DNA repair kinetics in irradiated undifferentiated and terminally differentiated cells

Abstract

The brains of male Fisher 344 rats bearing 80-150 mg intracerebral 9L/Ro tumors were irradiated with doses of 1,250-5,000 rads of x- or gamma-rays. At various times after irradiation, the cerebellum and tumor were excised, dissociated into single cells and the DNA from these cells sedimented through alkaline sucrose gradients in zonal rotors with slow gradient reorienting capability. Quantitation of the DNA repair kinetics demonstrated that the process in both tumor cells and neurons has a fast and slow phase. Although all other alternatives cannot be completely eliminated, we suggest that these two phases are most reasonably interpreted as representing repair of lesions in very accessible and less accessible regions of the genome rather than 1) repair of different types of lesions such as single- or double-strand breaks or 2) removal of immediate breaks and breaks induced during excision repair of latent base damage. The slow repair phase is saturable, but not inducible in both tumor cells and neurons. The data suggest that tumor cells restore their chromosomal DNA structure to the unirradiated state faster than neurons because 1) they contain more of the repair system per unit of DNA and 2) a larger proportion of their genetic material is comprised of very accessible regions. The data also suggest that the entire tumor cell genome may be accessible to the repair enzyme(s), while it is possible that a portion of the neuronal genome may be completely inaccessible.

Lethal and Potentially Lethal Lesions Induced by Radiation --- A Unified Repair Model

“…where 1 nPL(t') an_d 2 nPL(t') are given by equations (25) and (27), respectively. This is immediately integrated to give…”

“…Evidence exists at high doses that repair of double strand breaks shows the characteristics of saturation (27). It has yet to be established, however, what role, if any, saturation phenomena play in leading to cell lethality at doses less than 7 Gy where the shoulders of survival curves appear.…”

A Modified Alkaline Comet Assay for Measuring DNA Repair Capacity in Human Populations

“…Modified bases can also affect the outcome of this assay after their conversion into AP sites and SSBs by DNA repair pathways (39). At least two components of SSB repair kinetics have been identified using alkaline sucrose gradient sedimentation (40), alkali unwinding (41), alkaline elution (42) and the alkaline comet assay (30). The presence of these components is related to the different rates of repair for various types of DNA damage.…”

Effect of Dose Rate on Cell Killing and DNA Strand Break Repair in CHO Cells Exposed to Internal β-rays from Incorporated [3H]thymidine

“…The repair kinetics of these cell lines were found to be biphasic and the half-time of the slow component was measured to decrease with increasing dose for doses exceeding 25 Gy or 12 . 5 Gy, respectively (Wheeler and Wierowski 1983) . On the other hand, the increase in repair half-time measured for these two cell lines after split-dose experiments with decreasing time interval, which is comparable to an increase in dose rate, is far less pronounced than expected from single dose experiments (Wierowski et al .…”

“…Since the doses applied varied between 1 and 100 Gy, this result indicates that, at least for the cell lines tested, the repair kinetics of DNA strand breaks do not depend on dose in the range 1 to 100 Gy and, therefore, strand break repair is not exhausted or saturated when dose is increased . The only exceptions are intracerebral 9L tumour cells and cerebellar neurons (Wheeler and Wierowski 1983) . The repair curves of these cell lines were found to be a sum of two exponential components and the half-time of the slow component increased with increasing dose for doses exceeding 25 Gy or 12 .…”