Retroviral-mediated gene transfer and expression of human phenylalanine hydroxylase in primary mouse hepatocytes

Correction in female PKU mice by repeated administration of mPAH cDNA using phiBT1 integration system

Phenylketonuria (PKU) is a metabolic disorder secondary to a hepatic deficiency of phenylalanine hydroxylase (PAH) that predisposes affected children to develop severe and irreversible mental retardation. We have previously reported the complete and permanent correction of the hyperphenylalaninemic and hypopigmentation phenotypes in male, but not female, PKU mice after genome-targeted delivery of murine PAH (mPAH) complementary DNA (cDNA) in a phiBT1 bacteriophage integration system. Here we show that sequential administration of green fluorescent protein (GFP)- and red fluorescent protein (RFP)-expressing cassettes in the phiBT1 integration system led to distinct and non-overlapping populations of green and red fluorescent hepatocytes in vivo. The hyperphenylalaninemic and hypopigmentation phenotypes of female PKU mice were completely corrected after 10 weekly administrations of mPAH cDNA. Importantly, there was no apparent liver pathology in mice even after 10 consecutive administrations of the phiBT1 integration system. The results indicate that repeated administration of transgenes in the phiBT1 integration system can lead to their genome-targeted integration in a diverse population of hepatocytes and result in the elevation of transgene expression levels in a cumulative manner, which can be utilized to overcome insufficient transgene expression owing to low genome integration frequencies in a gene therapy paradigm for metabolic disorders.

Metabolic basis of sexual dimorphism in PKU mice after genome-targeted PAH gene therapy

We have previously reported a transgene delivery system based on phiBT1 bacteriophage integrase that results in targeted insertion of transgenes into mammalian genomes, and its use in the delivery of murine phenylalanine hydroxylase (PAH) complementary DNA (cDNA) into the hepatocytes of male phenylketonuria (PKU) mice, leading to a complete and permanent correction of their hyperphenylalaninemic phenotype. In this study, we report only partial phenotypic correction in female PKU mice, even though hepatic PAH activities in both sexes after gene treatment were similar. Daily injections of tetrahydrobiopterin (BH4), an essential co-factor for phenylalanine hydroxylation, in the gene-treated females led to complete correction of their PKU phenotype. After gonadectomy, serum phenylalanine levels in the gene-treated females were reduced to normal, whereas those in the gene-treated males remained unchanged. The sterile gene-treated PKU mice were subjected to daily sex hormone injections. Whereas the estradiol-treated sterile males developed hyperphenylalaninemia, the dihydrotestosterone-treated sterile females remained normal phenylalaninemic. The results indicate that it is estrogen that suppresses the steady-state levels of BH4 in mouse hepatocytes that became limiting, which is the underlying mechanism for the observed sexual dimorphism in PKU mice after PAH gene treatment. Livers of the PAH gene-corrected PKU mice also appeared normal and without apparent pathologies.

State-of-the-art 2003 on PKU gene therapy

Phenylketonuria (or PKU) is a well-known and widespread genetic disease for which many countries perform newborn screening, and life-long dietary restriction is still the ultimate and effective therapy. However, the diet is complicated, unpalatable, and expensive. The long-term effects of diet discontinuation in adults, except for the serious adverse effects of maternal hyperphenylalaninemia upon the developing fetus, have not been systematically studied, but cognitive decline and neurologic abnormalities have been anecdotally reported. Thus, alternative approaches for PKU therapy, including gene therapy, must be further explored. Here we summarize past present nonviral and viral gene transfer approaches, both in vitro studies and preclinical animal trials, to delivering the PAH gene into liver or other organs as potential alternatives to life-long phenylalanine-restricted dietary therapy.

Liver-directed gene transfer vectors

The ultimate goal of liver-directed gene therapy for genetic diseases is the stable expression of a therapeutic transgene in a significant proportion of hepatocytes. This article considers the various liver-directed gene transfer procedures studied so far. Performances and limitations of currently available vector systems are discussed with respect to their clinical relevance. Although some improvements have been reported, naked DNA and nonviral gene transfer vectors induce transient expression in only a limited number of cells. Clinical applications of retrovirus-mediated gene transfer are hampered by the need to induce hepatocyte division. First-generation adenovirus vectors are highly efficient; however, they induce an immune response leading to the rapid rejection of transduced cells. Promising new vector systems have emerged, including gutless adenovirus vectors, adeno-associated vectors, and lentivirus vectors. However, these systems are still poorly documented and their relevance to liver-directed gene therapy must be confirmed.

