Elucidation of the Fanconi Anemia Protein Network in Meiosis and Its Function in the Regulation of Histone Modifications

Abstract

Precise epigenetic regulation of the sex chromosomes is vital for the male germline. Here, we analyze meiosis in eight mouse models deficient for various DNA damage response (DDR) factors, including Fanconi anemia (FA) proteins. We reveal a network of FA and DDR proteins in which FA core factors FANCA, FANCB, and FANCC are essential for FANCD2 foci formation, whereas BRCA1 (FANCS), MDC1, and RNF8 are required for BRCA2 (FANCD1) and SLX4 (FANCP) accumulation on the sex chromosomes during meiosis. In addition, FA proteins modulate distinct histone marks on the sex chromosomes: FA core proteins and FANCD2 regulate H3K9 methylation, while FANCD2 and RNF8 function together to regulate H3K4 methylation independently of FA core proteins. Our data suggest that RNF8 integrates the FA-BRCA pathway. Taken together, our study reveals distinct functions for FA proteins and illuminates the male sex chromosomes as a model to dissect the function of the FA-BRCA pathway.

Keywords: DNA damage response; DNA repair; Fanconi anemia; X chromosome; epigenetics; germ cells; meiosis; meiotic sex chromosome inactivation; sex chromosomes; spermatogenesis.

Figure 1. The FA-BRCA Pathway Is Activated on the Sex Chromosomes during Meiosis

(A) Schematic of the FA-BRCA pathway. FA proteins analyzed in this study are shown in color. (B) Schematic of stages of meiotic prophase. (C, D, G, and H) Immunostains using indicated antibodies in meiotic chromosome spreads from wild-type mice. SYCP3 is a marker for meiotic chromosome axes. Substages are labeled to the left. Dashed squares border sex chromosomes and are magnified to the right. Arrowheads: selected FANCD2 foci present on synapsed autosomes. Consistent results were obtained with n = 3 independent mice. Scale bars, 5 µm. (E) Total number of FANCD2 foci on all chromosome axes (top) and on the sex chromosome axes (bottom) per spermatocyte for stages of meiotic prophase. Numbers of spermatocytes analyzed are noted above each graph. Bars represent means and SEMs. Data are aggregated from n = 6 wild-type adult mice. p values are derived from unpaired, two-tailed Student’s t tests: n.s., not significant, p > 0.05; *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001. Prophase spermatocyte stage abbreviations: L/Z, leptotene and zygotene; EP, early pachytene; MP, mid pachytene; LP, late pachytene; ED, early diplotene; LD, late diplotene. (F) Western blot analysis with three independent anti-FANCD2 antibodies (G33, E33, and Novus NB100-182 antibody: NB). K561R, PD20 cells expressing a mutated form of FANCD2 incapable of monoubiquitination; WT, PD20 cells complemented with wild-type FANCD2; Vector, PD20 cells containing empty vector. (I) Summary of temporal and spatial staining patterns for anti-FA protein antibodies on the sex chromosomes of wild-type mice. Axial, FA factors spread along XY axes. Domain, FA factors spread along XY axes and through XY chromatin. For comparison, FANCB results from our recent study (Kato et al., 2015) are summarized here. See also Figure S1.

Figure 2. FA Core-Dependent and -Independent Functions of the FA Pathway

(A–F) Immunostains using indicated antibodies in meiotic chromosome spreads from Fanca^−/− mice, Fancd2^−/− mice, and wild-type littermate controls. Stages are labeled above, genotypes are labeled to the left. Dashed squares border sex chromosomes and are magnified to the right. Consistent results were obtained with n = 3 independent littermate pairs for each mouse model. Scale bars, 5 µm. (G) Summary of temporal and spatial localization of anti-FA protein antibodies on the sex chromosomes in Fanca^−/− and Fancd2^−/− mice; summaries of localization in wild-type mice are shown in Figures 1I and S1C. See also Figure S2.

Figure 3. BRCA1 and MDC1 Regulate the Localization of FA Proteins in Meiosis

(A, C, D, F, H, and I) Immunostains using indicated antibodies in meiotic chromosome spreads from Brca1cKO mice, Mdc1^−/− mice, and wild-type or heterozygous littermate controls. Stages are labeled above; genotypes are labeled to the left. Dashed squares border sex chromosomes and are magnified to the right. Arrowheads: selected FANCD2 foci present on synapsed autosomes. Consistent results were obtained with n = 3 independent littermate pairs for each mouse model. Scale bars, 5 µm. (B and G) Total number of FANCD2 foci on all chromosome axes (top) and on the sex chromosome axes (bottom) per spermatocyte for stages of meiotic prophase. Numbers of spermatocytes analyzed are noted above each graph. Bars represent means and SEMs. Data are aggregated from n = 4 littermate pairs of Brca1 mice, n = 3 littermate pairs of Mdc1 mice. p values are derived from unpaired, two-tailed Student’s t tests: n.s., not significant, p > 0.05; *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001. L/Z, leptotene and zygotene; EP, early pachytene; MP, mid pachytene. (E and J) Summaries of temporal and spatial localization of anti-FA protein antibodies on the sex chromosomes inBrca1cKO and Mdc1^−/− mice; summaries of localization in wild-type mice are shown in Figures 1I and S1C. Spermatocytes from the Brca1cKO and Mdc1^−/− models undergo meiotic arrest and apoptosis after the mid pachytene stage, designated by “meiotic arrest.”

