Autoimmune endocrine ophthalmopathy and retrobulbar antigens

Endocrine Ophthalmopathy (EO) is based on autoimmune processes that lead to lymphocyte infiltration of the retrobulbar space. In this study, antigenic character of retrobulbar adipose, connective and muscle tissue as well as of cultured fibroblasts and myoblasts were examined. Samples were obtained from EO patients (n = 13, 8 fem., age 26-82 years, median 47 years) undergoing orbital decompression surgery. Retrobulbar and abdominal tissue from 7 controls (4 fem., 48 - 74 y) was investigated, too. Tissues were homogenized and the proteins were separated by SDS-PAGE according to molecular weight. In order to recover the separated proteins in soluble form, an electroelution technique was employed. Twenty-two separated soluble protein fractions were used as antigenic stimuli for autologous peripheral blood mononuclear cells (PBMC) separated by Ficoll gradient centrifugation. Subsequently, the proliferation of T cells was measured by [3H]-thymidine uptake. A marked T cell response to protein fractions with molecular weight of 6 - 10 kD and 19 - 26 kD was detected (p < 0.001). These autoantigens were found readily reproducible in adipose tissue in 8 out of 9 EO patients, stimulation index (SI) to antigen 6 - 10 kD 29 +/- 4.6 (mean +/- SEM); 19 - 26 kD 5 +/- 1.4 and in 3 out of 4 patients using retrobulbar eye muscle tissue (SI: 6 - 10 kD 23 +/- 4.2; 19 - 26 kD 6 +/- 2). Using the proteins of cultured fibroblasts as antigen, the autologous PBMC from 2 out of 4 tested EO-patients also responded (SI: 7 +/- 2; 4 +/- 1.4). Testing cultured retrobulbar myoblasts of an EO patient, a response to the 19 - 26 kD antigen was found only (SI: 8.0). In response to retrobulbar or muscle proteins, PBMC of 2 controls showed also a higher proliferation rate (SI: 16 +/- 3.5; 13 +/- 2.8), whereas, a response to abdominal adipose or muscle proteins (4 controls) was never found. Thus, two orbital antigens reacting with autologous T cells could be demonstrated and may play an important role in the immunopathogenesis of EO. According to these findings, retrobulbar fibroblast antigens are most likely the main T cell targets.

An improved retroviral vector encoding the herpes simplex virus thymidine kinase gene increases antitumor efficacy in vivo

Brain tumors have been treated clinically by intratumoral injection of cells that produce retroviral vectors encoding the herpes simplex virus thymidine kinase (HSV-TK) gene followed by systemic administration of the antiviral drug ganciclovir. In vitro and in vivo comparisons of two different HSV-TK vector producer clones, which were made using standard transfection and transinfection techniques, were conducted. The two clones, PA317/G1TkSvNa.53 (TK.53) and PA317/G1Tk1SvNa.7 (TK1.7), both used in clinical trials, differ with respect to sequences 3' to the HSV-TK stop codon. The retroviral construct used to generate the TK.53 vector producer cell clone contains an open reading frame encoding a portion of the herpes simplex virus glycoprotein H (gH), a potential polyadenylation site and a putative splice site in this region. These sequences were removed from the retroviral construct used to create the TK1.7 vector producer cell clone. Supernatants obtained from TK1.7 vector producer cells had 100- to 1000-fold higher titers (G418 or HAT) than did corresponding supernatants from TK.53 vector producer cells. A murine subcutaneous tumor model was used to assess transduction efficiency and antitumor activity of each vector producer cell clone. In vivo tumor cell transduction was 13- to 18-fold more efficient with TK1.7 cells as compared with TK.53 cells at equivalent doses. Complete tumor ablation was achieved using a 10-fold lower dose of TK1.7 cells as compared with TK.53 cells. These results suggest that TK1.7 cells combined with ganciclovir may provide a more potent antitumor response in humans.

