Cloning of human adenosine deaminase cDNA and expression in mouse cells

Gene therapy for severe combined immunodeficiencies and beyond

This review describes how gene therapy of severe combined immunodeficiency became a reality, primarily based on the expected selective advantage conferred by transduction of hematopoietic progenitor cells. Thus, it resulted in a progressive extension to the treatment of other primary immunodeficiencies.

Ex vivo retrovirally mediated gene therapy has been shown within the last 20 yr to correct the T cell immunodeficiency caused by γc-deficiency (SCID X1) and adenosine deaminase (ADA) deficiency. The rationale was brought up by the observation of the revertant of SCIDX1 and ADA deficiency as a kind of natural gene therapy. Nevertheless, the first attempts of gene therapy for SCID X1 were associated with insertional mutagenesis causing leukemia, because the viral enhancer induced transactivation of oncogenes. Removal of this element and use of a promoter instead led to safer but still efficacious gene therapy. It was observed that a fully diversified T cell repertoire could be generated by a limited set (<1,000) of progenitor cells. Further advances in gene transfer technology, including the use of lentiviral vectors, has led to success in the treatment of Wiskott–Aldrich syndrome, while further applications are pending. Genome editing of the mutated gene may be envisaged as an alternative strategy to treat SCID diseases.

Development of gene therapy: potential in severe combined immunodeficiency due to adenosine deaminase deficiency

The history of stem cell gene therapy is strongly linked to the development of gene therapy for severe combined immunodeficiencies (SCID) and especially adenosine deaminase (ADA)-deficient SCID. Here we discuss the developments achieved in over two decades of clinical and laboratory research that led to the establishment of a protocol for the autologous transplant of retroviral vector-mediated gene-modified hematopoietic stem cells, which has proved to be both successful and, to date, safe. Patients in trials in three different countries have shown long-term immunological and metabolic correction. Nevertheless, improvements to the safety profile of viral vectors are underway and will undoubtedly reinforce the position of stem cell gene therapy as a treatment option for ADA-SCID.

MolecularDefects inHumanSevereCombinedImmunodeficiency andApproaches toImmuneReconstitution

Mutations in nine different genes have been found to cause the human severe combined immunodeficiency syndrome. The products of three of the genes--IL-2RG, Jak3, and IL-7R alpha--are components of cytokine receptors, and the products of three more-RAG1, RAG2, and Artemis-are essential for effecting antigen receptor gene rearrangement. Additionally, a deficiency of CD3 delta, a component of the T-cell antigen receptor, results in a near absence of circulating mature CD3+ T cells and a complete lack of gamma/delta T cells. Adenosine deaminase deficiency results in toxic accumulations of metabolites that cause T cell apoptosis. Finally, a deficiency of CD45, a critical regulator of signaling thresholds in immune cells, also causes SCID. Approaches to immune reconstitution have included bone marrow transplantation and gene therapy. Bone marrow transplantation, both HLA identical unfractionated and T cell-depleted HLA haploidentical, has been very successful in effecting immune reconstitution if done in the first 3.5 months of life and without pretransplant chemotherapy. Gene therapy was highly successful in nine infants with X-linked SCID, but the trials have been placed on hold due to the development of a leukemic process in two of the children because of insertional oncogenesis.

Gene therapy of severe combined immunodeficiencies

The concept that the outcome of a devastating disease can be modified by inserting a transgene into abnormal cells is appealing. However, the gene-transfer technologies that are available at present have limited the success of gene therapy so far. Nevertheless, severe combined immunodeficiencies are a useful model, because gene transfer can confer a selective advantage to transduced cells. In this way, a proof of concept for gene therapy has been provided.

