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At least 595 records · Page 33Linked to original sources
PubMed2003

Renin similar to the submaxillary gland form is expressed in mouse mesangial cells: subcellular localization and all generation under control and glucose-stimulated conditions.

It was analyzed the forms of renin produced by a mouse immortalized mesangial cell line (MIC) and their ability to generate angiotensin II (AII). The synthesis, localization and secretion of renin and AII by MIC were evaluated under conditions of normal (10 mM) or high (30 mM) glucose concentration. Two major bands of 35 kDa and 70 kDa were observed in SDS-PAGE. The amino-terminal sequencing revealed the presence of prorenin and renin in these bands with higher homology with the submaxillary gland form of renin. Renin and AII were detected in cell lysate and in culture medium, indicating that MIC synthesize and secrete these peptides. Renin was localized in the cytoplasm while AII was seen predominantly inside the nucleus. High glucose induced an increase in the synthesis and secretion of renin and AII. Results suggest that MIC produce AII and a renin form similar to the submandibular. Intracellular AII may be directed at the nucleus and/or be secreted, indicating that AII may directly influences gene expression in these cells. The mechanisms of synthesis and secretion of renin and AII are potentially modified by high glucose concentration, suggesting a possible role of AII produced by mesangial cells in diabetic nephropathy.

Amino Acid Sequence↗
PubMed2003

Murine embryonic stem cell in vitro differentiation: applications to the study of vascular development.

The present review summarizes knowledge accumulated during the last decade concerning in vitro endothelial differentiation from embryonic stem (ES) cells. There is now growing evidence that ES cells may provide a powerful model system to determine the cellular and molecular mechanisms of vascular development. ES cells differentiate into the endothelial lineage by successive maturation steps recapitulating in vivo events observed in the embryo. Further maturation of ES-derived embryoid bodies either in three dimensional gels or in confrontation cultures with tumor spheroids can also provide a model of physiological or tumoral angiogenesis. The data obtained from experimental in vitro differentiation of genetically modified mouse ES cells highlight the potential and the complementarity of this model system to in vivo gene knock out studies. We also consider and discuss some of the potential applications of ES cell technology in vascular biology for future directions in basic research and medicine, by manipulation of differentiation and the generation of cell populations for analysis and transplantation for therapeutic use.

Animals↗
PubMed2026

From the Microscope to the Genome: A New Era in the Molecular Genetics of Epidermolysis Bullosa.

Epidermolysis bullosa (EB) is a heterogeneous group of inherited disorders characterised by skin fragility, caused by pathogenic variants in genes encoding structural components of the dermo-epidermal junction. With the advent of next-generation sequencing (NGS), the diagnostic paradigm has shifted from a morphological to a genotype-oriented approach. This review summarises the genetic architecture of EB, the types of mutations and genotype-phenotype relationships, the challenges in interpreting variants of unknown significance (VUS), and therapeutic strategies targeting specific mutational mechanisms, including read-through approaches, exon skipping and genome editing. The role of modifier genes and epigenetic factors in clinical variability is also discussed. The focus is on the translational potential of genomics for personalized therapy in EB. Overall, this review synthesizes the molecular basis of all four major EB types across 16+ classical genes, highlights the paradigm shift where NGS achieves a diagnostic yield exceeding 90%, and critically assesses recent therapeutic milestones-ranging from the first FDA-approved topical gene therapy to precision RNA and genome-editing modalities.

Humans↗
PubMed1992

A triple helix-forming oligonucleotide-intercalator conjugate acts as a transcriptional repressor via inhibition of NF kappa B binding to interleukin-2 receptor alpha-regulatory sequence.