Gene therapy for phenylketonuria

Classical phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of hepatic phenylalanine hydroxylase (PAH). Limitations of the current dietary treatment for PKU have led to the development of potential treatments based on somatic gene transfer. Three different vector systems have been examined. Vectors derived from a recombinant retrovirus or a DNA/protein complex can efficiently transduce the PAH cDNA into PAH-deficient hepatocytes in vitro, but the application of these vector systems is presently limited by their low transduction efficiency in vivo. In contrast, a vector derived from a recombinant adenovirus can restore 10%-80% of normal hepatic PAH activity into PAH-deficient mice, which completely normalizes serum phenylalanine levels. This treatment is transient and cannot be effectively re-administered due to the presence of neutralizing antibodies directed against the recombinant adenoviral vector. However, these findings suggest that PKU can be completely corrected by somatic gene therapy, and provide some direction for the future development of adenoviral vectors.

Regulated expression of artificial chimeric genes contained in retroviral vectors: implications for virus-directed enzyme prodrug therapy (VDEPT) and other gene therapy applications

Replication-defective retroviral vectors were created that contained chimeric genes composed of either the albumin (ALB) or the alpha-fetoprotein (AFP) transcriptional regulatory sequences linked to the coding domain of the thymidine kinase gene from Varicella zoster virus (VZV TK). These viruses were used to infect the human hepatoblastoma cell line, HepG2. Subsequent to infection, the infected cells were single-cell cloned. The level of expression of VZV TK from the chimeric genes correlated with the level of endogenous expression of ALB or AFP in most clones, indicating that the transcription of the chimeric VZV TK gene is controlled in a similar manner to the endogenous ALB or AFP genes, and that sites of viral integration are less important to overall gene expression. Most importantly, as the expression of the endogenous ALB gene was modified, so was expression of VZV TK from the ALB/VZV TK chimeric gene. This demonstrates that retroviruses can deliver a chimeric gene containing tissue-specific transcriptional regulatory sequences that can respond to endogenous cell regulatory signals resulting in regulated gene expression.

Somatic gene therapy for phenylketonuria and other hepatic deficiencies

Gene therapy is the delivery of genetic material to specific cell types of an organism to alter its physiology or function. This technology is being explored as a means of treating diseases caused by deficiencies of hepatic gene products. The two diseases being used as models for hepatic gene therapy are classical phenylketonuria (PKU) and haemophilia B. Vectors derived from adenoviruses can be used to completely correct these diseases in animal models. The phenotypic correction generated in these studies is transient, and cannot be duplicated by vector readministration. The transient nature of transgene expression results from the destruction of the virally-transduced cells by a cellular immune response directed against the late viral gene products that are also expressed in the target cells. The inability to repeatedly administer virus is caused by a humoral immune response directed against viral proteins present at the time of infusion. If the host immune response is suppressed, transgene expression can persist for 6 months or more. These findings suggest that host immunomodulation in combination with further modification of the adenoviral vector to reduce or eliminate late viral gene expression may permit long-term expression of potentially therapeutic gene products in mammalian liver.

Gene therapy: here to stay

Advances in biotechnology have brought gene therapy to the forefront of medical research. The feasibility of gene transfer was first demonstrated in experiments using tumour viruses. This led to the development of a variety of viral and nonviral methods for the genetic modification of somatic cells. Two main approaches emerged: in-vivo modification, in which gene transfer vehicles are delivered directly into patients, and ex-vivo manipulation, in which cells from the patient are grown in culture, genetically modified and then returned to the patient. In 1990, shortly after the safety of retrovirus-mediated gene transfer was demonstrated in patients with malignant melanoma, the first clinical trial of gene therapy was initiated for adenosine deaminase deficiency. Since then, the number of clinical protocols initiated worldwide has increased exponentially. Although some clinical trials now in progress are concerned with relatively rare inborn errors of metabolism, most are concerned with more commonly encountered cancers and infectious diseases. Preliminary results suggest that by the turn of the century the dream of treating diseases by replacing or supplementing the products of defective genes or introducing novel therapeutic genes will become a reality.

