Misrejoined , residual double strand DNA breaks and radiosensitivity in human tumor cell lines

The aim of the present study is to investigate whether differences between tumor cells in radiosensitivity are related to misrejoined- or residual DNA-double strand breaks.

An assay that allows measurement of absolute induction frequencies for DNA double strand breaks (DSBs) in defined regions in the genome, and that quantitates rejoining of correct DNA ends has been used to study repair of DSBs in three human tumor cell lines. DNA double-strand breaks (DSBs) were measured within a 3.5-Mbp Not 1 fragment on chromosome X of human tumor cell lines with different radiosensitivities. Correct rejoining of DSBs was measured by hybridization of single-copy DNA probe to Not 1 restriction fragments separated according to size by pulsed field gel electrophoresis (PFGE). Induction of DSBs is quantified from the decrease in the intensity of the hybridizing restriction fragment and an accumulation of a smear below the band. Rejoining of DSBs results in reconstitution of the intact restriction fragment only if correct DNA ends are joined. By comparing results from this technique with results from a conventional electrophoresis technique (FDR assay) that detects all rejoining events, it was possible to quantitate the misrejoining frequency after 50Gy of X irradiation. Residual breaks were measured 24h after irradiation.

In terms of clonogenic assay, squamous cell carcinoma cell line (4451) was the most radiosensitive, followed by the breast carcinoma cell line (BB) while the bladder carcinoma cell line (RT112) was the most radioresistant. Twenty-four hours after irradiation, 4451 cell line accumulated the highest level of residual (non-repairable) DSB followed by BB and then RT112 cell line, which showed the lowest level of residual DSB. This was the same rank as in the radiosensitivity assay. Regarding DSB misrejoining, RT112 cell line showed the highest percent of incorrectly repaired DSB, which does not agree with the results of the radiosensitivity assay.

From our data, it could be concluded that residual (non repairable) DSB is more important in terms of radiosensitivity than incorrectly repaired DSB.