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. 2016 Jun 14;15(11):2488-99.
doi: 10.1016/j.celrep.2016.05.031. Epub 2016 Jun 2.

FANCD2 Maintains Fork Stability in BRCA1/2-Deficient Tumors and Promotes Alternative End-Joining DNA Repair

Zeina Kais  1 , Beatrice Rondinelli  1 , Amie Holmes  1 , Colin O'Leary  1 , David Kozono  1 , Alan D D'Andrea  2 , Raphael Ceccaldi  3
Affiliations

FANCD2 Maintains Fork Stability in BRCA1/2-Deficient Tumors and Promotes Alternative End-Joining DNA Repair

Zeina Kais et al. Cell Rep. .

Abstract

BRCA1/2 proteins function in homologous recombination (HR)-mediated DNA repair and cooperate with Fanconi anemia (FA) proteins to maintain genomic integrity through replication fork stabilization. Loss of BRCA1/2 proteins results in DNA repair deficiency and replicative stress, leading to genomic instability and enhanced sensitivity to DNA-damaging agents. Recent studies have shown that BRCA1/2-deficient tumors upregulate Polθ-mediated alternative end-joining (alt-EJ) repair as a survival mechanism. Whether other mechanisms maintain genomic integrity upon loss of BRCA1/2 proteins is currently unknown. Here we show that BRCA1/2-deficient tumors also upregulate FANCD2 activity. FANCD2 is required for fork protection and fork restart in BRCA1/2-deficient tumors. Moreover, FANCD2 promotes Polθ recruitment at sites of damage and alt-EJ repair. Finally, loss of FANCD2 in BRCA1/2-deficient tumors enhances cell death. These results reveal a synthetic lethal relationship between FANCD2 and BRCA1/2, and they identify FANCD2 as a central player orchestrating DNA repair pathway choice at the replication fork.

