La double hélice d’ADN est une structure stable qui assure à la fois la sauvegarde et la transmission de l’information génétique. Pour accéder à cette information, une vaste famille d’enzymes multifonctionnelles appelées hélicases réalise la séparation des bases complémentaires de l’ADN. Certaines de ces hélicases sont associées chez l’homme à des syndromes de prédisposition au cancer. C’est le cas du syndrome de Bloom (BS), une maladie génétique à transmission récessive qui se traduit par une augmentation de l’instabilité génétique mais où aucun phénomène d’haplo-insuffisance ou de dominance négative n’est constaté chez les porteurs hétérozygotes. On reconnait pourtant que la protéine du syndrome de Bloom (BLM) adopte une structure multimérique in vitro mais sans que l’expression chez certains hétérozygotes d’une enzyme inactive ne soit considérée comme un facteur à risque. Pour expliquer ce paradoxe, nous avons étudié la structure de l’enzyme BLM et constater qu’elle fonctionne sous la forme d’un monomère, un résultat nouveau qui justifie mieux pourquoi ces formes inactives n’influence pas le degré de prédisposition au cancer. D’autre part, la toxicité cadmium est susceptible d’avoir un lien direct avec l’inactivation de l’hélicase BLM car les cellules exposées au cadmium présentent des analogies avec celles des patients atteints du syndrome de Bloom. Effectivement, nous avons observé qu’in vitro, de faibles concentrations de cadmium réduisent les activités de cette hélicase en induisant son oligomérisation. Ces travaux apportent des informations nouvelles sur le mécanisme moléculaire de l’hélicase BLM et soulignent son importance dans le maintien de l’intégrité du génome. / The DNA double helix is a stable structure that ensures both the protection and transmission of genetic information. To access this information, a large family of multifunctional enzymes called helicases performs the separation of complementary bases of DNA. Some of these helicases in humans are associated with cancer predisposition syndromes. This is the case of Bloom syndrome (BS), a recessive genetic disease that results in an increase in genetic instability but where no phenomenon of haploinsufficiency or dominant negative is found in carriers heterozygotes. Yet we recognize that the Bloom syndrome protein (BLM) adopts a multimeric structure in vitro, but the expression among some heterozygotes of an inactive enzyme is not considered as a risk factor. To explain this paradox, we studied the structure of the BLM and find that it works as a monomer, a new result which justifies why most inactive forms does not influence the degree of cancer predisposition. On the other hand, cadmium toxicity is potentially linked to the inactivation of the BLM helicase as cells exposed to cadmium present analogies with those of patients with Bloom syndrome. Indeed, we observed in vitro, that low concentrations of cadmium reduce helicase activity by promoting its oligomerization. These studies provide new information on the molecular mechanism of the BLM helicase and emphasize its importance in maintaining genome integrity.

Genomic instability driven by non-allelic homologous recombination (NAHR) provides a realistic mechanism that could account for the numerous chromosomal abnormalities that are hallmarks of cancer. We recently demonstrated that this type of instability could be assayed by analyzing the copy number variation of the human ribosomal RNA gene clusters (rDNA). Further, we found that gene cluster instability (GCI) was present in greater than 50% of the human cancer samples that were tested. Here, data is presented that confirms this phenomenon in the human GAGE gene cluster of those cancer patients. This adds credence to the hypothesis that NAHR could be a driving force for carcinogenesis. This data is followed by experimental results that demonstrate the same gene cluster instability in cultured cells that are deficient for the human BLM protein. Bloom’s Syndrome (BS) results from a genetic mutation that results in the abolition of BLM protein, one of human RecQ helicase. Studies of Bloom’s Syndrome have reported a 10-fold increase in sister chromatid exchanges during mitosis which has primarily been attributed to dysregulated homologous recombination. BS also has a strong predisposition to a broad spectrum of malignancies. Biochemical studies have determined that the BLM protein works in conjunction with TOPOIIIα and RMI1/RMI2 to function as a Holliday Junction dissolvase that suppress inadvertent crossover formation in mitotic cells. Because of the similarities in their biochemical activities it was suggested that another DNA helicase found in E. coli, the RecG DNA translocase, is the functional analog of BLM. RecG shares no sequence homology with BLM but it can complement both the sister chromatid exchange elevation and the gene- cluster instability phenotype caused by BLM deficiency. This indicates that the physiological function of BLM that is responsible for these phenotypes rests somewhere in the shared biochemical activities of these two proteins. These data taken together give new insights into the physiological mechanism of BLM protein and the use of Bloom’s Syndrome as a model for carcinogenesis.