A targeted mouse Brca1 mutation removing the last BRCT repeat results in apoptosis and embryonic lethality at the headfold stage

Abstract

A mouse model with a targeted mutation in the 3′ end of the endogenous Brca1 gene, Brca11700T, was generated to compare the phenotypic consequences of truncated Brca1 proteins with other mutant Brca1 models reported in the literature to date. Mice heterozygous for the Brca11700T mutation do not show any predisposition to tumorigenesis. Treatment of these mice with ionizing radiation or breeding with Apc, Msh-2 or Tp53 mutant mouse models did not show any change in the tumor phenotype. Like other Brca1 mouse models, the Brca11700T mutation is embryonic lethal in homozygous state. However, homozygous Brca11700T embryos reach the headfold stage but are delayed in their development and fail to turn. Thus, in contrast to Brca1null models, the mutant embryos do not undergo growth arrest leading to a developmental block at 6.5 dpc, but continue to proliferate and differentiate until 9.5 dpc. Homozygous embryos die between 9.5–10.5 dpc due to massive apoptosis throughout the embryo. These results indicate that a C-terminal truncating Brca1 mutation removing the last BRCT repeat has a different effect on normal cell function than does the complete absence of Brca1.

Introduction

Mutations in the BRCA1 tumor suppressor gene confer a strongly elevated risk to develop breast and ovarian cancer. Several studies have shed light on the function of the product of the BRCA1 gene. A variety of proteins have been shown to interact with BRCA1 directly or indirectly, although the physiological relevance of some of these interactions remains to be elucidated. However, two recurring themes are evident. BRCA1 appears to have a function in the maintenance of genomic integrity. BRCA1 co-localizes with RAD51, a protein involved in homologous recombination during meiosis and repair of dsDNA damage (Scully et al., 1997a,b). This and the direct interaction with the hRad50-hMre11-p95 complex, involved in non-homologues end joining (NHEJ) of dsDNA breaks, recombinational repair, checkpoint control, and in the formation and processing of meiotic DSBs (Zhong et al., 1999), point towards a function for BRCA1 in the repair of dsDNA breaks and maintenance of genomic integrity. Accordingly, murine cells devoid of normal Brca1 function have been found to be sensitive to γ-radiation (Shen et al., 1998) and show defects in homology-directed repair (Moynahan et al., 1999). Moreover, other data suggest a role for BRCA1 in transcription-coupled repair (Gowen et al., 1998). On the other hand, BRCA1 can function as a transcriptional co-activator (Chapman and Verma, 1996; Monteiro et al., 1996), apparently specific for p53 (Ouchi et al., 1998; Zhang et al., 1998). The recent identification of a large protein complex encompassing BRCA1 and several other proteins involved in the detection of and response to DNA damage suggests that BRCA1 may coordinate the response to different types of damage (Wang et al., 2000).

Since the identification of BRCA1 (Miki et al., 1994), over 800 different mutations have been described (http://www.nhgri.nih.gov/Intramural_research/Lab_transfer/Bic/). These are spread throughout the gene and include missense and truncating mutations or genomic deletions. Although stable expression of the mutant peptides has not been shown, different genotype-phenotype correlations have been described (Gayther et al., 1995; Grade et al., 1997; Sobol et al., 1996, 1997). The latter suggests that these proteins are stably expressed and may have a modulating effect on tumorigenesis. This also indicates that the mechanisms underlying BRCA1-mediated early-onset breast and ovarian cancer may differ among tumors with specific BRCA1 mutations.

A useful method to study the effect of BRCA1 mutations on physiological processes is the generation and analysis of mice with targeted mutations in the endogenous Brca1 gene. However, the Brca1 mutant mice described to date are either null mutants lacking any Brca1 derived protein, or interfere with the alternative splicing of exon 11 (Gowen et al., 1996; Hakem et al., 1996; Liu et al., 1996; Ludwig et al., 1997; Shen et al., 1998; Xu et al., 1999b). To study the effect of different BRCA1 mutations on normal cell function and predisposition to tumorigenesis additional mouse models are needed. Here, we report the generation and molecular and phenotypic analysis of Brca11700T, a mouse model carrying a truncating mutation in the 3′ end of the endogenous Brca1gene. This mutation removes the last BRCT repeat and interferes with the p53-specific co-activating domain, similar to previously reported human BRCA1 germline mutations (http://www.nhgri.nih.gov/Intramural_research/Lab_transfer/Bic/).