Adenosine deaminase: functional implications and different classes of inhibitors

Adenosine deaminase (ADA) is an enzyme of the purine metabolism which catalyzes the irreversible deamination of adenosine and deoxyadenosine to inosine and deoxyinosine, respectively. This ubiquitous enzyme has been found in a wide variety of microorganisms, plants, and invertebrates. In addition, it is present in all mammalian cells that play a central role in the differentiation and maturation of the lymphoid system. However, despite a number of studies performed to date, the physiological role played by ADA in the different tissues is not clear. Inherited ADA deficiency causes severe combined immunodeficiency disease (ADA-SCID), in which both B-cell and T-cell development is impaired. ADA-SCID has been the first disorder to be treated by gene therapy, using polyethylene glycol-modified bovine ADA (PEG-ADA). Conversely, there are several diseases in which the level of ADA is above normal. A number of ADA inhibitors have been designed and synthesized, classified as ground-state and transition-state inhibitors. They may be used to mimic the genetic deficiency of the enzyme, in lymphoproliferative disorders or immunosuppressive therapy (i.e., in graft rejection), to potentiate the effect of antileukemic or antiviral nucleosides, and, together with adenosine kinase, to reduce breakdown of adenosine in inflammation, hypertension, and ischemic injury.

Gene therapy for severe combined immunodeficiency caused by adenosine deaminase deficiency: improved retroviral vectors for clinical trials

Severe combined immunodeficiency (SCID) caused by adenosine deaminase deficiency (ADA-) is the first genetic disorder to be treated with gene therapy. Since 1990 when the first trial started for 2 patients with ADA- SCID, five clinical trials enrolling 11 patients have been conducted with different clinical approaches and the results obtained from these trials have recently been reported. According to these reports, T cell-directed gene transfer was useful in the treatment of ADA- SCID whereas the retroviral-mediated gene transfer to hematopoietic stem cells was insufficient for achievement of clinical benefits. This chapter reviews several crucial problems inherent in the current retroviral technology based on the clinical data observed in these pioneering ADA gene therapy trials and presents our new retroviral vector system for the next stem cell gene therapy.

Adenosine deaminase deficiency as the first target disorder in gene therapy

In the past decade, the advent of gene therapy has been acclaimed as a revolutionary medical intervention, embraced with great enthusiasm. However, recent disappointing results of the considerable clinical trials have also clearly demonstrated that such an initial expectation was an overestimation of gene therapy. There are only a few successful cases despite the 3000 patients who have been treated with various forms of gene therapy. Gene therapy for severe combined immunodeficiency (SCID) caused by adenosine deaminase (ADA) deficiency is one of the few such cases where results have been promising. In particular, peripheral T-lymphocytes-directed gene therapy provides further immunological improvements for patients with ADA-SCID receiving the PEG-ADA treatment whereas gene therapy targeting haematopoietic stem cell has so far proved insufficient for clinical benefits. This report will review crucial problems elucidated in the past five clinical trials for ADA-SCID and gives an outline of the next generation of stem cell gene therapy in Japan.

Preclinical aspects of lamivudine and famciclovir against hepatitis B virus

Chemotherapy for chronic hepatitis B virus (HBV) infection is inherently difficult for a variety of reasons that are related to unusual features of both HBV replication strategy and host cell metabolism. Previous attempts to treat chronic HBV infection using nucleoside analogues have been almost universally disappointing, but several recently developed nucleoside analogues have been identified as potent, non-toxic inhibitors of HBV replication. These fall into two broad categories: nucleosides having the 'unnatural' L-configuration, and deoxyguanosine analogues with modified-sugar configurations, represented by lamivudine and penciclovir respectively. Both lamivudine and penciclovir (in its orally available form, famciclovir) have progressed to phase III clinical trials against chronic HBV infection, with promising preliminary results. However, chemotherapy for chronic HBV is necessarily long term, which increases the risks for development of viral resistance and cumulative toxicity. Such risks might be minimized by the use of appropriate drug combinations, rational selection of which requires knowledge of the pharmacokinetics and mechanisms of action of the individual agents. An appreciation of cellular deoxynucleoside metabolism and its regulation, the complexities of which are still emerging, is an indispensable aid to understanding the biological activities of deoxynucleoside analogues. The modes of action of lamividine and penciclovir, and how these two deoxynucleoside analogues may interact in vitro and in vivo as inhibitors of HBV replication, are examined here in the context of cellular deoxynucleoside metabolism.

