Unscheduled DNA Synthesis in the Germ Cells of Male Mice Exposed In Vivo to the Chemical Mutagen Ethyl Methanesulfonate

Abstract

An unscheduled DNA synthesis has been clearly demonstrated in meiotic and postmeiotic germ cell stages of the male mouse after in vivo treatment with a 250 mg/kg dose of ethyl methanesulfonate. The germ cell stages showing unscheduled DNA synthesis range from early to middle meiotic prophase stages through early to middle spermatid stages. The initiation of this synthesis, taken to be repair of chemically damaged DNA in these germ cells, is rapid, beginning within 1 hour after the injection of ethyl methanesulfonate. Unscheduled DNA synthesis has not been detected in the most mature germ cell stages, which give rise to dominant lethal mutations, nor does it occur in germ cell stages where protamine has replaced the chromosomal histones.In the study ofchemical mutagenesis in a mammalian testsystem it is of great interest to know if the germ cells are capable of repairing damaged DNA (as well as other molecular targets), and the relationship between such a repair system, if it does exist, and the genetic consequences of such repair. Studies have been performed with a limited number of mammalian cell types (not including germ cells), which demonstrated repair of chemically damaged DNA. See, for example (1-4). However, in vivo repair of germ cell DNA after treatment with chemical agents has not been studied.It is possible to study DNA repair in meiotic and postmeiotic germ cells of male mice by making use of the wellstudied sequence of events that occurs during spermatogenesis and spermiogenesis (5-7). In developing male germ cells the last DNA synthesis takes place during a 14-hr period (7) in preleptotene spermatocytes. After DNA synthesis these spermatocytes continue to develop through a series of germ cell stages for about a 28-30 day period (5, 6) before the spermatids finally leave the testes and enter the caput epididymides. Two to three days later, the developing sperm reach the caudal epididymides, and, in two or three days more, they enter the vasa deferentia. If Recovery of Sperm Heads and DNA. At various times after treatment the animals were killed and the reproductive tracts were sectioned into three parts: the caput epididymides, the caudal epididymides, and the vasa deferentia (refer to Fig. 1). After pooling of the tissues from each region, the sperm heads were recovered by procedures already described (8).

DNA Repair Decline During Mouse Spermiogenesis Results in the Accumulation of Heritable DNA Damage

“…Our results confirm and extend prior studies of the DNA repair-deficient window during spermiogenesis [13][14][15][16][17][18][19][20]. The results of our experiments showed that the frequencies of zygotes with chromosomal aberrations were significantly higher when a total dose of 28 mg/kg DEB was given over days 14 through 8 before mating than when it was given as a single dose 14 days before mating (Table 2 and Figure 3A).…”

“…These results show that the last two weeks of mouse spermiogenesis, which corresponds to the period of spermatogenesis that is thought to be DNA repair-deficient in prior studies [13][14][15][16][17][18][19][20], is also the sensitive window for induction of chromosomal aberrations in sperm after paternal exposure to DEB, and that both sperm and late spermatids are unable to repair DEB-induced damage.…”

“…Over fifty years of germ cell mutagenesis have shown that the majority of mutagens induce their highest effect during the last few weeks of spermatogenesis before fertilization [11,12]. The high sensitivity of the postmeiotic period to mutagenic exposure has been associated with the reduced DNA repair capacity of late spermatids and sperm as compared to early spermatids and other spermatogenic cell types [13][14][15][16][17]. All major DNA repair pathways seem to be less functional in late spermatids and sperm [17][18][19][20].…”

Studies on chemical induction of chromosomal aberrations in postcopulation germ cells and zygotes of female mice. I. Comparative studies on the frequency of first-cleavage chromosomal aberrations and dominant-lethal mutations.

“…Further studieds are required to determine whether DNA repair plays a significant role in the modification of genetic damage in postcopulation-precleavage germ cells and zygotes as well as in spermatogenesis (Generoso et al 1979;Sega 1974;Tanaka and Katoh 1979 …”

Unscheduled DNA synthesis in the germ cells of male mice in vivo.

“…Kof man-Alf aro and Chandley (1971) reported the detection of UDS in germ cell cultures in vitro after UV and X-ray treatment, and Beikrich (1977) demonstrated UDS induced by chemicals. Sega et al (Sega 1974;Sega et al 1976;Sega 1976;Sega et al 1978), on the other hand, described the measurement of UDS in the germ cells of mice treated with X-rays and chemicals, and proposed the use of this procedure as a mammalian screening test system for the detection of mutagens.…”