The cyclin-G associated kinase (GAK) is a novel mitotic kinase and therapeutic target in diffuse large B-cell lymphoma

Diffuse large B cell lymphoma (DLBCL) is an aggressive non-Hodgkin lymphoma and the most frequently diagnosed hematologic malignancy in the United States. DLBCL exhibits significant molecular and clinical heterogeneity, and at least a third of patients are left uncured with standard frontline chemoimmunotherapy. As such, there is a critical need to identify novel targeted therapies to improve outcomes. We conducted a phenotypic screen of kinase inhibitors against DLBCL cell lines and non-malignant controls. We identified the cyclin G-associated kinase (GAK) as a tumor-selective, readily druggable target whose inhibition killed DLBCL cell lines, while sparing non-malignant blood cells. Upon investigation of GAK's cellular function, we discovered that inhibition results in G2/M-phase cell cycle arrest. Immunofluorescent confocal microscopy revealed significant chromosome misalignment and spindle distortion in DLBCL cells following GAK-inhibition, disrupting progression through mitosis. Analysis of RNA-seq data from clinical samples showed increased GAK expression associates strongly with RB1 deficiency in DLBCL cases, suggesting dependency on GAK for proper mitotic progression linked to retinoblastoma associated protein (RB) loss of function, a common DLBCL driver. In cell-cycle analyses and under microscopy, RB-deficient DLBCL cells treated with an exquisitely selective GAK inhibitor showed complete arrest at G2/M, pronounced distortion of mitotic spindles, and widespread chromosomal damage. Finally, in vivo studies of DLBCL xenograft-bearing NSG mice achieved a dramatic tumor-burden reduction in response to targeted GAK inhibition. These results reveal a novel cell cycle kinase suitable for therapeutic exploitation in DLBCL patients and linked to the common, undruggable biomarker of RB loss of function.