Transduction of rIL-2 expanded CD4+ and CD8+ ovarian TIL-derived T cell lines with the G1Na (neor) replication-deficient retroviral vector

We have expanded ovarian tumor-infiltrating lymphocytes (TIL) in low concentrations of recombinant interleukin-2 (rIL-2) to conduct intraperitoneal adoptive immunotherapy trials in patients with ovarian cancer. We have previously demonstrated that certain T cell lines and clones derived from ovarian TIL exhibit in vitro autologous tumor-specific cytotoxicity and/or cytokine production (interferon-gamma, tumor necrosis factor-alpha) preferentially in response to autologous tumor cells. Studies that utilize a marker gene introduced into the DNA of TIL can provide useful information on specific uptake or localization of TIL at tumor sites and on the survival of TIL in vivo. We have conducted a series of preclinical experiments in which we have successfully transfected TIL with G1Na, which encodes the gene for neomycin phosphotransferase (neoR). NeoR was detected in at least 10% of CD8+ cells (mean = 10.4%) and between 2.5 and 20% of CD4+ TIL (mean = 8.5%). Transduction of ovarian TIL with G1Na caused no substantial changes to the T cell phenotypes or in vitro cytotoxicities against ovarian and hematogenous tumor cell targets, or on the rIL-2 requirements of TIL for growth and proliferation. In addition, the intact G1Na provirus in transduced TIL cells was rescuable by replication-competent retrovirus and was transferred into the genome of NIH-3T3 fibroblasts, which were rendered resistant to G418. An enhanced polymerase chain reaction (PCR) procedure utilizing detection by ethidium bromide staining was developed. The enhanced PCR detected 1 in 100,000 neoR-labeled cells. Furthermore, detection of the G1Na genome in transduced TIL by in situ hybridization with an RNA probe provided evidence for expression of the neoR gene in transduced TIL. Results obtained from these studies suggest that ovarian TIL-derived T cell lines transduced with the neoR gene post infection with the G1Na retroviral vector can be utilized to examine the in vivo trafficking pattern of ovarian TIL-derived T cell lines expanded in low concentrations of rIL-2 and their survival.

Characterization of a replication-competent retrovirus resulting from recombination of packaging and vector sequences

A replication-competent retrovirus (RCR) was detected by S+/L- assays in three lots of retroviral vector G1Na that were harvested on consecutive days from a single culture of PA317/G1Na producer cells. Using a number of retrovirus-specific primer pairs, it was shown that this RCR was a novel recombinant created by exchanges between G1Na and helper sequence pPAM3 and was not an existing RCR introduced by cross-contamination. Sequencing of clones of DNA amplified in six independent PCR reactions confirmed that the 3' portion of this RCR was composed of retroviral envelope sequences unique to pPAM3 joined to a 3' long terminal repeat (LTR) unique to G1Na. Comparison of pPAM3 and G1Na sequences at the site corresponding to this junction revealed a short segment of patchy nucleotide identity (8 out of 10 bp), suggesting that these helper and vector sequences were joined by homologous recombination. Generation of RCR by exchanges between helper and vector sequences underscores the necessity of testing by efficient methods all retroviral vectors for the presence of RCR before their use. Production of 171 lots (855 liters) of various retroviral vectors that were free of RCR, including 42 lots of G1Na, however, indicates that the combination of exchanges required to generate an RCR are infrequent in this system.

Improved methods of retroviral vector transduction and production for gene therapy

To facilitate clinical applications of retroviral-mediated human gene transfer, retroviral vectors must be of high titer and free of detectable replication-competent retroviruses. The purpose of this study was to optimize methods of retroviral vector production and transduction. Studies were conducted using 22 retroviral vector producer cell lines. Inactivation of retroviral vectors was greater at 37 degrees C than at 32 degrees C. A 5- to 15-fold increase of vectors was produced at 32 degrees C compared to 37 degrees C; the vector increase at 34 degrees C was intermediate. For example, PA317/G1Na.40 grew to a titer of 1.8 x 10(7) cfu/ml at 32 degrees C, compared to 5.0 x 10(5) cfu/ml at 37 degrees C. The production of retroviral vectors was scalable achieving similar results in flasks, roller bottles, or a CellCube Bioreactor. Retroviral vectors were concentrated 15-24 times with vector recovery ranging from 91 to 96% in a Pellicon tangential flow filtration system. Retroviral supernatants were successfully lyophilized. The combination of glucose or sorbitol with gelatin resulted in recovery rates of 64-83%. In studies on transduction by retroviral vectors, centrifugation of vector supernatants onto target cells significantly increased transduction efficiency as measured by vector titration for G418 resistance, fluorescence-activated cell sorting (FACS), and polymerase chain reaction (PCR) analyses. The combination of the above methods has significantly increased the growth and transduction by this vector system.