Efficient retroviral-mediated gene transfer into primary culture of murine and human hepatocytes: expression of the LDL receptor

The ex vivo approach to hepatic gene therapy involves several steps, which include the isolation and culture of hepatocytes, followed by their transduction with a retrovirus. Subsequently, autologous hepatocytes are transplanted. The number of hepatocytes that can be transduced by retroviruses bearing the therapeutic gene is one of the limiting steps that can impair the success of this strategy. We presently describe an experimental approach that leads to improved transduction efficiency in mouse and human hepatocytes in vitro. By using a recombinant retrovirus bearing the Escherichia coli beta-galactosidase gene, we show that addition of growth factors to the cells, namely human hepatocyte growth factor (HGF), allows marked increase in the transduction efficiency in mouse (up to 80%) and human (40%) hepatocytes. Familial hypercholesterolemia (FH) is due to mutation in the low-density lipoprotein (LDL) receptor gene and results in a deficiency in LDL receptors. Transduction of the human LDL receptor cDNA under the transcriptional control of the L-type pyruvate kinase promoter-activator into mouse hepatocytes led to an elevated tissue-specific expression of the human protein. These results suggest that the ex vivo approach remains a promising alternative for hepatic gene therapy.

Gene therapy for phenylketonuria

Classical phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of hepatic phenylalanine hydroxylase (PAH). Three different vector systems have been developed to examine the potential of somatic gene therapy for the treatment of PKU. Recombinant retroviral vectors and DNA/protein complexes can efficiently transduce PAH-deficient hepatocytes in vitro, but their present application is limited by their low transduction efficiency in vivo. In contrast, infusion of a recombinant adenoviral vector expressing the human PAH cDNA into the portal circulation of PAH-deficient mice restores 10-80% of normal hepatic PAH activity and completely normalizes serum phenylalanine levels. At present, this effect is transient and re-administration has no further effect. However, this result suggests that PKU can be completely corrected by somatic gene therapy as more persistent vectors are developed.

Retrovirus-mediated gene transfer and expression of human ornithine delta-aminotransferase into embryonic fibroblasts

Ornithine delta aminotransferase (OAT) is a nuclear-encoded mitochondrial matrix enzyme that catalyzes the reversible transamination of ornithine to glutamate semialdehyde. In humans, genetic deficiency of OAT results in gyrate atrophy of the choroid and retina, a blinding chorioretinal degeneration usually beginning in late childhood. This disorder has been shown to be autosomal recessive, and is often caused by missense, nonsense, and/or frameshift mutations in the OAT gene. With the view of applying gene therapy, a Moloney murine leukemia virus (MoMLV)-based recombinant retrovirus vector has been constructed. The human OAT cDNA was placed under the control of the enhancer-promoter regulatory elements derived from the MoMLV long terminal repeat (LTR). The construct was transfected into the retroviral packaging cell lines GP + E - 86 and psi CRIP to produce virus particles. Supernatant from these OAT retrovirus producer cell lines were used to transduce mouse C57B1/6 embryonal fibroblasts. We showed that the recombinant retrovirus transfers the OAT gene to the recipient cells, which produce an OAT RNA transcript when analyzed by Northern blot. Western blot analysis and enzymatic assays confirmed the presence of an OAT polypeptide that has a high enzymatic activity in the transduced cell lines, even after a long period of time in vitro.

Hepatic gene therapy: efficient gene delivery and expression in primary hepatocytes utilizing a conjugated adenovirus-DNA complex

Receptor-mediated endocytosis is an effective method for gene delivery into target cells. We have previously shown that DNA molecules complexed with asialoglycoprotein can be efficiently endocytosed by primary hepatocytes and the internalized DNA can be released from endosomes by the use of a replication-defective adenovirus. Because the DNA and virus enter target cells independently, activity enhancement requires high concentrations of adenoviral particles. In this study, adenoviral particles were chemically conjugated to poly(L-lysine) and bound ionically to DNA molecules. Quantitative delivery to primary hepatocytes was achieved with significantly reduced viral titer when the asialoorosomucoid-poly(L-lysine) conjugate was included in the complex. The conjugated adenovirus was used to deliver a DNA vector containing canine factor IX to mouse hepatocytes, resulting in the expression of significant concentrations of canine factor IX in the culture medium. The results suggest that receptor-mediated endocytosis coupled with an efficient endosomal lysis vector should permit the application of targeted and efficient gene delivery into the liver for gene therapy of hepatic deficiencies.