Results

The Brca11700T mouse model was generated by inserting a neomycin resistance gene in exon 20 of the murine Brca1 gene (Figure 1a). The neomycin cassette was inserted in the same transcriptional orientation as Brca1 to avoid anti-sense transcriptional interference as previously reported for Apc (Smits et al., 1999). The introduction of this mutation predicts the expression of a 1700 amino acids Brca1 protein lacking the last BRCT repeat (Figure 1b) (Bork et al., 1997; Koonin et al., 1996). This affects both exon 11-containing and exon 11-deficient splice variants. It has been shown that a similar truncated protein, when overexpressed in vitro, has a dominant-negative effect on the ability of co-transfected wild type BRCA1 to function as a co-activator of p53 (Zhang et al., 1998). Using conventional targeting of embryonic stem (ES) cells followed by blastocyst injection, we generated chimeric mice and heterozygous mutant mice. Since antibodies recognizing murine Brca1 are not available, we tested the expression of the mutant allele at the mRNA level. RT–PCR analysis showed that the mutant transcript is expressed at similar levels as the wild type messenger (Figure 1d).

Figure 1
figure1

(a) Wild type Brca1 locus and Brca11700T targeting construct. Abbreviations: B: A: ApaI, BamHI, H: HindIII, S: SmaI. Figure is not drawn to scale. (b) Schematic representation of wild type and putative 1700T Brca1 proteins. Only relevant functional domains are depicted. The part encoded by exon 11 is shaded to emphasize that the Brca11700T mutation affects both exon 11-deficient and exon 11-containing splice variants. (c) Southern blot confirmation of correct targeting. Lanes 1 and 2: BamHI digests; lanes 3 and 4: HindIII digests; lanes 1 and 3: Brca1+/+; lanes 2 and 4: Brca1+/1770T. Probe is shown in (a). Additional internal probes and restriction enzymes were tested and found to give the predicted fragments (not shown). (d) Schematic representation of wild type and predicted 1700T transcripts. Primers 17-F, 21-R and PN5b were used in a 3 primer PCR to compare expression of wild type and mutant transcripts. Primers 17-F and 24-R were used to amplify the 3′ end of Brca1 to detect splicing artifacts as a result of the insertion of the neomycin resistance cassette in exon 20. (e) Expression analysis of the Brca11700T allele by RT–PCR. Amplification products are from wild type and heterozygous ES cell lines. See (d) for experimental design and primers. Lanes 1–3: RT–PCR products obtained with primers 17-F, 21-R and PN5b; lanes 4–6: primers 17-F and 24-R. Lanes 1 and 4: Brca1+/+; lanes 2 and 5: Brca1+/1700T; lanes 3 and 6: H2O (negative control). Results were confirmed using RNA derived from adult testis (not shown)

To test the viability of homozygous Brca11700T mice, we intercrossed heterozygous mice, but did not find any homozygous offspring out of more than 300 animals tested, indicating that homozygosity for this mutation is not compatible with post-natal life. We therefore analysed litters from heterozygous intercrosses to study the developmental stage at which embryonic lethality occurs (Figure 2). At E8.5, homozygous mutant embryos are already severely growth-retarded. The amniotic cavity and allantois are present but the embryonic component of the homozygous embryos appears small when compared to the extra-embryonic part (Figure 2a–c). At E9.5, the mutant embryos have continued to develop and headfold and heart structures are now clearly present. However, the tailbud of the mutant embryos is strongly enlarged while only a few somites can be observed, indicating that somitogenesis is strongly impaired (Figure 2d–f). At E10.5Brca11700T/1700T embryos have further increased in size, but failed to develop any further (Figure 2g–i). Microscopic sections show large numbers of apoptotic bodies. Notably, Chandler et al. (2001) showed that the Brca11700T embryonic lethality is rescued by breeding these mice with transgenic ones generated with bacterial artificial chromosomes (BAC) encompassing either the human BRCA1 or the murine Brca1 gene. These results confirm that the observed phenotype directly results from the targeted Brca11700T mutation.

