Chinese hamster ovary cell mutants resistant to DNA polymerase inhibitors. I. Isolation and biochemical genetic characterization

An aphidicolin-resistant mutant of Chinese hamster ovary cell with altered DNA polymerase and 3' exonuclease activities

A comparison was made of partially purified DNA polymerases alpha, delta, epsilon and from normal Chinese hamster ovary cells and mutant cells (JK3-1-2A) resistant to aphidicolin, araA, and araC. In vitro the pol alpha from the mutant cells (1) was resistant to aphidicolin and araCTP but was sensitive to araATP, (2) showed a 1.6 to 2.6-fold reduced specific activity, and (3) was more sensitive to carbonyldiphosphonate, DMSO and SJK 287-38 anti-pol alpha antibody inhibition, but was less sensitive to alkylphenyl nucleotide analogs BuPdGTP and BuAdATP. On the other hand, pol delta and pol epsilon of the mutant cells did not show increased aphidicolin-resistance but differed from the wild type enzymes with regard to their 3' exonuclease activity. The 3' exonuclease/DNA polymerase activity ratio was increased 6-fold for pol delta and 3.3-fold for pol epsilon for enzymes from the mutant cells in comparison to wild type values. It is suggested that these altered properties of the DNA polymerases of the alpha-family are responsible for in vivo aphidicolin resistance of the mutant cells. The higher 3' exonuclease activity may explain the observed antimutator phenotype of this cell line. In view of the proficient 3' exonuclease activities of the DNA pol delta and epsilon, the present aphr mutant is unique among all mammalian DNA polymerase mutants.

Aphidicolin-resistant Chinese hamster ovary cells possess altered DNA polymerases of the alpha-family

DNA polymerases alpha, delta and epsilon were partially purified and characterized from a wild type Chinese hamster ovary (CHO) cell line and from two aphidicolin-resistant mutant CHO cell lines (BR5 and BR5-20a). The main characteristics of the wild type and mutant DNA polymerases were compared in order to reveal differences in the properties of these enzymes responsible for the aphidicolin resistance of the mutant cell lines. Pol alpha's of the mutant cells show: (1) in vitro aphidicolin-resistance, (2) 1.5-3-fold lower specific activity than that of the wild type, (3) resistance to cytosine and adenosine arabinofuranoside 5'-triphosphate (araCTP and araATP), (4) altered resistance to carbonyldiphosphonate (COMDP) and to alkylphenyl nucleotide analogs (butylphenyl-dGTP and butylanilino-dATP), and (5) lower activity on poly(dA)/oligo(dT) template-primers. These changes in the biochemical properties of this enzyme may result from a mutation in pol alpha gene. Pol epsilon and delta of the mutant cells did not differ from the wild type enzymes with respect to aphidicolin resistance. However, the specific activities of these mutant enzymes were much higher (1.5 to 8-fold for pol epsilon and 4 to 20-fold for pol delta) in comparison to that of the wild type enzymes. Also in comparison to the wild type enzymes, the mutant pol epsilon showed changes in the template-primer preference; whereas the mutant pol delta was found to have altered sensitivity to other inhibitors. These results indicate that pol epsilon and pol delta are also altered as a secondary effect of mutation in the aphidicolin-resistant cells. It is suggested that these altered properties of the DNA pols of the alpha family are responsible for the in vivo aphidicolin resistance of the mutant cells.

Aphidicolin-resistant mutator strains of mouse teratocarcinoma

From among a series of stable, aphidicolin-resistant mutant strains of mouse teratocarcinoma, derived from a multipotent parental line (PSA-1-80), three were selected for further study on the basis of their comparatively high degrees of resistance and elevated frequencies of spontaneous forward mutation to 6-thioguanine and ouabain resistance. Fluctuation tests confirmed that they were mutator strains. Since each of the three mutants was isolated after multiple rounds of selection, and since a variety of biochemical abnormalities were observed, it is likely that a number of mechanisms, probably consisting of overlapping subsets, determine the phenotypes. Abnormalities in the metabolism of the nucleotide substrates for polymerization are likely to be of major importance in mutants designated Aph-2 and Aph-3, as there were marked alterations in the dCTP and dATP pool sizes. The specific activity of DNA polymerase alpha was also increased. For the case of Aph-3, which exhibited the greatest (400-fold) increase in resistance to aphidicolin, a mutation in the structural gene for DNA polymerase alpha may be an additional important component, since in vitro assays revealed that the isolated enzyme was resistant to aphidicolin. For the case of Aph-1 however, only minor alterations in dNTP pools were observed, and there was no increase in the specific activity of DNA polymerase alpha or in the aphidicolin resistance of the isolated DNA polymerase alpha, suggesting yet another mechanism(s) underlying the aphidicolin resistance/mutator phenotype. All three mutants formed subcutaneous tumors in syngeneic mice; both Aph-1 and Aph-2 were multipotent; whereas Aph-3 was nullipotent.(ABSTRACT TRUNCATED AT 250 WORDS)