Small-Molecule Polθ Inhibitors Provide Safe and Effective Tumour Radiosensitisation in Preclinical Models
Rodriguez-Berriguete et al., Clin Cancer Res. 2023
Around 50% of cancer patients receive radiotherapy, but despite major technical improvements in its administration, an increase in survival rates is usually limited by the radiosensitivity of adjacent tissues. One strategy for improvement of the outcome is selective radiosensitisation of the tumour cells. Tumour-specific radiosensitisation by suppressing the DNA damage response is a promising approach. Some DNA repair inhibitors are in clinical trials. These include poly(ADP-ribose) polymerase 1 (PARP1), ataxia telangiectasia-mutated (ATM), ataxia telangiectasiamutated and Rad3-related (ATR), DNA protein kinase, catalytic subunit (DNA-PKCS), checkpoint kinase 1 (CHK1) and WEE1 G2 checkpoint kinase (WEE1), to name a few. However, they are rarely combined with radiotherapy. A disadvantage is that these proteins are not specifically expressed in tumour cells and side effects are observed in the surrounding normal tissue. In contrast, the DNA repair enzyme DNA polymerase theta (Polθ or POLQ) is overexpressed in tumours only [1, 2]. POLθ could therefore represent an ideal tumour-specific target for radiation sensitisation [3].
POLθ plays a key role in the repair pathway known as microhomology-mediated end-joining (MMEJ, also called theta-mediated end-joining (TMEJ)). MMEJ is a repair pathway for DNA double-strand breaks (DSBs) that depends on the presence of short homologous sequences (two to four base pairs) at the break site [4]. MMEJ was previously considered only a backup pathway for repair by non-homologous end-joining (NHEJ) or homologous recombination (HR). Recent data show that MMEJ is also activated in parallel to NHEJ and HR. Like HR, MMEJ requires a 50-30 resection of the DNA break. However, in MMEJ the resection is shortened and serves to expose small microhomologies between the two single-stranded DNA strands to enable their annealing. In contrast to HR, which is considered error-free, MMEJ is an error-prone DSB repair pathway that is associated with deletions and the introduction of single-base errors [5]. POLθ deficiency is synthetically lethal in cells with defects in HR repair, suggesting that cancer cells that are deficient in HR, e.g. because of BRCA mutations, are more reliant on the MMEJ pathway than are other cancer cells [6].
Five POLθ inhibitors are currently being tested in phase I and II clinical trials with regard to their therapeutic success (https://clinicaltrials.gov/search?term=polymerase%20theta ). The study considers the drug as monotherapy, combined with PARP1 inhibitors or in patients with DNA repair defects. Their use combined with radiotherapy is not part of these clinical trials. However, there is increasing evidence that radiotherapy can offer great therapeutic benefits.
For example, the study by Rodriguez-Berriguete et al. [7] revealed two novel, specific allosteric inhibitors of the POLθ DNA polymerase domain radiosensitised tumour cells, especially in combination with fractionated radiation. In parallel, no radiosensitisation was observed in non-cancer cells. The study also showed that radiosensitisation caused by POLθ inhibition was most effective in the replication of cells, due to impaired DNA damage repair. Furthermore, it was shown that radiosensitisation was also effective under hypoxia. In vivo, the combination of POLθ inhibition and fractionated irradiation was also well tolerated and led to a significant reduction in tumour growth compared with the use of irradiation alone [7].
This observation has been supported by further studies. As long ago as 2010, Higgins et al. showed that knockdown of POLθ in tumour cells without mutations in the BRCA genes led to radiosensitisation [8]. This was manifested by increased amounts of unrepaired DSB and reduced cellular survival rates in several cancer cell lines. POLθ was also identified as a radiosensitiser for prostate cancer cell lines [9]. This suggests that inhibition of POLθ leads to radiosensitisation in a broad spectrum of tumours, independent of HR status. Rao et al. even observed an increase in POLθ expression after irradiation. Again, inhibition of POLθ led to an increase in amounts of unrepaired DSB and decreased cellular survival rates. This radiosensitising effect was also confirmed in a mouse model [10]. Even after high doses of linear-energy-transfer radiation, the inhibition of POLθ seemed to have a radiosensitising effect due to increased chromosomal instability [11]. Further studies investigated the combination of POLθ inhibition with other DNA repair inhibitors. A combination of POLθ with DNA-PKCS inhibition [12] and of POLθ with PARP inhibition [13] resulted in radiosensitisation.
The conclusion is that the radiosensitising potential of POLθ inhibition is very promising and should be tested in future clinical trials.
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13. Lin, X., et al., Talazoparib enhances resection at DSBs and renders HR-proficient cancer cells susceptible to Poltheta inhibition. Radiother Oncol, 2024. 200: p. 110475.
Marie Ronja Vogt and Kerstin Borgmann
Department of Radiotherapy and Radiation Oncology
Centre of Oncology
University Hospital Hamburg-Eppendorf
Hamburg, Germany
borgmann@uke.de
Kerstin Borgmann