Successful Peripheral T-Lymphocyte–Directed Gene Transfer for a Patient With Severe Combined Immune Deficiency Caused by Adenosine Deaminase Deficiency

Ten patients with adenosine deaminase deficiency (ADA−) have been enrolled in gene therapy clinical trials since the first patient was treated in September 1990. We describe a Japanese ADA− severe combined immune deficiency (SCID) patient who has received periodic infusions of genetically modified autologous T lymphocytes transduced with the human ADA cDNA containing retroviral vector LASN. The percentage of peripheral blood lymphocytes carrying the transduced ADA gene has remained stable at 10% to 20% during the 12 months since the fourth infusion. ADA enzyme activity in the patient's circulating T cells, which was only marginally detected before gene transfer, increased to levels comparable to those of a heterozygous carrier individual and was associated with increased T-lymphocyte counts and improvement of the patient's immune function. The results obtained in this trial are in agreement with previously published observations and support the usefulness of T lymphocyte-directed gene transfer in the treatment of ADA−SCID.

Successful Peripheral T-Lymphocyte–Directed Gene Transfer for a Patient With Severe Combined Immune Deficiency Caused by Adenosine Deaminase Deficiency

Ten patients with adenosine deaminase deficiency (ADA−) have been enrolled in gene therapy clinical trials since the first patient was treated in September 1990. We describe a Japanese ADA− severe combined immune deficiency (SCID) patient who has received periodic infusions of genetically modified autologous T lymphocytes transduced with the human ADA cDNA containing retroviral vector LASN. The percentage of peripheral blood lymphocytes carrying the transduced ADA gene has remained stable at 10% to 20% during the 12 months since the fourth infusion. ADA enzyme activity in the patient's circulating T cells, which was only marginally detected before gene transfer, increased to levels comparable to those of a heterozygous carrier individual and was associated with increased T-lymphocyte counts and improvement of the patient's immune function. The results obtained in this trial are in agreement with previously published observations and support the usefulness of T lymphocyte-directed gene transfer in the treatment of ADA−SCID.

Cell surface adenosine deaminase: much more than an ectoenzyme

During the last 10 years, adenosine deaminase (ADA), an enzyme considered to be cytosolic, has been found on the cell surface of many cells, therefore it can be considered an ectoenzyme. EctoADA, which seems to be identical to intracellular ADA and has a globular structure, does not interact with membranes but with membrane proteins. Two of these cell surface receptors for ectoADA have been identified: CD26 and A1 adenosine receptors (A1R). Apart from degradation of extracellular adenosine another functional role of ectoADA has been assigned. EctoADA is able to transmit signals when interacting with either CD26 or A1R. In this way, it acts as a co-stimulatory molecule which facilitates a variety of specific signalling events in different cell types. The heterogeneous distribution of the enzyme in the nervous system indicates that ectoADA may be a neuroregulatory molecule. On the other hand, ectoADA might act as a bridge between two different cells thus raising the possibility that it may be important for the development of the nervous system.

Construction of eukaryotic expression vector pBlacz and its expression both in vitro and in vivo

A novel eukaryotic expression vector pBlacZ was constructed, which was transfected into the cell lines of NIH/3T3, COS-1, CHO and the primary culture of murine dermatic fibroblasts in vitro, and also into the murine subcutaneous layer and skeletal muscles of rats in vivo. It was detected that the gene expression vector could encode the E. Coli beta-galactosidase effectively in all these histocytes. The results suggested that pBlacZ, as a novel expression vector, might have certain value of application.