Cell transplantation in liver-directed gene therapy

Somatic cell gene therapy is a new field of biomedical research that encompasses a variety of traditional basic research and clinical disciplines. This new approach to therapeutics has the potential to prevent, treat, or cure a variety of inherited and acquired diseases. Two divergent strategies of hepatocyte transplantation are being employed in animal models and clinical trials in an attempt to correct genetic deficiencies. Allogeneic hepatocyte transplantation has two main advantages over autologous cell transplantation. First, invasive surgical procedures are not required in the recipient. Second, allogeneic cells can be administered repetitively, so that multiple harvests are not necessary. The major drawbacks to allogeneic hepatocyte transplants are rejection and the risks of immunosuppression. Although there is no clinical experience with the treatment of genetic disease by allogeneic hepatocyte transplantation, a variety of animal models have been characterized, including the Gunn rat (UDP-glucuronosyl transferase deficient), the Nagase analbuminemic rat, and the Watanabe heritable hyperlipidemic rabbit (LDL receptor deficient). The use of genetically corrected autologous cells represents a different and more elegant approach to the correction of inherited disease. A segment of liver is harvested from the affected individual. Recombinant retroviruses are used to transduce normal genes--with a variety of promoter/enhancer constructs--into the patients own hepatocytes. The genetically corrected hepatocytes are then transplanted back into the patient. This approach, known as ex vivo gene therapy, eliminates the risk of rejection and the need for immunosuppression. The safety and efficacy of this approach has been proven in a variety of preclinical animals models, including Watanabe rabbits, dogs, and Papio spp. A clinical trial for the treatment of familial hypercholesterolemia is currently in progress. A number of approaches for the reintroduction of hepatocytes into the recipient have been proposed, including catheter-mediated delivery into the inferior mesenteric vein, the umbilical vein, or into the spleen. Candidate diseases, which are likely to result in the first clinical trials include familial hypercholesterolemia, ornithine transcarbamylase deficiency, Crigler-Najjar syndrome, alpha 1-antitrypsin deficiency, and phenylketonuria.

Transduction of primary human hepatocytes with amphotropic and xenotropic retroviral vectors

Experiments in animal models suggest that it is feasible to consider hepatic gene therapy using a strategy in which hepatocytes would be isolated by partial hepatectomy, transduced with recombinant retroviral vectors containing genes of therapeutic importance, and then transplanted back into the patient by autologous hepatocellular transplantation. The application of this strategy in clinical trials will require adapting these methods to human cells. We describe the transduction of primary human hepatocytes with two forms of retroviral vectors: amphotropic vectors, which have been used previously in clinical trials, and xenotropic vectors, which have a different host range. Human hepatocytes were harvested from organs preserved in Belzer's solution and were cultivated in a serum-free, tyrosine-free, hormonally defined medium. These cells proliferated for 3-5 days in culture, exhibited characteristic hepatocyte morphology, and expressed liver-specific functions, including phenylalanine hydroxylase, alpha 1-antitrypsin, and glutamine synthase. Transduction with an amphotropic LNL6 retroviral vector resulted in stable incorporation of the provirus into 1% of the cells as estimated by semiquantitative PCR. Consistently higher transduction efficiencies (as much as 10% of the cells) were observed with a xenotropic N2 vector. These data support the feasibility of using LNL6 as a marker gene in clinical trials of hepatocellular transplantation. These data also suggest that the efficiency of transducing hepatocytes with amphotropic vectors in animal models may not accurately reflect the utility of these vectors for human applications. Consideration should be given to the use of xenotropic vectors for optimizing the efficiency of transduction for human applications.

Concepts and strategies for human gene therapy

Methods of modern molecular genetics have been developed that allow stable transfer and expression of foreign DNA sequences in human and other mammalian somatic cells. It is therefore no surprise that the methods have been applied in attempts to complement genetic defects and correct disease phenotypes. Two decades of research have now led to the first clinically applicable attempts to introduce genetically modified cells into human beings to cure diseases caused at least partially by genetic defects. We discuss here some of the strategies being followed for both in vitro and in vivo application of therapeutic gene transfer and summarize some of the technical and conceptual difficulties associated with somatic-cell gene therapy.