Figure 2
figure2

Analysis of homozygous Brca11700T embryos. (ac) 8.5 d.p.c. (df) 9.5 d.p.c. (gi) 10.5 d.p.c. Abbreviations: T/T: homozygous Brca11700T, wt: wild type, ac: amniotic cavity, al: allantois, am: amnion, e: embryonic component, ee: extra-embryonic component, h: heart, hf: headfolds, imp: implantation site, s: somites. Arrows indicate the boundary between embryonic and extra-embryonic components

The embryonic phenotype in the Brca11700T mouse model is clearly different from that reported for other Brca1 knockout mouse models. Embryos homozygous for null mutations are extremely growth retarded at E6.5 and have died by E7.5 (Hakem et al., 1996; Ludwig et al., 1997). In these embryos p21 expression is upregulated, accompanied by a cell proliferation blockage. No apoptosis could be detected in these models, suggesting that a cell cycle arrest resulting in a proliferation block is the primary cause of embryonic lethality. In contrast, Brca11700T/1700T cells continue to proliferate up to E9.5, as evaluated by BrdU incorporation (Figure 3a). These results were confirmed using an antibody against the Ki67 proliferation marker (not shown). At E10.5 proliferation can no longer be detected (Figure 3b). In contrast, an apoptotic response is activated between E9.5 and E10.5 as observed by using antibodies against the PARP p85 apoptotic fragment and by morphological examination of pyknotic cells (Figure 3c,d). This was confirmed using the TUNEL assay (not shown). Another feature of homozygous null embryos is the lack of mesoderm induction. The morphology of Brca11700T/1700T embryos already strongly suggests that in this model mesoderm can be formed. Expression of Brachyury, a marker for early undifferentiated mesoderm, can be detected at E8.5 and E9.5. However, its expression appears to be reduced when compared with stage-matched wild type embryos (Figure 4).

Figure 3
figure3

Analysis of proliferation and apoptosis, by BrdU incorporation and PARP p85 immunohistochemistry respectively, in homozygous Brca11700T embryos. (a,b) BrdU staining (c,d) p85 staining (a,c) 9.5 d.p.c. (b,d) 10.5 d.p.c. Results were confirmed with α Ki67 antibodies (proliferation) and TUNEL assay (apoptosis ) (not shown)

Figure 4
figure4

Whole amount RNA in situ hybridization analysis showing expression of Brachyury in homozygous Brca11700T embryos (a,b) and stage-matched and age-matched (littermates) wild type embryos (c, d, e)

Heterozygous Brca11700T mice have been followed for 2 years on an inbred 129Ola (n=40) and mixed C57BL/6JIco X 129Ola background (n=25), and do not show any susceptibility to tumor formation when compared to their age-matched wild type littermates (n=38 for 129Ola, n=22 for mixed background). This is in accordance with previously published results with other Brca1 mouse mutants (Gowen et al., 1996; Hakem et al., 1996; Liu et al., 1996; Ludwig et al., 1997; Shen et al., 1998). In view of the role played by BRCA1 in genomic integrity and repair of dsDNA breaks, we exposed 7-week-old Brca1+/+ (n=23) and Brca1+/1700T (n=27) mice to γ-radiation (5 Gy full body irradiation). This treatment dramatically increases the rate of tumor formation in mammary and ovarian tissue of Apc1638N mice (Houven van Oordt et al., 1997, 1999). Mice were sacrificed at 1 year of age or when moribund. We could not detect any change in the rate or distribution of tumors in Brca1+/1700T mice compared to wild type controls.

