Despite the high importance of protecting the genome in the germline, paradoxically, hundreds of DNA double strand breaks (DSBs) are generated by SPO11, in meiotic germ cells. These DSBs serve an important function as they initiate meiotic homologous recombination which is not only necessary to repair the DSBs, but also to promote the pairing of homologous chromosomes. At least one repair event per chromosome pair creates crossovers which is required for reliable chromosome segregation during cell division. Given that efficient repair of high numbers of DSBs is not straightforward, some of the meiotic germ cells fail to repair all of the SPO11-induced DSBs. Oocytes with persistent DSBs are eliminated by apoptosis before entering dictyate arrest, which is a resting stage of oocytes that can last up to 50 years in humans. The oocyte surveillance system that triggers cell death in response to DSBs has been characterized in recent years. It involves a member of the p53 protein family, TAp63α, that is preferentially produced in resting oocytes . This p63 isoform adopts an inactive and only dimeric conformation . ATM- and/or ATR-mediated detection of DSBs activates the DNA damage response kinase CHK2, which phosphorylates TAp63α at S582 . While this phosphorylation is not sufficient to activate TAp63α, it recruits the kinase CK1 . Sequential addition of four more phosphate groups results in activation of TAp63α to the tetrameric state  and initiation of apoptosis .
While many molecular details of this p63-mediated quality control mechanism are well-understood in oocytes, TAp63α’s function in the male germ line has remained poorly explored. Comparison of the transcriptomes of wild type and p63+/- mouse germ cells suggested that p63 is involved in regulating an apoptotic program . Elevated p53 and TAp63 levels also were observed in spermatocytes of Trip13mod/mod mice, where DSB repair was delayed and spermatocytes were eliminated in mid prophase due to defective meiotic recombination and misregulation of HORMAD1/2 proteins that activate meiotic prophase checkpoints [8,9,10]. Interestingly, whereas TAp63-deficiency did not prevent apoptosis in Trip13mod/mod spermatocytes, TAp63-deficiency enabled the expression of a late meiotic marker that is normally repressed in Trip13mod/mod spermatocytes. This suggests that the quality control of spermatocytes involves TAp63, albeit the role of TAp63 seems to be redundant and/or limited in male meiosis . Interestingly, male germ cells of humans and great apes express a modified form of TAp63α, GTAp63α , which has a 37 amino acid N-terminal extension that is thought to inhibit p63’s function . This extension corresponds to a LTR of the human endogenous retrovirus 9 (ERV9). DNA damage triggers activation of GTAp63α which initiates apoptosis .
Whereas these listed observations suggest a similar role of p63 in male and in female germ cells, our recent mouse study cast doubts on p63’s importance for the quality control of spermatocytes. We created a heterozygous mouse expressing one wild type TAp63α allele and one TAp63β allele . TAp63β is characterized by a shorter C-terminus that lacks the inhibitory domain, resulting in constitutively tetrameric and active p63 . Consequently, female heterozygous mice had lost all oocytes by P7 even in the absence of any DNA damage. This result is consistent with the observation that p63 mutations that result in truncated proteins lacking its C-terminal inhibitory domain cause premature ovarian insufficiency in female patients . Surprisingly, male heterozygous mice are fertile. If TAp63α activation promoted germ cell elimination in both male and female germ cells, then the constitutively active TAp63β form would be expected to trigger germ cell apoptosis in both sexes. The observed fertility in the TAp63α/TAp63β mouse model suggests that p63 is differentially regulated in male and female germ cells. These results might reflect the different developmental schemes of female and male germ cells that require distinct types of quality control mechanisms (Fig. 1). While oocytes are limited in number, divide in a highly asymmetric manner with only one daughter cell surviving and are arrested for years in a tretraploid state in prophase I, spermatocytes are constantly produced in high quantities, divide symmetrically and stay in prophase only for hours. Mechanistically, a potential explanation for sexual differences of p63’s role is rooted in differences between sex chromosomes in females and males, and in particular, the limited synapsis of the X and Y sex chromosomes in spermatocytes. Unsynapsed chromosomal regions promote ATR activity leading to the transcriptional silencing of unsynapsed sex chromosomes, which is essential for spermatocyte survival beyond meiotic prophase [10, 16]. It follows that ATR is constitutively active throughout meiotic prophase even in normal spermatocytes. Recombination defects and persistent DSBs on autosomes disrupt efficient sex chromosome silencing, which leads to spermatocyte apoptosis independent of p53 or p63 [11, 16]. Given these circumstances, the higher base level of ATR activity might make it necessary to keep TAp63α at low levels or not expressed at all. It is also possible that TAp63α activation occurs but does not contribute to apoptotic pathways in spermatocytes. Importantly, p63 loss enabled advancement in the meiotic transcriptional program, but p63 loss did not prevent apoptosis in DSB repair-defective Trip13mod/mod spermatocytes . Together with the TAp63α/TAp63β phenotype these observations suggest that male quality control in mice does not rely on p63-dependent apoptotic pathways and other factors possibly play decisive roles in spermatocytes. The situation in humans, however, might be different as the isoform that is selectively expressed in male germ cells of humans and great apes, GTAp63α , forms more stable dimers . The N-terminal domain of the human protein might stabilize the inactive state on the background of a higher basal activity of the ATR kinase, which, counterintuitively, might allow the use of human GTAp63α for apoptosis induction if excess ATR/ATM signaling is present from damaged DNA. Clearly more research on the role of the different p63 isoforms in the male germ line is necessary to understand its role in quality control.
