Identification et caractérisation des cibles transcriptionnelles de ETV6, un facteur de transcription impliqué dans la leucémie de l’enfant

Acute lymphoblastic leukemia (ALL) accounts for approximately 25% of all paediatric cancers. Approximately 85% of ALL cases happen in immature B precursors lymphocytes (pre-B ALL). Pre-B ALL is a sporadic cancer that leads to the massive proliferation of leukemia pre-B lymphocytes in the bone marrow. During the past 30 years, the scientific community has developed efficient therapeutic treatments against paediatric ALL, reaching more than 80% cure rate. However, these treatments lack specificity because of the lack of knowledge regarding the molecular mechanisms involved in the initiation and progression of paediatric pre-B ALL. In other words, we know little about the aetiology of this disease. Approximately 25% of children affected with pre-B ALL carry the chromosomal translocation t(12;21)(p13;q22) involving the ETV6 and AML1 genes. This translocation originates in utero and leads to the expression of the transcriptional chimera ETV6-AML1, which alone is insufficient to trigger the development of pre-B ALL. Therefore, other genetic events would be required to initiate paediatric leukemogenesis. The deletion of the residual allele of ETV6 is frequently observed at the diagnosis of pre-B ALL t(12;21)+. This leads to the complete inactivation of ETV6 in leukemia pre-B lymphocytes. ETV6 is a transcriptional repressor of the Ets family. My research hypothesis is that ETV6 acts as a tumour suppressor gene in paediatric pre-B ALL. The inactivation of ETV6 would lead to the deregulated expression of its transcriptional targets and, consequently, would favour the initiation and progression of paediatric leukemogenesis. Given the fact that only very few ETV6 transcriptional targets are known, I initially performed chromatin immunoprecipitation experiments and luciferase assays that enabled the identification of six novel transcriptional targets: TP53 (p53 and Δ133p53), SPHK1, IL-18, PTGER4 and LUM. The ETV6-mediated transcriptional regulation involves both functional domains: PNT (protein interactions) and ETS (DNA-binding domain). These functional domains facilitate the recognition of consensus EBS sites located in a region close to the basal promoter, a molecular mechanism used according to the target promoter and cellular context. Functional studies using leukemia pre-B lymphocytes were useful to understand the role of the ETV6 transcriptional targets during cell proliferation, induction of apoptosis and cell migration through the CXC12/CXCR4 signalling pathway. These functional studies showed the implication of SPHK1, IL-18 and PTGER4 genes during paediatric leukemogenesis. This study is one of the first to support the role of ETV6 as a tumour suppressor gene in paediatric pre-B ALL. Once ETV6 is inactivated, the increased expression of its transcriptional targets favours the proliferation and survival of leukemia pre-B lymphocytes in the bone marrow. The identification of new genes involved in the development of paediatric pre-B ALL opens new doors to the development of therapeutic treatments with a better specificity toward the aetiology of the disease.

Keyphrases: paediatric cancer, acute lymphoblastic leukemia, oncogenesis, ETV6 transcription factor, transcriptional regulation, functional studies