1039 - The proto-oncogene TCL1A deregulates cell cycle and genomic stability in CLL

The oncoprotein T-cell leukemia/lymphoma 1A (TCL1A) is a 14 kDa adapter protein that is causally implicated in various B-cell and T-cell malignancies. TCL1A was initially discovered in T-prolymphocytic leukemia (T-PLL), where chromosomal abnormalities cause an increase in TCL1A expression, considered the initiating event in this aggressive disease. In chronic lymphocytic leukemia (CLL), TCL1A is overexpressed in the majority of patients, and high expression correlates with aggressive disease features and inferior clinical outcomes. Despite extensive research on TCL1A's oncogenic functions in stemness, survival, and signaling pathways (e.g., enhancing the activity of the serine/threonine kinase AKT), its full spectrum of effector functions and the molecular concept of TCL1A-mediated leukemogenesis remain unclear. Therefore, we aimed to systematically characterize undiscovered TCL1A effector molecules, affected pathways, and their influence on TCL1A-induced CLL pathogenesis. We observed in mouse models of subcellular-site specific TCL1A-induced lymphomagenesis, that TCL1A exerts a strong transforming impact via nuclear topography. In proteomic screens of TCL1A-bound molecules in CLL cells and B-cell-lymphoma lines, we identified regulators of cell cycle and DNA repair pathways as novel TCL1A interactors, particularly enriched under induced DNA damage and mitosis. We identified as a novel TCL1A interactor the mitotic checkpoint protein CDC20, which is crucial in the regulation of timely separation of the sister chromatids during mitosis. CDC20 functions as a coactivator of the anaphase-promoting complex/cyclosome (APC/C), thereby driving mitotic exit, but is also part of the mitotic checkpoint complex (MCC), inhibiting the APC/C. Via in-silico modeling and experimental validations, we defined the crucial domains mediating the direct TCL1A-CDC20 interaction and identified an abrogating effect of TCL1A on the engagement of CDC20 with the MCC protein MAD2. According to a regulatory impact of TCL1A on the activity of the CDC20-containing mitotic checkpoint and anaphase-promoting complexes during mitotic progression, TCL1A overexpression accelerated cell-cycle transition in B-cell-lymphoma lines, impaired apoptotic damage responses in association with pronounced chromosome mis-segregation and caused cellular aneuploidy in Eµ-TCL1A mice.
Interestingly, among hematopoietic cancers, CDC20 levels seem particularly low in CLL. CDC20 expression negatively correlated with TCL1A, and lower expression marked more aggressive and genomically instable disease and cellular phenotypes. Importantly, knock-down of Cdc20 in TCL1A-initiated murine CLL promoted aneuploidy and leukemic acceleration.
Taken together, our data give significant novel insights into the molecular mechanisms of TCL1A-induced leukemogenesis. We propose that the cross-talks between enhanced survival signaling, and deregulation of the DNA damage response and the cell cycle checkpoints, drive genomic instability and malignant transformation. Through this comprehensive characterization of TCL1A’s function, we gained a deeper understanding of the mechanisms involved in CLL pathogenesis, which could also potentially be applied to other malignancies harboring TCL1A overexpression. It will also help uncover potential vulnerabilities that could be exploited for more targeted treatment approaches.