Further characterization of the radiosensitivity of the scid mouse.

PURPOSE: To further characterize the radiation response of the scid mutation. MATERIALS AND METHODS: X-ray induced chromosomal aberrations and cell killing were analysed using various in vivo or in vitro cell systems. RESULTS: Using low LET X-irradiation a reverse dose-rate effect was found for killing of differentiated and differentiating spermatogonia and the chromosomal hyperradiosensitivity of scid mice was extended to the meiotic prophase. Most striking was the observation made in vitro with synchronized established cell lines that, contrary to the situation in wild-type cells, scid cells display high levels of both chromatid- and chromosome type aberrations when irradiated during the G1-phase of the cell cycle. A time-course for induction of micronucleated polychromatic erythrocytes (MPCE) was determined for scid mice using flow analysis. No significant differences with wild-type mice were recorded. The chromosomal radiosensitivity at the G1 stage in scid cells was 4.3 times higher than in control CB-17 cells whereas G2 sensitivity differed only by a factor of 1.3. CONCLUSIONS: The reportedly normal radiosensitivity for MPCE in scid mice together with previous findings of hypo- or normal radiation sensitivity of scid cells could be explained by the induction of highly lethal chromatid-type damage at the G1 stage of the cell cycle leading to selective elimination of aberration-carrying cells. The differences in chromosomal radiosensitivity between wild-type and scid for the G1 and G2 stage of the cell cycle correlate with variation in the rates of DNA double-strand break (dsb) repair in scid cells during the cell cycle found by others.

Non-random chromosomal involvement in radiation-induced translocations in mouse spermatogonial stem cells.

X-ray induced chromosomal translocations were studied in mouse spermatogonial stem cells by meiotic analysis at the spermatocyte stage many cell generations after induction. Using a composite DNA probe for mouse chromosomes 1, 11 and 13, in combination with fluorescence in situ hybridization, the involvement of these three chromosomes in translocation formation was recorded. The obtained results at various sampling times ranging from 75 to 320 days after irradiation show no significant differences in the participation pattern of the painted chromosomes in multivalent formation with increasing sampling time. Pooling of the data at the different dose levels of 5 and 7 Gy indicated significant overrepresentation of the specifically stained bivalents in translocation formation. This suggests that clonal proliferation cannot be held responsible for the observed non-randomness. Experiments with the DNA-repair inhibitor 3-aminobenzamide showed no change in the recorded overrepresentation, indicating that it is probably not the repair of lesions that is causing this phenomenon but rather non-random induction.

Mitotic and meiotic detection of radiation-induced translocations in mouse stem cell spermatogonia using fluorescence in situ hybridization.

The efficiency of two methods of detection of translocations induced in mouse stem cell spermatogonia by X-ray doses of 2, 5 and 7 Gy was compared: classical multivalent analysis at diakinesis-metaphase I of meiosis and observation via fluorescence in situ hybridization analysis of mitotic or meiotic stages. Specific DNA libraries for chromosomes 1, 11 and 13 were used. The results obtained indicate that (a) chromosomes 1, 11 and 13 are more involved in multivalent formation than expected on the basis of DNA content and (b) if the mitotic FISH analysis data are corrected for the observed over-representation, the frequencies of induced translocations are similar to those recorded in the classical multivalent studies, suggesting equal scoring efficiencies in both systems.

X-ray-induced chromosomal aberrations and cell killing in somatic and germ cells of the scid mouse.

To characterize further the radiosensitivity of severe combined immunodeficiency (scid) mice, the induction of micronuclei (MN) in polychromatic erythrocytes as well as cell killing and translocation induction in stem cell spermatogonia was studied. Scid mice turned out to be clearly hypersensitive for X-ray-induced cell killing of both bonemarrow cells and spermatogonial stem cells. The frequencies of recorded micronuclei in polychromatic erythrocytes were comparable with that reported for the normal mouse, whereas the recovery of translocations was extremely low in the scid mouse. The dose-response relationship for induced translocations was bell shaped with a maximum of about 0.5% around doses of 0.5-1.5 Gy X-rays.