Oligonucleotide-directed triplex formation within upstream regulatory sequences is envisioned as a potential tool for gene inhibition. However, this approach requires that triple helix-forming oligonucleotides are chemically modified, so that the triplex is stable under physiological conditions. Here, we have compared several chemical modifications of an oligonucleotide, targeted to a natural 15-base pair homopyrimidine.homopurine sequence located in the upstream regulatory region of the gene encoding the interleukin-2 receptor alpha chain (p55, IL-2 R alpha). Methylation of the cytosines strongly stabilized the triplex. Further attachment of an intercalating agent (acridine) dramatically increased the stability of the triplex, as assessed by Tm measurements or by band shift assays. Furthermore, the acridine-derivatized oligonucleotide was more efficient in competing away high affinity DNA-binding proteins, as assessed by restriction enzyme inhibition assays. Using a novel footprinting assay, we have further shown that the interaction of the methylcytosine-substituted, acridine-derivatized oligonucleotide with a plasmidic target, harboring the IL-2 R alpha regulatory region, remains highly sequence specific, occurs at physiological pH and is independent of the superhelicity of the plasmid. Acridine derivatization did not impair the exquisite target specificity of triplex formation, since the derivatized oligonucleotide inhibited the binding of nuclear proteins to the overlapping NF kappa B enhancer sequence on an IL-2 R alpha target and not on the related human immunodeficiency virus long terminal repeat target. Finally, the oligonucleotide inhibited the NF kappa B-dependent tax-induced transcriptional activation of the IL-2 R alpha chloramphenicol acetyltransferase construct in live cells, whereas it did not have any effect on a human immunodeficiency virus long terminal repeat chloramphenicol acetyltransferase construct. We conclude that this modified oligonucleotide acts as a transcriptional repressor for the IL-2 R alpha gene via triple helix formation with regulatory sequences.

Acridines↗
PubMed2006

Evaluation of allergenicity of genetically modified soybean protein extract in a murine model of oral allergen-specific sensitization.

BACKGROUND: With the development of genetically modified crop plants there has been a growing interest in the approaches available to assess the potential allergenicity of novel gene products. For additional assessment of the potential allergenicity of expressed proteins, informative data can be generated using animal models. Soybean is one of the major source of protein in human and animal nutrition, and has also been well characterized as a major allergenic source. Advances in biotechnology have resulted in an increasing number of genetically engineered foods, and among these soybean is one of the most widespread. OBJECTIVE: To develop and characterize a murine model of IgE-mediated soybean sensitization induced by intragastric immunization, in the presence of Cholera Toxin, with wild-type soybean extract (wt-SE) or with genetically modified soybean extract (gm-SE). METHODS: Balb/c mice born in our animal facilities, from females fed on soy-free food, were fed with the same soy-free food and used in all the experiments. Mice were sensitized by gavages with soybean extracts, and allergen-specific IgE and IgG responses were studied by direct ELISA and ELISA inhibition. Antigen-specific cell proliferation and cytokine production were evaluated in spleen cell cultures. Results Sensitization with both soybean extracts induced high levels of antigen-specific IgE and IgG1 and low levels of specific IgG2a. Both wt-SE and gm-SE were able to inhibit the binding of specific IgE from mice immunized with gm-SE to the same antigen used for the ELISA coating. A comparable proliferative response was obtained with the homologous as well as with the heterologous extracts. CONCLUSION: In sensitized mice, we observed a predominantly T-helper type 2 (Th2)-type immune response, with increased soybean-specific IgE and IgG1 antibodies and a concomitant increase of IL-4 and IL-5 production. RESULTS: obtained by specific IgE ELISA inhibition and by antigen-specific T cell proliferation demonstrated that wt-SE and gm-SE shared B and T epitopes. The present murine model of soybean sensitization established by the oral route should provide valuable information about risk assessment for food allergy from new proteins of genetically modified foods.

Administration, Oral↗
PubMed1996

IL-2 gene therapy of solid tumors: an approach for the prevention of signal transduction defects in T cells.