Five missense mutations at the adenosine deaminase locus (ADA) detected by altered restriction fragments and their frequency in ADA - patients with severe combined immunodeficiency (ADA - SCID)

Severe combined immunodeficiency (SCID) is a heterogeneous syndrome, due to X-linked and autosomal recessive defects. A significant proportion of the autosomal recessive forms of SCID are due to mutations at the adenosine deaminase (ADA) locus. Nine different mutations at the ADA locus, including 7 missense point mutations, have been reported in children with ADA-SCID. We could detect 5 of the 7 missense mutations associated with ADA-SCID by alterations in restriction fragments utilizing standard restriction digestion of genomic DNA and hybridization of radiolabelled ADA genomic probes to Southern transfers. We additionally developed more rapid nonradioactive methods employing digestion of genomic DNA amplified by PCR that also detected all 5 mutations. Using these methods, we have examined a sample of 45 ADA-SCID chromosomes and report that these 5 missense mutations account for one third of the ADA--chromosomes studied, with 2 mutations being relatively common.

Construction and expression of an adenosine deaminase::lacZ fusion gene

A eukaryotic expression vector was constructed in which the coding nucleotide sequences (ADA) of human adenosine deaminase (ADA) were fused in frame with the coding sequences of the bacterial gene lacZ encoding beta-galactosidase (beta Gal). This ADA::lacZ fusion gene was anticipated to encode a hybrid protein that has retained the biological functions of both proteins. Transfection of mammalian cells with the fusion gene resulted in the synthesis of both ADA and beta Gal. Cells expressing the gene could therefore be detected with the histochemical staining procedure that relies on the conversion of the indicator, XGal, by beta Gal. In addition, the transfected cells could be sorted on a fluorescence-activated cell sorter with the use of a vital staining procedure described for the selection of beta Gal-producing cells. Cell lines that harbored the fusion gene were tested for ADA overexpression by exposing them to the cytotoxic adenosine analog 9-beta-D-xylofuranosyl adenine (Xyl-A), in the presence of the ADA inhibitor deoxycoformycin (dCF). Resistance to Xyl-A/dCF was observed in the lines carrying ADA::lacZ and moreover, the fraction of cells that survived a stringent selection for ADA overexpression also exhibited significantly increased levels of beta Gal, which confirmed the direct linkage between ADA and lacZ expression. The use of this and other fusion genes might be useful in the development of gene-therapy protocols where they could help to meet the demand for versatile methods to detect and select cells with newly introduced genes.

Isolation and characterization of a variant dihydrofolate reductase cDNA from methotrexate-resistant murine L5178Y cells

Dihydrofolate reductase (DHFR) cDNA sequences were isolated from a methotrexate-resistant mouse L5178Y cell line previously shown to contain methotrexate-resistant dihydrofolate reductase enzyme activity. Specifically-primed reverse transcription products were amplified using the polymerase chain reaction and then cloned into a mammalian expression plasmid. Candidate clones were identified by restriction analysis and then functionally tested by transfection into mouse 3T3 fibroblasts, selecting for methotrexate-resistant colonies. Sequence analysis of the cDNA clones demonstrated the substitution of tryptophan (TGG) in place of the wild-type phenylalanine (TTC) at codon 31. Sequencing of PCR-amplified genomic DNA extracted from the drug-resistant L5178Y cells confirmed the tryptophan codon at position 31. Transfection of mammalian tissue culture cells with expression plasmids containing the trp31 DHFR sequence resulted in substantial methotrexate-resistant colony formation. Recombinant trp31 DHFR enzyme activity expressed in stably-transfected Chinese hamster ovary cells was approximately 20-fold less sensitive to methotrexate inhibition than wild-type mouse DHFR enzyme activity. We conclude that the cloned Trp31 DHFR sequence encodes an enzyme substantially resistant to methotrexate which confers a drug-resistance phenotype to cells in which it is expressed.

