Immune responses against adenoviral vectors and their transgene products: a review of strategies for evasion

Secretion of thymidine kinase to increase the effectivity of suicide gene therapy results in the loss of enzymatic activity

Low efficiency of gene transfer is one of the major limitations of gene therapy. A solution to this problem may be transmission; by modification of the transgene, the gene product can be secreted and internalized by the surrounding cells. Cancer gene therapy using the herpes simplex thymidine kinase (HSV-TK) suicide gene is a promising treatment, and TK has been used in clinical trials with some success. However, this kind of therapy has limited efficacy due to the low level of gene transfer reached. A modified TK protein, capable of migrating from the producing cell to neighboring cells, would result in a greater proportion of cells affected by the treatment. As a first step towards transmission, we constructed a secretory form of HSV-TK by including the Igkappa leader peptide in the gene. An endoplasmatic reticulum export signal was added to the construct to further improve its secretion. Secretion and protein production in cancer cells, the enzymatic activity of the modified proteins and the ability of the modified TK to sensitize cancer cells to ganciclovir were tested. Addition of the Igkappa leader resulted in high levels of secretion of HSV-TK, with up to 70% of the total amount of protein secreted. Inclusion of an ER export signal did not further improve secretion. The enzyme activity of the secreted TK however, was decreased when compared to native TK. This study is the first to report on secretion of TK, and provides a first step in a novel strategy to improve the efficiency of cancer gene therapy. The loss of function in secreted TK however, may present a major hurdle in the development of a transmitted form of TK.

How not to be seen: immune-evasion strategies in gene therapy

The development of efficient and safe vectors for gene delivery paved the way for evolution of gene therapy as a new modality for treatment of various inherited disorders and for cancer. The current vectors, viral and non-viral, have their limitations. Innate and adaptive immune responses to vector particles and components may restrict the efficiency of gene transfer and the persistence of expression of the transgene. Results from preclinical studies in animals and more recently data from clinical studies have demonstrated the potential impact of the cellular and the humoral immune response on the therapeutic efficacy. Not only the vector components, but also the transgene products may induce an immune response that negatively affects the therapeutic efficacy. The induction of a cytotoxic T-cell response to transgene-encoded peptides, as well as the production of antibodies directed against secreted proteins have been reported in preclinical and clinical studies, and these may thwart those applications that require long-term expression. Here we will review some of the options to blunt the acquired immune responses to transgene-encoded polypeptides.

The histone deacetylase inhibitor FK228 given prior to adenovirus infection can boost infection in melanoma xenograft model systems

A major limitation of adenovirus type 5-mediated cancer gene therapy is the inefficient infection of many cancer cells. Previously, we showed that treatment with low doses of the histone deacetylase inhibitor FK228 (FR901228, depsipeptide) increased coxsackie adenovirus receptor (CAR) levels, histone H3 acetylation, and adenovirus infection efficiencies as measured by viral transgene expression in cancer cell lines but not in cultured normal cells. To evaluate FK228 in vivo, the effects of FK228 therapy in athymic mice bearing LOX IMVI or UACC-62 human melanoma xenografts were examined. Groups of mice were treated with FK228 using several dosing schedules and the differences between treated and control animals were determined. In mice with LOX IMVI xenografts (n = 6), maximum CAR induction was observed 24 h following a single FK228 dose of 3.6 mg/kg with a 13.6 +/- 4.3-fold (mean +/- SD) increase in human CAR mRNA as determined by semiquantitative reverse transcription-PCR analysis. By comparison, mouse CAR levels in liver, kidney, and lung from the same animals showed little to no change. Maximum CAR protein induction of 9.2 +/- 4.8-fold was achieved with these treatment conditions and was associated with increased histone H3 acetylation. Adenovirus carrying a green fluorescent protein (GFP) transgene (2 x 10(9) viral particles) was injected into the xenografts and GFP mRNA levels were determined. A 7.4 +/- 5.2-fold increase in GFP mRNA was found 24 h following adenovirus injection into optimally FK228-treated mice (n = 10). A 4-fold increase in GFP protein-positive cells was found following FK228 treatment. These studies suggest that FK228 treatment prior to adenovirus infection could increase the efficiency of adenovirus gene therapy in xenograft model systems.