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Figures

Figure 1
Figure 1. Upregulated FANCD2 protects replication forks in BRCA2-deficient tumor cells
(A) FANCD2 gene expression in subtypes of ovarian, breast and uterine cancers. For each tumor group, expression values are represented as the mean of z-scores. (B) FANCD2 immunoblot in HeLa cells after siRNA depletion of BRCA2, at indicated time points after UV treatment and cellular fractionation. Immunoblot showing BRCA2 depletion efficiency. (C) Quantification of baseline and damage (HU)-induced FANCD2 foci in HeLa cells after siRNA depletion of BRCA2. Representative images are shown. Satistics were performed on n≥150 cells per condition and expressed as relative to siScr that was conventionally set to 1. (D) Baseline and damage (HU)-induced FANCD2, TopBP1 and PCNA immunofluorescence in HeLa cells after siRNA depletion of BRCA2. Representative images are shown. Representative images are shown. Satistics were performed on n≥150 cells per condition and expressed as relative to siScr that was conventionally set to 1. (E, F) Schematic for the labeling of FANCD2-deficient (PD20) cells with ldU and CIdU for fork degradation (E) and for fork restart experiments (F). Scatter dot plot for CIdU to ldU ratio of cells expressing indicated cDNA and transfected with indicated siRNA. Satistics were performed on n≥100 fibers per condition and expressed as relative to EV, that was conventionally set to 1. Representative images are shown. Data in C–F represent mean ± s.e.m. over n=3 independent experiments. Data in A, C–F were analyzed using Student’s t test. Abbreviations: ON, over-night; ns, non significant; EV, empty vector; WT, wild-type.
Figure 2
Figure 2. FANCD2 maintains genomic stability in BRCA1 and BRCA2-deficient tumors
(A) Clonogenic formation of A2780 cells expressing the indicated shRNA together with the indicated siRNA. Immunoblot showing knockdown efficiencies of siRNAs and representative images for clonogenic formation are shown. (B) Chromosome breakage analysis of A2780 cells expressing the indicated shRNA and treated with MMC. (C) Clonogenic formation of A2780 cells expressing the indicated shRNA under increasing concentration of MMC. Survival is shown as relative to the untreated sample (MMC 0 ng/mL). Immunoblot showing silencing efficiency of shRNAs. (D) Chromosome breakage analysis of BRCA2-deficient (VU 423) cells transfected with the indicated siRNA. Representative images are shown. Arrows indicate chromosomal aberrations. Data in A–D represent mean ± s.e.m. over n=3 independent experiments and were analyzed by using the Chi-squared test for trend in proportions (A) or Student’s t test (B, D). Data in A, D are displayed as relative to siScr samples, while in B they are displayed as relative to shBRCA2 sample.
Figure 3
Figure 3. BRCA1 and BRCA2-deficient tumor cells are hyperdependent on monoubiquitinated FANCD2 for survival
(A) Clonogenic formation of a panel of 18 breast cell lines transfected with Scr or FANCD2 siRNA. Percent survival of cells transfected with siFANCD2 versus siScr is plotted. The genetic status of the cell line analyzed is indicated (Nle: normal, non-transformed mammary epithelial cell line; TNBC: triple negative breast cancer; HRP: HR-proficient; HRD: HR-deficient). FANCD2 gene expression in the 58 breast cell lines from the CCLE collection that includes 16 of the 18 breast cell lines tested in Figure 3A. Cell lines are grouped based on their genetic status. For each group, expression values are represented as the mean of z-scores. (Hor. pos.: hormone positive breast cancer). A nonparametric test (Mann-Withney-Wilcoxon) was used to compare the value of the HRD TNBC group with the rest of the groups. (B) Growth of MDA-MB-436 cells expressing doxycycline (dox)-inducible FANCD2 or scrambled (Scr) shRNA in athymic nude mice. Immunoblot showing silencing efficiency. (C) Relative tumour volumes (RTV) for individual mice treated in (B) after five weeks of doxycycline (dox) treatment. Each group represents n≥10 tumors from n≥6 mice while each dot represents data from one tumor. Data are shown as mean ± s.e.m. Student’s t test was used to compare each sample to “shFANCD2; dox”. (D) Clonogenic formation of FANCD2-deficient (PD20) cells expressing empty vector (EV) or FANCD2 cDNA constructs and transfected with BRCA2 siRNA. Percent survival of cells transfected with BRCA2 versus Scr siRNA is plotted. An Immunoblot shows expression of FANCD2 cDNA constructs. Representative images are shown. (E) Clonogenic formation of FANCD2-deficient (PD20) cells expressing EV or FANCD2 cDNA constructs and transfected with BRCA2 siRNA in increasing concentration of MMC. Percent survival relative to non-treated cells (MMC 0 ng/mL) is plotted. Data in A, D–E represent mean ± s.e.m. over n=3 independent experiments. Data in D was analyzed using the Chi-squared test for trend in proportions. Abbreviations: ns, non significant; EV, empty vector; WT, wild-type.
Figure 4
Figure 4. Monoubiquitinated FANCD2 is required for Polθ and CtIP foci formation and for alt-EJ
(A) Quantification of damage (UV)-induced Polθ foci in HeLa cells after siRNA depletion of BRCA2. (B) End-joining reporter assay in EJ2-U2OS cells transfected with the indicated siRNA. (C) FANCD2 immunofluorescence of HeLa cells transfected with GFP-tagged full length Polθ and subjected to UV damage. Representative images are shown. (D) FANCD2 and γH2AX immunofluorescence in HeLa cells transfected with GFP-tagged full length Polθ after laser micro-irradiation. Representative images are shown. (E) FANCD2 and CtIP immunofluorescence in HeLa cells transfected with GFP-tagged full length Polθ after laser micro-irradiation. Representative images are shown. (F) Quantification of Polθ localization at laser micro-irradiation sites in HeLa cells transfected with GFP-tagged full length Polθ and with indicated siRNA. γH2AX immunofluorescence serves as a marker of laser-induced DNA breaks. Representative images are shown. (G) Quantification of baseline and damage (UV)-induced Polθ foci in HeLa cells transfected with indicated siRNA. (H, I) Quantification of Polθ foci (H) and CtIP foci (I) formation in FANCD2-deficient (PD20) cells expressing EV, wild-type or K561R FANCD2 cDNA constructs. Representative images are shown. (J) End-joining reporter assay in EJ2-U2OS cells expressing indicated cDNAs and transfected with Scr or 5′UTR-FANCD2 siRNA. Data in A–B, F–J represent mean ± s.e.m. over n=3 independent experiments. Data in A–B, F–G, J are displayed as relative to siScr transfected sample, which value is set to 1 and were analyzed using Student’s t test. Data in H and I were analyzed using the Chi-squared test for trend in proportions. Abbreviations: ns, non significant; EV, empty vector; WT, wild-type.
Figure 5
Figure 5. FANCD2 overexpression in BRCA1-mutated breast cell line confers resistance to PARP inhibitor
(A) Clonogenic formation assay of BRCA1-mutated (MDA-MB-436) breast cell line expressing EV or FANCD2 cDNA constructs treated with increasing concentration of PARPi. FANCD2 immunoblot showing cDNA constructs expression. Representative images for clonogenic formation assays are shown. Survival of each sample is expressed as percent relative to non-treated (PARPi: 0 μM). (B) Quantification of baseline and damage (IR)-induced RAD51 foci in HeLa and BRCA1-mutated (MDA-MB436) cells expressing indicated cDNA. Data are displayed as relative to unirradiated HeLa cells. (C) Schematic for the labeling of MDA-MB-436 cells with ldU and CIdU for fork degradation experiments. Scatter dot plot for CIdU to ldU ratio upon HU treatment for BRCA1-mutated (MDA-MB-436) breast cell line expressing indicated cDNA. Satistics were performed on n≥100 fibers per condition and expressed as relative to EV, that was conventionally set to 1. (D) Model for FANCD2 functions in BRCA1- and BRCA2-deficient tumors. FANCD2 functions in HR and alt-EJ repair pathways and also participates in genomic maintenance by protecting fork stability. Although all pathways function in cycling cells, HR is the predominant pathway under normal conditions and the alt-EJ pathway is though to play only a minor role (left panel). Inactivation of BRCA1 or BRCA2 has no consequence on cell survival but induces both the localization of FANCD2 to stalled forks and the hyperactivation of the alt-EJ pathway (middle panel). As a consequence, loss of both BRCA1/2 and FANCD2 pathways induces cell death, defining a synthetic lethal interaction between the BRCA and the FA pathways (right panel). Data in A–C represent mean ± s.e.m. over n=3 independent experiments and were analyzed using Student’s t test. Abbreviations: ns, non significant; EV, empty vector; WT, wild-type.
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