Myosin-IIa Is Required for Leukemia Cell Extravasation and Its Inhibition Reduces Leukemia Dissemination and Prolongs Survival in a Mouse Model of Acute Lymphoblastic Leukemia

Background: Leukemia affects approximately 45,000 people each year in the USA with more than 20,000 fatalities. Many leukemia patients experience initial remission but often relapse, with the relapsing leukemia affecting organs such as the central nervous system (CNS). The CNS acts as a sanctuary site allowing leukemia cells to escape treatments such as tyrosine kinase inhibitors (TKIs) and chemotherapy. Leukemia dissemination is a complex process requiring leukemia cells to exit the blood circulation by extravasation and invade target tissues. To extravasate, leukemia cells cross through vascular endothelial walls in a process called trans-endothelial migration, which requires cytoskeletal remodeling. However, the specific cytoskeletal effectors of leukemia extravasation are not fully known. Notably, leukemia dissemination correlates negatively with survival rates.

Goal: Our goal was to validate Myosin-IIA, a class II myosin motor protein, as a molecular target to inhibit lymphoid leukemia cell extravasation and hinder leukemia dissemination, particularly infiltration into the CNS. Myosin-IIA has been shown to play a role in cell migration. We recently showed that this myosin is more profoundly required for activated lymphocyte entry into the CNS than for homeostatic entry into secondary lymphoid organs. This suggests that Myosin-IIA may be a promising candidate to prevent leukemia infiltration into the CNS without completely inhibiting homeostatic lymphocyte trafficking.

Results: For this study, we used a mouse model of Bcr-Abl driven B cell acute lymphoblastic leukemia (B-ALL), which closely recapitulates Philadelphia chromosome positive human leukemias. We used shRNA interference to inhibit Myosin-IIA protein expression in the leukemia cells. Myosin-IIA depletion did not affect baseline apoptosis of the leukemia cells but did result in a small but significant reduction in their growth rate. Myosin-IIA was key in promoting leukemia cell migration in response to the chemokine CXCL12. Expression of Myosin-IIA was also critical for leukemia cells to complete trans-endothelial migration through brain-derived endothelial cells in an in vitro model of extravasation. In addition, our data suggested that inhibition of Myosin-IIA reduces the ability of leukemia cells to disseminate in vivo, including reducing leukemia infiltration into the CNS. Finally, compared to control leukemia cells, inhibition of Myosin-IIA significantly prolonged survival in an in vivo leukemia transfer model.

Conclusion: Our data suggest that Myosin-IIA regulates leukemia migration thus making this myosin a promising target to inhibit leukemia dissemination in vivo, particularly into the CNS. Given the serious side-effects of cranial irradiation and (intrathecal or systemic) chemotherapy, the ability to inhibit CNS infiltration of leukemia cells can be a valuable therapeutic tool to improve the efficacy of current therapies by reducing the relapse frequency following TKI and chemotherapy treatments.