The identification of tumor-associated antigens has focused attention on the mechanisms that underlie the failure of T cells to destroy tumor cells. A deeper understanding of the process of signal transduction following the binding of ligand by the T cell receptor can help to identify underlying defects that may be involved. Gene therapy using tumor cells genetically modified to express cytokines or surface determinants is a promising technique for stimulating antitumor responses. A potential pitfall in its application to cancer, however, is that some patients' T cells are immune suppressed and may resist stimulation by such genetically engineered vaccines. Recent studies have demonstrated that T cells from tumor-bearing patients exhibit abnormalities in signal transduction events, possibly rendering them unable to respond to activation signals. Gene therapy with interleukin 2 secreting tumor cells in an animal model has been shown effective in preventing the onset of signaling defects. A more precise definition of the molecular mechanisms that enable cytokine-secreting tumor cells to stimulate specific antitumor responses may make it feasible to optimize immunotherapeutic approaches resulting in better clinical results.

Animals↗
PubMed2003

RNA interference in mammalian cells by chemically-modified RNA.

RNA interference (RNAi) is proving to be a robust and versatile technique for controlling gene expression in mammalian cells. To fully realize its potential in vivo, however, it may be necessary to introduce chemical modifications to optimize potency, stability, and pharmacokinetic properties. Here, we test the effects of chemical modifications on RNA stability and inhibition of gene expression. We find that RNA duplexes containing either phosphodiester or varying numbers of phosphorothioate linkages are remarkably stable during prolonged incubations in serum. Treatment of cells with RNA duplexes containing phosphorothioate linkages leads to selective inhibition of gene expression. RNAi also tolerates the introduction of 2'-deoxy-2'-fluorouridine or locked nucleic acid (LNA) nucleotides. Introduction of LNA nucleotides also substantially increases the thermal stability of modified RNA duplexes without compromising the efficiency of RNAi. These results suggest that inhibition of gene expression by RNAi is compatible with a broad spectrum of chemical modifications to the duplex, affording a wide range of useful options for probing the mechanism of RNAi and for improving RNA interference in vivo.

Blotting, Western↗
PubMed2003

[Expression of a DNA fragment encoding the active domain of human TNF related apoptosis inducing ligand in pichia pastoris].

Human tumor necrosis factor-related apoptosis-inducing ligand(TRAIL) is a member of the tumor necrosis factor (TNF) family of ligands which has been reported in 1995. The TRAIL protein induces apoptosis of certain types of target cells, such as transformed cells that include but are not limited to cancer cells and virus-infected cells but the normal cells. It is a type II transmembrane protein and the extracellular domain of TRAIL is the functional domain in induction of cell apoptosis. A gene fragment encoding for the active domain of TRAIL was modified with oligo-nucleotide directed mutagenesis according to the characters of Pichia pastoris expressing vector. Arginine at the position of 149 corresponding to the amino acid residue 531 which might be a potential Kex2 protease processing sites was substituted with Lysine to prevent the expressed protein from the digestion by the protease. After proved with DNA sequencing. the modified gene fragment coding soluble TRAIL domain was inserted into the Pichia pastoris expression vector pPIC9K in the same reading frame with alpha-factor secreting signal peptide. The recombinant plasmid pPIC9K - TRAIL was transferred into P. pastoris cell by spheroplast transformation. The recombinant yeasts were identified by antibiotic G418 and Southern dot blot. The transformants (His+ Mut(s)) containing multi-copy gene fragment of TRAIL were selected with increasing concentration of G418 and induced with 0.5% methanol in shaking flask to expression the active domain of TRAIL. After inducing for 3 - 4 days, the proteins in the culture supernatant was assayed with SDS-PAGE and Western blot. Two expressed protein bands whose appearant molecular weight were 19kD and 38kD, respectively, could be specifically recognized by polyclonal antibodies against human TRAIL. The 38kD protein might be a dimers of TRAIL in the culture supernatant. The amount of expressed foreign protein made up to 36% of the total proteins in the culture suprenatant. Biological activity assay, in vitro, indicated that the expressed protein could induce tumor cells apoptosis.

