Bloom syndrome (BS) is a rare human genetic disease in which patients exhibit growth retardation, immunodeficiency, infertility, photosensitivity, and predisposition to cancer. The gene defective in BS has recently been cloned (named BLM) and was found to belong to an evolutionarily conserved helicase family, called RecQ. The recombinant BLMp protein has been shown to possess helicase activity in vitro, suggesting that BS could be caused by a defect in a DNA metabolic reaction such as replication or repair. Interestingly, the BLM gene belongs to the helicase family, like the genes mutated in Werner Syndrome and Rothmund-Thomson syndrome (RTS). All three diseases have some common features, such as genetic instability and predisposition to cancer. But each disease has its own distinctive symptoms. For example, WS patients prematurely display many age-related features, including osteoporosis, atherosclerosis, diabetes and cataracts, which are not observed in BS or RTS. Also, WS individuals do not show immunodeficiency or photosensivity like BS patients. To help understand the molecular mechanism of these human diseases, we are isolating the protein complexes containing each gene product. To investigate the mechanism of BS, we isolated from human HeLa extracts three complexes containing the helicase BLM defective in BS. Interestingly, one of the complexes, termed BRAFT, also contains five of the Fanconi anemia (FA) complementation group proteins (FA). FA resembles BS in genomic instability and cancer predisposition, but most of its gene products have no known biochemical activity and the molecular pathogenesis of the disease is poorly understood. BRAFT displays a DNA-unwinding activity that requires the presence of BLM, so that complexes isolated from BLM-deficient cells lack such an activity. The complex also contains topoisomerase IIIa and replication protein A, proteins that are known to interact with BLM and could facilitate unwinding of DNA. We find that BLM complexes isolated from a FA cell line have a lower molecular mass. Our study provides the first biochemical characterization of a multiprotein FA complex and suggests a connection between the BLM and FA pathways of genomic maintenance. The findings that FA proteins are part of a DNA-unwinding complex imply that FA proteins may participate in DNA repair. We showed that BLAP75 is a component of all three BLM complexes from HeLa cells. Using siRNA knockdown techniques, we showed that BLAP75 is essential for BLM complex stability in vivo. Consistent with a role in BLM-mediated processes, BLAP75 co-localized with BLM in subnuclear foci in response to DNA damage, and its depletion impaired the recruitment of BLM to these foci. Depletion of BLAP75 by siRNA also resulted in deficient phosphorylation of BLM during mitosis, as well as defective cell proliferation. Moreover, cells depleted of BLAP75 displayed an increased level of sister-chromatid exchange, similar to cells depleted of BLM by siRNA. Thus, BLAP75 is an essential component of the BLM-associated cellular machinery that maintains genome integrity. After our work was published, two other labs used genetic approaches to show that the yeast homolog of BLAP75, named RMI1, is also a component of RecQ helicase-Topo IIIa complex, and is required for maintaining genome stability. Thus, biochemistry in human and genetics in yeast have reached the same conclusion. Together, these data suggest that BLAP75/RMI1 and its homologs in various species have a conserved function in guarding the genome. It was shown previously that BLM, together with its evolutionarily conserved binding partner topoisomerase III (hTOPO III ), can process a toxic DNA intermediate generated in DNA repair reactions into a non-toxic product by a mechanism termed dissolution. In a collaboration with I. Hickson, G. Brown, and L. Lis labs, it was found that RMI1 (new name for BLAP75) can strongly promote the dissolution catalyzed by hTOPO III by recruiting this enzyme to the toxic intermediate. This study demonstrates that BLM, hTOPO III and BLAP75/RMI1 function as a molecular machine that maintains genome stability by efficiently processing the toxic intermediates generated during DNA repair. Identification of this machine and its biochemical activity should provide new means to screen drugs and could eventually contribute to the development of cancer therapies. We are currently examining the importance of two new BLM complex components, RMI2 and BLAP250, in genome maintenance. We found that RMI2, which interacts with RMI1 (BLAP75) through two OB-fold domains similar to those in RPA. The resulting complex, named RMI, differs from RPA in that it lacks obvious DNA binding activity. Nevertheless, RMI stimulates the dissolution of a homologous recombination intermediate in vitro and is essential for the stability, localization, and function of the BLM complex in vivo. Notably, inactivation of RMI2 in chicken DT40 cells results in an increased level of sister-chromatid exchange (SCE)--the hallmark feature of Bloom syndrome cells. Epistasis analysis revealed that RMI2 and BLM suppress SCE within the same pathway. A point mutation in the OB-domain of RMI2 disrupts the association between BLM and the rest of the complex, and abrogates the ability of RMI2 to suppress elevated SCE. Our data suggest that multi-OB-fold complexes mediate two modes of BLM action: via RPA-mediated protein-DNA interaction and via RMI-mediated protein-protein interactions. We have now identified BLAP250 in BLM complexes immunopurified by both BLM and RMI1 antibodies using mass spectrometry. Reciprocal immunopurification with a BLAP250 antibody also yielded BLM, TopoIIIa, RMI1 and RMI2. These data indicate that BLAP250 is an integral component of the BLM complexes. We inactivated BLAP250 in chicken DT40 cells. Preliminary data showed that BLAP250 mutant cells exhibit a level of sister-chromatid exchange (SCE) indistinguishable from that of wildtype cells, suggesting that BLAP250 is dispensable for BLM to suppress SCE. Interestingly, BLAP250 mutant cells are hypersensitive to several DNA replication inhibitors, hinting that BLAP250 may play a role in replication. We collaborated with Mike Seidmans group at NIA, and found that BLAP250 colocalizes with BLM at replication forks blocked by interstrand crosslinking drugs. In an ongoing collaboration with Yves Pommiers lab at NCI, we found that replication fork recovery occurs at a reduced rate in BLAP250 mutant cells compared to wildtype cells. The data suggest that BLAP250 may play an important role in protecting genome stability during replication. We will continue to analyze the roles of BLAP250 and BLM in DNA replication.
