The p53-like Protein CEP-1 Is Required for Meiotic Fidelity in C. elegans

Abstract

The passage of genetic information during meiosis requires exceptionally high fidelity to prevent birth defects and infertility. Accurate chromosome segregation during the first meiotic division relies on the formation of crossovers between homologous chromosomes and a series of precisely controlled steps to exchange genetic information. Many studies have hinted at a role for p53 in meiosis, but how it functions in this process is poorly understood. Here, we have identified a cooperative role for the p53-like protein CEP-1 and the meiotic protein HIM-5 in maintaining genome stability in the C. elegans germline. Loss of cep-1 and him-5 results in synthetic lethality that is dependent on the upstream DNA damage checkpoint but independent of the downstream core apoptotic pathway. We show that this synthetic lethality is the result of defective crossover formation due to reduced SPO-11-dependent double-strand breaks. Using cep-1 separation-of-function alleles, we show that cep-1 and him-5 also suppress inappropriate activation of the nonhomologous end joining (NHEJ) pathway. This work reveals an ancestral function for the p53 family in ensuring the fidelity of meiosis and establishes CEP-1 as a critical determinant of repair pathway choice.

Figure 1. cep-1 and him-5 Cooperate to Maintain Chromosome Stability during Meiosis

(A) Structural features of CEP-1 protein showing the N-terminal domain (N-term), DNA-binding domain (DBD), oligomerization domain (OD), and C-terminal sterile alpha motif (SAM). The gk138 allele is a 1,630-bp in-frame deletion spanning the DBD that is predicted to generate a truncated protein retaining the OD and SAM domains. The lg12501 allele is a 1,231-bp frameshift deletion that removes part of the DBD and OD. (B) Quantification of lethality among F1 progeny of cep-1 and him-5 single mutants and cep-1;him-5 double mutants. Data are represented as mean ± SEM; n, broods counted. Statistical comparisons between him-5 single mutants and cep-1;him-5 double mutants were performed using the two-tailed Mann-Whitney test; 95% CI. ***p < 0.0001, **p = 0.002. See also Table S1. (C) Representative confocal images of DAPI-stained diakinesis chromosomes of young adult animals (day 1 or ~24 hr post-L4 stage) of the indicated genotypes. The scale bar represents 5 µm. See also Figure S1. (D) Quantification of DAPI-stained bodies in diakinesis oocytes of young adult animals (day 1) of the indicated genotypes. n, number of oocytes.

Figure 2. Synthetic Lethality with him-5 Is Independent of Apoptosis but Involves the DDR Checkpoint

(A) Simplified schematic of the DNA damage checkpoint pathway in the C. elegans germline. (B) Quantification of total lethality among F1 progeny of ced-3(n717) and him-5(e1490) single mutants and ced-3(n717);him-5(e1490) double mutants. Data are represented as mean ± SEM; n, broods counted. Statistical comparisons between combined lethality of ced-3(n717) and him-5(e1490) single mutants and ced-3(n717);him-5(e1490) double mutants were performed using the two-tailed Mann-Whitney test; 95% CI. p = 0.3508. See also Figure S2A. (C) Quantification of apoptotic corpses in cep-1(lg12501) and him-5(e1490) single and cep-1(lg12501);him-5(e1490) double mutants under physiological conditions. Data are represented as mean ± SEM; n, germlines counted. Statistical comparisons between wild-type and him-5 single mutants were performed using the two-tailed Mann-Whitney test; 95% CI. p = 0.1704. (D) Quantification of total lethality among F1 progeny of hus-1(op244), him-5(e1490), and atm-1(gk186) single mutants and hus-1(op244);him-5(e1490) and atm-1(gk186);him-5(e1490) double mutants. Data are represented as mean ± SEM; n, broods counted. Statistical comparisons between genotypes were performed using the two-tailed Mann-Whitney test; 95% CI. him-5(e1490) versus hus-1(op244); him-5(e1490), ***p < 0.0001;him-5(1490) versus atm-1(gk186);him-5(e1490), *p = 0.0159. (E) Representative confocal image of DAPI-stained diakinesis chromosomes of young adult (Day 1) hus-1(op244);him-5(e1490) mutants. Scale bar represents 5 µm. See also Figure S2B. (F) Representative confocal image of DAPI-stained diakinesis chromsomes of young adult (Day 1) atm-1(gk186);him-5(e1490) mutants. Scale bar represents 5 µm. See also Figure S2C.

