Abstract
The human T-lymphotropic virus type 1 (HTLV-1) was the first retrovirus associated with a human malignancy. It infects 5 to 10 million individuals worldwide and is responsible, in 5% of cases, for a rare but highly aggressive and incurable cancer named Adult T-cell Leukemia (ATL). Among viral proteins implicated in the infection and cancer onset, antisense protein HBZ (HTLV-1 bZIP factor) plays a major role. It is encoded on the complementary strand of proviral genome from the 3'-LTR and exists under two proteins isoforms: usHBZ (unspliced) and HBZ_SP1 (spliced). If there are no doubts about HBZ functions in HTLV-1 associated pathogenesis and oncogenesis, very few studies have distinguished the two isoforms so far. We reveal in the first part of this thesis work that both usHBZ and HBZ_SP1 have oncogenic properties, but that HBZ_SP1 is ~300 times more expressed than usHBZ in ATL patients' samples. Three HBZ-splicing factors have been identified: hnRNPs A1, H1 and E1; and A1 seems to be part of a regulation loop to maintain HBZ splicing toward HBZ_SP1 isoform. Various minigene-assays also revealed that HBZ_SP1 modulates splicing by promoting exon inclusion. In the second part, we approached a transcription factor overexpressed in ATL: Fra-2. We managed to describe experimentally for the first time the expression of two isoforms (X1 and X4) in ATL patients' samples. X4 is the most oncogenic isoforms of all, given its cell proliferation, transformation and migration properties, and also gives a drug resistance advantage to cells by promoting drug efflux. Fra-2 X4 minigene construction then revealed that splicing factor RNPS1 seemingly interacts with HBZ, and induces Fra-2 splicing toward the X4 oncogenic isoform. As HBZ_SP1 also promotes exon inclusion, its role in the inclusion of the exon 4 of Fra-2 remains to be confirmed. This work contributes to a better understanding of the role of different oncogenic proteins isoforms in cell transformation and underlines the importance of studying alternative splicing regulation in infections and cancers.