Apoptosis↗
PubMed2005

Pharmacogenomics of responsiveness to interferon IFN-beta treatment in multiple sclerosis: a genetic screen of 100 type I interferon-inducible genes.

OBJECTIVES: Interferon IFN-beta is indicated for the treatment of multiple sclerosis. A significant proportion of patients show a poor clinical response to therapy. Type I interferon exerts its effect at least partially through interaction of specific transcription factors with interferon-stimulated response elements (ISREs), mostly located in promoter regions of its target genes. We hypothesized that polymorphisms may occur within or close to ISRE elements, altering type I interferon inducibility and ultimately leading to a modified clinical response in carriers. METHODS: We selected 100 ISRE-containing genes and sequenced their promoter regions in small genomic deoxyribonucleic acid pools of responding and nonresponding patients, as well as healthy control subjects. A selection of polymorphisms discovered by this approach was scrutinized subsequently in a collection of individual deoxyribonucleic acid samples. RESULTS: We identified 4 genes containing polymorphisms associated with response to recombinant IFN-beta: IFNAR1 (P = .036), LMP7 (P = .002; odds ratio [OR], 6.37 [95% confidence interval (CI), 1.84-24.1]), CTSS (P = .02; OR, 0.38 [95% CI, 0.18-0.84]), and MxA (P = .015; OR, 3.37 [95% CI, 1.11-11.4]). Logistic regression analysis with treatment outcome used as the dependent variable and genotype as the independent variable revealed 2 genes, LMP7 (OR, 0.55 [95% CI, 0.34-0.89]) and MxA (OR, 0.41 [95% CI, 0.19-0.88]), that were independently associated with treatment response. CONCLUSIONS: Our work confirms and extends previous indications for a polygenic mechanism involved in bringing about responsiveness to recombinant IFN-beta. The identification of 2 genes active in the antigen processing and presentation cascade; that is, LMP7, coding for the proteasome subunit beta, and CTSS, coding for cathepsin S; as potential response modifiers may identify this pathway as being of particular relevance to phenotypic expression of response heterogeneity.

Chromosome Mapping↗
PubMed2005

Kv4 potassium channel subunits control action potential repolarization and frequency-dependent broadening in rat hippocampal CA1 pyramidal neurones.

A-type potassium channels regulate neuronal firing frequency and the back-propagation of action potentials (APs) into dendrites of hippocampal CA1 pyramidal neurones. Recent molecular cloning studies have found several families of voltage-gated K(+) channel genes expressed in the mammalian brain. At present, information regarding the relationship between the protein products of these genes and the various neuronal functions performed by voltage-gated K(+) channels is lacking. Here we used a combination of molecular, electrophysiological and imaging techniques to show that one such gene, Kv4.2, controls AP half-width, frequency-dependent AP broadening and dendritic action potential propagation. Using a modified Sindbis virus, we expressed either the enhanced green fluorescence protein (EGFP)-tagged Kv4.2 or an EGFP-tagged dominant negative mutant of Kv4.2 (Kv4.2g(W362F)) in CA1 pyramidal neurones of organotypic slice cultures. Neurones expressing Kv4.2g(W362F) displayed broader action potentials with an increase in frequency-dependent AP broadening during a train compared with control neurones. In addition, Ca(2)(+) imaging of Kv4.2g(W362F) expressing dendrites revealed enhanced AP back-propagation compared to control neurones. Conversely, neurones expressing an increased A-type current through overexpression of Kv4.2 displayed narrower APs with less frequency dependent broadening and decreased dendritic propagation. These results point to Kv4.2 as the major contributor to the A-current in hippocampal CA1 neurones and suggest a prominent role for Kv4.2 in regulating AP shape and dendritic signalling. As Ca(2)(+) influx occurs primarily during AP repolarization, Kv4.2 activity can regulate cellular processes involving Ca(2)(+)-dependent second messenger cascades such as gene expression and synaptic plasticity.

