RP1 evaluates the hypothesis that transcription-blocking DNA lesions are highly effective in the therapeutic targeting of CLL cells. The DNA repair group of Björn Schumacher teamed up with the translational group of Marco Herling to exploit transcription-coupled nucleotide excision repair (TC-NER) mechanisms to improve the therapeutic options for CLL. The hypothesis was derived from three specific characteristics. (1) CLL cells are typically residing in the G0/G1 phase and might therefore be less susceptible to conventional chemotherapy that often relies on inflicting DNA lesions that impair DNA replication. In contrast, TC-NER functions independently of the cell cycle to maintain integrity of actively transcribed genes. (2) In the current front-line CLL therapy the nucleoside analogue fludarabine exerts cytotoxic effects by poisoning the gap-filling step that completes the NER reaction. (3) Cellular responses to transcription-blocking lesions are independent of p53 or ATM and might therefore offer opportunities to effectively target clinically problematic chemo-resistant CLL cases. During the first funding period, we systematically conducted proof-of-concept studies. IlludinM, its derivative ferrocen-IM, and trabectedin were tested as “TC-NER active” substances. IlludinM and ferrocen-IM inflict DNA lesions that are recognized by TC-NER. Trabectedin has been demonstrated to lead to highly cytotoxic lesions once its adducts are acted upon by TC-NER resulting in irreparable strand breaks. Confirming our hypothesis, those TC-NER active substances induced cell death in CLL cells that was independent of p53 or ATM and also effectively evoked in therapy-resistant CLL cells. Moreover, we determined synergies between fludarabine and TC-NER active substances. As an already clinically approved substance, we validated trabectedin in TCL1-driven murine models of CLL and determined its superior efficacy. We further performed small-molecule compound screens in TC-NER-deficient C. elegans that allows for the efficient selection of substances that trigger replication- and cell cycle-independent DNA damage responses, in vivo. We isolated inhibitors of Topoisomerase II (Topo II) and DNA polymerase α-primase (DNA polα). Genetic data from the nematode support the role of such types of thus far replication-associated factors in DNA repair in postmitotic cell types.
We propose in phase-II of the project to (1) investigate the mechanisms of the p53/ATM-independent apoptosis in CLL cells induced by the TC-NER active compounds, (2) establish the mechanisms of Topo II and DNA polα inhibition-mediated replication-independent DNA damage response, and to (3) assess the potential clinical relevance of Topo II and DNA polα inhibition in CLL cells and mouse models in combination with TC-NER active substances. We expect to reveal new insights that will improve CLL therapy particularly of those CLL cases that are resistant to currently available treatment options.