Structural and functional analysis of the murine adenosine deaminase gene

We describe the structural and functional analysis of cosmid clones that span the entire murine adenosine deaminase gene. Functional analysis indicated that these clones are capable of encoding murine adenosine deaminase activity when introduced into human cell lines. Structural analysis revealed that the gene consists of 12 exons distributed over approximately 25 kb. The exact size of each exon and the sequence of each exon/intron junction were determined. The results show that the 1056-nucleotide open reading frame for adenosine deaminase extends from exon 1 to exon 11, and that exon 12 contains untranslated sequences only. During the course of these investigations, we discovered that a gene encoding an abundant 1.3-kb polyadenylated transcript overlaps the 3' end of the murine adenosine deaminase gene and is transcribed from the opposite strand.

Sequence of full length cDNA for human S-adenosylhomocysteine hydrolase

Two cDNA clones for human S-adenosylhomocysteine hydrolase isolated from a placental cDNA library were sequenced. Each contained a sequence of 1299 nucleotides encoding a 432 amino-acid protein of MW 47,660. Clone 16-1 contained 47 nucleotides 5' of the coding region, and a 780 nucleotide 3' flanking region terminating in a poly A tail. In addition, a 101 nucleotide unprocessed intron interrupted the coding sequence at nucleotide 854 (second base of codon 285). Clone 20-1 contained 43 nucleotides 5' and 742 nucleotides 3' flanking the uninterrupted coding region. Besides the intron, the clones differed in one position of the coding sequence and at two positions of the 3' non-coding region. The cDNAs for human and rat S-adenosylhomocysteine hydrolase were identical at 91.5% of positions in the coding sequence and showed 70% homology in the 3' non-coding regions. Human and rat S-adenosyl-homocysteine hydrolases are identical at 97% of amino-acid residues, and the Dictyostelium and human enzymes at 75%.

Normal and mutant human adenosine deaminase genes

Adenosine deaminase (ADA) deficiency in humans is one cause of severe combined immunodeficiency. When ADA fails to catalyze the deamination of adenosine and deoxyadenosine, the levels of deoxyadenosine that accumulate are toxic to lymphoid cells. Patients with complete ADA deficiency (e.g., with less than 5% normal ADA catalytic activity) lack both B- and T-lymphocyte function. B-lymphoblast cell lines derived from patients with ADA deficiency have been analyzed at multiple levels. Blot hybridization and S1 nuclease analysis of ADA messenger RNA (mRNA) indicates that the majority of ADA-deficient cell lines have ADA mRNA in the same abundance and size as in normal cell lines. Sequence analysis of ADA cDNAs derived from these mRNAs shows that the majority of mutations are single base changes that alter the amino acid sequence. Expression analysis proves that these point mutations lead to deficiency of ADA catalytic activity. Several cell lines have mutations that alter mRNA transcription or processing. These include a point mutation in one allele of an ADA-deficient cell line that leads to deletion of exon 4 during mRNA splicing. In addition, two cell lines are homozygous for large deletions of the gene that are the result of homologous recombination. Subjects with partial ADA deficiency have undetectable ADA activity in their erythrocytes, variable activity in their lymphoid cells, and normal immunological function. Analysis of the ADA catalytic activity of partially deficient cell lines indicates that the mutations involved affect protein stability. However, the mutations causing partial ADA deficiency are as yet undefined.

Adenosine deaminase gene expression is regulated posttranscriptionally in the nucleus

The housekeeping enzyme adenosine deaminase (ADA) shows a large variation in tissue-specific expression ranging from 1 Iu in red blood cells to 880 Iu in thymocytes. We investigated the acute lymphocytic leukemic cell line Molt-4 (660 Iu ADA/g protein) and the promyelocytic cell line HL-60 (38 Iu ADA/g protein) as a model system to determine the levels at which the tissue-specific expression of ADA is regulated. From our results it can be concluded that the almost 20-fold difference in ADA expression between Molt-4 and HL-60 is the result of differences in the post-transcriptional processing and/or stability of ADA pre-mRNA within the nucleus.

