The extreme radiosensitivity of the squamous cell carcinoma SKX is due to a defect in double-strand break repair.
OBJECTIVE
Squamous cell carcinomas (SCCs) are characterized by moderate radiosensitivity. We have established the human head & neck SCC cell line SKX, which shows an exceptionally high radiosensitivity. It was the aim of this study to understand the underlying mechanisms.


METHODS
Experiments were performed with SKX and FaDu, the latter taken as a control of moderate radiosensitivity. Cell lines were grown as xenografts as well as cell cultures. For xenografts, radiosensitivity was determined via local tumour control assay, and for cell cultures using colony assay. For cell cultures, apoptosis was determined by Annexin V staining and G1-arrest by BrdU labelling. Double-strand breaks (DSBs) were detected by both constant-field gel electrophoresis (CFGE) and gammaH2AX-foci technique; DSB rejoining was also assessed by in vitro rejoining assay; chromosomal damage was determined by G01-assay.


RESULTS
Compared to FaDu, SKX cells are extremely radiosensitive as found for both xenografts (TCD(50) for 10 fractions 46.0Gy [95% C.I.: 39; 54 Gy] vs. 18.9 Gy [95% C.I.: 13; 25Gy]) and cell cultures (D(0.01); 7.1 vs. 3.5Gy). Both cell lines showed neither radiation-induced apoptosis nor radiation-induced permanent G1-arrest. For DSBs, there was no difference in the induction but for repair with SKX cells showing a higher level of both, slowly repaired DSBs and residual DSBs. The in vitro DSB repair assay revealed that SKX cells are defective in nonhomologous endjoining (NHEJ), and that more than 40% of DSBs are rejoined by single-strand annealing (SSA). SKX cells also depicted a two-fold higher number of lethal chromosomal aberrations when compared to FaDu cells.


CONCLUSIONS
The extreme radiosensitivity of the SCC SKX seen both in vivo and in vitro can be ascribed to a reduced DNA double-strand break repair, resulting from a defect in NHEJ. This defect might be due to preferred usage of other pathways, such as SSA, which prevents efficient endjoining.
Kasten-Pisula
Ulla
U
Laboratory of Radiobiology & Experimental Radiooncology, University Medical Center Hamburg - Eppendorf, Hamburg, Germany.
Menegakis
Apostolos
A
Brammer
Ingo
I
Borgmann
Kerstin
K
Mansour
Wael Y
WY
Degenhardt
Sarah
S
Krause
Mechthild
M
Schreiber
Andreas
A
Dahm-Daphi
Jochen
J
Petersen
Cordula
C
Dikomey
Ekkehard
E
Baumann
Michael
M
D000818:Animals; D017209:Apoptosis / Q000528:radiation effects; D002294:Carcinoma, Squamous Cell / Q000235:genetics* / Q000532:radiotherapy; D002869:Chromosome Aberrations / Q000528:radiation effects; D053903:DNA Breaks, Double-Stranded; D004260:DNA Repair / Q000528:radiation effects; D005260:Female; D016193:G1 Phase / Q000528:radiation effects; D006258:Head and Neck Neoplasms / Q000235:genetics / Q000532:radiotherapy; D006801:Humans; D008297:Male; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D011836:Radiation Tolerance / Q000235:genetics*; D011879:Radiotherapy Dosage; D014407:Tumor Cells, Cultured / Q000528:radiation effects