The radiobiology of HPV-positive and HPV-negative head and neck squamous cell carcinoma

Abstract

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide, with reported incidences of ~800 000 cases each year. One of the critical determinants in patient response to radiotherapy, particularly for oropharyngeal cancers, is human papillomavirus (HPV) status where HPV-positive patients display improved survival rates and outcomes particularly because of increased responsiveness to radiotherapy. The increased radiosensitivity of HPV-positive HNSCC has been largely linked with defects in the signalling and repair of DNA double-strand breaks. Therefore, strategies to further radiosensitise HPV-positive HNSCC, but also radioresistant HPV-negative HNSCC, have focussed on targeting key DNA repair proteins including PARP, DNA-Pk, ATM and ATR. However, inhibitors against CHK1 and WEE1 involved in cell-cycle checkpoint activation have also been investigated as targets for radiosensitisation in HNSCC. These studies, largely conducted using established HNSCC cell lines in vitro, have demonstrated variability in the response dependent on the specific inhibitors and cell models utilised. However, promising results are evident targeting specifically PARP, DNA-Pk, ATR and CHK1 in synergising with radiation in HNSCC cell killing. Nevertheless, these preclinical studies require further expansion and investigation for translational opportunities for the effective treatment of HNSCC in combination with radiotherapy.

Keywords: DNA damage; DNA repair; head and neck cancer; human papillomavirus; ionising radiation; radiation biology.

Fig. 1.

The DNA damage signalling pathway. Accumulation of DNA damage and replication stress leads to activation of ATM- and ATR-dependent signalling pathways mediated through the CHK2 and CHK1 effector kinases, respectively. Phosphorylation of the CDC25 phosphatases (leading to inactivation or ubiquitylation-dependent degradation) or phosphorylation-dependent stabilisation of the p53 tumour suppressor protein ultimately leads to cell-cycle arrest at either the G₂M or G₁S checkpoints, respectively allowing for DNA damage repair. WEE1 is also a target of CHK1 kinase activity, leading to G2/M checkpoint activation.

Fig. 2.

The BER pathway. BER is instigated by damage-specific DNA glycosylases that excise the damaged DNA base. Subsequently, APE1 promotes strand incision at the abasic site to generate a SSB containing a 5′-dRP end, which promotes PARP-1 binding. In short patch BER, single nucleotide incorporation and 5′-dRP removal is stimulated by Pol β, and DNA ligation by XRCC1-Lig IIIα. In contrast, in long-patch BER and following single nucleotide addition by Pol β, there is a polymerase switch to Pol δ/ε that insert 2–8 nucleotides and promote strand displacement. The flap structure is removed by FEN-1 with PCNA, and finally Lig I seals the SSB.

Fig. 3.

DNA DSB repair pathways. NHEJ can be divided into canonical NHEJ (C-NHEJ) and alternative NHEJ (A-NHEJ). C-NHEJ is stimulated by recruitment of the Ku70/80 heterodimer, DNA-PKcs and end-processing factors to the DSB ends. DNA ligation is promoted by XRCC4-Lig IV. In contrast for A-NHEJ, PARP-1 binds to the resected DNA ends generated by MRN and CtIP containing regions of micro-homology, and following DNA synthesis, ligation is performed by XRCC1-Lig IIIα or Lig I. During HR and following MRN/CtIP action in a BRCA1-dependent process, RPA binds to the single-stranded DNA regions generated. Further recruitment of BRCA2 and the RAD51 recombinase that displaces RPA to form a nucleoprotein filament promotes strand invasion, DNA synthesis and Holliday Junction formation and resolving.