The occurrence of mammary tumors in mice with a conditional mutation removing exon 11 during lactation suggests that inactivation of the wild type Brca1 allele is a rate-limiting event for tumorigenesis in heterozygous Brca1 mutant mice (Xu et al., 1999a). As previously shown for the Apc gene in Apc+/−Msh2−/− mice (Smits et al., 2000), the frequency at which second hits occur can be increased in a mismatch repair deficient (Msh2−/−) genetic background. We therefore crossed the Brca11700T mice with Msh2Δ7N mice (Smits et al., 2000), but again found no effect of the Msh2 deficiency on the phenotype of Brca11700T heterozygous mice (n=11 for Brca1+/1700T/Msh2−/−, n=9 for Brca1+/+/Msh2−/−).

Brca11700T mice were also bred with Apc1638N mice (Fodde et al., 1994; Smits et al., 1998) because of their low incidence of mammary tumors (Houven van Oordt et al., 1997, 1999). However, no differences in tumor incidence were observed by comparison of Apc+/1638N/Brca1+/+ (n=13) mice with compound Apc+/1636N/Brca1+/1700T mice (n=15) when analysed at 1 year of age or when moribund.

Finally, we crossed Brca11700T mice with Tp53 knockout mice (Jacks et al., 1994). Tp53−/− animals develop T-cell lymphomas at 3–5 months of age. In heterozygous mice, sarcomas but also other neoplastic lesions have been observed between 10 and 18 months. The heterozygous Brca11700T mutation had no additional effect on the phenotype of Tp53−/− mice (n=8 for Brca1+/1700T/Tp53−/−, n=5 for Brca1+/+/Tp53−/−). We also studied tumor incidence in compound heterozygous Brca1+/1700T/Tp53+/−. Since the two genes are localized 20 cM apart on mouse chromosome 11, it is essential to distinguish between the mutant alleles in the cis and trans phases. In the Brca1+/1700T/Tp53+/−cis mice, loss of the wild type chromosome will result in a cell where only the putative Brca11700T protein is expressed in a p53-deficient background. Therefore, we first bred single heterozygous mice to generate trans compound heterozygous animals (n=22) which were subsequently bred to wild type C57BL/6JIco mice to select for meiotic recombinants with both mutant alleles in cis (n=26). However, we could not detect any difference in tumor incidence in the Brca1+/1700T/Tp53+/−cis or trans phases at 10 months of age when compared with age-matched Brca1+/+/Tp53+/− (n=32) mice.

Discussion

The observed differences in embryonic lethality among mouse models carrying different Brca1 targeted mutations provide clues for the understanding of the function of BRCA1. When compared with Brca1 null models, homozygous Brca11700T embryos show a delay in embryonic lethality and, in contrast, continue to proliferate and differentiate until an apoptotic response is activated. Hence, specific Brca1 mutations are associated with different molecular and cellular phenotypic consequences.

Brca1 homozygous null embryos are extremely growth retarded at E 6.5 and die at E 7.5 (Hakem et al., 1996; Ludwig et al., 1997). The absence of Brca1 in these embryos is likely to result in accumulation of DNA damage. Accordingly, p53 becomes activated leading to up-regulation of p21 and the accompanying cell proliferation block. Additional mouse Brca1 targeted alleles have been generated aimed at exon 11: Liu et al. (1996) reported an antisense insertion of a neomycin cassette within exon 11. The corresponding homozygous embryos show a similar phenotype as the null mice. In two other cases, 5′ regions of exon 11 including the splice acceptor sites have been replaced with a neomycin resistance gene (Gowen et al., 1996; Shen et al., 1998). While the model reported by Shen et al. strongly resembles the null allele in terms of timing of embryonic lethality, the mouse generated by Gowen and colleagues dies between E10.5 and E13.5 due to neuroepithelial abnormalities. In a fourth model, exon 11 has been removed using Cre-Lox technology (Xu et al., 1999b). In the latter case, named Brca1Δ11, the homozygous embryos die at E12.5–E18.5. Unfortunately, the phenotype of the latter embryos has not been reported in great details. In view of the phenotypic differences among the above exon 11 mutants, it is difficult to speculate on the molecular consequences of these mutant Brca1 alleles. This is further complicated by the physiological alternative splicing of exon 11 (Thakur et al., 1997; Wilson et al., 1997). Hence we will limit the discussion to the comparison between Brca11700T and the null models. It should also be noted that differences in the genetic background of the Brca1 mutant embryos are not likely to account for phenotypic differences. The present data have been obtained with C57BL/6J X 129Ola backcrosses. The two null models described previously have been obtained with identical (Hakem et al., 1996) or similar (C57BL/6J X 129Sv, Ludwig et al., 1997) backcrosses and are therefore comparable.

