There are a number of conditions in humans, where individuals succumb to tumours much earlier than is seen in the general population. These conditions are commonly referred to as ‘cancer predisposition’ or ‘genome instability’ disorders, and are caused by mutations in key players responsible for the maintenance of genome integrity. In addition, such mutations can cause developmental disorders or premature ageing. One such genetic condition is Bloom syndrome, caused by mutations in the BLM gene, which, in my old laboratory, we used as a model to study the relationship between genome instability and the development of cancer. I contributed to the discovery and analysis of how BLM participates in the safeguarding of DNA replication (1-3), and its early (4-6) and late (7-12) roles in homologous recombination repair.

Genetic data from various organisms suggests that RecQ helicases participate in homologous recombination (13). In many organisms RecQ mutants show a hyper-recombination phenotype, which in BS cells is manifested as an increase in the frequency of sister chromatid exchanges (SCEs) and inter-homologue recombination. We hypothesised that BLM would be involved in the preservation of the fidelity of homologous recombination, and the rejection of homeologous, that is not fully homologous, recombination intermediates, as the genetic interaction between Sgs1 and members of the mismatch repair pathway in yeast suggests (Myung et al. Nat.Genet 27:113). We described that BLM physically interacts with MSH6, one of the human homologues of MutS (5). We also showed that BLM preferentially unwinds DLoops that are resulting from a 5’ strand invasion event (4). Interestingly, human RAD51 preferentially catalyses 5’ strand invasion (Mazin et al. EMBO J. 19:1148), but as 5’ invading tails are not substrates for polymerase extension, they are non-processible and should be rejected. It was somewhat disappointing to find that the human MutS homologue does not influence the helicase activity of BLM (5), even on DLoop substrates that contain mismatches (4).

In addition to its involvement in the early events of homologous recombination, BLM also participates late, in the processing phase of HR intermediates. Together with topoisomerase IIIα (TOP3A) it catalyses a newly discovered process termed double Holliday junction dissolution (Wu & Hickson Nature 426:870) (7). This reaction is stimulated by a newly discovered OB-fold containing heterodimer, called RMI. The RMI complex contains RMI1, which binds to TOP3A and modifies its DNA binding properties (9). Stimulation of TOP3A requires direct protein-protein interaction between RMI1 and TOP3A, and we successfully mapped the surface of interaction between the two proteins (11). Dissolution of double Holliday junction is a two-step process; first, BLM catalyses the convergent migration of the branches of double Holliday junctions, while TOP3A relieves the resulting torsional stress. In the final step, TOP3A decatenates the resulting hemicatenane (11;12); in the orthologous budding yeast system we showed that RMI1 is essential for this last step (8). The other member of the RMI heterodimer, RMI2 is essential for the stability of the RMI complex, however, it does not seem to stimulate dissolution on its own (10).

Our pioneering biochemical studies were finally proven in vivo by the laboratory of West,S.C. They proved our original hypothesis that in the absence of BLM (and TOP3A) catalysed double Holliday junction dissolution, the cells resort to double Holliday junction resolution catalysed by the structure-specific DNA endonucleases GEN1, MUS81/EME1 and SLX1/SLX4 (Wechsler et al. Nature 471:642).

   1.   Bachrati,C.Z. and Hickson,I.D. (2008) RecQ helicases: guardian angels of the DNA replication fork. Chromosoma, 117, 219-233.

   2.   Selak,N., Bachrati,C.Z., Shevelev,I., Dietschy,T., van Loon,B., Jacob,A., Hubscher,U., Hoheisel,J.D., Hickson,I.D. and Stagljar,I. (2008) The Bloom’s syndrome helicase (BLM) interacts physically and functionally with p12, the smallest subunit of human DNA polymerase delta. Nucleic Acids Res., 36, 5166-5179.

   3.   Jiao,R., Bachrati,C.Z., Pedrazzi,G., Kuster,P., Petkovic,M., Li,J.L., Egli,D., Hickson,I.D. and Stagljar,I. (2004) Physical and Functional Interaction between the Bloom’s Syndrome Gene Product and the Largest Subunit of Chromatin Assembly Factor 1. Mol.Cell.Biol., 24, 4710-4719.

   4.   Bachrati,C.Z., Borts,R.H. and Hickson,I.D. (2006) Mobile D-loops are a preferred substrate for the Bloom’s syndrome helicase. Nucleic Acids Res., 34, 2269-2279.

   5.   Pedrazzi,G., Bachrati,C.Z., Selak,N., Studer,I., Petkovic,M., Hickson,I.D. and Stagljar,I. (2003) The Bloom’s syndrome helicase directly interacts with the human mismatch repair protein MSH6. Biol.Chem., 384, 1155-1164.

   6.   Popuri,V., Bachrati,C.Z., Muzzolini,L., Mosedale,G., Costantini,S., Giacomini,E., Hickson,I.D. and Vindigni,A. (2008) The Human RecQ helicases, BLM and RECQ1, display distinct DNA substrate specificities. J.Biol.Chem., 283, 17766-17776.

   7.   Bachrati,C.Z. and Hickson,I.D. (2009) Dissolution of double Holliday junctions by the concerted action of BLM and topoisomerase IIIalpha. Methods Mol.Biol., 582, 91-102.

   8.   Cejka,P., Plank,J.L., Bachrati,C.Z., Hickson,I.D. and Kowalczykowski,S.C. (2010) Rmi1 stimulates decatenation of double Holliday junctions during dissolution by Sgs1-Top3. Nat.Struct.Mol.Biol., 17, 1377-1382.

   9.   Wu,L., Bachrati,C.Z., Ou,J., Xu,C., Yin,J., Chang,M., Wang,W., Li,L., Brown,G.W. and Hickson,I.D. (2006) BLAP75/RMI1 promotes the BLM-dependent dissolution of homologous recombination intermediates. Proc.Natl.Acad.Sci.U.S.A., 103, 4068-4073.

10.   Xu,D., Guo,R., Sobeck,A., Bachrati,C.Z., Yang,J., Enomoto,T., Brown,G.W., Hoatlin,M.E., Hickson,I.D. and Wang,W. (2008) RMI, a new OB-fold complex essential for Bloom syndrome protein to maintain genome stability. Genes Dev., 22, 2843-2855.

11.   Yang,J., Bachrati,C.Z., Ou,J., Hickson,I.D. and Brown,G.W. (2010) Human topoisomerase IIIalpha is a single-stranded DNA decatenase that is stimulated by BLM and RMI1. J.Biol.Chem., 285, 21426-21436.

12.   Yang,J., Bachrati,C.Z., Hickson,I.D. and Brown,G.W. (2012) BLM and RMI1 Alleviate RPA Inhibition of TopoIIIalpha Decatenase Activity. PLoS.One., 7, e41208.

13.   Bachrati,C.Z. and Hickson,I.D. (2003) RecQ helicases: suppressors of tumorigenesis and premature aging. Biochem.J., 374, 577-606.