INDUCTION AND REPAIR OF DNA SINGLE-STRAND BREAKS AND DNA-BASE DAMAGE AT DIFFERENT CELLULAR STAGES OF SPERMATOGENESIS OF THE HAMSTER UPON IN-VITRO EXPOSURE TO IONIZING-RADIATION

Alkaline elution has been used for quantitative detection of DNA damage caused by ionizing radiation in unlabeled. somatic and germ cells. Both the induction and subsequent repair have been studied for two classes of DNA damage, viz. single-strand breaks (SSB), and base damage (BD) recognized by the gamma-endonuclease activity in a cell-free extract of Micrococcus luteus bacteria. The high sensitivity of the assay permitted the measurement of induction and repair of SSB and BD after in vitro exposure of hamster germ cells in different cellular stages of spermatogenesis (spermatocytes, round and elongated spermatids), and of bone-marrow cells, to biologically- relevant doses (0-8 Gy) of Co-60 gamma-rays. A dose-dependent increase was observed for both types of lesions, which was similar for most cell types. The elongated spermatids, however, showed a lower induction frequency of SSB (and perhaps BD). Spermatocytes, round spermatids and bone-marrow cells had normal, fast repair of the SSB when compared with the repair reported for cultured rodent cells and human lymphocytes. In contrast, the elongated spermatids showed hardly any SSB repair. The initial rate of repair of BD in spermatocytes and bone-marrow cells was in the same range as that for SSB, but only 60-70% of the initial BD was repaired within 1 h, whereas after that period no SSB were detectable. The round spermatids hardly repaired any BD within the first hour after irradiation, but after 7 h only a few BD could be detected. In elongated spermatids repair of BD could not be measured due to a high background level of this type of damage.