BRCA1 deficient embryonic stem cells display a decreased homologous recombination frequency and an increased frequency of non-homologous recombination that is corrected by expression of a brca1 transgene

BRCA1 is a nuclear phosphoprotein that has been classified as a tumor suppressor based on the fact that women carrying a mutated copy of the BRCA1 gene are at increased risk of developing breast and ovarian cancer. The association of BRCA1 with RAD51 has led to the hypothesis that BRCA1 is involved in DNA repair. We describe here the generation and analysis of murine embryonic stem (ES) cell lines in which both copies of the murine homologue of the human BRCA1 gene have been disrupted by gene targeting. We show that exogenous DNA introduced into these BRCA1 deficient cells by electroporation is randomly integrated into the genome at a significantly higher rate than in wild type ES cells. In contrast, integration of exogenous DNA by homologous recombination occurs in BRCA1 deficient cells at a significantly lower rate than in wild type controls. When BRCA1 expression is re-established at 5-10% of normal levels by introduction of a Brca1 transgene into BRCA1 deficient ES cells, the frequency of random integration is reduced to wild type levels, although the frequency of homologous recombination is not significantly improved. These results suggest that BRCA1 plays a role in determining the response of cells to double stranded DNA breaks.

Determination of the contribution of cysteinyl leukotrienes and leukotriene B4 in acute inflammatory responses using 5-lipoxygenase- and leukotriene A4 hydrolase-deficient mice

Arachidonic acid metabolism by 5-lipoxygenase leads to production of the potent inflammatory mediators, leukotriene (LT) B4 and the cysteinyl LT. Relative synthesis of these subclasses of LT, each with different proinflammatory properties, depends on the expression and subsequent activity of LTA4 hydrolase and LTC4 synthase, respectively. LTA4 hydrolase differs from other proteins required for LT synthesis because it is expressed ubiquitously. Also, in vitro studies indicate that it possesses an aminopeptidase activity. Introduction of cysteinyl LT and LTB4 into animals has shown LTB4 is a potent chemoattractant, while the cysteinyl LT alter vascular permeability and smooth muscle tone. It has been impossible to determine the relative contributions of these two classes of LT to inflammatory responses in vivo or to define possible synergy resulting from the synthesis of both classes of mediators. To address this question, we have generated LTA4 hydrolase-deficient mice. These mice develop normally and are healthy. Using these animals, we show that LTA4 hydrolase is required for the production of LTB4 in an in vivo inflammatory response. We show that LTB4 is responsible for the characteristic influx of neutrophils accompanying topical arachidonic acid and that it contributes to the vascular changes seen in this model. In contrast, LTB4 influences only the cellular component of zymosan A-induced peritonitis. Furthermore, LTA4 hydrolase-deficient mice are resistant to platelet-activating factor, identifying LTB4 as one mediator of the physiological changes seen in systemic shock. We do not identify an in vivo role for the aminopeptidase activity of LTA4 hydrolase.

Characterization of Brca1 deficient mice

BRCA1 is a nuclear phosphoprotein that is expressed in a cell cycle regulated manner in virtually all normal dividing cells. Inheritance of a mutated copy of the BRCA1 gene increases a woman's risk for developing breast and ovarian cancer (1-3). Since the tumors that arise in these individuals consistently fail to express the wild-type allele, BRCA1 is believed to encode a tumor suppressor. Loss of the remaining functional BRCA1 allele, therefore, is one of the steps leading to neoplastic transformation of some types of epithelial cells. The isolation of the murine homologue of the human BRCA1 gene opened up the possibility of using a powerful genetic approach to study the role of this gene in both normal development and tumor formation. This genetic approach involves in vitro manipulation of the genome of embryonic stem (ES) cells, stable tissue culture cell lines derived from mouse blastocysts. After introducing mutations into the murine homologue of the BRCA1 gene Brca1 in these cell lines, four groups have generated mouse lines carrying the same mutations (4-7). Surprisingly, mice carrying a single mutant Brca1 allele do not display the increased risk for breast tumors seen in humans carrying similar mutations. However, while loss of BRCA1 appears to be a one of the many events involved in tumorgenesis in humans, these mouse lines demonstrate that gene expression is essential for development; as homozygosity for each of the Brca1 mutations results in postimplantation embryonic lethality. The survival of Brca1 deficient embryos is extended by one or two days in the absence of p53 and p21 (7,8).

