Development of a mouse cell line containing the PhiX174 am3 allele as a target for detecting mutation

Three origins of phiX174 am3 revertants in transgenic cell culture

Transgenic systems for measuring mammalian mutagenesis often use recoverable viral vectors. We hypothesize that mutations in these transgenic systems can arise from three different origins of DNA damage and replication errors and that these three origins of mutations (in vivo, ex vivo, and in vitro) can be differentiated in the PhiX174 am3, cs70 single burst assay (SBA) on the basis of burst size (BS). In vivo mutations are fixed in the animal, ex vivo mutations are fixed in bacterial cells during recovery of the phage, and in vitro revertants arise during the first replications of nonmutant phages under selective conditions. PX-2 cells, derived from a homozygous embryo of a PhiX174 transgenic mouse, were treated with vehicle or N-ethyl-N-nitrosourea (ENU). An algorithm was developed to estimate the BS that resulted in the highest induced revertant frequency; the estimate was 56. In vivo revertants were defined as having BS >55, ex vivo revertants as having a BS of 13-56, and in vitro revertants as having a BS of <14. The frequencies of in vivo revertants at 0, 100, and 200 mg/kg ENU were 0.06, 0.36, and 4.10 x 10(-6) (dose response, P = 0.004); ex vivo revertants were 0.36, 0.46, and 0.41 x 10(-6) (P = 0.37), and in vitro revertants were 0.39, 0.46, and 0.41 x 10(-6) (P = 0.55), respectively. These results show that only in vivo revertants reflect mutagen treatment. They also provide a basis for identifying PhiX174 am3 revertants induced in vivo and may increase the sensitivity of the assay for in vivo mutation.

Characterization of mutant spectra generated by a forward mutational assay for gene A of Phi X174 from ENU-treated transgenic mouse embryonic cell line PX-2

The sensitivity of in vivo transgenic mutation assays benefits from the sequencing of mutations, although the large number of possible mutations hinders high throughput sequencing. A forward mutational assay exists for Phi X174 that requires an altered, functional Phi X174 protein and therefore should have fewer targets (sense, base-pair substitutions) than forward assays that inactivate a protein. We investigated this assay to determine the number of targets and their suitability for detecting a known mutagen, N-ethyl-N-nitrosourea (ENU). We identified 25 target sites and 33 different mutations in Phi X174 gene A after sequencing over 350 spontaneous and ENU-induced mutants, mostly from mouse embryonic cell line PX-2 isolated from mice transgenic for Phi X174 am3, cs70 (line 54). All six types of base-pair substitution were represented among both the spontaneous and ENU-treated mutant spectra. The mutant spectra from cells treated with 200 and 400 microg/ml ENU were both highly different from the spontaneous spectrum (P < 0.000001) but not from each other. The dose trend was significant (P < 0.0001) for a linear regression of mutant frequencies (R(2) = 0.79), with a ninefold increase in mutant frequency at the 400 microg/ml dose. The spontaneous mutant frequency was 1.9 x 10(-5) and the spontaneous spectrum occurred at 11 target base pairs with 15 different mutations. Thirteen mutations at 12 targets were identified only from ENU-treated cells. Seven mutations had highly significant increases with ENU treatment (P < 0.0001) and 15 showed significant increases. The results suggest that the Phi X174 forward assay might be developed into a sensitive, inexpensive in vivo mutagenicity assay.