Cellular immunity and retrobulbar fibroblasts in Graves' ophthalmopathy

In Graves' ophthalmopathy (GO), retrobulbar connective tissue is infiltrated by T cells whose role in the pathogenesis of the disease was investigated in the present work. The aims included firstly to characterize subsets of blood lymphocytes and of sessile lymphocytes cloned from a retroorbital tissue specimen. Second, in counterstimulation assays, the ability of patients' T cells to influence cultivated retrobulbar fibroblasts and in turn the enhancement of lymphocyte proliferation by retrobulbar fibroblasts was investigated. Blood lymphocytes of 16 GO patients and 12 controls isolated by density gradient centrifugation and retrobulbar fibroblasts obtained from orbital decompression were alternately exposed to irradiation of 4000 rad (to suppress proliferation of either cell type), then cocultivated for 48 h. Subsequently, the cells (250,000 lymphocytes, 5000 retrobulbar fibroblasts) were incubated with [3H]thymidine for 24 h. A stimulation index representing the degree of proliferation in comparison with a control was determined. Screening of 62 retrobulbar lymphocyte clones by cell-ELISA revealed a CD4/CD8 ratio of 8.2, contrasting with a normal ratio of 2.1 in peripheral lymphocytes (as obtained by FACS analysis). Incubation of patient's lymphocytes with autologous retrobulbar fibroblasts resulted in a markedly elevated stimulation index (9.7) compared to incubation of lymphocytes with retrobulbar fibroblasts from controls (2.5). In another assay, the degree of stimulation amounted to 2.2 vs. -0.8. Retrobulbar fibroblasts were stimulated twice as much by lymphocytes of the same patient compared to retrobulbar fibroblasts from controls. The autologous reactions demonstrated suggest the presence of autoreactive T lymphocytes in GO patients directed against retrobulbar fibroblasts, which, in addition, are stimulated by these lymphocytes.

Glycosaminoglycan antibodies in endocrine ophthalmopathy

An increased accumulation of glycosaminoglycans (GAG) in the orbita has been reported in endocrine ophthalmopathy (EO). In this study we investigated whether antibodies directed against GAG are present in the sera of 52 EO patients and 47 healthy controls. Three out of 52 patients exhibited low titers of antinuclear antibodies and all patients were negative for antibodies against extractable nuclear antigens. Isotype IgG antibodies were detected by means of an ELISA using hyaluronic acid and dermatan sulfate as antigens. Values were expressed as optical density at 405 nm. In comparison to the control group (0.445, 0.364, 0.588; median, 25th, 75th percentile) significantly (p < 0.001) higher hyaluronic acid antibody levels were found in EO patients (0.626, 0.473, 0.801). Untreated patients had significantly higher hyaluronic acid antibody values compared to controls (0.770 vs 0.437, median, p < 0.005). Thyroid status or antithyroid treatment did not influence binding of hyaluronic acid antibodies. When screening for dermatan sulfate antibodies no such difference could be observed between patients and controls. Thus, the detection of GAG antibodies in EO patients emphasizes the important role of glycosaminoglycans in the pathogenesis of thyroid eye disease.