Long-term expression of a retrovirally introduced beta-galactosidase gene in rodent cells implanted in vivo using biodegradable polymer meshes

Grafts of various types of cells have been performed using bioresorbable polymer matrices. These synthetic fibers are degraded by hydrolysis into normal metabolic intermediates and induce a number of events that are conductive to healing and/or repair, the most important of which may be angiogenesis. The use of biodegradable meshes to deliver genetically altered cells was studied. A beta-galactosidase gene was inserted into Long-Evans rat bone marrow stromal (BMS) cells or fibroblasts derived from C57BL/6J mouse embryos using the retroviral vector LNL-SLX beta gal. Expression was monitored using X-gal staining. X-gal+ cells from monolayer cultures were seeded onto either polyglycolic acid (PGA) or polyglactin (PGL) biodegradable meshes and grown to confluence. Two types of grafts were performed: (1) embryonic C57BL/6J mouse fibroblasts (EMF) into either nude mice or adult C57BL/6J mice, and (2) Long-Evans rat BMS into Long-Evans rats. Beta-Galactosidase activity was found for up to 152 days for EMF in nude mice, 123 days for EMF in adult C57BL/6J mice, and 90 days for grafts of syngeneic BMS cells into Long-Evans rats. Noninfected cells grafted using the same methods did not stain with X-gal.

Treatment of inherited neurometabolic diseases: the future

The past 10 years' experience with bone marrow transplantation from normal, immunologically compatible donors indicates its possible use in various neurometabolic diseases, particularly in a patient who has not suffered irreparable brain damage. This experience may be a prelude to treatment by somatic gene therapy. This can be applied as an autologous bone marrow transplant, grafting the patient's own stem cells inserted with the normal gene. Although somatic gene therapy will be relatively easy for tissues with dividing cells, its application to target tissues with little or no cell division may pose difficulties. Meanwhile, techniques for the preservation, culture, and grafting of fetal neurons in humans have been developed and have been used in the treatment of Parkinson's disease. These procedures could readily be transferred to the treatment of other neurodegenerative diseases that cause significant morbidity, but ethical, legal, and religious considerations must be taken into account. All these efforts promise novel and improved management of inborn neurometabolic errors.

Reconstitution of enzymatic activity in hepatocytes of phenylalanine hydroxylase-deficient mice

Phenylketonuria (PKU) is a metabolic disorder secondary to a deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH). The recent creation of a mouse strain for PAH deficiency has provided an excellent model system to explore the possibility of its phenotypic correction by hepatic gene therapy. A recombinant retrovirus containing the mouse PAH cDNA under the transcriptional control of the human CMV promoter was constructed and used to transduce hepatocytes isolated from PAH-deficient mice. Viral-transduced hepatocytes produced dramatically higher levels of mouse PAH mRNA as compared to control mock-infected hepatocytes. The PAH mRNA was translated efficiently into PAH protein that is capable of converting phenylalanine to tyrosine in vitro. These results demonstrate that the PAH-deficient mouse hepatocytes can be readily reconstituted by retroviral-mediated gene transduction, which is a crucial step towards somatic gene therapy for PKU.

Expression of human alpha 1-antitrypsin in dogs after autologous transplantation of retroviral transduced hepatocytes

The liver represents an excellent organ for gene therapy since many genetic disorders result from the deficiency of liver-specific gene products. We have previously demonstrated that transgenic mouse hepatocytes can be heterologously transplanted into congenic recipients where they survived indefinitely and continued to function as hepatocytes. Here we demonstrate the autologous transplantation of retrovirally transduced canine hepatocytes. At least 1 x 10(9) hepatocytes or 5% of the liver mass can be transplanted by the portal vasculature. In two animals we have transplanted hepatocytes transduced with a retroviral vector containing the human alpha 1-antitrypsin cDNA under transcriptional control of the cytomegalovirus promoter. Both animals had significant human alpha 1-antitrypsin in the serum for 1 month. Although the serum levels of human alpha 1-antitrypsin eventually fell due to inactivation of the cytomegalovirus promoter, PCR analysis demonstrated that a significant fraction of transduced hepatocytes migrated to the liver and continued to survive in vivo. The results suggest that gene therapy of hepatic deficiencies may be achieved by hepatocellular transplantation after genetic reconstitution with the use of promoters of cellular genes that are active in the normal liver.

Hepatocellular transplantation in acute hepatic failure and targeting genetic markers to hepatic cells