TERT promoter-driven adenovirus vector for cancer gene therapy via systemic injection

Adenovirus vectors (Adv) are used widely in cancer gene therapy research. However, the clinical application of Adv currently is limited to local, intratumoral administration; systemic administration leads to redundant transgene expression in the liver and subsequent hepatotoxicity. Here we replaced the conventional cytomegalovirus (CMV) promoter of Adv with a tumor-specific telomere reverse transcriptase (TERT) promoter, to restrict expression of the Adv-transduced transgene to tumor tissue alone. We evaluated the therapeutic and side effects after systemic administration of Adv expressing herpes simplex virus thymidine kinase (Ad-HSVtk) in mice bearing Meth-A tumors. Although systemically injected CMV promoter-driven Ad-HSVtk lacked therapeutic effect, mice injected with 2x10(11) viral particles containing TERT promoter-driven Ad-HSVtk showed inhibited tumor growth and prolonged survival with minimal side effects. Our results suggest that Adv in which transgene expression is driven by the TERT promoter are a promising prototype of tumor-targeting vectors for effective and safe cancer gene therapy.

Targeted delivery of NK4 to multiple lung tumors by bone marrow-derived mesenchymal stem cells

Most advanced solid tumors metastasize to different organs. However, no gene therapy effective for multiple tumors has yet been developed. Since a unique characteristic of bone marrow-derived mesenchymal stem cells (MSCs) is that they migrate to tumor tissues, we wanted to determine whether MSCs could serve as a vehicle of gene therapy for targeting multiple tumors. First, we confirmed that mouse MSCs preferentially migrate to multiple tumors of the lung in the Colon-26 (C-26) lung metastasis model. Next, MSCs were efficiently transduced with NK4, an antagonist of hepatocyte growth factor (HGF), by an adenoviral vector with an RGD motif. MSCs expressing NK4 (NK4-MSCs) strongly inhibited development of lung metastases in the C-26 lung metastasis model after systemic administration via a tail vein. Treatment with NK4-MSCs significantly prolonged survival of the C-26-tumor-bearing mice by inhibiting tumor-associated angiogenesis and lymphangiogenesis and inducing apoptosis of the tumor cells. MSC-based gene therapy did not induce the severe adverse effects induced by conventional adenoviral vectors. These results indicate that MSCs can serve as a vehicle of gene therapy for targeting multiple lung metastatic tumors.

Long-Term toxicity studies in Canine of E10A, an adenoviral vector for human endostatin gene

E10A, a recombinant adenovirus type 5 vector carrying the human endostatin gene, may be a promising gene therapy drug in the treatment of solid tumors by antiangiogenesis, but a preclinical safety evaluation of E10A has not yet been performed. With high and low doses equivalent to 30 and 7.5 times the human curative dose, respectively, intramuscular injections of E10A were given once daily, 6 days/week, for 3 months, followed by a 1-month recovery period. As of 4 months, all experimental animals appeared generally healthy: normal behavior and eating habits, no nausea, vomiting, or salivation, no abnormal changes in urination or defecation, and increased body weight with the time of experiment. Urinalysis, hemogram, blood biochemistry, electrocardiogram, macroscopic and microscopic studies of organs and tissues were done before treatment, at month 3 of treatment, and 1 month posttreatment. At all time points, no significant abnormal toxic effects were noted. Preliminary investigation of E10A immunotoxicity in dogs indicated that anti-adenoviral antibodies were generated, in a dose- and time-independent manner, after E10A injection. Our data demonstrated that, long term, high-dose intramuscular administration of recombinant human endostatin-carrying adenovirus (E10A) was not notably toxic and might be safe for clinical therapeutic use, although additional long-term toxicity studies by other administration routes are still necessary.

