Abstract
Mutations in BLM in Bloom Syndrome patients predispose them to multiple types of cancers. Here we report that BLM is recruited in a biphasic manner to annotated DSBs. BLM recruitment is dependent on the presence of NBS1, MRE11 and ATM. While ATM activity is essential for BLM recruitment in early phase, it is dispensable in late phase when MRE11 exonuclease activity and RNF8-mediated ubiquitylation of BLM are the key determinants. Interaction between polyubiquitylated BLM and NBS1 is essential for the helicase to be retained at the DSBs. The helicase activity of BLM is required for the recruitment of HR and c-NHEJ factors onto the chromatin in S- and G1-phase, respectively. During the repair phase, BLM inhibits HR in S-phase and c-NHEJ in G1-phase. Consequently, inhibition of helicase activity of BLM enhances the rate of DNA alterations. Thus BLM utilizes its pro- and anti-repair functions to maintain genome stability.
Highlights
Mutations in BLM in Bloom Syndrome patients predispose them to multiple types of cancers
Concomitant with other DNA damage response (DDR) factors, the levels of endogenous BLM increased after double-strand breaks (DSBs) generation as early as 30 min after 4-OHT treatment which became more pronounced after 4 h (Supplementary Figure 2A)
BLM colocalized with NBS1, γH2AX, and pSer1981 ATM foci (Fig. 1a, Supplementary Figure 2B), further indicating that BLM is an integral component of the DDR
Summary
Mutations in BLM in Bloom Syndrome patients predispose them to multiple types of cancers. In contrast to stalled replication forks that are recognized by the ATR/Chk[1] axis, DSBs are known to be recognized by MRE11–RAD50–NBS1 (MRN) complex, which recruits ATM and allows its optimal activation. Site-specific cleavage in the human genome has been achieved by fusing the restriction enzyme AsiSI to a modified estrogen receptor (ER) hormone-binding domain, which only binds to 4-hydroxy tamoxifen (4-OHT), an active metabolite of tamoxifen. This has led to the generation of the U2OS–AsiSI–ER system, which has been used previously to obtain the high-resolution profiling of γH2AX around DSBs in the human genome. It was inferred that γH2AX distribution was dependent on gene transcription[21]
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