Hypersensitivity of Primordial Germ Cells to Compromised Replication-Associated DNA Repair Involves ATM-p53-p21 Signaling

Abstract

Genome maintenance in germ cells is critical for fertility and the stable propagation of species. While mechanisms of meiotic DNA repair and chromosome behavior are well-characterized, the same is not true for primordial germ cells (PGCs), which arise and propagate during very early stages of mammalian development. Fanconi anemia (FA), a genomic instability syndrome that includes hypogonadism and testicular failure phenotypes, is caused by mutations in genes encoding a complex of proteins involved in repair of DNA lesions associated with DNA replication. The signaling mechanisms underlying hypogonadism and testicular failure in FA patients or mouse models are unknown. We conducted genetic studies to show that hypogonadism of Fancm mutant mice is a result of reduced proliferation, but not apoptosis, of PGCs, resulting in reduced germ cells in neonates of both sexes. Progressive loss of germ cells in adult males also occurs, overlaid with an elevated level of meiotic DNA damage. Genetic studies indicated that ATM-p53-p21 signaling is partially responsible for the germ cell deficiency.

Endogenous DNA Damage Leads to p53-Independent Deficits in Replicative Fitness in Fetal Murine Fancd2 −/− Hematopoietic Stem and Progenitor Cells

“…This is entirely consistent with an operational model of diminished, but not eliminated in utero HSPC function, and experimentally supported by fetal deficits in Fancd2 − / − that are not due to apoptosis but rather result from diminished proliferative capacity (Alvarez et al., 2015, Shen et al., 2013, Zhang et al., 2011). Our observations are further consistent with spontaneous developmental deficits that arise during rapid mitotic cycles in Fancc and Fancm germ cell proliferation (Luo et al., 2014, Nadler and Braun, 2000). …”

“…This is consistent with a case report of compromised cord blood clonogenicity and observations of decreased bone marrow progenitor content in young FA patients (Auerbach et al., 1990, Ceccaldi et al., 2012). Indeed, rapid proliferation during experimental transplantation of murine FA bone marrow, progenitor proliferation in rapidly dividing FANCD2- and FANCA-depleted human embryonic stem cells (Tulpule et al., 2010), or primordial germ cell turnover (Luo et al., 2014) all reveal proliferation deficits following replication stress. During development, definitive murine HSPCs experience a rapid 33-fold expansion in the fetal liver (FL) between E12.5 and E16.5 (Ema and Nakauchi, 2000, Mikkola and Orkin, 2006), after which hematopoietic function transitions to the bone marrow where HSPCs assume a more quiescent postnatal phenotype (Copley et al., 2013, Morrison and Spradling, 2008).…”

Correction of a pathogenic gene mutation in human embryos

“…By contrast, HDR efficiency in iPSCs was significantly lower and primarily achieved through an exogenous DNA template. This striking difference implies that human gametes and embryos employ a different DNA damage response system, perhaps reflecting the evolutionary importance of maintaining germline genome integrity 32 . If, as seems likely, gametes and zygotes endure an increased number of DSBs during meiotic recombination and segregation, an efficient genome repair capacity would be critical 33 and unique zygotic DNA repair machinery might rely entirely on maternal oocyte factors deposited and stored during maturation since zygotes are transcriptionally silent.…”

BRCA Mutations, DNA Repair Deficiency, and Ovarian Aging1

“…In Fanconi anemia (FA), a genetic disease of DSB repair, females experience early menopause and gonadal failure [15]. Ovaries of Fanconi-gene mutant mice are hypoplastic and exhibit greatly reduced numbers of primordial follicles [16]. Interestingly, FANCD1, one of the genes responsible for FA, is none other than the BRCA2 gene and the activated version of another gene involved in FA, FANCD2, interacts with BRCA1 [17].…”