Molecular characterization and functional analysis of poor-prognosis B-cell leukemias

The overall aim of this thesis consisted of expanding the current understanding of the genetic basis and physiopathology of aggressive B-cell leukemias, namely in chronic lymphocytic leukemia (CLL) subtypes and in B-cell prolymphocytic leukemia (B-PLL). CLL, the most common form of adult leukemia in the West, is characterized by an accumulation of monoclonal B cells (CD20+, CD5+ and CD23+) in the peripheral blood, bone marrow, and secondary lymphoid organs. CLL is a highly heterogeneous disease, with a large panel of genetic alterations leading to variable clinical outcomes. Gain of the short arm of chromosome 2 (2p gain) is a frequent chromosomal abnormality in CLL and in other malignancies. Our group has reported that 2p gain was associated with drug refractoriness and poor prognosis factors such as unmutated IGHV and 11q deletion. Using cytogenetic and molecular analyses, we have notably identified a minimal region of gain which encompasses among others XPO1 and REL. In my main thesis project, functional analysis of the role of REL, using three complementary strategies of pharmacological inhibition, gene knockout and transcriptional activation, led to its identification as a key player driving cell survival in CLL. Moreover, I developed several CLL cellular models that allow the overexpression of any gene, alone or in combination, in order to further investigate the roles of REL and XPO1 in CLL and identify potential oncogenic cooperation driving phenotypic features of 2p gain CLL. Finally, we have analyzed the hierarchy and the clonal evolution of the chromosomal abnormalities in 2p gain CLL. CLL with 17p deletion, del(17p), is associated with a lack of response to standard treatment and thus the worst clinical outcome. Our findings showed that del(17p) and 8q24 gain have a synergistic impact on outcome, therefore patients with this “double-hit” CLL have a particularly poor prognosis. B-PLL is an aggressive leukemia, usually resistant to standard chemo-immuno therapy, defined by the presence of prolymphocytes in peripheral blood exceeding 55% of lymphoid cells. We described the cytogenetic and molecular features of a large cohort of 34 B-PLL cases, as well as their in vitro response to novel targeted drugs. Altogether, this work enabled a better understanding of CLL and B-PLL, as well as paving the way for the development of novel therapeutic strategies.