Figure 4. RNF8 Regulates the FA-BRCA Pathway

(A, C, and E) Immunostains using indicated antibodies in meiotic chromosome spreads from Rnf8^−/− mice and wild-type littermate controls. Stages are labeled above, genotypes are labeled to the left. Dashed squares border sex chromosomes and are magnified to the right. Consistent results were obtained with n = 9 independent littermate pairs. Scale bars, 5 µm. (B) Total number of FANCD2 foci on all chromosome axes (top) and on the sex chromosome axes (bottom) per spermatocyte for stages of meiotic prophase. Numbers of spermatocytes analyzed are noted above each graph. Bars represent means and SEMs. Data are aggregated from n = 4 littermate pairs. p values are derived from unpaired, two-tailed Student’s t tests: n.s., not significant, p > 0.05; *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001. L/Z, leptotene and zygotene; EP, early pachytene; MP, mid pachytene; LP, late pachytene; ED, early diplotene; LD, late diplotene. (D) Categorical staining patterns for BRCA2 accumulation on sex chromosomes in pachytene and diplotene spermatocytes. Numbers of spermatocytes analyzed are noted above each graph. Accumulation patterns: Full domain, covers entirety of XY axes and chromatin; Partial domain, covers XY axes and portions of XY chromatin; Axial accumulation, covers XY axes; No accumulation, depletion from XY axes and chromatin. Data are aggregated from n = 6 littermate pairs. p values are derived from Pearson’s chi-square test: n.s., not significant, p > 0.05; *p ≤ 0.05; **p < 0.01; ***p ≤ 0.001. (F) Summary of temporal and spatial localization of anti-FA protein antibodies on the sex chromosomes in Rnf8^−/− mice; summaries of localization in wild-type mice are shown in Figures 1I and S1C. (G) Model of two-step amplification of FANCD2 foci on the sex chromosomes.

Figure 5. FANCD2 Colocalizes with RAD51 at Sites of Persistent DSBs

(A–C) Immunostains using indicated antibodies in meiotic chromosome spreads from Spo11^−/− mice and control littermates. Stages of meiotic prophase are labeled above images; genotypes are labeled to the left of images. Dashed boxes border selected nuclear regions and are magnified below. Arrowheads: co-localization of FANCD2 and RAD51. Consistent results were obtained with n = 3 independent littermate pairs. PAR, pseudo-autosomal region. Scale bars, 5 µm. See also Figure S3.

Figure 6. FANCD2 Cooperates with the BRCA1-MDC1-RNF8 Axis to Regulate FANCM on the Sex Chromosomes

(A, C, E, and G) Immunostains using indicated antibodies in meiotic chromosome from Fancd2^−/− (A), Brca1cKO (C), Mdc1^−/− (E), and Rnf8^−/− mice (G), and corresponding wild-type littermate controls. Stages are labeled above; genotypes are labeled to the left. Dashed squares border sex chromosomes and are magnified to the right. Consistent results were obtained with n = 5 Fancd2, n = 4 Brca1, n = 4 Mdc1, and n = 4 Rnf8 littermate pairs. Scale bars, 5 µm. (B, D, F, and H) Categorical staining patterns for FANCM accumulation on sex chromosomes of Fancd2 (B), Brca1 (D), Mdc1 (F), and Rnf8 (H) spermatocytes. Numbers of spermatocytes analyzed are noted above each graph. Accumulation scored according to criteria described in the legend for Figure 4D. Data are aggregated from n = 5 Fancd2, n = 4 Brca1, n = 4 Mdc1, and n = 4 Rnf8 littermate pairs. p values are derived from Pearson’s chi-square tests: n.s., not significant, p > 0.05; *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001. See also Figures S4 and S5.

Figure 7. FANCD2 Regulates H3K4me2 Independently of FA Core Factors, whereas FA Core Factors and FANCD2 Cooperate to Regulate H3K9 Methylation

(A, C, E, G, I, and K) Immunostains using indicated antibodies in meiotic chromosome spreads from Fanca^−/− mice, Fancd2^−/− mice, and corresponding wild-type littermate controls. Stages are labeled above, genotypes are labeled to the left. Dashed squares border sex chromosomes and are magnified to the right. Scale bars, 5 µm. (B, D, F, H, J, and L) Quantification of H3K4me2 (B and D), H3K9me2 (F and H), and H3K9me3 (J and L) relative mean fluorescence intensity (RMFI) on sex chromosomes (XY) and autosome regions (Au.) in pachytene (P) and diplotene (D) spermatocytes. Numbers of spermatocytes analyzed are noted above each graph. Bars represent means and SEMs. Data are aggregated from n = 4 Fanca littermate pairs (B, F, and J), and n = 3 Fancd2 littermate pairs (D, H, and L). p values, indicated in the panels, are derived from one-way ANOVA and Tukey’s method posttest. (M) Model of the FA-DDR network on the sex chromosomes. See the text for details. See also Figures S6 and S7.