In vitro synthesis of glycosaminoglycans in endocrine ophthalmopathy

The effects of humoral and cell-mediated immunity on the glycosaminoglycan synthesis of retrobulbar fibroblasts was evaluated in patients with endocrine ophthalmopathy. After incubation with IgG and sera, secreted glycosaminoglycans, radiolabeled with D-6-3H-glucosamine and 35sulfate, were precipitated with cetylpyridinium chloride and ethanol. Hyaluronic acid synthesis of human retrobulbar fibroblasts after incubation with sera and IgG and after co-culture with lymphocytes was assessed by means of a radiometric test. Patients' IgG, compared to controls', accounted for a higher secretory stimulation of porcine retrobulbar fibroblasts (as measured by cetylpyridinium chloride precipitation) after 24 and 48 h. Contrasting with 24 h incubation time, glycosaminoglycan values after 48 h were increased two to threefold. Patients' and controls' sera caused earlier and stronger, yet indistinguishable glycosaminoglycan production. Non-sulfated hyaluronic acid was the preponderant glycosaminoglycan secreted into the media by retrobulbar fibroblasts. As assessed with the radiometric test, incubation with patients' and controls' sera and IgG did not reveal a significant difference in stimulating the hyaluronic synthesis of patients' and controls' retrobulbar fibroblasts. When measuring the hyaluronic acid synthesis of controls' and patients' retrobulbar fibroblasts after co-cultivation of lymphocytes, however, patients' lymphocytes had a marked ability to increase the hyaluronic acid concentration compared to controls' lymphocytes. The hyaluronic acid concentration after incubation of a patient's retrobulbar fibroblasts with autologous lymphocytes was markedly more elevated than the intrinsic hyaluronic acid production of retrobulbar fibroblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Humoral immunity and retrobulbar fibroblasts in endocrine ophthalmopathy

The exact role of retrobulbar fibroblasts in the immunopathogenesis of endocrine ophthalmopathy still remains to be elucidated. To evaluate the in vitro influence of humoral immunity on retrobulbar fibroblasts, the effects of immunoglobulin G as well as of the sera of 50 euthyroid patients with endocrine ophthalmopathy and 30 controls on both porcine and human (patients' and controls') retrobulbar fibroblasts were measured by means of several assays: a colorimetric test involving a heterocyclic chemical, a tetrazolium bromide, was applied to quantify the activity of mitochondrial dehydrogenases; the incorporation of 3H-thymidine was determined as a sensitive parameter for cell proliferation, and an enzyme-linked immunosorbent assay was to reveal specific binding of antibodies to the cells. There was consistently no significant difference between patients' (untreated or treated) and controls' IgG to bind to, to activate or to stimulate the proliferation of porcine and human (patients and controls) retrobulbar fibroblasts. The effects of patients' heat-inactivated and non-inactivated sera were indistinguishable from those of the controls. Incubation of autologous sera, however, led to an activation of retrobulbar fibroblasts which was both higher than the median caused by the patients' group and that engendered by incubation of autologous IgG. Yet, a significant role that humoral immunity might play directly on retrobulbar fibroblasts could not be detected in the experiments conducted in this study.

Glycosaminoglycans in thyroid-associated ophthalmopathy

Glycosaminoglycan (GAG) accumulation in the retrobulbar space of patients with thyroid-associated ophthalmopathy (TAO) has been documented in a number of immunohistochemical studies. In order to gain further insight into possible immunopathogenic mechanisms, the influence of humoral immunity on retrobulbar fibroblasts (RF) as GAG producing cells as well as on GAGs themselves was investigated. The effect of lymphocytes on hyaluronic acid (HA) synthesis of RF as well as in turn the influence of RF on lymphocytes were evaluated. In search of methods which would facilitate management of patients with TAO and allow assessment of disease activity, GAGs were determined in both urine and plasma. Immunoglobulin G (IgG) of patients with TAO were found to markedly stimulate the 3H-GAG secretion of RF. Patients with TAO exhibited significantly greater antibody values directed against HA than controls. Preliminary results concerning the influence of lymphocytes on RF indicate a tendency for patients' lymphocytes to increase the synthesis of HA. Furthermore, these lymphocytes in turn were stimulated more by irradiated autologous RF than by irradiated heterologous RF. Urine and plasma GAG determination proved to be suitable for the routine assessment of disease activity and outcome of therapy. In conclusion, GAGs seem to play an important role in the pathogenesis of the disease and their measurement may provide aid to the endocrinological evaluation of patients with TAO.

