Methylation of DNA and protamine by methyl methanesulfonate in the germ cells of male mice

Abstract

The molecular dosimetry of methyl methanesulfonate (MMS) in the germ cells of male mice has been investigated. The mice were injected i.p. with 100 mg/kg of [ 3H]MMS and methylations per sperm head, per deoxynucleotide, and per unit of protamine were then determined over a 3-week period. The methylations per sperm head paralleled the dominant lethal frequency curve for MMS, reaching a maximum of between 22 and 26 million methylations per vas sperm head 8–11 days after treatment. Methylation of sperm DNA was greatest at 4 h (the earliest time point studied) after treatment, with 16.6 methylations/10 5 deoxynucleotides. DNA methylation gradually decreased during the subsequent 3-week period. The methylation of germ-cell DNA did not increase in the stages most sensitive to MMS (late spermatids → early spermatozoa) and was not correlated with the dominant lethal frequency curve for MMS. However, methylation of protamine did increase in the germ-cell stages most sensitive to MMS, and showed an excellent correlation with the incidence of dominant lethals produced by MMS in the different germ-cell stages. The pattern of alkylation produced by MMS in the developing germ-cell stages of the mouse is similar to that found for EMS. However, for equimolar exposures, MMS alkylates the germ cells 5–7 times more than does EMS.