Similar repair of O6-methylguanine in normal and ataxia-telangiectasia fibroblast strains. Deficient repair capacity of lymphoblastoid cell lines does not reflect a genetic polymorphism

The control of O6-methylguanine-DNA methyltransferase (MGMT) activity in mammalian cells: a pre-molecular view

Human peripheral blood lymphocytes (PBLs) can have a range of O6-methylguanine-DNA methyltransferase (MGMT) activities. PBLs from some individuals may have almost no MGMT activity. Such individuals have most often been subject to malignancy or to immunodeficiency disease. Long-term lymphoblastoid lines (LCLs) prepared from PBLs of normal subjects by Epstein-Barr virus (EBV) transformation have MGMT activities which are in general somewhat higher than the PBLs from which they derive. Such cultures are therefore generally MGMT-positive. Only in rare cases, and generally from patients with low MGMT activity, are freshly obtained lines with very low activity obtained. There is however a 4-fold range of MGMT activity over which multiple lines derived from the same PBL sample can be found. Long-term cultivation can lead to LCLs with low activity as well as to lines of high activity. On rare occasions an MGMT-positive line may, within a few divisions, give a negative line. Some (but not all) MGMT-negative (or very low) lines have been known to gain (some) activity. Chinese hamster ovary (CHO) cell lines are in general very low in MGMT activity. Lines of higher activity can be selected by treatment with mutagenic crosslinking alkylating agents. Chinese hamster lines with high MGMT activity can be obtained by transfection with human DNA from MGMT-positive cells. Lines with significant activity can also be obtained by transfection of CHO cells with human DNA from MGMT-negative (or very low) cells. Resistance to MNNG treatment can be acquired without the acquisition of significant MGMT activity. Crosses of lines of high and low MGMT activity give equivocal results. Hybrids of low x low activity have no activity. Crosses of positive x positive strains give varied results. It has not been possible to identify MGMT-positive hybrids as including one particular chromosome by this type of experiment. There is no evidence for a general adaptive effect on MGMT synthesis greater than the variation within the cell cycle.

Flow cytometric analysis of X-ray sensitivity in ataxia telangiectasia

Flow cytometric analysis of 5-bromodeoxyuridine (BrdU) incorporation during DNA synthesis was used to characterize the effects of X-rays on cell-cycle kinetics in the DNA-repair deficiency disease ataxia telangiectasia (AT). Cultured fibroblasts from homozygotes (at/at), heterozygotes (at/+) and normal controls (+/+) were either: (1) irradiated, cultured, then pulsed with BrdU and harvested, or (2) pulsed with BrdU, irradiated, cultured and then harvested. Cells were then fixed and stained with both a fluoresceinated monoclonal antibody against BrdU to identify S-phase cells and with propidium diiodide to measure total DNA content. Irradiation of +/+ and at/+ cells induced a similar, transient G2/M arrest detectable within 8 h, which subsequently delayed by 6-8 h the passage of cells into G1 and depleted early S phase. In contrast, at/at cells failed to arrest in G2/M phase and entered the next cell cycle without pausing to repair radiation-induced damage. X-Rays also blocked entry of +/+ G1 cells into S phase, subsequently reducing the total S-phase population. This effect was not observed in at/at cells. These cell-cycle responses to radiation may be of diagnostic use and ultimately may help explain the basic defect in AT.

Instability of Mex- phenotype in human lymphoblastoid cell lines

Three lymphoblastoid cell lines (LCLs) had extremely low activities of O6-alkylguanine-DNA alkyltransferase (O6-AGT), and were classified as Mex-. They were highly sensitive to cell killing by 1-(4-amino-2-methyl-5-pyrimidinyl)-methyl-3-(2-chloroethyl)-3-nitrosoure a hydrochloride (ACNU), whereas NMO2, a Mex+ LCL with a high O6-AGT activity, was resistant to the agent. Small fractions of these Mex- LCLs survived the treatment with 10 micrograms/ml of ACNU for 24 h, and the surviving cells were found to be resistant to subsequent treatments with the agent. In addition, they contained O6-AGT activities comparable to that of NMO2 and were therefore regarded as Mex+. These results suggest that the Mex- phenotype in LCLs is unstable.