A new 440-kD isoform is the major ankyrin in neonatal rat brain

This report describes initial characterization of a 440-kD isoform of brain ankyrin (ankyrinB) representing an alternatively spliced mRNA product of the gene encoding the major isoform of ankyrin in adult human brain (Otto, E., M. Kunimoto, T. McLaughlin, V. Bennett, J. Cell Biology. 114:241-253). Northern and immunoblot analyses indicate that 440-kD ankyrinB includes the spectrin and membrane-binding domains as well as a regulatory domain of the major 220-kD isoform. 440-kD ankyrinB contains, in addition, a sequence of a predicted size of 220 kD which is inserted between the regulatory domain and spectrin/membrane-binding domains. 440-kD ankyrinB has properties expected of a peripherally associated membrane-skeletal protein: it is exclusively present in the particulate fraction of brain homogenates, is extracted with NaOH, and remains associated with Triton-X-100-resistant structures. Expression of 440-kD ankyrinB in rat brain began at birth before other ankyrins could be detected, peaked 10 d after birth, and then decreased progressively to 30% of the maximum in adults. Expression of the 220-kD ankyrinB and ankyrinR (erythroid ankyrin) began approximately 10 d after the 440-kD isoform, increased rapidly between 10 and 15 d after birth, and finally achieved their maximal levels in adults. 440-kD ankyrinB is present in approximately equivalent amounts in all regions of neonatal brain while in adult brain it is present in highest levels in cerebellum and lowest in brain stem. 440-kD ankyrinB was localized by immunofluorescence in regions of neonatal and adult brain containing primarily dendrites and unmyelinated axons. 440-kD ankyrinB thus may play a specialized role in neuronal processes.

Primary structure and domain organization of human alpha and beta adducin

Adducin is a membrane-skeletal protein which is a candidate to promote assembly of a spectrin-actin network in erythrocytes and at sites of cell-cell contact in epithelial tissues. The complete sequence of both subunits of human adducin, alpha (737 amino acids), and beta (726 amino acids) has been deduced by analysis of the cDNAs. The two subunits have strikingly conserved amino acid sequences with 49% identity and 66% similarity, suggesting evolution by gene duplication. Each adducin subunit has three distinct domains: a 39-kD NH2-terminal globular protease-resistant domain, connected by a 9-kD domain to a 33-kD COOH-terminal protease-sensitive tail comprised almost entirely of hydrophilic amino acids. The tail is responsible for the high frictional ratio of adducin noted previously, and was visualized by EM. The head domains of both adducin subunits exhibit a limited sequence similarity with the NH2-terminal actin-binding motif present in members of the spectrin superfamily and actin gelation proteins. The COOH-termini of both subunits contain an identical, highly basic stretch of 22 amino acids with sequence similarity to the MARCKS protein. Predicted sites of phosphorylation by protein kinase C include the COOH-terminus and sites at the junction of the head and tail. Northern blot analysis of mRNA from rat tissues, K562 erythroleukemia cells and reticulocytes has shown that alpha adducin is expressed in all the tissues tested as a single message size of 4 kb. In contrast, beta adducin shows tissue specific variability in size of mRNA and level of expression. A striking divergence between alpha and beta mRNAs was noted in reticulocytes, where alpha adducin mRNA is present in at least 20-fold higher levels than that of beta adducin. The beta subunit thus is a candidate to perform a limiting role in assembly of functional adducin molecules.

Purkinje cell degeneration associated with erythroid ankyrin deficiency in nb/nb mice

Mice homozygous for the nb mutation (Chromosome 8) have a severe hemolytic anemia and develop a psychomotor disorder at 6 mo of age. The nb/nb mice are deficient in erythroid ankyrin (Ank-1) but, until the present study, the role of Ank-1 and of Ank-2 (brain ankyrin) in disease genesis was unknown. In normal erythroid tissues, we show that two major transcripts are expressed from Ank-1, and one of these is also present at high levels in the cerebellum. By in situ hybridization and immunocytochemistry, Ank-1 localizes to the cerebellar Purkinje cells and, to a lesser extent, the granule cells. In nb/nb mice, Ank-1 transcripts are markedly reduced in both erythroid and neural tissue, and nb/nb Purkinje cells and granule cells are nearly devoid of Ank-1. The neurological syndrome appears concurrently with a dramatic loss of Purkinje cells. Ank-2 maps to Chromosome 3 and its expression is unaffected by the nb mutation. We conclude that Ank-1 is specifically required for Purkinje cell stability and, in its absence, Purkinje cell loss and neurological symptoms appear.