The prostaglandin receptor EP4 triggers remodelling of the cardiovascular system at birth

Survival of newborn placental mammals depends on closure of the ductus arteriosus (DA), an arterial connection in the fetus which directs blood away from the pulmonary circulation and towards the placenta where oxygenation occurs. Here we show that morphological changes resulting in closure of the DA in mice are virtually identical to those observed in larger mammals, including humans, and that maintenance of the DA in the open, or patent, state in fetal mice is dependent on prostaglandin synthesis. This requirement is absent in mice lacking the prostaglandin E2 EP4 receptor (EP4(-/-) mice). In EP4(-/-) mice of the 129 strain, remodelling of the DA fails to occur after birth, resulting in a left-to-right shunt of blood and subsequently in death. This suggests that the neonatal drop in prostaglandin E2 that triggers ductal closure is sensed through the EP4 receptor. In contrast, 5% of EP4(-/-) mice of mixed genetic background survive, and selective breeding of these mice leads to a 21% survival rate, suggesting that alleles at other loci can provide an alternative mechanism for ductal closure.

Anaphylaxis mediated through a humanized high affinity IgE receptor

Mast cells and basophils, which are activated by IgE and allergens through the high affinity IgE receptor (Fc epsilon RI), play a prominent role in anaphylaxis in the mouse. Mice deficient in this receptor become resistant to passive anaphylaxis. As a first step in developing an in vivo model that more closely mimics the IgE-mediated responses in man, we used a combination of transgenic and embryonic stem cell technology to generate a mouse line in which the murine Fc epsilon RI alpha-chain has been replaced with its human homologue. We demonstrate here that these mice express a tetrameric high affinity IgE receptor, in which the human alpha-chain associates with the murine beta- and gamma-chains, and that upon triggering with relevant Ag, this receptor mediates the initiation of the expected intracellular events. In addition, we show that the human alpha-chain restores an anaphylactic response to the nonresponsive alpha-deficient parental mouse line. This "humanized" mouse represents a potentially important model system, not only for studying the role of IgE in human immune responses, but also for testing potential therapeutic reagents that can interfere with responses mediated through the human Fc epsilon RI receptor.

Anaphylaxis mediated through a humanized high affinity IgE receptor

Mast cells and basophils, which are activated by IgE and allergens through the high affinity IgE receptor (Fc epsilon RI), play a prominent role in anaphylaxis in the mouse. Mice deficient in this receptor become resistant to passive anaphylaxis. As a first step in developing an in vivo model that more closely mimics the IgE-mediated responses in man, we used a combination of transgenic and embryonic stem cell technology to generate a mouse line in which the murine Fc epsilon RI alpha-chain has been replaced with its human homologue. We demonstrate here that these mice express a tetrameric high affinity IgE receptor, in which the human alpha-chain associates with the murine beta- and gamma-chains, and that upon triggering with relevant Ag, this receptor mediates the initiation of the expected intracellular events. In addition, we show that the human alpha-chain restores an anaphylactic response to the nonresponsive alpha-deficient parental mouse line. This "humanized" mouse represents a potentially important model system, not only for studying the role of IgE in human immune responses, but also for testing potential therapeutic reagents that can interfere with responses mediated through the human Fc epsilon RI receptor.

A murine model of cystic fibrosis

We have generated a mouse line in which the cystic fibrosis transmembrane conductance regulator (CFTR) gene has been mutated by gene targeting. Like human cystic fibrosis (CF) patients, mice lacking a functional CFTR gene, referred to as CFTR(-/-) mice, show increased numbers of goblet cells and obstruction of glands with inspissated eosinophilic secretions. The obstruction of glands often results in the destruction of gland-containing tissues in these animals. However, unlike the case in human CF patients, the most severe pathological changes in these mice were found, on preliminary analysis, to be confined to the intestinal tract and gallbladder. Although respiratory failure is the primary cause of death among humans with CF, we found only minor pathological alterations in the lungs and upper airways of our CFTR(-/-) animals. Possible explanations for the apparent lack of respiratory disease are the young age at which the animals were examined and the pathogen-free environment in which they were housed. In this manuscript, we examine the respiratory and other organ systems of CFTR(-/-) mice that have survived to adulthood. We also report on initial experiments in which CFTR(-/-) mice have been exposed to bacterial pathogens, and we present data on a single animal that displayed severe respiratory disease.

Altered inflammatory responses in leukotriene-deficient mice

Leukotrienes have been implicated in the regulation of immune responses, including inflammation and immediate hypersensitivity reactions. Here, we describe the phenotypic analysis of leukotriene-deficient mice generated by inactivation of the 5-lipoxygenase (5LO) gene. These 5LO(-/-) mice were unable to synthesize detectable levels of leukotrienes and were more resistant to lethal anaphylaxis induced by platelet-activating factor. The intensity of an acute inflammatory response induced by arachidonic acid was similar in 5LO(-/-) mice and controls. However, the response in 5LO(-/-) mice, but not in controls, could be virtually eliminated by a cyclooxygenase inhibitor. These data suggest that inflammatory responses are modulated by arachidonic acid metabolites through a variety of interconnected mechanisms. This has important implications for understanding the early events of an inflammatory response and for designing drugs for use in therapeutic intervention.

