HNSCC cell lines positive for HPV and p16 possess higher cellular radiosensitivity due to an impaired DSB repair capacity.

Background and purpose: When treated by radiotherapy, patients with squamous cell carcinomas of the
head and neck (HNSCC) positive for HPV and p16INK4a possess a clearly favorable prognosis as compared
to those with HPV-negative HNSCC. The aim of this work was to study whether the better outcomes
might be caused by an enhanced cellular radiosensitivity.
Materials and methods: The radiation response of five HPV/p16INK4a-positive and five HPV-negative cell
lines was characterized with regard to cellular radiosensitivity by colony formation assay. Furthermore
G1- and G2-arrest, apoptosis and residual DNA double-strand breaks (DSB) were analyzed by the colcemid-
based G1-efflux assay, propidium iodide staining, the detection of PARP cleavage, the fluorescencebased
detection of caspase activity and the immunofluorescence staining of cH2AX and 53BP1 foci.
Results: On average, the cellular radiosensitivity of the HNSCC cell lines positive for HPV and p16INK4a was
higher as compared to the sensitivity of a panel of five HPV-negative HNSCC cell lines (SF3 = 0.2827 vs.
0.4455). The higher sensitivity does not result from increased apoptosis or the execution of a permanent
G1-arrest, but is rather associated with both, elevated levels of residual DSBs and extensive G2-arrest.
Conclusions: Increased cellular radiosensitivity due to compromised DNA repair capacity is likely to contribute
to the improved outcome of patients with HPV/p16INK4a-positive tumors when treated by
radiotherapy.