Action Potentials↗
PubMed1995

Epidermal growth factor induction of human papillomavirus type 16 E6/E7 MRNA in tumor cells involves two AP-1 binding sites in the viral enhancer.

The early genes E6 and E7 from human papillomaviruses (HPVs) play a key role in the development of cervical cancer. Modulation of E6 and E7 gene expression may alter tumour progression; therefore, modifiers of viral transcription such as hormones or growth factors are potential risk factors in cancer development. We have analysed the effects of epidermal growth factor (EGF) on E6/E7 mRNA from human papillomavirus type 16 (HPV-16) by Northern blot in two cell lines, SiHa cervical carcinoma cells, and HPK IA, an HPV-16-immortalized keratinocyte cell line. E6/E7 mRNA is EGF-inducible in SiHa cells, with the earliest response after 2 h. In contrast, in HPK IA cells no increase in E6/E7 RNA is observed, suggesting a differential EGF response of viral transcription in tumour cells compared with keratinocytes. We demonstrate that the cell type-specific HPV-16 enhancer is a target of EGF-induced signals, as its activity is amplified by EGF in SiHa cell transfections. However, when transfected into HPK IA keratinocytes, the viral enhancer shows no EGF response. The enhancer contains two binding sites for the transcription factor AP-1, a potential mediator of the EGF signalling cascade. Enhancer subfragments with single AP-1 binding sites are also EGF-responsive in SiHa cells. Mutating either AP-1 site in the complete enhancer decreases the EGF response, whereas a double mutation causes a complete loss of EGF regulation, suggesting that the EGF induction of HPV-16 early transcription requires AP-1 activation. We conclude that alterations of EGF responsiveness that increase viral oncogene expression may contribute to cervical cancer progression.

Base Sequence↗
PubMed1995

Positive-selection vector with enhanced lytic potential based on a variant of phi X174 phage gene E.

A cloning vector, pUH89, allowing positive selection of recombinant Escherichia coli clones by insertional inactivation of the modified lysis gene E of bacteriophage phi X174, was developed. Ten unique cloning sites were introduced into gene E by site-directed mutagenesis. To achieve efficient expression of the mutagenized gene, the combined lac and tac promoters were used. Additional restriction sites in the flanking sequences allow screening for transcription terminators and the excision of several cartridges suitable for vector construction.

Amino Acid Sequence↗
PubMed2003

Metastatic Breast Tumour Regression Following Treatment by a Gene-Modified Vaccinia Virus Expressing MUC1 and IL-2.

MUC1 is expressed by glandular epithelial cells. It is overexpressed in the majority of breast tumours, making it a potential target for immune therapy. The objectives of the present study were to evaluate the anti-tumour activity and tolerance of repeated administration of TG1031 (an attenuated recombinant vaccinia virus containing sequences coding for human MUC1 and the immune stimulatory cytokine IL-2) in patients with MUC1-positive metastatic breast cancer. This was an open-label, randomised study comparing two dose levels, 5 x 10E6 and 5 x 10E7, with 14 patients in each arm. The treatment was administered intramuscularly every 3 weeks for the first 4 doses and every 6 weeks thereafter, until progression. Two patients had a partial tumour regression ( > 50%), and 15 patients had stable disease as their best overall response until at least the 5th injection. Partial regression lasted for 11 months in one patient and for 12 months in the second patient who then underwent surgical resection of her hepatic metastases. The most frequent adverse events included inflammation at injection site: 7 patients, itching or pain at injection site: 5 patients, and moderate fever: 6 patients. One responding patient developed antinuclear, anti-DNA, and increased anti-TPO antibodies after the fifth injection, and which resolved at the end of treatment. The treatment regimes were well tolerated with a low toxicity profile. Although clinical efficacy remains limited, this study demonstrates the potential use of MUC1-based immune therapy in breast cancer.

Journal Article↗
PubMed2002

A limited association of OGG1 Ser326Cys polymorphism for adenocarcinoma of the lung.

