Hijacking Germ Cells for Cancer: Examining a 'Dead End' in Male Germ Cell Development
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Germ cells represent the immortal line: they are guardians of a totipotent genome and are essential for the genetic survival of an individual organism and ultimately a species. An error at any stage in development (specification, migration, colonization, differentiation, adult maintenance) can lead to one of two disastrous outcomes: (1) germ cell death or (2) unchecked growth and proliferation leading to tumorigenesis. The work in this dissertation utilizes a classic mouse model (<italic>Ter</italic>) resulting in both of these phenotypes to further explore the molecular mechanisms important for development of germ cells.
A homozygous nonsense mutation (<italic>Ter</italic>) in murine <italic>Dnd1</italic> (<italic>Dnd1<super>Ter/Ter</super></italic>) results in a significant (but not complete) early loss of primordial germ cells (PGCs) prior to colonization of the gonad in both sexes and all genetic backgrounds tested. The same mutation also leads to testicular teratomas only on the 129/SvJ background. Male mutants on other genetic backgrounds ultimately lose all PGCs with no incidence of teratoma formation. It is not clear how these PGCs are lost, develop into teratomas, or what factors directly control the strain-specific phenotype variation.
Work here demonstrates that <italic>Dnd1</italic> expression is restricted to germ cells and that the <italic>Ter</italic> mutant defect is cell autonomous. The early loss of germ cells is due in part to BAX–mediated apoptosis which also affects the incidence of tumorigenesis on a mixed genetic background. Moreover, tumor formation is-specific to the male developmental pathway and not dependent on sex chromosome composition of the germ cell (XX vs. XY). Despite normal initiation of the male somatic pathway, mutant germ cells fail to differentiate as pro–spermatogonia and instead prematurely enter meiosis.
Results here also reveal that, on a 129/SvJ background, many mutant germ cells fail to commit to the male differentiation pathway, instead maintain expression of the pluripotency markers, NANOG, SOX2, and OCT4, and initiate teratoma formation at the stage when male germ cells normally enter mitotic arrest. RNA immunoprecipitation experiments reveal that mouse DND1 directly binds a group of transcripts that encode negative regulators of the cell cycle, including <italic>p27Kip1</italic>, which is not translated in <italic>Dnd1<super>Ter/Ter</super></italic> germ cells. Additionally, overexpression of DND1 in a teratocarcinoma cell line leads to significant alteration of pathways controlling the G1/S checkpoint and the RB tumor suppressor protein. This strongly suggests that DND1 regulates mitotic arrest in male germ cells through regulation of cell cycle genes, serving as a gatekeeper to prevent the activation of a pluripotent program leading to teratoma formation. Furthermore, strain–specific morphological and expression level differences possibly account for sensitivity to tumor development.
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