A study of the mechanisms involved in the increased sensitivity of mouse myeloid leukemic cells to the induction of chromosome aberrations

X ray-induced chromosome aberrations were analyzed in in vitro normal mouse RFM bone marrow cells and in an RFM mouse myeloid leukemia cell line, ML-1. Chromosome aberrations were analyzed in cells that were irradiated in G₁of the cell cycle. Dicentrics and terminal deletions were induced in greater frequencies in ML-1 cells than in normal cells, and chromatid-type aberrations were induced only in ML-1 cells.

The induction of chromatid-type aberrations in ML-1 and not in normal cells suggested: 1) a difference between the normal and ML-1 cells in cell cycle progression after irradiation and/or 2) a difference between the cells in the repair of X ray-induced DNA damage. Flow cytometric analyses showed that cells of both cell types in G₂ at the time of irradiation were similarly delayed in progression into mitosis. However, normal cells in G₁ were delayed in progression into and through the S phase while the ML-1 cells progressed without delay into the S phase. The X ray-induced delay in G₁ normal cells may serve as a protective mechanism allowing extra time for DNA repair before DNA replication in the S phase. It was concluded that the lack of a cell cycle delay after irradiation in G₁ML-1 cells allowed cells to progress into the S phase with unrepaired DNA damage where a misrepair event results in the formation of chromatid-type aberrations.

These results indicated that the ML-1 cells had possible repair deficiencies resulting in X ray-induced DNA damage which persisted until the cells entered the S phase. Experiments utilizing the excision repair inhibitor cytosine arabinoside (ara C) showed that the ML-1 cells had a slower rate of excision repair resynthesis than the normal cells.

In order to determine if the differential sensitivity between ML-1 and normal cells was present for cells in other cell cycle phases, aberrations were analyzed in cells that were treated in G₂ with X rays and ara C. ML-1 cells had a higher frequency of aberrations induced by X rays, and ara C, and an increased sensitivity to a combined ara C and X ray treatment compared to normal cells. This suggests that the DNA in ML-1 cells has fragile sites. These results provide an explanation of the mechanisms leading to the increased sensitivity to chromosome aberration formation in the ML-1 cells.