Increased tumor localization and reduced immune response to adenoviral vector formulated with the liposome DDAB/DOPE

We aimed to increase the efficiency of adenoviral vectors by limiting adenoviral spread from the target site and reducing unwanted host immune responses to the vector. We complexed adenoviral vectors with DDAB-DOPE liposomes to form adenovirus-liposomal (AL) complexes. AL complexes were delivered by intratumoral injection in an immunocompetent subcutaneous rat tumor model and the immunogenicity of the AL complexes and the expression efficiency in the tumor and other organs was examined. Animals treated with the AL complexes had significantly lower levels of beta-galactosidase expression in systemic tissues compared to animals treated with the naked adenovirus (NA) (P<0.05). The tumor to non-tumor ratio of beta-galactosidase marker expression was significantly higher for the AL complex treated animals. NA induced significantly higher titers of adenoviral-specific antibodies compared to the AL complexes (P<0.05). The AL complexes provided protection (immunoshielding) to the adenovirus from neutralizing antibody. Forty-seven percent more beta-galactosidase expression was detected following intratumoral injection with AL complexes compared to the NA in animals pre-immunized with adenovirus.

CONCLUSIONS

Complexing of adenovirus with liposomes provides a simple method to enhance tumor localization of the vector, decrease the immunogenicity of adenovirus, and provide protection of the virus from pre-existing neutralizing antibodies.

Adenoviral urokinase-type plasminogen activator (uPA) gene transfer enhances venous thrombus resolution

INTRODUCTION

There is an increase in the natural level of urokinase-type plasminogen activator (uPA) activity within the thrombus during venous thrombus resolution. The use of uPA as a thrombolytic agent in the treatment of acute iliofemoral deep vein thrombosis is not suitable for all patients. This study aimed to determine whether thrombus resolution could be enhanced by upregulating uPA expression using adenoviral gene transfer as an alternative method of delivery.

METHODS

The production of functional uPA by an adenoviral gene construct (ad.uPA) was confirmed by a colorimetric substrate assay and fibrin plate lysis. Thrombus was formed in the inferior vena cava of wild-type mice and injected, 48-hours after induction, with either a control virus at 10(8) plaque-forming units (pfu) or ad.uPA at 10(7) or 10(8) pfu. Thrombi were removed and weighed 7 days after treatment. Activity of metalloproteinase (MMP) 2 and 9 was measured by zymography and the release of vascular endothelial growth factor (VEGF) and D-dimer levels by enzyme-linked immunoabsorbent assay. The results were expressed as a mean +/- SEM. Values were standardized for wet weight or for soluble protein content (mg/sol protein).

RESULTS

Treatment with ad.uPA reduced thrombus weight by twofold compared with thrombi treated by control virus (15.1 +/- 1.1 mg vs 7.4 +/- 1.3 mg, P = .004). Urokinase activity (17 +/- 3 pg/mg wet weight) was detected in all treated thrombi, but there was no dose-dependent effect. D-dimer activity was increased twofold after treatment with ad.uPA (1.7 +/- 0.15 ng/mg of sol protein vs 0.8 +/- 0.1 ng/mg of sol protein, P = .0015) and was associated with a reduction in thrombus size (P = .03). Urokinase overexpression did not affect the activity of MMP2, MMP9, or VEGF in the thrombus.

CONCLUSION

Increasing urokinase activity within the thrombus significantly enhanced natural thrombus resolution by a fibrinolytic action. Therapeutic delivery of ad.uPA in patients may provide a novel method of treating deep vein thrombosis.

CLINICAL RELEVANCE

The use of urokinase as a thrombolytic agent in the treatment of acute iliofemoral deep vein thrombosis is not suitable for all patients. This study aimed to determine whether thrombus resolution could be enhanced by upregulating urokinase expression using adenoviral gene transfer as an alternative method of therapeutic delivery. The study shows that by increasing urokinase activity within the thrombus, natural thrombus resolution can be significantly enhanced. The delivery of ad.uPA in patients may provide a novel method of treating deep vein thrombosis.

