uncover the mechanism by which posttranslational modification with small ubiquitin-like modifier (SUMO) proteins promotes DNA repair (see also the associated commentary by Boulton). Morris et al. detected increases in the abundance of SUMO1 as well as in that of SUMO2 or SUMO3 or both (SUMO2/3), and these proteins associated with sites of DNA damage [as indicated by the presence of γ-histone 2AX (γ-H2AX)]. Biochemical assays revealed that BRCA1 isolated from cells treated with hydroxyurea, cisplatin, or heat shock was conjugated to SUMO2 and, to a lesser extent, SUMO1. Depletion of the SUMO E3 ligases PIAS1 [protein inhibitor of activated signal transducer and activator of transcription (STAT) 1] and PIAS4 with small interfering RNAs (siRNAs) decreased BRCA1 colocalization with γ-H2AX and sumoylation of BRCA1 in hydroxyurea-treated cells. In addition, in vitro assays showed that BRCA1 that was sumoylated at Lys119 exhibited increased ubiquitylation activity compared with unmodified BRCA1. Depletion of PIAS1 or PIAS4 impaired DNA repair by homologous recombination or by nonhomologous end joining, leading to reduced cell survival after treatment with cisplatin.

In an independent study, Galanty et al. noticed that SUMO1 and 53BP1 colocalized in irradiated U2OS cells. Furthermore, the authors used lasers to micro-irradiate narrow linear areas across nuclei and found that endogenous SUMO1 and SUMO2/3 accumulated in these “laser lines.” The accumulation of SUMO1 at laser lines was prevented by transfection of siRNAs directed against 53BP1 or PIAS4, whereas that of SUMO2/3 was blocked by transfection of siRNAs directed against BRCA1, PIAS1, or PIAS4. Furthermore, siRNA-mediated knockdown of PIAS4 decreased sumoylation of 53BP1 after irradiation and accumulation of 53BP1 at laser lines. siRNA-mediated depletion of PIAS1 or PIAS4 decreased sumoylation of BRCA1 and accumulation of BRCA1 in γ-H2AX–positive cells after exposure to ionizing radiation, although ubiquitylation of H2A was decreased only with knockdown of PIAS4. Similar to Morris et al., Galanty et al. found that PIAS1 or PIAS4 was required for efficient DNA repair by homologous recombination or by nonhomologous end joining and that depletion of either SUMO E3 ligase led to decreased cell survival after exposure to ionizing irradiation. Thus, sumoylation of DNA repair proteins by PIAS1 and PIAS4 promotes cellular responses to double-stranded breaks.

J. R. Morris, C. Boutell, M. Keppler, R. Densham, D. Weekes, A. Alamshah, L. Butler, Y. Galanty, L. Pangon, T. Kiuchi, T. Ng, E. Solomon, The SUMO modification pathway is involved in the BRCA1 response to genotoxic stress. Nature 462, 886–890 (2009). [Online Journal]

Y. Galanty, R. Belotserkovskaya, J. Coates, S. Polo, K. M. Miller, S. P. Jackson, Mammalian SUMO E3-ligases PIAS1 and PIAS4 promote responses to DNA double-strand breaks. Nature 462, 935–939 (2009).[Online Journal]

S. J. Boulton, DNA repair: A heavyweight joins the fray. Nature 462, 857–858 (2009). [Online Journal]

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