OVEREXPRESSION OF WILD-TYPE RRAS2 DRIVES B-CELL
CHRONIC LYMPHOCYTIC LEUKEMIA (B-CLL)
Clara L. Oeste1, Alejandro M. Hortal1, Claudia Cifuentes1, Miguel Sánchez Alcoceba2, Marcos
González Díaz2, Balbino Alarcón1
1Cell Biology and Immunology Department, Centro de Biología Molecular Severo Ochoa, Madrid,
Spain.
2Haematology Service. Salamanca University Hospital, Salamanca, Spain.
Objectives: The small GTPase RRAS2 is closely related to the Ras subfamily of proteins and
has a similar, or even higher, transformation capacity than classical Ras proteins. Instead of
being mutated, RRAS2 has been found overexpressed in the wild-type form in human cancer
cell lines and freshly isolated tumors, including lymphoid malignancies. Studies from our
laboratory have previously reported that mice defective in the small GTPase RRAS2 have
fewer B cells due to defective homeostatic proliferation and survival signals. We therefore
set out to explore the possible effects on lymphoid function of overexpressing wild-type
RRAS2.
Methods: To do so, we generated knock-in mice overexpressing the wild-type form of RRAS2
either ubiquitously (R26-Rras2 Sox2Cre) or B-cell-specifically (R26-Rras2 Mb1Cre). These
mice are monitored by periodic bleeding and staining of leukemic markers by flow cytometry.
We also were provided with CLL patient blood samples, in which we checked RRAS2 levels by
flow cytometry and qPCR.
Results: Surprisingly, in these mouse lines, we have observed the emergence of spontaneous
B-CLL in which CD19+ CD5+ lymphocytes accumulate in the spleen, peripheral blood, lymph
nodes and bone marrow, starting at an early age and in 100% of mice in both lines. These
mature CD19+ CD5+ B cells from R26-Rras2 Mb1Cre mice were isolated and adoptively
transferred into wild-type mice, where they readily expanded as well, conferring the wild-type
mice a CLL phenotype. Therefore, this is the first model of leukemia development by
overexpression of the wild-type form of a Ras family member. We have hence also performed
analyses of more than 150 CLL patient samples to assess RRAS2 levels in human leukemia.
Our qPCR data shows that human CLL lymphocytes (CD19+ CD5+) of untreated patients very
significantly overexpress unmutated RRAS2, with a mean expression of more than 8-fold
higher (n=75, p<0.0001) in full-blown CLL and 3-fold higher (n=25, p=0.0003) in monoclonal
B lymphocytosis (MBL) when compared to healthy donors. We also find a correlation
between RRAS2 mRNA expression levels and the number of malignant CD19+ CD5+ cells in
blood.
Conclusions: Taken together, these results highlight the role of unmutated RRAS2 as an
oncogenic driver and set this gene forth as a leukemic driver, a putative prognostic marker
and a plausible new chemotherapeutic target for B-CLL.
POSTER 19