Analysis of the heterogeneity of the biological responses to native and mutant human interleukin-6

The structure-function relationships of the biological activities of mutant varieties of the pleiotropic cytokine interleukin-6 (human) were measured by three assays: induction of immunoglobulin M (IgM) secretion from an Epstein-Barr virus-transformed human B cell line and induction of fibrinogen secretion from either a human hepatoma cell line or a rat hepatoma cell line. The biological effects of the cytokine were characterized by three parameters as determined by a novel analysis: effectiveness (the maximal response attainable), efficiency (the concentration yielding a half-maximal response), and complexity (a measure of heterogeneity and feedback control). Substitution of serine for cysteine was associated with a reduction in the effectiveness of interleukin-6 in both fibrinogen secretion assays. In the assay with human hepatoma cells, there was also a profound reduction in efficiency. Serine substitution in the human IgM synthesis assay appears mainly to reduce the efficiency. Deletion of amino acids 4 to 23 increased the efficiency in the rat hepatoma assay. The complexity parameter suggests the presence of multiple receptor classes or negative feedback in all three assays. Use of the proposed sequential approach to the analysis of dose-response relations in bioassays provides a more useful quantitative assessment of activities as well as more insight into the complexity of the reactions.

An animal model for cystic fibrosis made by gene targeting

Cystic fibrosis results from defects in the gene encoding a cyclic adenosine monophosphate-dependent chloride ion channel known as the cystic fibrosis transmembrane conductance regulator (CFTR). To create an animal model for cystic fibrosis, mice were generated from embryonic stem cells in which the CFTR gene was disrupted by gene targeting. Mice homozygous for the disrupted gene display many features common to young human cystic fibrosis patients, including failure to thrive, meconium ileus, alteration of mucous and serous glands, and obstruction of glandlike structures with inspissated eosinophilic material. Death resulting from intestinal obstruction usually occurs before 40 days of age.

Role of disulfide bonds in biologic activity of human interleukin-6

We have examined the functional importance of the two disulfide bonds formed by the four conserved cysteines of human interleukin (IL-6). Using a bacterial expression system, we have synthesized a series of recombinant IL-6 mutants in which the constituent cysteines of the first (Cys45-Cys51), second (Cys74-Cys84), or both disulfide bonds of recombinant human interleukin-6 were replaced by other amino acids. Each mutant was partially purified and tested in four representative bioassays. While mutants lacking Cys45 and Cys51 retained activity similar to nonmutated recombinant IL-6, the activity of mutants lacking Cys74 and Cys84 was significantly reduced, especially in assays involving human cell lines. These results indicate that the first disulfide bond of human interleukin-6 is not required for maintenance of normal biologic activity. However, the fact that mutants lacking Cys45 and Cys51 were more active than corresponding cysteine-free mutants indicates that the disulfide bond formed by these residues contributes to biologic activity in the absence of the second disulfide bond. Competition binding studies with representative mutants indicate that their affinity for the human IL-6 receptor parallels their biologic activities on human cells.

Effects of site-specific mutations on biologic activities of recombinant human IL-6

To examine structure-activity relationships of human IL-6, we have determined the effects of specific mutations on the biologic activity of a human rIL-6 expressed in bacteria. Three types of mutants were examined: 1) a variant that contains serines in place of the four naturally occurring cysteines; 2) a series of cysteine-containing deletion mutants, each having a single internal 20 amino acid deletion; and 3) a cysteine-free variant containing a single 20 amino acid deletion. The mutants of the second type constitute a set of nonoverlapping, adjacent deletions spanning amino acids 4 through 183 of the 184 amino acids in natural human IL-6. All of the mutants were expressed, along with the full length, cysteine-containing analogue, in Escherichia coli as fusion proteins, joined to beta-galactosidase through a collagen linker. This system allows microgram quantities of the rIL-6 variants to be partially purified from small bacterial cultures without chromatographic or refolding steps. Each of the rIL-6 variants was released from the beta-galactosidase fusion protein with collagenase, and the recovered rIL-6 was quantitated by laser densitometry of Coomassie-stained, SDS polyacrylamide gels. The sp. ac. of each of the rIL-6 variants was determined using four assays: induction of IgM secretion from an EBV transformed human B cell line, induction of fibrinogen secretion from a human hepatoma cell line, induction of fibrinogen secretion from a rat hepatoma cell line, and induction of proliferation of a murine hybridoma cell line. Replacement of cysteines with serines reduced activity relative to cysteine-containing rIL-6 to about 20% in the rat hepatoma assay and about 3% in the mouse hybridoma assay, whereas activity in both of the human cell lines was reduced to less than 0.1%. These data suggest that the murine and rat cell lines are less selective than the human cell lines in their requirements for recognition of biologically active IL-6. Each of the deletions, except that of amino acids 4 through 23, resulted in loss of activity in all four assays. These results suggest that the information necessary for activity is not contained within any one portion of the IL-6 molecule, but rather that multiple segments of the protein are required for each of the biologic activities that we tested.

