Paysage génomique de la leucémie aiguë lymphoblastique de l’enfant

Acute lymphoblastic leukemia (ALL) is a complex disease with a multi-factorial etiology. It represents the most frequent pediatric cancer and despite a significant increase of survival rate, about 15% of the patients still do not respond to current treatment protocols and over 2/3 of survivors experience long-term treatment related side effects. To reduce these numbers, a better understanding of the underlying causes of ALL is needed. Through the analysis of next-generation sequencing (NGS) data obtained from the established Quebec cALL (QcALL) cohort of the Sainte-Justine hospital, I have been particularly concerned about the genomic determinants that contribute to different phases of ALL (predispositions, onset/progress and relapses). First, I developed an analysis tool (SNooPer) based on a machine learning algorithm integrating both normal and tumor NGS data of the patient to identify somatic mutations from low-pass sequencing. This tool, combined to in silico predictive analysis and to adequate functional validations, allowed us to characterize rare or recurrent events involved in the leukemogenesis process. Through the analysis of pre-B ALLs, I have been able to identify several rare driver genes which had never been associated to ALL before (ACD, DOT1L, HCFC1). The functional study of the identified mutation in ACD, a member of the shelterin complex, showed a concomitant lengthening of the telomeres and decreased apoptosis levels in leukemia cells. Besides the interest aroused by the discovery of these new drivers, I wanted to demonstrate the importance of low-frequency somatic events to establish the generally underestimated interindividual specificity and identify all cellular functions involved. During this work, I also identified new recurrent driver events in T-cell ALL (T-ALL), particularly among poorly characterized immature T-ALL patients. For example, I demonstrated the impact of a recurrent mutation in U2AF1, member of the spliceosome, on alternative splicing of cancer-relevant genes, further suggesting the importance of aberrant splicing in leukemogenesis. I also identified two new X-linked tumor suppressors, MED12 and USP9X, never associated to T-ALL before and obtained results supporting a potential role for these genes in the male-biased sex ratio observed in T-ALL (ratio male:female =1.22). Finally, through the longitudinal study of pre-B cALLs who suffered one or multiple relapses, I analyzed the clonal architecture and evolution of the tumors. I identified two distinct evolution patterns governing either early or late relapses: on one hand a highly dynamic pattern, sustained by a defect of DNA repair processes, illustrating the quick emergence of fitter clones - and on the other hand, a quasi-inert evolution pattern suggesting the escape from dormancy of neoplastic stem cells likely spared from initial cytoreductive therapy. Overall, this thesis contributed to the characterization of genomic determinants that constitute the inter- and intra-tumor variability, participate in leukemogenesis and/or in resistance mechanisms. This new knowledge will contribute to refine patient stratification and treatment.

Keyphrases: drivers, Childhood acute lymphoblastic leukemia, Genomic, Rare or recurrent driver mutations, Somatic mutations, Next-generation sequencing, Machine learning, Relapse, Clonal architecture and dynamics, Genetic susceptibility