Severe combined immune deficiency due to a homozygous 3.2-kb deletion spanning the promoter and first exon of the adenosine deaminase gene

We have investigated the structural gene for adenosine deaminase (ADA) in a female infant with ADA deficiency associated severe combined immune deficiency (ADA-SCID) disease and her family by DNA restriction-fragment-length analysis. In this family a new ADA-specific restriction-fragment-length variant was detected, which involves a 3.2-kb deletion spanning the ADA promoter as well as the first exon. It was found that the patient, who was born to a consanguineous couple, was homozygous and both her parents and her brother were heterozygous for the deletion. No ADA-specific mRNA could be detected by hybridization in fibroblasts derived from this patient. Thus the patient was established to be homozygous for a true null ADA allele. In the light of the apparently normal development of most tissues except the lymphoid tissue the above finding directly questions the classification of ADA as a 'housekeeping' enzyme.

Adenosine deaminase mRNA expression is regulated posttranscriptionally during differentiation of HL-60 cells

The expression of the enzyme adenosine deaminase (ADA) decreases in the course of the differentiation of the human promyelocytic leukemic cell line HL-60, dependent on the pathway chosen. Differentiation to monocytes as induced by the phorbol ester TPA leads to a 50% reduction of enzyme activity. Induction to myeloid cells as induced by DMSO has a slower and less extensive (75% remaining activity) effect. The reduction in ADA enzymatic activity is preceded by a 5-10 fold reduction in ADA-specific mRNA which is also more rapid during TPA-induced differentiation. In contrast, c-myc mRNA expression is both in TPA- and DMSO-induced differentiation reduced to less then 5% of its initial level within 4h. Nuclear run-on analysis revealed that the reduction of c-myc-mRNA expression during both TPA- and DMSO-induced differentiation could be ascribed to the abolition of transcription of the third exon, whereas no change in the transcription of the first exon could be observed. No change could be detected in the rate of transcription of either the 5' and 3' parts of the ADA gene during TPA- and DMSO-induced differentiation, indicating that the expression of the ADA gene in HL-60 is controlled at a posttranscriptional level.

Expression of human adenosine deaminase in nonhuman primates after retrovirus-mediated gene transfer

Primate bone marrow cells were infected with a retroviral vector carrying the genes for human adenosine deaminase (h-ADA) and bacterial neomycin resistance (neor). The infected cells were infused back into the lethally irradiated donor animals. Several monkeys fully reconstituted and were shown to express the h-ADA and neor genes at low levels in their recirculating hematopoietic cells for short periods of time.

Separation of human from mouse and monkey adenosine deaminase by ion-exchange chromatography following retroviral-mediated gene transfer

A method for the chromatographic separation of human adenosine deaminase (ADA) from murine and monkey ADA is described. This procedure was developed in order to detect the expression of low or moderate levels of human ADA following retroviral-mediated gene transfer of cloned human ADA gene sequences into both mouse and monkey cells. Protein separation was achieved on a Mono Q (HR 5/5) anion-exchange column using the Pharmacia fast protein liquid chromatography system and was found to be a highly reproducible method yielding enzymatically active protein. An increasing linear gradient extending from 0.05 to 0.5 M potassium chloride (pH 7.5) was used to elute the enzyme. Under these conditions, most human ADA does not bind to the column and elutes in the low-salt buffer (0.05 M KCl), while murine ADA elutes at 0.12 M KCl and monkey ADA at 0.15 M KCl. The column fractions were assayed for ADA activity, and the characteristic isozyme banding patterns for human, mouse, and monkey ADA were confirmed by starch gel electrophoresis. This procedure allows the rapid and reproducible separation of human ADA from that of other species and yields partially purified enzymatically active protein.