Orthotopic liver transplantation (OLT) represents the only therapeutic option for many patients with end-stage liver disease as well as many inborn genetic errors of hepatic metabolism. Despite dramatic progress in methods for OLT, the utilization of this procedure is limited by its considerable morbidity and mortality, by a chronic shortage of organs for transplant, and by difficulty arranging funding for many patients. Many children with fulminant hepatic failure do not receive OLT because this technology is unavailable or unaffordable. Hepatocellular transplantation (HCT), in which isolated, heterologous hepatocytes from a donor liver would be infused into the diseased organ in order to provide essential hepatic functions, could provide a much needed therapeutic alternative to OLT in the treatment of some causes of hepatic insufficiency. Experiments in animals have demonstrated that several genetic deficiencies of hepatic metabolism as well as experimental induced hepatic failure in animals can be reversed by HCT. Despite this experience, HCT has never been attempted in human subjects. This protocol represents the first proposed clinical trial of HCT. We are proposing a clinical trial in which HCT would be attempted as a therapeutic intervention in children with acute hepatic failure who have no other medical or surgical options. This proposal is intended to establish surgical methods for HCT and to evaluate the feasibility of this procedure for treating hepatic disease in humans. It is our expectation that HCT may provide short-term support for patients awaiting organ availability, a "bridge to recovery" allowing patients with fulminant hepatic failure to recover, or a long-term repopulation of the patient's liver with healthy donor cells. One of the major limitations of many animal studies in HCT is that, since the donor hepatocytes are often indistinguishable from those of the host, it has often been difficult to demonstrate a clear correlation between engraftment and the therapeutic effect. In order to verify engraftment independent of the therapeutic response, we propose to "mark" the donor hepatocytes by transducing these cells with a recombinant retroviral vector (LNL6) carrying a marker gene (NEO-R, neomycin phosphoribosyl transferase). The presence of this marker will enhance the ability to identify transplanted cells in the host using assays for the NEO-R gene or transcribed NEO-R mRNA. The LNL6 vector has been approved for human use and has been used as a marker gene for transplanted cells in human subjects without any reported adverse effects. We would like to emphasize that this is a proposal with therapeutic intent.(ABSTRACT TRUNCATED AT 400 WORDS)

Towards liver-directed gene therapy: retrovirus-mediated gene transfer into human hepatocytes

Liver-directed gene therapy is being considered in the treatment of inherited metabolic diseases. One approach we are considering is the transplantation of autologous hepatocytes that have been genetically modified with recombinant retroviruses ex vivo. We describe, in this report, techniques for isolating human hepatocytes and efficiently transducing recombinant genes into primary cultures. Hepatocytes were isolated from tissue of four different donors, plated in primary culture, and exposed to recombinant retroviruses expressing either the LacZ reporter gene or the cDNA for rabbit LDL receptor. The efficiency of gene transfer under optimal conditions, as determined by Southern blot analysis, varied from a maximum of one proviral copy per cell to a minimum of 0.1 proviral copy per cell. Cytochemical assays were used to detect expression of the recombinant derived proteins, E. coli beta-galactosidase and rabbit LDL receptor. Hepatocytes transduced with the LDL receptor gene expressed levels of receptor protein that exceeded the normal endogenous levels. The ability to isolate and genetically modify human hepatocytes, as described in this report, is an important step towards the development of liver-directed gene therapies in humans.

Employment of fibroblasts for gene transfer: applications for grafting into the central nervous system

Genetic modification of primary skin fibroblasts offers a new approach to the focal delivery of deficient transmitter-specific enzymes (e.g., TH) or trophic substances (e.g., NGF) to the damaged or diseased CNS. Although fibroblasts are unable to provide anatomical corrections to defective neural connectivity, they can serve as biological pumps for the enzymes and growth factors in vivo. The capability of genetically engineered cells to ameliorate disease phenotypes in animal models of CNS disorders may ultimately results in the restoration of function. At this time, primary skin fibroblasts appear to be a convenient cellular population for the application of gene transfer and intracerebral grafting for the animal model of Parkinson's disease. It is now important for future investigations to provide data concerning the long-term stable expression of the transgene product (e.g., TH) following intracerebral implantation, as well as determining optimal conditions for the survival of primary cells grafted into the nervous system.

Regulatory concerns in human gene therapy

Gene therapy in humans is now being undertaken in an investigational setting. Such therapy involves the administration of biological products to human patients. A document entitled, "Points to Consider in Human Somatic Cell Therapy and Gene Therapy" has been prepared by the Center for Biologics Evaluation and Research (CBER) of the Food and Drug Administration (FDA) and is published elsewhere in this issue. This paper provides explanatory material about the CBER regulatory process and the scientific and regulatory basis for the requests for data in that document.

Human gene therapy

Recent advances in molecular genetics have made possible the use of retroviral "vectors" to transfer cloned human genes into somatic cells. With this new technology, the genetic defect underlying many recessive inherited disorders can probably be corrected by inserting a normal gene into the patient's hematopoietic stem cells. This article reviews the design and safety of the viral vectors and the results of in vivo studies in mice and large animals that have led to the first human trials. Other target cells for gene transfer, such as endothelial cells, fibroblasts, keratinocytes, and hepatocytes, are also discussed. The use of recombinant retroviruses for gene transfer in vivo is still a new area of research, but the feasibility of "gene therapy" for genetic disorders is rapidly gaining medical and scientific acceptance.