Isolation and characterization of cDNAs encoding human brain ankyrins reveal a family of alternatively spliced genes

Ankyrins are a family of membrane-associated proteins that can be divided into two immunologically distinct groups: (a) erythrocyte-related isoforms (ankyrinR) that have polarized distributions in particular cell types; and (b) brain-related isoforms (ankyrinB) that display a broader distribution. In this paper, we report the isolation and sequences of cDNAs related to two ankyrinB isoforms, human brain ankyrin 1 and 2, and show that these isoforms are produced from alternatively spliced mRNAs of a single gene. Human brain ankyrin 1 and 2 share a common NH2-terminus that is similar to human erythrocyte ankyrins, with the most striking conservation occurring between areas composed of a repeated 33-amino acid motif and between areas corresponding to the central portion of the spectrin-binding domain. In contrast, COOH-terminal sequences of brain ankyrin 1 and 2 are distinct from one another and from human erythrocyte ankyrins, and thus are candidates to mediate protein interactions that distinguish these isoforms. The brain ankyrin 2 cDNA sequence includes a stop codon and encodes a polypeptide with a predicted molecular mass of 202 kD, which is similar to the Mr of the major form of ankyrin in adult bovine brain membranes. Moreover, an antibody raised against the conserved NH2-terminal domain of brain ankyrin cross-reacts with a single Mr = 220 kD polypeptide in adult human brain. These results strongly suggest that the amino acid sequence of brain ankyrin 2 determined in this report represents the complete coding sequence of the major form of ankyrin in adult human brain. In contrast, the brain ankyrin 1 cDNAs encode only part of a larger isoform. An immunoreactive polypeptide of Mr = 440 kD, which is evident in brain tissue of young rats, is a candidate to be encoded by brain ankyrin 1 mRNA. The COOH-terminal portion of brain ankyrin 1 includes 15 contiguous copies of a novel 12-amino acid repeat. Analysis of DNA from a panel of human/rodent cell hybrids linked this human brain ankyrin gene to chromosome 4. This result, coupled with previous reports assigning the human erythrocyte ankyrin gene to chromosome 8, demonstrates that human brain and erythrocyte ankyrins are encoded by distinct members of a multigene family.

Specific 33-residue repeat(s) of erythrocyte ankyrin associate with the anion exchanger

Erythrocyte ankyrin contains an 89-kDa domain (residues 2-827) comprised almost entirely of 22 tandem repeats of 33 amino acids which are responsible for the high affinity interaction of ankyrin with the anion exchanger (Davis, L., and Bennett, V. (1990) J. Biol. Chem. 265, 10589-10596). The question of whether the repeats are equivalent with respect to binding to the anion exchanger was addressed using defined regions of erythrocyte and brain ankyrins expressed in bacteria. The conclusion is that the repeats are not interchangeable and that the 44 residues from 722 to 765 are essential for high affinity binding between erythrocyte ankyrin and the anion exchanger. Residues 348-765 were active whereas a polypeptide of the same size (residues 305-721) but missing the 44 residues was not active. The difference between the active and inactive polypeptides was not caused by the degree of folding based on circular dichroism spectra. The 44 residues from 722 to 765 were not sufficient for binding since deletions of residues from 348 to 568 resulted in a 10-fold loss of activity. However, the role of residues 348-568 may be at the level of folding rather than a direct contact since the deleted sequences were not active in the absence of 722-765 and since circular dichroism spectra revealed significant loss of structure in the smaller polypeptides. Further evidence that the 33-residue repeats are not equivalent in ability to bind to the anion exchanger is that a region of human brain ankyrin containing 18 33-residue repeats with 67% overall sequence identity to erythrocyte ankyrin was 8-fold less active than a region of erythrocyte ankyrin containing only 12 repeats. The fact that the anion exchanger binds to certain repeats suggests that the other 33-amino acid repeats could interact with proteins distinct from the anion exchanger and provide ankyrin with the potential for considerable diversity in association with membrane proteins as well as cytoplasmic proteins. Tubulin was identified as one example of a protein that can interact with ankyrin repeats that are not recognized by the anion exchanger.

