ROR1 Expression Accelerates Leukemia Development in RORxTCL1 Transgenic Mice

Abstract Abstract 3905 ROR1 is a receptor tyrosine kinase-like orphan receptor and an oncofetal protein that is expressed on chronic lymphocytic leukemia (CLL) B cells, but not on normal B cells or most other adult tissues. The leukemia-associated expression of ROR1 suggests that it potentially could contribute to the development and/or progression of CLL. To investigate its functional significance in the development and/or progression of CLL, we generated B6 mice transgenic (Tg) for human ROR1 (hROR1) under the control of the murine Ig promoter/enhancer, which drives B-cell-restricted expression of hROR1. These mice developed increased numbers of splenic B lymphocytes relative to that of control littermates lacking the hROR1 transgene; a few of these animals also developed hROR1+/CD5+/B220low B cell leukemia resembling human CLL at 15–18 months of age. We crossed hROR1-Tg mice with B6 Eμ-TCL1-Tg mice (TCL1), which at 7 months of age develop CD5+B220low leukemia B cells detectable in the blood and resembling human CLL cells except lacking expression of mouse ROR1. We found the F1 animals with both transgenes (ROR1XTCL1) developed hROR1+/CD5+/B220low B-cell CLL at a significantly younger median age than did littermate-control mice having either transgene alone. Comparison of the number of CD5+B220low leukemia B cells in ROR1xTCL1 or TCL1 Tg mice at five, six, and seven months of age demonstrated that ROR1xTCL1 Tg mice have significantly higher median percentages of circulating leukemic cells in the blood compared to that of age-matched TCL1 Tg mice (Welchs’s t test based on the average of all 3 measurements, p=0.038). ROR1xTCL1 leukemia B cells also expressed higher levels of hROR1 than non-leukemia B cells of ROR1 Tg mice. To confirm that ROR1 enhances leukemia-cell expansion, we adoptively transferred CD5+B220low leukemia B cells from ROR1xTCL1 Tg mice into syngeneic ROR1-Tg mice. Adoptive transfer of equal numbers (1×105) of leukemia B cells from ROR1xTCL1 or TCL1 Tg mice resulted in transfer of leukemia in both cases without requiring prior conditioning of recipient animals. However, animals engrafted with ROR1xTCL1 leukemia cells developed more aggressive disease and more marked splenomegaly than did animals engrafted with TCL1 leukemia cells. Moreover, eight weeks after adoptive transfer, animals engrafted with ROR1xTCL1 leukemia cells had ≥4-fold greater median numbers of neoplastic B cells in the spleen (e.g. 4 × 108) than did animals engrafted with TCL1 leukemia cells. In vivo labeling studies with bromodeoxyuridine (BrdU), measurement of the relative proportions of Ki-67-positive leukemia cells from either type of adoptive host animals, and use of the terminal deoxynucleotidyl transferase (dUTP) nick end-labeling (TUNEL) assay to detect cells that had undergone apoptosis. These studies revealed that animals engrafted with hROR1+ leukemia cells had significantly lower-rates of leukemia-cell turnover due to higher rates of cell proliferation and lower rates of apoptosis than that animals engrafted with TCL1 leukemia cells. These studies indicate that hROR1 can accelerate development of de novo B-cell leukemia, lower the rate of leukemia-cell turnover, and enhance disease progression in this model system. Disclosures: No relevant conflicts of interest to declare.