Figure 3. cep-1 and him-5 Promote Crossover Formation

(A) Representative confocal images of DAPI-stained diakinesis chromosomes of young adult animals of the indicated genotypes. The scale bar represents 5 µm. See also Figures S3A and S3B. (B) Quantification of DAPI-stained bodies in diakinesis oocytes of young adult animals of the indicated genotypes. n, number of oocytes. Data for cep-1;him-5 double mutants were taken from Figure 1D. (C) Pachytene and diakinesis nuclei of wild-type (1), cep-1(gk138) (2), cep-1(lg12501) (3), him-5(e1490) (4), cep-1(gk138);him-5(e1490) (5), and cep-1(lg12501);him-5(e1490) (6) stained with antibody against the chromosome axis protein HIM-3. See also Figures S3C–S3G. (D) Frequency distribution of ZHP-3 foci in late pachytene and diplotene nuclei of the indicated genotypes. Statistical comparisons between genotypes were performed using the two-tailed Mann-Whitney test; 95% CI. Wild-type versus him-5(e1490), p < 0.0001;him-5(e1490) versus cep-1(lg12501);him-5(e1490), p = 0.0070.

Figure 4. cep-1 and him-5 Regulate Meiotic DSB Formation

(A) Quantification of RAD-51 foci in the meiotic germlines of worms of the indicated genotypes and schematic of the C. elegans germline. Each gonad was divided into four meiotic zones (TZ/EP, transition zone/early pachytene; MP, mid-pachytene; LP/diplotene, late pachytene). Error bars represent SD. See also Table S2. (B) Quantification of DAPI-stained bodies in diakinesis oocytes of wild-type worms 24 hr post-IR treatment. See also Figure S4A. (C) Quantification of DAPI-stained bodies in diakinesis oocytes of cep-1(gk138) and cep-1(lg12501) single mutants 24 hr post-IR treatment. See also Figure S4B. (D) Quantification of DAPI-stained bodies in diakinesis oocytes of him-5(e1490) single mutants 24 hr post-IR treatment. See also Figure S4C. (E) Quantification of DAPI-stained bodies in diakinesis oocytes of cep-1(gk138);him-5(e1490) and cep-1(lg12501);him-5(e1490) double mutants 24 hr post-IR treatment. See also Figure S4D. (F) Representative confocal images of DAPI-stained diakinesis chromosomes of adult animals of indicated genotypes. The scale bar represents 5 µm. Image for cep-1 single mutant was taken from Figure 1C. Green triangle indicates aberrant, poorly condensed chromosomes. Yellow triangle indicates chromosome fragments. Light blue triangle indicates condensed bodies with aggregations. See also Figure S1. (G) Quantification of DAPI-stained bodies in diakinesis oocytes of young adult animals of indicated genotypes. n, number of oocytes. Data for cep-1 single mutants were taken from Figure 1C.

Figure 5. cep-1 and him-5 Prevent Inappropriate NHEJ Activity

(A) Diakinesis nuclei of day 4 animals of the indicated genotypes labeled with FISH probes to chromosomes V (yellow) and X (magenta). DNA is shown in green. White arrow indicates a single DAPI body containing both FISH probes, demonstrating a fusion between nonhomologous chromosomes. Red triangle indicates chromosome fragments. White triangle indicates a single DAPI body containing probes for chromosomes V and X. The scale bar represents 1 µm. See also Table S3. (B) Representative confocal images of DAPI-stained diakinesis chromosomes of cep-1(gk138);cku-80(ok1861);him-5(e1490) triple mutants 24 hr after mock (0 Gy) or IR (10 Gy) treatment. Image of cep-1(gk138);him-5(e1490) mutant is shown for comparison. Scale bar represents 5 µm. See also Figures S5A–S5C. (C) Representative confocal images of DAPI-stained diakinesis chromosomes of cep-1(lg12501);cku-80(ok1861);him-5(e1490) triple mutants 24 hr after mock (0 Gy) or IR (10 Gy) treatment. Image of cep-1(lg12501);him-5(e1490) mutant is shown for comparison. Scale bar represents 5 µm. See also Figures S5A–S5C. (D) Quantification of DAPI-stained bodies in diakinesis oocytes of cep-1(gk138);him-5(e1490) double and cep-1(gk138);cku-80(ok861);him-5(e1490) triple mutants 24 hr after mock (0 Gy) or IR (10 Gy) treatment. (E) Quantification of DAPI-stained bodies in diakinesis oocytes of cep-1(lg12501);him-5(e1490) double and cep-1(lg12501);cku-80(ok861);him-5(e1490) triple mutants 24 hr after mock (0 Gy) or IR (10 Gy) treatment.

Figure 6. cep-1 and him-5 Cooperate to Maintain Genome Stability during Meiosis

Schematic highlighting the regulatory node consisting of HIM-5 and CEP-1, which regulates SPO-11-dependent break formation and ensures fidelity of repair by suppressing the error-prone NHEJ pathway. The dashed lines represent novel roles described in this study.