Regulated expression at high copy number allows production of a growth-inhibitory oncogene product in Drosophila Schneider cells

The Drosophila metallothionein promoter (Mtn) was used to obtain efficient, regulated expression of foreign gene products inserted in high copy numbers into Drosophila melanogaster Schneider 2 cells. An expression unit comprised of a reporter gene [Escherichia coli galactokinase (galK)] fused to the Mtn promoter was stably introduced into Schneider 2 cells in up to several hundred copies per cell in a single transfection-selection event. This system contrasts dramatically with other eukaryotic systems that permit only a few copies of a gene to be stably inserted in a single transfection-selection event. The transfected Drosophila S2 cell lines expressed high levels of both galK mRNA and protein in response to metal induction. Most important, and in contrast to mammalian cells, expression remained fully regulated even at high copy number, with low basal expression maintained in the absence of inducer. This regulated system was used to obtain efficient expression in Drosophila cells of an otherwise lethal or growth-inhibitory gene product, the human H-ras oncogene. The ability to obtain regulated high-level expression of potentially lethal foreign proteins is unique to the Drosophila cell system.

Increased incidence of CAD gene amplification in tumorigenic rat lines as an indicator of genomic instability of neoplastic cells

It has been hypothesized that genomic instability is an important component of tumorigenesis. In an attempt to establish this relationship, we determined the frequencies with which two nontumorigenic and four tumorigenic rat liver epithelial cell lines underwent a particular type of genetic instability, gene amplification. By exposing cells to N-(phosphonoacetyl)-L-aspartate (PALA), a drug which specifically inhibits the aspartate transcarbamylase activity of the multifunctional CAD enzyme and selects for amplification of the CAD gene, we observed a striking parallel between the ability of these cell lines to become resistant to this drug and the ability of these same cells to form tumors after injection into day-old syngeneic rats. Cells of one highly tumorigenic line became resistant to PALA greater than 70 times more often than those of a non-tumorigenic line. Molecular analyses of eight independent PALA-resistant subclones confirmed that, in each case, this resistance was due to amplification of the CAD gene. Thus, our results demonstrate the relationship between tumorigenicity and at least one measure of genomic instability, CAD gene amplification. The method developed in this study provides a quantitative, rapid indicator of tumorigenicity and should prove useful in trying to elucidate the underlying basis of genomic instability in neoplastic cells.

Metallothionein gene duplications and metal tolerance in natural populations of Drosophila melanogaster

A search for duplications of the Drosophila melanogaster metallothionein gene (Mtn) yielded numerous examples of this type of chromosomal rearrangement. These duplications are distributed widely--we found them in samples from four continents, and they are functional--larvae carrying Mtn duplications produce more Mtn RNA and tolerate increased cadmium and copper concentrations. Six different duplication types were characterized by restriction-enzyme analyses using probes from the Mtn region. The restriction maps show that in four cases the sequences, ranging in size between 2.2 and 6.0 kb, are arranged as direct, tandem repeats; in two other cases, this basic pattern is modified by the insertion of a putative transposable element into one of the repeated units. Duplications of the D. melanogaster metallothionein gene such as those that we found in natural populations may represent early stages in the evolution of a gene family.