Different models can be envisaged for the mechanisms underlying the lethality of homozygous Brca11700T embryos depending on the specific Brca1 function that is affected. On one hand, repair of DNA damage might be unaffected since the domains interacting with RAD50 and RAD51 are encoded by exon 11, upstream of the targeted exon 20 region. On the other hand, DNA repair might be impaired because the C-terminal BRCT repeat, deleted in Brca11700T, is often found in proteins involved in response to DNA damage (Bork et al., 1997). In fact, from the present data we can conclude that a single BRCT repeat is not sufficient for Brca1 function in the mouse. BRCA1 also interacts directly with BRCA2 via its C-terminus (Chen et al., 1998). BRCA2 function is also linked to DNA damage repair, as shown by its direct interaction with Rad51, the sensitivity to γ-irradiation of Brca2−/− embryos, and the chromosomal aberrations in cells only expressing a truncated Brca2 protein (Sharan et al., 1997; Yu et al., 2000). If BRCA2 function is dependent on its interaction with BRCA1, the truncation of the C-terminus in Brca11700T may result in a repair defect.

Another function of the BRCA1 C-terminus likely to be impaired in the Brca11700T mouse model, is the p53 co-activating activity. In vitro studies have shown that an analogous mutation in the human BRCA1 cDNA results in loss of the p53 co-activating function, and may exert a dominant-negative effect (Zhang et al., 1998). However, it is not likely that disturbance of the p53 co-activation domain by itself can account for the lethality of Brca11700T embryos. If lack of p53 co-activation was the only cause of the observed in utero lethality, complete p53 deficiency (in a Brca1 wild type background) should have a phenotype of at least comparable severity. However, Tp53−/− mice are viable except for a small subset of the homozygous embryos (Sah et al., 1995). Moreover, the fact that p53 deficiency partially rescues the embryonic lethality of Brca1null/null mice indicates that p53 is functional in these embryos, and that not all p53 responses are Brca1-dependent (Hakem et al., 1997; Ludwig et al., 1997).

In a unifying hypothesis, repair of DNA damage is impaired in Brca11700T homozygous mice due to the lack of the last BRCT repeat and/or to the disturbed interaction with Brca2, leading to accumulation of DNA damage. In normal cells p53 is activated by DNA damage and can bind to the Brca1 exon 11-encoded binding domain to become co-activated by the C-terminus of full-length Brca1. However, since the latter domain is absent in Brca11700T, p53 could be sequestered by the mutant protein and thereby functionally inactivated. Hence, a mutant BRCA1 protein might have a dominant-negative effect on both the maintenance of the genomic integrity and on the response to the resulting instability. This would explain why the embryonic lethality of homozygous Brca11700T resembles that of compound Brca1null/null/Tp53−/− embryos in timing and gross morphology. The observed apoptosis in the E10.5 Brca11700T embryos should then result either from a delayed (or impaired) p53 response or from a p53-independent mechanism. We note, however, that this putative dominant-negative nature of the Brca11700T protein on p53 function does not suffice to trigger tumorigenesis in the presence of the wild type Brca1 allele.

In conclusion, the embryonic lethality in homozygous Brca11700T embryos and the phenotypic differences with null embryos suggest that mutant Brca1 proteins may have a different effect on cell function than complete absence of Brca1. In particular, since p53 status is an important prognostic factor in breast cancer (Blaszyk et al., 2000), the effect of truncating BRCA1 mutations on p53 function may have predictive clinical value for BRCA1 mutation carriers.