The 8-oxoguanine DNA glycosylase (OGG1) repairs DNA by removing 8-hydroxyguanine, a highly mutagenic oxidative DNA adduct. Recently, the gene for OGG1 was cloned and several polymorphisms have been reported. Because environmental carcinogens produce 8-hydroxyguanine residues that potentially cause oncogenic mutations by mismatching to this modified base, the capacity to repair these lesions can be involved in cancer susceptibility. This study investigated the association between OGG1 Ser326Cys polymorphism and risk of the lung adenocarcinoma for Japanese by a prevalent case-control study in Japan. The subjects comprised 138 cases and 241 non-cancer outpatients as controls. OGG1 gene polymorphism was genotyped by a PCR-CTPP (polymerase chain reaction with confronting two-pair primers) method. The distribution of OGG1 Ser326Cys genotype among controls (Ser/Ser, 28.3%; Ser/Cys, 49.2%; and Cys/Cys, 22.5%) was not different from that among cases (Ser/Ser, 29.0%; Ser/Cys, 51.4%; and Cys/Cys, 24.0%). The sex-age adjusted odds ratio (OR) was 1.06 with 95% confidence interval (CI) 0.64-1.76 for Ser/Cys genotype and 0.81 with 0.44-1.52 for Cys/Cys genotype. The ORs according to the interval between diagnosis and study enrollment were also examined because the polymorphism was a potential prognostic factor of lung cancer. The ORs of Ser/Cys and Cys/Cys genotypes in the cases less than 3 years after diagnosis were higher than overall ORs; 1.86 (95%CI, 0.91-3.77), and 1.46 (0.64-3.35), respectively. The OR for smoking was not statistically different among genotype, though the sample size was too small to detect even a moderate interaction. This study supported the first study by Sugimura et al (Cancer Epidemiol Biomarkers Prev, 1999; 8: 669-674), that the association of OGG1 Ser326Cys polymorphism was limited for the risk of lung adenocarcinoma.

Adenocarcinoma↗
PubMed2002

Gene therapy: reality or myth for the global bleeding disorders community?

In recent years, five different human gene therapy clinical studies have been initiated in the United States, covering a broad spectrum of gene transfer technologies. Both in vivo and ex vivo studies have been performed, and a variety of target organs/tissues have been studied. The results of this early human clinical research indicate that there is still much to be done before a safe and effective gene therapy procedure becomes commercially available, but there is strong evidence that the obstacles that remain will not prove insurmountable. Small increases in circulating clotting factor levels have been achieved in some patients without significant side-effects, providing proof of the principle that gene therapy can provide a therapeutic benefit for patients with haemophilia. Still unclear is whether gene therapy, when it becomes available, will be accessible to the global haemophilia community. As seen with recombinant and other high purity factor concentrates, technological advances do not always lead to improvements in care for the majority of the world's haemophilia patients. In fact, advances in technology can potentially increase the gulf in care if newer processes displace existing manufacturing technologies. A modified vaccine production model should be considered to make gene therapy more widely available to those who presently have inadequate access to treatment.

Blood Coagulation Disorders↗
PubMed1995

Systemic delivery of a recombinant protein by genetically modified mesothelial cells reseeded on the parietal peritoneal surface.

