Clyde S. Montgomery

Supermutagenicity of ethylnitrosourea in the mouse spot test: comparisons with methylnitrosourea and ethylnitrosourethane.

The effects of the 3 related compounds ethylnitrosourea (ENU), methylnitrosourea (MNU), and ethylnitrosourethane (NEC) were studied in the mouse spot test. ENU induces a large number of recessive spots (RS) and, due to its low toxicity, can be applied at relatively high doses. This combination of properties makes it the most efficient spot-test mutagen, as shown in a comparison with 16 other chemicals, even though, on the basis of molarity, it is not the most potent one. The ENU mutation frequency in cells at risk, calculated per locus, per unit of applied dose, is roughly similar for melanocyte precursors (in the spot test) and spermatogonial stem cells (in the specific-locus test). MNU which, due to its high embryotoxicity, could be tested only at a low dose, is clearly mutagenic, and dose extrapolations indicate it to be more potent than ENU. NEC, though it could be tested at higher molarities than ENU, is only weakly mutagenic. The spot test, in addition to mutational data, also yields information on cytotoxicity (white midventral spots), embryotoxicity, and teratogenicity. The toxicity and teratogenicity findings parallel earlier results in the rat. For all endpoints studied. ENU is more effective than NEC. Relative to MNU, ENU is less toxic, less teratogenic, and less mutagenic in the spot test; but it is much more carcinogenic (transplacentally) and more mutagenic in spermatogonia. We propose that MNU is more effective in inducing gross chromosomal damage than is ENU, while ENU induces relatively more gene mutations. The spot test scores both types of mutational damage, while mostly the latter type is recovered from spermatogonia.

Use of the mouse spot test to investigate the mutagenic potential of triclosan (Irgasan®DP300)☆

Triclosan, a chlorophenoxyphenol used in several commercial products, was tested in the mouse in vivo somatic mutation test (spot test) by intraperitoneal injection on day 9.25 or 10.25 postconception. Although the dose range tested overlapped the toxic, the frequency of presumed somatic mutations was not significantly greater in the experimental groups than in the methanol-injected controls; and the results rule out with 95% confidence a spot incidence 5 or more times greater than the control incidence. These findings fail to confirm the claim by Fahrig et al. (1978) that triclosan is mutagenic in the spot test.

The incidence of sex-chromosome anomalies following irradiation of mouse spermatogonia with single or fractionated doses of X-rays

Abstract Attempts to recover XO offspring resulting from 600 R irradiation of spermatogonia proved negative. X-Rays were administered either in a single dose or in 100+ 500 R fractions separated by 24 h, and controls were strictly comparable in all respects. Altogether 14016 offspring were scored, including a small group derived from irradiated mature spermatozoa. The breeding scheme allowed phenotypic detection of paternal or maternal sex-chromosome loss, paternal nondisjunction, and certain translocations. All phenotypically exceptional mice were examined cytologically and through breeding tests. Similar tests of the mothers of presumed O/X p exceptionals revealed that in 9 of 14 cases there was a pre-existing XO condition, indicating the importance of performing such a test. Two of the 3a X m /O-appearing mice were probably X m /O///X m /X p mosaics, with the integument predominantly XO and the germinal tissue predominantly X/X. The frequency of paternal sex-chromosome loss was 2.4 · 10 −3 in the controls and 2.0 · 10 −3 in the two irradiated groups, where X m /Os must therefore be assumed to be of spontaneous origin. Since translocation experiments provide evidence that chromosome breaks are induced by irradiation of spermatogonia, the failure to recover XOs is explained in one of two possible ways. (1) Breakage in spermatogonia does not lead to recoverable single-chromosome loss, either because no sister-chromatid joining occurs, or, if it does, because this leads to cell-division failure. (2) Alternatively, sex chromosome loss does occur, but resulting X/O and O/Y cell progeny is inviable in the testis—a suggestion supported by evidence from natural and artificial mosaics. The results of the present experiment lend further support to our earlier suggestion that most spontaneously occurring X m /O mice are the result of events occurring after sperm entry into the egg. The spontaneous frequency of paternal sex-chromosome loss has ranged over two orders of magnitude in various reports. On the other hand, the frequencies of spontaneous X m loss (O/X p daughters of X/X females/total daughters) and of paternal nondisjunction (2 · X m /X p /Y frequency)