Additional Gene Therapy with rAAV-wt-p53 Enhanced the Efficacy of Cisplatin in Human Bladder Cancer Cells

OBJECTIVE

Gene therapy is defined as the treatment of an acquired or inherited disease by transfer of genetic material. The most common strategies in gene therapy of bladder cancer are corrective, inductive and cytotoxic gene therapy. Mutations in the p53 tumor suppressor gene are the most common molecular genetic abnormalities in bladder cancer and p53 gene transfer in the human bladder cancer cell line by adenoviral or other vectors was demonstrated to be cytotoxic. However, so far there has been no report of adeno-associated virus-2 vector-mediated p53 gene deliveries in bladder cancer. In this study, wild-type p53 cDNA was transfected into the bladder cancer cells, using the adeno-associated virus-2 vector, and the capability of rAAV-wt-p53 gene therapy in bladder cancer was evaluated in vitro.

METHOD

Bladder cancer cell lines 5637 were transduced with adeno-associated virus-2 vectors containing wild-type human p53 gene (rAAV-wt-p53). Gene expression and transcriptional activation of p53 was determined by Western blot analysis. The cellular growth inhibition and apoptosis of rAAV-mediated p53 transfection were assessed by flow cytometry. The combination effect of rAAV-wt-p53 and cisplatin was measured by MTT assay.

RESULTS

The virus rAAV efficiently enters the cells and expresses its gene products. The gene product of rAAV-wt-p53 is cytotoxic to bladder cancer cells. The bladder cell line 5637 was found to experience a synergistic killing effect when rAAV-wt-p53 was used in combination with cisplatin.

CONCLUSION

rAAV-mediated p53 gene transfer could offer a powerful novel therapeutic approach in bladder cancer.

Gene therapy using adenovirus-mediated full-length anti-HER-2 antibody for HER-2 overexpression cancers

PURPOSE

Therapeutic monoclonal antibody is increasingly applied in many clinical applications, although complicated technologies and high cost still limit their wide applications. To obtain the sustained serum antibody concentration with one single injection and lower the cost of antibody protein therapy, an adenovirus-mediated full-length antibody gene therapy was developed.

EXPERIMENTAL DESIGN

Full-length antibody light-chain and heavy-chain sequences were linked with internal ribosome entry site and constructed into adenoviral vector under the control of cytomegalovirus promoter. Antibody expression in vitro and in vivo were tested with ELISA, and its antitumor efficacy was evaluated in SKOV-3-inoculated nude mice.

RESULTS

Ad5-TAb-generated anti-HER-2 antibody presented the similar binding specificity with commercial trastuzumab. A single i.v. injection of 2 x 10(9) plaque-forming units of Ad5-TAb per mouse resulted in not only a sustained over 40 microg/mL serum antibody level for at least 4 weeks but also significant tumor elimination in the ovarian cancer SKOV-3-inoculated nude mice.

CONCLUSIONS

An in vivo full-length antibody gene delivery system allows continuous production of a full-length antibody at high concentration after a single administration. Bioactive antibody macromolecules can be generated via gene transfer in vivo. All the data suggest that this novel adenovirus-mediated antibody gene delivery can be used for the exploitation of antibodies, without being hampered by the sophisticated antibody manufacture techniques and high cost, and, furthermore, can shorten the duration and reduce the expense of antibody developments.

Adenovirus-mediated p53 tumor suppressor gene therapy of osteosarcoma

The clinical outcome for osteosarcoma (OS) remains discouraging despite efforts to optimize treatment using conventional modalities including surgery, radiotherapy and chemotherapy. Novel therapeutic approaches based on our expanding understanding of the mechanisms of tumor cell killing have the potential to alter this situation. Tumor suppressor gene therapy aims to restore the function of a tumor suppressor gene lost or functionally inactivated in cancer cells. One such molecule, the p53 tumor suppressor gene plays a critical role in safeguarding the integrity of the genome and preventing tumorigenesis. Introduction of wild-type (wt) p53 into transformed cells has been shown to be lethal for most cancer cells in vitro, but clinical trials of p53 gene replacement have had limited success. Analysis of these clinical trials highlighted the insufficient efficacy of current vectors and low proapoptotic activity of wt p53 as a single agent in vivo. In this review, a contemporary summarization of the current status of adenovirus-mediated p53 gene therapy of OS is presented. Advancement in our understanding of p53 tumor suppressor activity, the molecular biology of chemoresistant OS, and recent advances in tumor targeting with adenoviral vectors are also addressed. Based on these parameters, prospects for future investigations are proposed.