Materials and methods

Generation of the Brca11700T mouse model

A 8 kb genomic HindIII/ApaI fragment encompassing exon 20 and exon 21 was derived from a P1 clone containing the complete murine Brca1 gene (Bennett et al., 1995). This fragment was employed for the generation of the targeting construct by inserting a neomycin resistance gene under control of a PGK promoter and PGK poly A signal in a unique SmaI site in exon 20. A HA-tag was inserted in frame with the Brca1 coding sequence upstream the PGK promoter. The resulting replacement-type targeting vector was used for transfection of E14 ES cells (129Ola) according to standard procedures (Fodde et al., 1994). Correctly targeted clones were identified by PCR (not shown) and Southern blot analysis. Targeted clones were tested for karyotype and morphology and one clone was employed for the generation of chimeric mice through blastocyst injection.

Expression of the Brca11700T allele

Since different antibodies recognizing the HA-tag could not reproducibly show expression of the mutant protein, RNA was isolated from wild type and Brca1+/1700T ES lines or from testis from adult mice and reverse transcribed using random priming. An exon 17 forward primer (17-F: TTTgCTgAAAAATACCgCCT) was used in combination with exon 21 (21-R: AATAgACCTGTAggCCCTTgAA) and PGK promoter (PN5b: CTAAAgCgCAgGCTCCAgACT) reverse primers in a 3 primer PCR to simultaneously amplify the wild type and Brca11700T alleles. To exclude shorter RT–PCR products generated by alternative splicing, the exon 17 forward primer was also used in combination with an exon 24 reverse primer (24-R: TTggAgTCTTgTggCTCACTA).

Mouse strains

Brca11700T mice were bred on an inbred 129Ola background. These mice were used for breeding with Apc+/1638N C57Bl/6JIco (N22) females to generate F1 compound heterozygous mice. For all other experiments mice were backcrossed to C57Bl/6JIco (B6). N4 backcross generation mice were used for the radiation experiment. Mice were irradiated with 5 Gy full body radiation at 7 weeks of age (Smart 225, Andrex; dose rate 0.1 Gy/min.). N3 Brca1+/1700T males were bred with N6 Msh2+/Δ7N females to generate compound heterozygous mice which were subsequently intercrossed to generate Msh2-deficient mice (Brca1+/+ and Brca1+/1700T). N5 Brca1+/1700T males were crossed with N12 Tp53+/− females to generate trans Brca1+/1700T/Tp53+/− mice. These were intercrossed to generate p53-deficient mice (Brca1+/+ and Brca1+/1700T) or crossed to B6 wild type mice to generate cis Brca1+/1700T/Tp53+/− mice. Mice used for the analysis of embryonic lethality were N5 or further backcross generations.

Analysis of embryonic lethality

Embryos were isolated from timed matings and genotyped using PCR on yolk sac or Reicherts membrane with an intron 19 forward primer (gCTggCCTggACATgAgTgTA) in combination with an intron 20 reverse primer (gTTCTgTCACATAAAgAgggACT) specific for the wild type allele and the PGK promoter primer (see above) for the mutant allele. Embryos were fixed o/n at 4°C in 4% PFA, dehydrated and embedded in paraffin for sectioning. Sections were stained using standard hematoxin/eosin staining. For detection of proliferation in embryos, pregnant mice were injected with BrdU (0.1 mg/g bodyweight) 1 h prior to sacrifice. Immunohistochemistry with α-BrdU antibodies (Roche Molecular Biochemicals) was performed on paraffin sections after heat antigen retrieval and denaturation. BrdU staining results were confirmed with antibodies against the Ki67 proliferation marker (Novacastra). Apoptosis was detected using antibodies recognizing the apoptotic p85 fragment of PARP (Promega) and the TUNEL assay (Roche Molecular Biochemicals) according to the supplier's protocol. Whole mount RNA in situ hybridizations were done as described (Wilkinson, 1992) with a probe recognizing Brachyury (Wilkinson et al., 1990)

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Acknowledgements

The authors would like to thank Shantie Jagmohan-Changur, Ron Smits, Heleen van der Klift, and Nandy Hofland for technical assistance and critical reading of the manuscript.

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Correspondence to Riccardo Fodde.

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Keywords

  • Brca1 1700T
  • mouse model
  • hereditary breast cancer
  • p53
  • genotype-phenotype correlation
  • DNA damage

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