Transcription of ribosomal DNA (rDNA) by RNA Polymerase I (Pol I) is elevated in cancer cells to accommodate for their increased rate of protein production and cell growth. We have established that rDNA transcription plays a regulatory role in cellular checkpoints and thus represents a viable target for chemotherapeutic treatment. Our data demonstrate that CX-5461, a first-in-class inhibitor of Pol I transcription, is effective at treating cancer in a range of tumours models. However, all tumours are susceptible to relapse through acquired resistance.
To understand how resistance to CX-5461 is mediated, mice were transplanted with Eμ-Myc lymphoma cells and upon engraftment were treated with CX-5461 continuously until relapse, following a true remission period. The CX-5461 resistant tumours were then sequenced by whole exome sequencing. Strikingly, DNA Topoisomerase IIα (Top2α) was the only recurringly mutated gene isolated in all tumours, with mutations spanning the entire gene.
DNA Topoisomerases (Topos) are essential for resolving topological constraints in the DNA by cleaving and re-ligating the DNA. Inhibiting this process can trigger a DNA damage response (DDR), which may lead to growth arrest, apoptosis and cell death. Since Topo expression is elevated in tumour cells, Topo inhibitors are commonly used in cancer treatment. However, the effectiveness of Topo inhibitors is limited as the resulting global DNA damage can cause secondary malignancies, as well as damage to normal tissue.
We will present data to suggest that the mutations lead to reduced Top2α expression, which is sufficient to cause resistance to CX-5461. We hypothesise that CX-5461 can cause Top2α to become trapped at the rDNA loci, thereby eliciting a localised DNA damage-like response, and this response is dependant on the abundance of Top2α. This highlights the potential of Pol I inhibitors as targeted DNA damaging agents that don’t produce the cytotoxicity typically associated with Top2 inhibitors.