Abstract
Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is an oncoembryonic protein expressed on chronic lymphocytic leukemia (CLL) that can serve as a receptor for Wnt5a, which can promote leukemia cell migration, proliferation, and survival. We found Wnt5a could induce ROR1 to complex with DOCK2 (dedicator of cytokinesis 2) and induce activation of Rac1/2; these effects could be blocked by cirmtuzumab, a humanized anti-ROR1 monoclonal antibody. We find that silencing DOCK2 specifically impaired the capacity of Wnt5a to induce activation of Rac1/2 or enhance CLL cell proliferation. We generated truncated forms of ROR1 and found the cytoplasmic proline-rich domain (PRD) of ROR1 was required for Wnt5a to induce ROR1 to complex with DOCK2 and activate Rac1/2 in the CLL cell-line MEC1. We introduced single amino acid substitutions of proline (P) to alanine (A) in the ROR1-PRD at potential binding sites for the Src-homology 3 domain of DOCK2. In contrast to wild-type ROR1, or other ROR1 P→A variants, ROR1P808A was unable to recruit DOCK2 in response to Wnt5a. Moreover, unlike MEC1 cells transfected with wild-type ROR1 or ROR1 with P→A substitutions at positions 784, 826, or 841, MEC1 cells transfected to express ROR1P808A did not have a growth advantage over MEC1 cells that do not express ROR1. This study reveals that the recruitment of DOCK2 may be critical for the capacity of Wnt5a to enhance CLL proliferation, which may contribute to the observed increased tendency for disease progression in patients who have CLL cells that express high levels of ROR1.