Long-term persistence of gene expression from adeno-associated virus serotype 5 in the mouse airways

Recombinant adeno-associated virus vectors based on serotype 2 (rAAV2) have been used to deliver transgenes to the airways in a variety of pre-clinical and clinical studies. Gene transfer in these studies has been severely restricted by the basolateral localization of rAAV2 receptors. Here, we studied vectors constructed from the AAV5 genome and capsid, which utilize N-linked sialic acid-containing receptors found on the apical surface of airway epithelial cells. We investigated gene transfer efficacy and duration of transgene expression following delivery of rAAV5/5 vectors to the mouse respiratory tract. Robust, dose-dependent transgene expression was observed in the epithelium lining the nose for at least 32 weeks, and for at least 52 weeks in the lung. Importantly, in the lung, transgene expression mediated by rAAV5/5 was 40-fold greater than by rAAV2/2 vectors. A distinct cellular preference for rAAV5/5-mediated transduction was observed, with transgene expression being predominantly restricted to sustentacular cells of the olfactory epithelium in the nose and alveolar type II cells in the lung. Administration of rAAV5/5 vectors to both the nose and lungs led to the rapid development of rAAV5/5-neutralizing antibodies, suggesting that repeated administration may be severely hampered by host immune responses.

Canine adenovirus vectors for lung-directed gene transfer: efficacy, immune response, and duration of transgene expression using helper-dependent vectors

A major hurdle to the successful clinical use of some viral vectors relates to the innate, adaptive, and memory immune responses that limit the efficiency and duration of transgene expression. Some of these drawbacks may be circumvented by using vectors derived from nonhuman viruses such as canine adenovirus type 2 (CAV-2). Here, we evaluated the potential of CAV-2 vectors for gene transfer to the respiratory tract. We found that CAV-2 transduction was efficient in vivo in the mouse respiratory tract, and ex vivo in well-differentiated human pulmonary epithelia. Notably, the in vivo and ex vivo efficiency was poorly inhibited by sera from mice immunized with a human adenovirus type 5 (HAd5, a ubiquitous human pathogen) vector or by human sera containing HAd5 neutralizing antibodies. Following intranasal instillation in mice, CAV-2 vectors also led to a lower level of inflammatory cytokine secretion and cellular infiltration compared to HAd5 vectors. Moreover, CAV-2 transduction efficiency was increased in vitro in human pulmonary cells and in vivo in the mouse respiratory tract by FK228, a histone deacetylase inhibitor. Finally, by using a helper-dependent CAV-2 vector, we increased the in vivo duration of transgene expression to at least 3 months in immunocompetent mice without immunosuppression. Our data suggest that CAV-2 vectors may be efficient and safe tools for long-term clinical gene transfer to the respiratory tract.

Gutless adenovirus: last-generation adenovirus for gene therapy

Last-generation adenovirus vectors, also called helper-dependent or gutless adenovirus, are very attractive for gene therapy because the associated in vivo immune response is highly reduced compared to first- and second-generation adenovirus vectors, while maintaining high transduction efficiency and tropism. Nowadays, gutless adenovirus is administered in different organs, such as the liver, muscle or the central nervous system achieving high-level and long-term transgene expression in rodents and primates. However, as devoid of all viral coding regions, gutless vectors require viral proteins supplied in trans by a helper virus. To remove contamination by a helper virus from the final preparation, different systems based on the excision of the helper-packaging signal have been generated. Among them, Cre-loxP system is mostly used, although contamination levels still are 0.1-1% too high to be used in clinical trials. Recently developed strategies to avoid/reduce helper contamination were reviewed.

