Regulation and localization of the Bloom syndrome protein in response to DNA damage

Bloom syndrome (BS) is an autosomal recessive disorder characterized by a h igh incidence of cancer and genomic instability. BLM, the protein defective in BS, is a RecQ-like helicase, presumed to function in DNA replication, r ecombination, or repair. BLM localizes to promyelocytic leukemia protein (P ML) nuclear bodies and is expressed during late S and G2. We show, in norma l human cells, that the recombination/repair proteins hRAD51 and replicatio n protein (RP)-A assembled with BLM into a fraction of PML bodies during la te S/G2. Biochemical experiments suggested that BLM resides in a nuclear ma trix-bound complex in which association with hRAD51 may be direct. DNA-dama ging agents that cause double strand breaks and a G2 delay induced BLM by a p53- and ataxia-telangiectasia mutated independent mechanism. This inducti on depended on the G2 delay, because it failed to occur when G2 was prevent ed or bypassed. It coincided with the appearance of foci containing BLM, PM L, hRAD51 and RP-A, which resembled ionizing radiation-induced foci. After radiation, foci containing BLM and PML formed at sites of single-stranded D NA and presumptive repair in normal cells, but not in cells with defective PML. Our findings suggest that BLM is part of a dynamic nuclear matrix-base d complex that requires PML and functions during G2 in undamaged cells and recombinational repair after DNA damage.