Effects of site-specific mutations on biologic activities of recombinant human IL-6

To examine structure-activity relationships of human IL-6, we have determined the effects of specific mutations on the biologic activity of a human rIL-6 expressed in bacteria. Three types of mutants were examined: 1) a variant that contains serines in place of the four naturally occurring cysteines; 2) a series of cysteine-containing deletion mutants, each having a single internal 20 amino acid deletion; and 3) a cysteine-free variant containing a single 20 amino acid deletion. The mutants of the second type constitute a set of nonoverlapping, adjacent deletions spanning amino acids 4 through 183 of the 184 amino acids in natural human IL-6. All of the mutants were expressed, along with the full length, cysteine-containing analogue, in Escherichia coli as fusion proteins, joined to beta-galactosidase through a collagen linker. This system allows microgram quantities of the rIL-6 variants to be partially purified from small bacterial cultures without chromatographic or refolding steps. Each of the rIL-6 variants was released from the beta-galactosidase fusion protein with collagenase, and the recovered rIL-6 was quantitated by laser densitometry of Coomassie-stained, SDS polyacrylamide gels. The sp. ac. of each of the rIL-6 variants was determined using four assays: induction of IgM secretion from an EBV transformed human B cell line, induction of fibrinogen secretion from a human hepatoma cell line, induction of fibrinogen secretion from a rat hepatoma cell line, and induction of proliferation of a murine hybridoma cell line. Replacement of cysteines with serines reduced activity relative to cysteine-containing rIL-6 to about 20% in the rat hepatoma assay and about 3% in the mouse hybridoma assay, whereas activity in both of the human cell lines was reduced to less than 0.1%. These data suggest that the murine and rat cell lines are less selective than the human cell lines in their requirements for recognition of biologically active IL-6. Each of the deletions, except that of amino acids 4 through 23, resulted in loss of activity in all four assays. These results suggest that the information necessary for activity is not contained within any one portion of the IL-6 molecule, but rather that multiple segments of the protein are required for each of the biologic activities that we tested.

Development of a vector system for the expression of bioengineered proteins

The low natural abundance of many proteins is a major factor in preventing their development as therapeutic or diagnostic tools. To circumvent this barrier, we have used synthetic oligonucleotide technology to construct a gene based on the sequence of a cDNA for human interleukin 6 (IL-6). The synthetic gene encodes a cysteine-free, bioengineered rIL-6 protein, which is expressed at high concentrations in Escherichia coli as a tripartite fusion protein. Cleavage of the fusion protein with collagenase (EC 3.4.24.8) releases a 23-kDa rIL-6 protein that can be easily purified to homogeneity. This rIL-6 protein displays a range of biological activities similar to those of natural human IL-6, as demonstrated by its ability to (a) protect cells from viral infection and (b) stimulate the synthesis of fibrinogen in rat FAZA cells.

High-level expression of a bioengineered, cysteine-free hepatocyte-stimulating factor (interleukin 6)-like protein

Hepatocyte-stimulating factor, interferon-beta 2, B-cell stimulation factor 2, and hybridoma/plasmacytoma growth factor are identical proteins presently referred to as interleukin 6 (IL-6). Through the use of synthetic oligonucleotide technology, we have constructed a biologically active recombinant IL-6 (rIL-6) gene based on the sequence of a human IL-6 cDNA. The synthetic gene encodes a cysteine-free, bioengineered rIL-6 protein that is expressed at high levels in Escherichia coli as a tripartite fusion protein. Cleavage of the fusion protein with collagenase releases a 23-kDa rIL-6 protein that can be easily purified to homogeneity. We show that the rIL-6 protein displays a range of biological activities similar to those of natural human IL-6, as demonstrated by its ability to (i) protect cells from viral infection, (ii) stimulate the synthesis of fibrinogen in rat FAZA 967 cells, and (iii) induce the terminal differentiation of B cells, resulting in elevated secretion of immunoglobulin.