SUMMARYHuman germ cell tumors show a strong sensitivity to genetic background similar to Dnd1Ter/Ter mutant mice, where testicular teratomas arise only on the 129/SvJ genetic background. The introduction of the Bax mutation onto mixed background Dnd1Ter/Ter mutants, where teratomas do not typically develop, resulted in a high incidence of teratomas. However, when Dnd1 Ter/Ter ; Bax -/-double mutants were backcrossed to C57BL/6J, no tumors arose. Dnd1Ter/Ter germ cells show a strong downregulation of male differentiation genes including Nanos2. In susceptible strains, where teratomas initiate around E15.5-E17.5, many mutant germ cells fail to enter mitotic arrest in G0 and do not downregulate the pluripotency markers NANOG, SOX2 and OCT4. We show that DND1 directly binds a group of transcripts that encode negative regulators of the cell cycle, including p27Kip1 and p21
Cip1. P27Kip1 and P21 Cip1 protein are both significantly decreased in Dnd1 Ter/Ter germ cells on all strain backgrounds tested, strongly suggesting that DND1 regulates mitotic arrest in male germ cells through translational regulation of cell cycle genes. Nonetheless, in C57BL/6J mutants, germ cells arrest prior to M-phase of the cell cycle and downregulate NANOG, SOX2 and OCT4. Consistent with their ability to rescue cell cycle arrest, C57BL/6J germ cells overexpress negative regulators of the cell cycle relative to 129/SvJ. This work suggests that reprogramming of pluripotency in germ cells and prevention of tumor formation requires cell cycle arrest, and that differences in the balance of cell cycle regulators between 129/SvJ and C57BL/6 might underlie differences in tumor susceptibility.
KEY WORDS: Germ cell, Cell cycle, Testicular teratoma, MouseRegulation of male germ cell cycle arrest and differentiation by DND1 is modulated by genetic background sex-specific reprogramming process that leads to the downregulation of pluripotent markers, and by E12.5 they can no longer be efficiently induced to form EG cells (Anderson et al., 2000;Molyneaux et al., 2001). Between E12.5 and E15.5, germ cells in the testis begin differentiation as pro-spermatogonia, and enter mitotic arrest in G0 until near birth (Durcova-Hills and Capel, 2008;Matsui, 1998;McLaren, 1984;Western et al., 2008). Cell cycle regulators associated with the initiation of mitotic arrest include p27 Kip1 (Cdkn1b), p21 Cip1 (Cdkn1a), p16 Ink4a (Cdkn2a) and p15
Ink4b(Cdkn2b) (Western et al., 2008). Survival and commitment of germ cells to male fate is dependent on expression of Fgf9 (fibroblast growth factor 9) in somatic cells (DiNapoli et al., 2006), which upregulates Nanos2 (nanos homolog 2) in germ cells (Barrios et al., 2010; Bowles et al., 2010). Null mutations of Nanos2 lead to transient upregulation of meiotic markers (typical of female germ cells during this stage of fetal development) and subsequent germ-cell death, but do not lead to teratomas (Suzuki and Saga, 2008). Recent work has demonstrated that DND1 can bind uridine-rich regions in the 3Ј untranslated region (UTR) ...