Evaluation of relative promoter strength in primary hepatocytes using optimized lipofection

For most genetic deficiencies manifested in the liver, maximization of gene expression in hepatocytes will be an important factor in achieving successful gene therapy. A rapid, highly efficient, and nontoxic method for transfecting DNA into hepatocytes was used to compare directly promoter strengths of various cellular and viral promoters. Conditions are described here for transfecting 5-10% of primary hepatocytes using the positively charged liposomes, Lipofectin. Cells are not damaged by this method as they continue to transcribe genes controlled by liver specific promoters and can survive for over 2 weeks in culture. We find that the cytomegalovirus, SR alpha, and beta-actin promoters are more active than the SV40, RSV, RNA polymerase II, albumin, alpha 1-antitrypsin, or phosphoenolpyruvate carboxykinase promoters. A simple TK promoter and a TK promoter with the polyoma enhancer (MCI) were almost completely inactive. This information will be useful in the construction of vectors designed to express genes efficiently in primary hepatocytes for purposes of gene therapy, although the stability of expression from these promoters will need to be demonstrated in hepatocytes in vivo.

Persistent gene expression after retroviral gene transfer into liver cells in vivo

The development of liver-directed gene therapy protocols depends upon the ability to transfer genes into a large number of liver cells such that the genes are expressed persistently. We used a retroviral vector to transfer the gene for neomycin phosphotransferase (neo) into mouse liver cells in vivo. Direct injection of the retrovirus preparation into mitotically active (regenerating) liver parenchyma resulted in efficient gene transfer, with neo sequences detectable in the livers of every animal tested 10 weeks to 6 months later. The neo gene was expressed for at least 3 months. This methodology may eventually be applicable to the treatment of human disease.

High-velocity mechanical DNA transfer of the chloramphenicolacetyl transferase gene into rodent liver, kidney and mammary gland cells in organ explants and in vivo

Mouse and rat liver, kidney and mammary gland explants were bombarded with high-velocity microprojectiles carrying a chloramphenicolacetyl transferase gene under different promoters (pTAT-cat, p chi-Casein-cat, p beta-Casein-cat). The expression of a CAT gene was revealed in all organ explants 24 h after transfection. The most pronounced expression was found when a TAT-CAT construction was used. In experiments in vivo rat liver was bombarded in situ with microprojectiles carrying pTAT-cat DNA. A marked activity of the CAT gene was detected 24 h after the bombardment.

Overexpression of the multidrug resistance gene mdr3 in spontaneous and chemically induced mouse hepatocellular carcinomas

Overexpression of a family of plasma membrane glycoproteins, known as P-glycoproteins, is commonly associated with multidrug resistance in animal cells. In rodents, three multidrug resistance (mdr or pgp) genes have been identified, but only two can confer the multidrug resistance phenotype upon transfection into animal cells. Using the RNase protection method, we demonstrated that the levels of three mdr gene transcripts differ among mouse tissues, confirming a previous report that the expression of these genes is tissue specific (J.M. Croop, M. Raymond, D. Huber, A. DeVault, R. J. Arceci, P. Gros, and D. E. Housman, Mol. Cell. Biol. 9:1346-1350, 1989). The levels of mdr transcripts were determined for mouse liver tumors spontaneously arising in both C3H/HeN and transgenic animals containing the hepatitis B virus envelope gene and for tumors induced by two different carcinogenic regimens in C57BL/6N and B6C3-F1 mice. The mdr3 gene was overexpressed in all 22 tumors tested. Our results demonstrate that overexpression of the mdr3 gene in mouse liver tumors does not require exposure of the animals to carcinogenic agents and suggest that its overexpression is associated with a general pathway of hepatic tumor development. The overexpression of the mdr3 gene, which is the homolog of human mdr1 gene, in hepatocellular carcinomas may be responsible for the poor response of these tumors to cancer chemotherapeutic agents.