To evaluate the ability of genetically modified peritoneal mesothelial cells to deliver recombinant proteins to the systemic circulation, we used our previously described mesothelial cell-based ex vivo gene therapy strategy. Rat primary peritoneal mesothelial cells, isolated from parietal peritoneum by enzymatic digestion, were stably transfected (using strontium phosphate DNA co-precipitation) with the plasmid pSVTKgh to express a secreted reporter gene product, human growth hormone (hgh). Such hgh-secreting mesothelial cells were reseeded on the denuded peritoneal surface of syngeneic recipients and delivery of the reporter gene product to the systemic circulation was monitored by analysis of serum samples for the presence of hgh at various times after mesothelial cell implantation. Polymerase chain reaction (PCR) analysis demonstrated that the hgh-transfected mesothelial cells repopulated the denuded areas and remained attached there for at least 12 weeks. Moreover, these genetically modified mesothelial cells continued to express the reporter gene product in vivo and secreted hgh in sufficient quantity to be detected in the systemic circulation (ie statistically significant amounts of hgh could be measured in the serum of cyclosporine A-treated rats for at least 2 months; Mann-Whitney test, P < 0.05). Our results demonstrate the successful, sustained, systemic delivery of a recombinant protein by genetically modified peritoneal mesothelial cells following their reattachment to the peritoneal surface, and suggest the potential of ex vivo mesothelial cell-mediated gene therapy for the treatment of inherited or acquired disorders requiring delivery of therapeutic proteins to the circulation.

Animals↗
PubMed2004

Signal sequence analysis of expressed sequence tags from the nematode Nippostrongylus brasiliensis and the evolution of secreted proteins in parasites.

BACKGROUND: Parasitism is a highly successful mode of life and one that requires suites of gene adaptations to permit survival within a potentially hostile host. Among such adaptations is the secretion of proteins capable of modifying or manipulating the host environment. Nippostrongylus brasiliensis is a well-studied model nematode parasite of rodents, which secretes products known to modulate host immunity. RESULTS: Taking a genomic approach to characterize potential secreted products, we analyzed expressed sequence tag (EST) sequences for putative amino-terminal secretory signals. We sequenced ESTs from a cDNA library constructed by oligo-capping to select full-length cDNAs, as well as from conventional cDNA libraries. SignalP analysis was applied to predicted open reading frames, to identify potential signal peptides and anchors. Among 1,234 ESTs, 197 (~16%) contain predicted 5' signal sequences, with 176 classified as conventional signal peptides and 21 as signal anchors. ESTs cluster into 742 distinct genes, of which 135 (18%) bear predicted signal-sequence coding regions. Comparisons of clusters with homologs from Caenorhabditis elegans and more distantly related organisms reveal that the majority (65% at P < e-10) of signal peptide-bearing sequences from N. brasiliensis show no similarity to previously reported genes, and less than 10% align to conserved genes recorded outside the phylum Nematoda. Of all novel sequences identified, 32% contained predicted signal peptides, whereas this was the case for only 3.4% of conserved genes with sequence homologies beyond the Nematoda. CONCLUSIONS: These results indicate that secreted proteins may be undergoing accelerated evolution, either because of relaxed functional constraints, or in response to stronger selective pressure from host immunity.

Animals↗
PubMed1994

Homologous recombination and gene replacement at the dihydrofolate reductase-thymidylate synthase locus in Toxoplasma gondii.

To investigate the feasibility of genomic transgene expression and gene targeting in Toxoplasma gondii, parasites have been transfected with constructs differing in the length of contiguous genomic sequence spanning the dihydrofolate reductase-thymidylate synthase (DHFR-TS) gene. We have previously reported that vectors derived from a DHFR-TS cDNA 'minigene' containing mutations in the DHFR coding sequence confer pyrimethamine resistance to transfected parasites (Donald and Roos, 1993). Stably resistant parasite clones arise at high frequency, generally by virtue of transgene integration into parasite chromosomes at locations scattered throughout the genome. In contrast, using a vector which contains 8 kb of contiguous genomic sequence (vs. < 2 kb for the cDNA-derived vectors), approximately half of the integration events occur by homologous recombination. Homologous recombination appears to occur at even higher frequency when a 16 kb genomic clone is used. Circular plasmids were more efficient than linearized molecules at producing homologous recombination in this system, integrating by reciprocal crossing-over to produce a duplication of the DHFR-TS locus. Double crossing-over (or gene conversion) was also observed at low frequency, resulting in complete allelic replacement in this haploid stage of the parasite. The ability to produce either homologous or non-homologous recombinants, by the selection of appropriate transformation constructs, has considerable genetic potential.

Alleles↗