Impact of transgene expression on drug metabolism following systemic adenoviral vector administration

BACKGROUND

Systemic administration of a first-generation adenovirus expressing E. coli beta-galactosidase (AdlacZ) alters expression and function of two hepatic drug-metabolizing enzymes, cytochrome P450 (CYP) 3A2 and 2C11, for 14 days. The objective of these studies was to determine how the transgene cassette influences CYP expression and function.

METHODS

Sprague-Dawley rats were given 5.7 x 10(12) viral particles (vp)/kg of either: AdlacZ, Ad expressing murine erythropoietin (Epo), Ad without a transgene (Null), or phosphate-buffered saline (Vehicle). Hepatic CYP protein expression, activity, mRNA and alanine aminotransferase (ALT) levels were analyzed 0.25, 1, 4, and 14 days following a single intravenous injection.

RESULTS

Administration of Epo did not alter CYP3A2 activity, but induced RNA levels by a factor of 2 at 4 and 14 days (P< or =0.01). This vector suppressed CYP2C11 activity levels by 45% at 1 day (P< or =0.05) and RNA levels throughout the study period (P< or =0.05). The Null vector suppressed CYP3A2 activity by 36, 63, 34, and 45% at 0.25, 1, 4 and 14 days, respectively (P< or =0.05). CYP2C11 activity was suppressed 1 day after administration (41%) and RNA levels were suppressed at 6 h (53%) and 1 day (36%, P< or =0.05). In contrast, AdlacZ suppressed both CYP3A2 and 2C11 at all time points.

CONCLUSIONS

The immunogenic and biological nature of the transgene cassette can influence changes in CYP3A2, but not the 2C11 isoform. The shift in transcription and translation of protein for maintenance of physiologic homeostasis to production of viral proteins and transgene product and their associated toxicity during viral infection may explain our observations.

Gene therapy for neurodegenerative and ocular diseases using lentiviral vectors

Gene therapy holds great promise for the treatment of a wide range of inherited and acquired disorders. The development of viral vector systems to mediate safe and long-lasting expression of therapeutic transgenes in specific target cell populations is continually advancing. Gene therapy for the nervous system is particularly challenging due to the post-mitotic nature of neuronal cells and the restricted accessibility of the brain itself. Viral vectors based on lentiviruses provide particularly attractive vehicles for delivery of therapeutic genes to treat neurological and ocular diseases, since they efficiently transduce non-dividing cells and mediate sustained transgene expression. Furthermore, novel routes of vector delivery to the nervous system have recently been elucidated and these have increased further the scope of lentiviruses for gene therapy application. Several studies have demonstrated convincing therapeutic efficacy of lentiviral-based gene therapies in animal models of severe neurological disorders and the push for progressing such vectors to the clinic is ongoing. This review describes the key features of lentiviral vectors that make them such useful tools for gene therapy to the nervous system and outlines the major breakthroughs in the potential use of such vectors for treating neurodegenerative and ocular diseases.

Ex vivo transduction of human dermal tissue structures for autologous implantation production and delivery of therapeutic proteins

Systemic delivery of therapeutic proteins through gene transfer approaches has been carried out mostly by ex vivo transduction of single cells or by direct in vivo injection of an expression vector. In this work an intact miniature biopsy of human dermis (microdermis) is harvested and transduced ex vivo by a viral vector encoding a gene for the therapeutic protein. The microdermis preserves its three-dimensional structure and viability during the ex vivo manipulations. Furthermore, upon transduction with adenoviral and adeno-associated viral vectors the microdermis secretes recombinant human erythropoietin (hEPO). Biochemical analysis of the secreted hEPO showed similarity to the clinically approved recombinant hEPO. Subcutaneous implantation of microdermal hEPO into SCID mice exhibited hEPO secretion in the blood circulation and preserved elevated hematocrit for several months, demonstrating the technology's potential for sustained delivery of protein therapeutics.