Efficient retrovirus-mediated transfer and expression of a human adenosine deaminase gene in diploid skin fibroblasts from an adenosine deaminase-deficient human

Skin fibroblasts might be considered suitable recipients for therapeutic genes to cure several human genetic diseases; however, these cells are resistant to gene transfer by most methods. We have studied the ability of retroviral vectors to transfer genes into normal human diploid skin fibroblasts. Retroviruses carrying genes for neomycin or hygromycin B resistance conferred drug resistance to greater than 50% of the human fibroblasts after a single exposure to virus-containing medium. This represents at least a 500-fold increase in efficiency over other methods. Transfer was achieved in the absence of helper virus by using amphotropic retrovirus-packaging cells. A retrovirus vector containing a human adenosine deaminase (ADA) cDNA was constructed and used to infect ADA-fibroblasts from a patient with ADA deficiency. The infected cells produced 12-fold more ADA enzyme than fibroblasts from normal individuals and were able to rapidly metabolize exogenous deoxyadenosine and adenosine, metabolites that accumulate in plasma in ADA-deficient patients and are responsible for the severe combined immunodeficiency in these patients. These experiments indicate the potential of retrovirus-mediated gene transfer into human fibroblasts for gene therapy.

Transient expression of murine interferon-alpha genes in mouse and monkey cells

The coding regions of murine interferon-alpha (IFN-alpha) genes were combined with promoter and 3'-noncoding sequences from other eukaryotic genes. Transient expression of these fusion genes was achieved in monkey COS cells and in a mouse cell line (TOP cells) expressing polyoma virus (Py) large T antigen constitutively. The efficiency of the different expression plasmids was determined by measuring the amount of IFN secreted into the medium. Replacement of the 3'-noncoding region of an IFN-alpha gene by that of the rabbit beta-globin gene resulted in a fourfold higher IFN-alpha production. The SV40 early promoter and the Moloney murine leukemia virus (MoMLV) long terminal repeat (LTR) produced similar amounts of IFN-alpha in COS cells. However, a tandem combination of the SV40 enhancer/early promoter and the mouse metallothionein-I promoter appeared fivefold more active than the SV40 early promoter. In TOP cells the MoMLV LTR was found to be threefold more active than the Py early promoter.

Adenosine deaminase: characterization and expression of a gene with a remarkable promoter

Cosmid clones containing the gene for human adenosine deaminase (ADA) were isolated. The gene is 32 kb long and split into 12 exons. The exact sizes and boundaries of the exon blocks including the transcription start sites were determined. The sequence upstream from this cap site lacks the TATA and CAAT boxes characteristic for eukaryotic promoters. Nevertheless, we have shown in a functional assay that a stretch of 135 bp immediately preceding the cap site has promoter activity. This 135-bp DNA fragment is extremely rich in G/C residues (82%). It contains three inverted repeats that allow the formation of cruciform structures, a 10-bp and a 16-bp direct repeat and five G/C-rich motifs (GGGCGGG) disposed in a strikingly symmetrical fashion. Some of these structural features were also found in the promoter region of other genes and we discuss their possible function. Knowledge of the exact positions of the intron-exon boundaries allowed us to propose models for abnormal RNA processing that occurs in previously investigated ADA-deficient cell lines.

Introduction of sequences encoding functional human adenosine deaminase into mouse cells using a retroviral shuttle system

A retroviral packaging system was used to generate a murine virus carrying sequences encoding human adenosine deaminase (ADA). To this end, human ADA cDNA was inserted into the retroviral shuttle vector pZIP-NeoSV(X)1. This vector provides all of the cis-acting sequences necessary for the efficient packaging and transmission of the viral genome as well as a selectable gene for G418 resistance. Transfection of this recombinant plasmid into cells that provide essential virus products (psi-2 cells) yielded cell lines that stably produced virions carrying the coding sequence of human ADA. We have used these virions to infect NIH3T3 cells, which after 48 h synthesized catalytically active human ADA. Furthermore, G418-resistant cell lines were obtained from the virus-infected NIH3T3 cells that stably produced the human ADA enzyme.