Genomic alterations may initiate cancer development as the consequence of endogenous or exogenous DNA damaging factors. Defects in DNA repair mechanisms may also facilitate cancer progression as well as accumulation of mutations which favor cancer cell survival. However, DNA repair pathways in cancer cells can be considered as their Achilles heel which are possible targets in order to compromise their survival. For instance, it has been demonstrated recently that inhibition of a protein called MTH1 via RNA interference (RNAi) or chemical inhibitors can stop tumor growth and triggers cell death by increasing the load of oxidative DNA damage. MTH1 is a hydrolase which converts 8-oxo-dGTP into 8-oxo-dGMP in order to prevent incorporation of oxidatively damaged nucleotides into DNA. In addition, DNA glycosylases which recognize and remove mismatched or damaged nucleotide pairs in DNA can also participate in repair of 8-oxo-dG, such as MUTYH repairing A:8-oxo-dG pair. The goal of the current study was to investigate the importance of MUTYH activity upon MTH1 depletion. The current study tried to answer whether simultaneous knock-down of MTH1 and MUTYH sensitizes cancer cells to oxidative stress and increases cell death. Both enzymes were simultaneously depleted in T cell acute lymphoblastic leukemia cells using RNAi. Then, we analyzed the efficiency of gene and protein knock-down by quantitative real-time-PCR and western blotting, respectively. Induction of cell death was also assessed by flow cytometric analysis of cell cycle. Afterwards, the effect of the treatments on DNA repair pathways was studied by analysis of gene expression of several DNA glycosylases and DNA polymerases using qRT-PCR. The results showed that concurrent depletion of both enzymes led to synergistic induction of cell death. Down-regulation of NEIL1 DNA glycosylase as well as POLQ and POLH DNA polymerases mRNAs adapted their DNA repair pathways to cope with induced damages under these conditions. Finally, the results of this study suggest that dual suppression of MTH1 and MUTYH may provide a new approach to reduce survival of T cell ALL.

Les leucémies aiguës (LA) sont un groupe hétérogène d'hémopathies malignes dues à latransformation oncogénique clonale des cellules souches hématopoïétiques (CSH). On distingue les LA myéloblastiques etlymphoblastiques (LAL). Les LAL sont classées selon le type de précurseur lymphoïde atteint, leur degré de maturité et leurs anomalies cytogénétiques.Le traitement permettant d'obtenir 80 à 90 % de rémission complète (RC) comporte une chimiothérapie d'induction, une consolidation et une intensification(intensification retardée ou allogreffe de CSH selon la situation pronostique). Néanmoins la survie globale à long terme n'est que de 40 à 50 %, du fait de la survenue de rechutes et de la toxicité des traitements. Différents groupes pronostiques basés sur la cytogénétique et la biologie moléculaire se dégagent,pouvant bénéficier de thérapeutiques adaptées. Dans les LAL à chromosome philadelphie (LAL à Ph),antérieurement de mauvais pronostic, les inhibiteurs de tyrosine kinase (ITK) permettent d'obtenir 80% de RCavec cependant un taux de rechute non négligeable. Nous avons démontré qu'une intensification thérapeutique par autogreffe chez des patients avec une maladie résiduelle indétectable permettait une survie à long terme prolongée avec une toxicité moindre que celle de l'allogreffe. En montrant l'implication de l'autotaxine dans les mécanismes de résistance aux ITK dans les LAL à Ph, nous ouvrons la voie à l'utilisation potentielle de nouvelles thérapeutiques. Dans les LAL T, groupe considéré de bon pronostic, un tiers des patients rechute. Nous avons démontré que l'absence de mutation de Notch et/ou FBXW7 ou la présence de mutations de RAS ou PTEN était de mauvais pronostic identifiant un sous-groupe de LAL T dont le traitement devait être renforcé. Nos travaux ont ainsi contribué à l'identification des groupes pronostiques dans les LAL et à l'adaptation des traitements afin d'améliorer les chances de survie. / Acute leukemias are a heterogeneous groups of malignant hematological diseases due to the clonaloncogenic transformation of hematopoietic stem cells(HSTs). We distinguish acute myeloblastic leukaemiafrom acute lymphoblastic leukemia (ALL). ALLs are classified according to the type of lymphoid precursoraffected, its degree of maturity, and with associated cytogenetic abnormalities.Treatment incorporating induction therapy,consolidation, and intensification – delayedintensification or allogeneic stem cell transplantation(SCT) according to prognostic factors – enable 80 to 90% of complete remission (CR). Nevertheless, long-termoverall survival is only 40 to 50% because of relapseand treatment-related toxicity. Different prognosticgroups based on cytogenetic abnormalities andmolecular biology are emerging and patients from eachprognostic group can benefit from adapted therapies.In chromosome Philadelphia-positive ALL (Ph+ ALL) which used to be of particular bad prognosis, tyrosinekinase inhibitors (TKIs) enables 80% of CR but with ahigh-relapse risk. We demonstrated that high-dosetherapy followed by autologous SCT enables prolongedlong-term survival with less drug-related toxicity ascompared to allogeneic SCT in patients with undetectable minimal residual disease. By showing the implication of autotaxine in the resistance to TKIs inPh+ LAL, we enable the use of novel therapeutics inclinical practice.T-cell ALL is considered of poor prognosis as one thirdof patients relapse. In this group of patients we showedthat the absence of a Notch and/or a FBXW7 mutation or the presence of mutations in RAS or PTEN identified a subgroup of patients in whom the treatmentmust be intensified. Our research has contributed to the identification of prognostic groups in ALL and to theadjustment of treatment according to potential survival.