Anti-tumor immune responses following neoadjuvant immunotherapy with a recombinant adenovirus expressing HSP72 to rodent tumors

Gene modification of tumor cells is commonly utilized in various strategies of immunotherapy preventive both as treatment and a means to modify tumor growth. Gene transfer prior to surgery as neoadjuvant therapy has not been studied systematically. We addressed, whether direct intra-tumoral injection of a recombinant adenovirus expressing the immunomodulatory molecule, heat shock protein 72 (ADHSP72), administered prior to surgery could result in sustainable anti-tumor immune responses capable of affecting tumor progression and survival in a number of different murine and rat tumor models. Using intra-dermal murine models of melanoma (B16), colorectal carcinoma (CT26), prostate cancer (TrampC2) and a rat model of glioblastoma (9L), tumors were treated with vehicle or GFP expressing adenovirus (ADGFP) or ADHSP72. Tumors were surgically excised after 72 h. Approximately 25-50% of animals in the ADHSP72 treatment group but not in control groups showed sustained resistance to subsequent tumor challenge. Tumor resistance was associated with development of anti-tumor cellular immune responses. Efficacy of ADHSP72 as neoadjuvant therapy was dependent on the size of the initial tumor with greater likelihood of immune response generation and tumor resistance associated with smaller tumor size at initial treatment. ADHSP72 neoadjuvant therapy resulted in prolonged survival of animals upon re-challenge with autologous tumor cells compared to ADGFP or vehicle control groups. To study the effects on tumor progression of distant metastases, a single tumor focus of animals with multifocal intra-dermal tumors was treated. ADHSP72 diminished progression of the secondary tumor focus and prolonged survival, but only when the secondary tumor focus was <50 mm3 . Our results indicate that gene modification of tumors prior to surgical intervention may be beneficial to prevent recurrence in specific circumstances.

Therapeutics of hearing loss: expectations vs reality

With the completion of the sequencing of the human genome, the field of medicine is undergoing a dramatic and fundamental change. The identification of our genes and the proteins they encode and the mechanisms of mutations that are pathogenic will allow us to devise revolutionary new ways to diagnose, treat and prevent the thousands of disorders that affect us. Certainly, disorders of the auditory system are no exception. Revealing the molecular mechanisms of hearing and understanding the role of each player in the intricate auditory network could enable us to employ gene- or cell-based therapy to cure or prevent hearing loss. To this end, much emphasis has been placed on the identification and characterization of genes involved in human deafness, as well as research on mouse models for deafness. Ultimately, the effect of genomics on medicine will be dramatic, providing us with the ability to cure sensory defects, a tangible goal that is now within our reach.

Liver-directed gene expression employing synthetic transcriptional control units

AIM: To generate and characterize the synthetic transcriptional control units for transcriptional targeting of the liver, thereby compensating for the lack of specificity of currently available gene therapeutic vector systems.

METHODS: Synthetic transcriptional control unit constructs were generated and analyzed for transcriptional activities in different cell types by FACS quantification, semi-quantitative RT-PCR, and Western blotting.

RESULTS: A new bifunctionally-enhanced green fluorescent protein (EGFP)/neo^r fusion gene cassette was generated, and could flexibly be used both for transcript quantification and for selection of stable cell clones. Then, numerous synthetic transcriptional control units consisting of a minimal promoter linked to “naturally” derived composite enhancer elements from liver-specific expressed genes or binding sites of liver-specific transcription factors were inserted upstream of this reporter cassette. Following liposome-mediated transfection, EGFP reporter protein quantification by FACS analysis identified constructs encoding multimerized composite elements of the apolipoprotein B100 (ApoB) promoter or the ornithin transcarbamoylase (OTC) enhancer to exhibit maximum transcriptional activities in liver originating cell lines, but only background levels in non-liver originating cell lines. In contrast, constructs encoding only singular binding sites of liver-specific transcription factors, namely hepatocyte nuclear factor (HNF)1, HNF3, HNF4, HNF5, or CAAT/enhancer binding protein (C/EBP) only achieved background levels of EGFP expression. Finally, both semi-quantitative RT-PCR and Western blotting analysis of Hep3B cells demonstrated maximum transcriptional activities for a multimeric 4xApoB cassette construct, which fully complied with the data obtained by initial FACS analysis.

CONCLUSION: Synthetic transcriptional control unit constructs not only exhibit a superb degree of structural compactness, but also provide new means for liver-directed expression of therapeutic genes.