Germ cell pluripotency, premature differentiation and susceptibility to testicular teratomas in mice

Abstract

SUMMARYTesticular teratomas result from anomalies in germ cell development during embryogenesis. In the 129 family of inbred strains of mice, teratomas initiate around embryonic day (E) 13.5 during the same developmental period in which female germ cells initiate meiosis and male germ cells enter mitotic arrest. Here, we report that three germ cell developmental abnormalities, namely continued proliferation, retention of pluripotency, and premature induction of differentiation, associate with teratoma susceptibility. Using mouse strains with low versus high teratoma incidence (129 versus 129-Chr19 MOLF/Ei ), and resistant to teratoma formation (FVB), we found that germ cell proliferation and expression of the pluripotency factor Nanog at a specific time point, E15.5, were directly related with increased tumor risk. Additionally, we discovered that genes expressed in pre-meiotic embryonic female and adult male germ cells, including cyclin D1 (Ccnd1) and stimulated by retinoic acid 8 (Stra8), were prematurely expressed in teratoma-susceptible germ cells and, in rare instances, induced entry into meiosis. As with Nanog, expression of differentiation-associated factors at a specific time point, E15.5, increased with tumor risk. Furthermore, Nanog and Ccnd1, genes with known roles in testicular cancer risk and tumorigenesis, respectively, were co-expressed in teratoma-susceptible germ cells and tumor stem cells, suggesting that retention of pluripotency and premature germ cell differentiation both contribute to tumorigenesis. Importantly, Stra8-deficient mice had an 88% decrease in teratoma incidence, providing direct evidence that premature initiation of the meiotic program contributes to tumorigenesis. These results show that deregulation of the mitotic-meiotic switch in XY germ cells contributes to teratoma initiation.

Testicular teratomas: an intersection of pluripotency, differentiation and cancer biology

“…This is consistent with findings that ES cells transit rapidly through the cell cycle, but slow down upon differentiation (White et al, 2005). Also in accord with this idea, the maintenance of mitotic activity beyond the normal stage of mitotic arrest is associated with teratoma susceptibility in Dmrt1 -/-, Pten null, and 129-Chr19 MOLF mice (Heaney et al, 2012, Kimura et al, 2003, Krentz et al, 2009. Interestingly, in Pten deleted mice, 129-Chr19 MOLF strain, and in Dnd1Ter/Ter mutants, some male germ cells express early markers of meiosis (Cook et al, 2011, Heaney et al, 2012, Kimura et al, 2003.…”

“…Also in accord with this idea, the maintenance of mitotic activity beyond the normal stage of mitotic arrest is associated with teratoma susceptibility in Dmrt1 -/-, Pten null, and 129-Chr19 MOLF mice (Heaney et al, 2012, Kimura et al, 2003, Krentz et al, 2009. Interestingly, in Pten deleted mice, 129-Chr19 MOLF strain, and in Dnd1Ter/Ter mutants, some male germ cells express early markers of meiosis (Cook et al, 2011, Heaney et al, 2012, Kimura et al, 2003. However, no overlap has been reported between cells expressing meiotic markers and cells expressing tumor markers; thus, it is unclear whether meiosis is one of the possible differentiation pathways when mitotic arrest is blocked or delayed, or whether an initial entry into meiosis is a first step toward teratoma formation.…”

“…Active repression of the genes associated with pluripotency is required at the stage when fetal male germ cells undergo mitotic arrest and embark on a program of differentiation as spermatogonia (Western, 2009). Failure to repress pluipotency (specifically, expression of Sox2, Nanog and Oct4), and/or failure to maintain cell cycle arrest may render germ cells susceptible to tumor formation during this critical window of development and perhaps at later stages (Cook et al, 2011, Heaney et al, 2012. However, it is not clear whether either of these events is causative.…”

“…Stevens's transplantation experiments suggested that germ cells are most susceptible to teratoma transformation prior to E12.5 (Stevens, 1964). However, the earliest stage when clusters of precursor cells have been identified by elevated levels of E-cadherin and up-regulation of pluripotent markers in vivo is E15.5 (Cook et al, 2011, Heaney et al, 2012. Because likely differences in growth rates make it difficult to determine when a transformation event initiated, the period of susceptibility has not been firmly established.…”

“…However, no overlap has been reported between cells expressing meiotic markers and cells expressing tumor markers; thus, it is unclear whether meiosis is one of the possible differentiation pathways when mitotic arrest is blocked or delayed, or whether an initial entry into meiosis is a first step toward teratoma formation. Heterozygous deletion of Stra8, a gene associated with meiotic entry, exerted a protective effect in the context of the 129-Chr19 MOLF strain, leading to the suggestion that premature initiation of the meiotic program may promote tumorigenesis (Heaney et al, 2012).…”