Suh EK, Yang A, Kettenbach A, Bamberger C, Michaelis AH, Zhu Z, et al. p63 protects the female germ line during meiotic arrest. Nature. 2006;444:624–8.
Deutsch GB, Zielonka EM, Coutandin D, Weber TA, Schafer B, Hannewald J, et al. DNA damage in oocytes induces a switch of the quality control factor TAp63alpha from dimer to tetramer. Cell. 2011;144:566–76.
Bolcun-Filas E, Rinaldi VD, White ME, Schimenti JC. Reversal of female infertility by Chk2 ablation reveals the oocyte DNA damage checkpoint pathway. Science. 2014;343:533–6.
Tuppi M, Kehrloesser S, Coutandin DW, Rossi V, Luh LM, Strubel A, et al. Oocyte DNA damage quality control requires consecutive interplay of CHK2 and CK1 to activate p63. Nat Struct Mol Biol. 2018;25:261–9.
Gebel J, Tuppi M, Chaikuad A, Hotte K, Schroder M, Schulz L, et al. p63 uses a switch-like mechanism to set the threshold for induction of apoptosis. Nat Chem Biol. 2020;16:1078–86.
Kerr JB, Hutt KJ, Michalak EM, Cook M, Vandenberg CJ, Liew SH, et al. DNA damage-induced primordial follicle oocyte apoptosis and loss of fertility require TAp63-mediated induction of Puma and Noxa. Mol Cell. 2012;48:343–52.
Wang H, Yuan QQ, Niu MH, Zhang WH, Wen LP, Fu HY, et al. Transcriptional regulation of P63 on the apoptosis of male germ cells and three stages of spermatogenesis in mice. Cell Death Dis. 2018;9:76.
Wojtasz L, Daniel K, Roig I, Bolcun-Filas E, Xu H, Boonsanay V, et al. Mouse HORMAD1 and HORMAD2, two conserved meiotic chromosomal proteins, are depleted from synapsed chromosome axes with the help of TRIP13 AAA-ATPase. PLoS Genet. 2009;5:e1000702.
Daniel K, Lange J, Hached K, Fu J, Anastassiadis K, Roig I, et al. Meiotic homologue alignment and its quality surveillance are controlled by mouse HORMAD1. Nat Cell Biol. 2011;13:599–610.
Wojtasz L, Cloutier JM, Baumann M, Daniel K, Varga J, Fu J, et al. Meiotic DNA double-strand breaks and chromosome asynapsis in mice are monitored by distinct HORMAD2-independent and -dependent mechanisms. Genes Dev. 2012;26:958–73.
Marcet-Ortega M, Pacheco S, Martinez-Marchal A, Castillo H, Flores E, Jasin M, et al. p53 and TAp63 participate in the recombination-dependent pachytene arrest in mouse spermatocytes. PLoS Genet. 2017;13:e1006845.
Beyer U, Moll-Rocek J, Moll UM, Dobbelstein M. Endogenous retrovirus drives hitherto unknown proapoptotic p63 isoforms in the male germ line of humans and great apes. Proc Natl Acad Sci USA. 2011;108:3624–9.
Pitzius S, Osterburg C, Gebel J, Tascher G, Schafer B, Zhou H, et al. TA*p63 and GTAp63 achieve tighter transcriptional regulation in quality control by converting an inhibitory element into an additional transactivation domain. Cell Death Dis. 2019;10:686.
Lena AM, Rossi V, Osterburg S, Smirnov A, Osterburg C, Tuppi M, et al. The p63 C-terminus is essential for murine oocyte integrity. Nat Commun. 2021;12:383.
Coutandin D, Osterburg C, Srivastav RK, Sumyk M, Kehrloesser S, Gebel J, et al. Quality control in oocytes by p63 is based on a spring-loaded activation mechanism on the molecular and cellular level. Elife. 2016;5:e13909.
Royo H, Polikiewicz G, Mahadevaiah SK, Prosser H, Mitchell M, Bradley A, et al. Evidence that meiotic sex chromosome inactivation is essential for male fertility. Curr Biol. 2010;20:2117–23.
The research was funded by the DFG (DO 545/18-1), the AIRC grant (IG-22206 to EC) and Ministry of Health (IDI-IRCCS, RC to EC). Open Access funding enabled and organized by Projekt DEAL.
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The authors declare no competing interests.
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Candi, E., Melino, G., Tóth, A. et al. Mechanisms of quality control differ in male and female germ cells. Cell Death Differ 28, 2300–2302 (2021). https://doi.org/10.1038/s41418-021-00818-8
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