A DNA segment controlling metal-regulated expression of the Drosophila melanogaster metallothionein gene Mtn

Cloned fragments of DNA including the Drosophila melanogaster metallothionein gene Mtn and different amounts of 5' flanking sequences were introduced into flies by P-element-mediated germ line transformation. Comparison of RNA levels in different transformants revealed that metal-regulated and tissue-specific expression of Mtn requires no more than 373 base pairs upstream of the initiation site of transcription. Transformants having an additional, transcribed copy of Mtn could tolerate increased concentrations of cadmium, indicating that Mtn expression is directly related to this phenotype. In separate experiments, these D. melanogaster promoter sequences were fused to the coding sequences of the herpes simplex virus thymidine kinase (TK) gene. After transfection of this fusion into baby hamster kidney cells, increases in TK activity and accumulation of TK RNA were inducible by metals. A series of 5' and 3' deletions showed that D. melanogaster sequences from -130 to -6 were sufficient to confer metal-regulated expression to the TK gene. The function of the D. melanogaster metallothionein promoter in mammalian cells indicates that the mechanism controlling metal regulation is evolutionarily conserved.

Structure and expression of a tandem duplication of the Drosophila metallothionein gene

A strain of cadmium-resistant Drosophila containing a chromosomal duplication of the metallothionein gene was isolated. This duplication is stably inherited in the absence of selective pressure, and larvae homozygous for it can produce approximately twice as much metallothionein RNA as wild-type larvae. The entire duplication was cloned within a 5.7-kilobase fragment; this fragment contained a direct, tandem repeat of 2.2 kilobases of DNA: 228 bases of 5' flanking DNA, the entire transcription unit, and 1.4 kilobases of 3' flanking sequences. The 3' region of the first repeated unit is joined to the 5' region of the second unit by a 6-base-pair segment we define as the novel joint. This joint forms part of a 10-base-pair inverted repeat of a segment within the 3' region of the first unit. Comparison of the sequences of the 5' and 3' boundaries revealed no extensive regions of similarity at a position corresponding to the novel joint, thus suggesting that a mechanism other than homologous recombination was involved in the origin of this duplication.

Effects of heavy metals on Drosophila larvae and a metallothionein cDNA

Drosophila melanogaster larvae reared on food containing radioactive cadmium retained over 80% of it, mostly in the intestinal epithelium. The majority of this radioactivity was associated with a soluble protein of less than 10,000 molecular weight. Synthesis of this cadmium-binding protein was induced by the metal as demonstrated by incorporation of radioactive cysteine. Most copper ingested by larvae was also found to associate with a low molecular weight, inducible protein, but some of it was found in an insoluble fraction. Zinc was unable to, or very inefficient at, binding or inducing the synthesis of a similar protein. A D. melanogaster cDNA clone was isolated based on its more intense hybridization to copies of RNA sequences from copper-fed larvae than from control larvae. This clone showed strong hybridization to mouse metallothionein-I cDNA at reduced stringency. Its nucleotide sequence includes an open-reading segment which codes for a 40-amino acid protein; this protein was identified as metallothionein based on its similarity to the amino-terminal portion of mammalian and crab metalloproteins. The ten cysteine residues present occur in five pairs of near-vicinal cysteines (Cys-X-Cys). This cDNA sequence hybridized to a 400-nucleotide polyadenylated RNA whose presence in the cells of the alimentary canal of larvae was stimulated by ingestion of cadmium or copper; in other tissues this RNA was present at much lower levels. Mercury, silver, and zinc induced metallothionein to a lesser extent. Whether (any of) the protein(s) discussed above correspond(s) to that coded by this RNA sequence has not yet been determined.

Molecular and cytogenetic characterization of a metallothionein gene of Drosophila

A chromosomal DNA segment containing the metallothionein gene was isolated from a genomic library of Drosophila melanogaster using a previously characterized cDNA of this species as a probe. A segment of 1543 base pair (bp) was sequenced and found to include the cDNA sequence interrupted by one small intron. Several lines of evidence indicate that there is a single copy of the metallothionein gene (Mtn) in Drosophila; any other related genes, if they occur, must be sufficiently different that they are not detectable by our probe, even under hybridization conditions of reduced stringency. According to in situ hybridization and deletion mapping, Mtn is located in the right arm of the third chromosome in region 85E10-15. Within 300 bases upstream of the apparent site of transcription initiation, there are several short intervals very similar to the 12-bp segments considered to be responsible for metal regulation in mammalian systems.