The Scaffolding Protein NEDD9 Regulates Chronic Lymphocytic Leukemia Cell Migration Via the CXCR4 - CXCL12 Axis and Promotes Disease Progression

Despite the undeniable vast improvement in CLL treatment strategies, resistance to novel compounds such as ibrutinib and venetoclax already emerged and posed a challenge in many aggressive cases. The fundamental role of the homing process in CLL progression and presumably relapse prompted us to analyze the impact of a crucial regulator of chemokine response, migration and lymphocyte homing - namely NEDD9 - on CLL pathogenesis. The scaffold protein NEDD9 is frequently upregulated and hyperphosphorylated in different cancer entities, with its deregulation being associated with poor clinical outcome and therapy resistance. In B cells, activation of integrin- and the B cell receptor signaling pathways leads to hyperphosphorylation of NEDD9, predominantly by Src family kinases, promoting cell adhesion, migration and chemotaxis.

To elucidate the functional relevance of NEDD9 in CLL pathogenesis in vivo, Eµ-TCL1 transgenic mice were crossbred with Nedd9 deficient mice. CLL burden was monitored in the peripheral blood of Nedd9-proficient (TCL1^tg/wt Nedd9^wt/wt) versus Nedd9-deficient (TCL1^tg/wt Nedd9^-/-) mice every two months over a year, revealing a significantly lower proportion of CLL per total B cells (CLL/B cells) in the peripheral blood in TCL1^tg/wt Nedd9^-/- mice at four and six months of age. CLL onset was clearly delayed in TCL1^tg/wt Nedd9^-/- mice in comparison to TCL1^tg/wt Nedd9^wt/wt mice. The infiltration of CLL cells into the spleen and bone marrow was significantly reduced in TCL1^tg/wt Nedd9^-/- mice at three and ten months, accompanied by significantly longer overall survival of the TCL1^tg/wt Nedd9^-/- group. Particularly, this eminent role of Nedd9 in CLL pathogenesis could be largely attributed to Nedd9 expression in B cells. Using a conditional Nedd9 knockout mouse exclusively in B cells (TCL1^tg/wt CD19Cre^tg/wt Nedd9^flfl (TCN)), we observed a highly similar phenotype of TCN mice to the TCL1^tg/wt Nedd9^-/- mice, including significantly delayed CLL onset, lower proportion of CLL per total B cells (CLL/B cells) in the peripheral blood, and reduced hepatosplenomegaly in TCN mice compared to the control TCL1^tg/wt CD19Cre^wt/wt Nedd9^flfl (TN) mice. In summary, our mouse data suggest that Nedd9 deficiency significantly delayed CLL onset and progression, particularly in the early stages of CLL. Moreover, Nedd9 deficiency significantly decreased the accumulation of CLL cells both in typical leukemic homing organs such as spleen and bone marrow as well as in the peripheral blood in two independent mouse models and significantly prolonged survival of the TCL1^tg/wt Nedd9^-/- mice.

The strongly reduced capacity of Nedd9-deficient CLL cells to migrate and home to the lymphoid niche prompted us to investigate the underlying mechanistic signaling pathway upon Nedd9 loss. For this purpose, we examined surface expression levels of prominent cell trafficking mediators on Nedd9-proficient and -deficient CLL cells, and found a consistently reduced level of CXCR4 on the surface of TCL1^tg/wt Nedd9^-/-cells. In the transwell assay, CLL cells isolated from TCL1^tg/wt Nedd9^-/- mice showed a dramatic reduction in migration towards CXCL12 compared to their wild type counter parts. In line with the results of the murine CLL cells, NEDD9-depleted MEC1 cells (shNedd9) also showed decreased CXCR4 levels and dramatic reduction in migration towards CXCL12 compared to the control cell line (shNT).

Collectively, we provide the first direct evidence that genetic targeting of Nedd9 in vivo impairs CLL cell adhesion, migration and chemotaxis, resulting in decreased CLL cell infiltration into secondary lymphoid organs and the bone marrow. These observations could serve as basis for the development of new treatment strategies, targeting a scaffold protein to impair the homing process of CLL cells, a prerequisite for their survival and expansion within the microenvironment of protective niches.