Overexpression of the multidrug resistance gene mdr3 in spontaneous and chemically induced mouse hepatocellular carcinomas

Overexpression of a family of plasma membrane glycoproteins, known as P-glycoproteins, is commonly associated with multidrug resistance in animal cells. In rodents, three multidrug resistance (mdr or pgp) genes have been identified, but only two can confer the multidrug resistance phenotype upon transfection into animal cells. Using the RNase protection method, we demonstrated that the levels of three mdr gene transcripts differ among mouse tissues, confirming a previous report that the expression of these genes is tissue specific (J.M. Croop, M. Raymond, D. Huber, A. DeVault, R. J. Arceci, P. Gros, and D. E. Housman, Mol. Cell. Biol. 9:1346-1350, 1989). The levels of mdr transcripts were determined for mouse liver tumors spontaneously arising in both C3H/HeN and transgenic animals containing the hepatitis B virus envelope gene and for tumors induced by two different carcinogenic regimens in C57BL/6N and B6C3-F1 mice. The mdr3 gene was overexpressed in all 22 tumors tested. Our results demonstrate that overexpression of the mdr3 gene in mouse liver tumors does not require exposure of the animals to carcinogenic agents and suggest that its overexpression is associated with a general pathway of hepatic tumor development. The overexpression of the mdr3 gene, which is the homolog of human mdr1 gene, in hepatocellular carcinomas may be responsible for the poor response of these tumors to cancer chemotherapeutic agents.

Expression of human factor IX in rabbit hepatocytes by retrovirus-mediated gene transfer: potential for gene therapy of hemophilia B

Hemophilia B (Christmas disease) is a chromosome X-linked blood clotting disorder which results when factor IX is deficient or functionally defective. The enzyme is synthesized in the liver, and the existence of animal models for this genetic disease will permit the development of somatic gene therapy protocols aimed at transfer of the functional gene into the liver. We report the construction of an N2-based recombinant retroviral vector, NCMVFIX, for efficient transfer and expression of human factor IX cDNA in primary rabbit hepatocytes. In this construct the human cytomegalovirus immediate early promoter directs the expression of factor IX. Hepatocytes were isolated from 3-week-old New Zealand White rabbits, infected with the recombinant virus, and analyzed for secretion of active factor IX. The infected rabbit hepatocytes produced human factor IX that is indistinguishable from enzyme derived from normal human plasma. The recombinant protein is sufficiently gamma-carboxylated and is functionally active in clotting assays. These results establish the feasibility of using infected hepatocytes for the expression of this protein and are a step toward the goal of correcting hemophilia B by hepatic gene transfer.

Pahhph-5: a mouse mutant deficient in phenylalanine hydroxylase

Mutant mice exhibiting heritable hyperphenylalaninemia have been isolated after ethylnitrosourea mutagenesis of the germ line. We describe one mutant pedigree in which phenylalanine hydroxylase activity is severely deficient in homozygotes and reduced in heterozygotes while other biochemical components of phenylalanine catabolism are normal. In homozygotes, injection of phenylalanine causes severe hyperphenylalaninemia and urinary excretion of phenylketones but not hypertyrosinemia. Severe chronic hyperphenylalaninemia can be produced when mutant homozygotes are given phenylalanine in their drinking water. Genetic mapping has localized the mutation to murine chromosome 10 at or near the Pah locus, the structural gene for phenylalanine hydroxylase. This mutant provides a useful genetic animal model affected in the same enzyme as in human phenylketonuria.

Clinical application of somatic gene therapy in inborn errors of metabolism

Rapid advances in recombinant DNA and gene transfer technologies provide the potential for somatic gene therapy of inborn errors of metabolism in which the genetically defective function will be restored by transfer of a normal gene into somatic cells. The therapeutic potential and safety of gene therapy has been explored in cultured cells and experimental animals, but therapeutic clinical trials have not yet been proposed or performed. The technologies which may make somatic gene replacement therapy feasible need to be considered and criticised from a clinical perspective. Clinical trials will be necessary to determine the efficacy of somatic gene therapy and address concerns about safety.

Progress toward human gene therapy

Current therapies for most human genetic diseases are inadequate. In response to the need for effective treatments, modern molecular genetics is providing tools for an unprecedented new approach to disease treatment through an attack directly on mutant genes. Recent results with several target organs and gene transfer techniques have led to broad medical and scientific acceptance of the feasibility of this "gene therapy" concept for disorders of the bone marrow, liver, and central nervous system; some kinds of cancer; and deficiencies of circulating enzymes, hormones, and coagulation factors. The most well-developed models involve alteration of mutant target genes by gene transfer with recombinant pathogenic viruses in order to express new genetic information and to correct disease phenotypes--the conversion of the swords of pathology into the plowshares of therapy.