Tissue-specific gene expression in medullary thyroid carcinoma cells employing calcitonin regulatory elements and AAV vectors

Improved cell-specificity of adeno-associated viral vectors for medullary thyroid carcinoma using calcitonin gene regulatory elements

Targeted gene therapy using recombinant adeno-associated virus (rAAV) vectors is a potential therapeutic strategy for treating cancer, and tissue-specific promoters may help with tissue targeting. Medullary thyroid carcinoma (MTC) is a disease of the calcitonin secreting thyroid C cells, and calcitonin is highly expressed in MTC tumors compared to other cells. To target MTC cells, we evaluated an rAAV serotype 2 vector (rAAV2-pM+104-GFP) containing a modified calcitonin/calcitonin gene related peptide promoter (pM+104) and a green fluorescent protein (GFP) reporter gene. In vitro transduction experiments comparing the MTC TT cell line with non-MTC cell lines demonstrated that rAAV2-pM+104-GFP infection yielded significantly (p < 0.05) higher GFP expression in TT cells than in non-MTC cell lines (HEK293 and HeLa), and significantly higher expression than in TT cells infected with the positive control rAAV2-pCBA-GFP vector. The rAAV2-pCBA-GFP control vector included a well-characterized, ubiquitously expresses control promoter, the chicken beta actin promoter with a cytomegalovirus enhancer (pCBA). In vivo experiments using a TT cell xenograft tumor mouse model showed that tumors directly injected with 2 x 1010 vg of rAAV2-pM+104-GFP vector resulted in GFP expression detected in 21.7% of cells, 48 hours after the injection. Furthermore, GFP expression was significantly higher for rAAV-pM+104-GFP treatments with a longer vector treatment duration and higher vector dose, with up to 52.6% (q < 0.05) GFP cells detected 72 hours after injecting 1x 1011 vg/tumor. These data show that we have developed an rAAV vector with improved selectivity for MTC.

Transcriptional targeting of gene expression in breast cancer by the promoters of protein regulator of cytokinesis 1 and ribonuclease reductase 2

For cancer gene therapy, cancer-specific over- expression of a therapeutic gene is required to reduce side effects derived from expression of the gene in normal cells. To develop such an expression vector, we searched for genes over-expressed and/or specifically expressed in cancer cells using bioinformatics and have selected genes coding for protein regulator of cytokinesis 1 (PRC1) and ribonuclease reductase 2 (RRM2) as candidates. Their cancer-specific expressions were confirmed in both breast cancer cell lines and patient tissues. We compared each promoter's cancer-specific activity in the breast normal and cancer cell lines using the luciferase gene as a reporter and confirmed cancer-specific expression of both PRC1 and RRM2 promoters. To test activities of these promoters in viral vectors, the promoters were also cloned into an adeno-associated viral (AAV) vector containing green fluorescence protein (GFP) as the reporter. The GFP expression levels by these promoters were various depending on cell lines tested and, in MDA-MB-231 cells, GFP activities derived from the PRC1 and RRM2 promoters were as strong as that from the cytomegalovirus (CMV) promoter. Our result showed that a vector containing the PRC1 or RRM2 promoter could be used for breast cancer specific overexpression in gene therapy.

Thyroid cancer molecular signaling pathways and use of targeted therapy

Several agents are currently being tested that target thyroid molecular signaling and cancer cell biology. The pathways involved include but are not limited to the Ras pathway, vascular endothelial growth factor and epidermal growth factor receptors and antibodies, angiogenesis inhibitors, tyrosine kinase inhibitors, heat shock protein inhibitors, demethylating agents, histone deacetylase inhibitors, and gene therapy. Each of these targeted approaches holds promise for our future ability to treat patients with thyroid cancer unresponsive to traditional therapy.

Technology insight: gene therapy and its potential role in the treatment of medullary thyroid carcinoma

Metastatic medullary thyroid cancer (MTC) responds poorly to conventional treatments with chemotherapy and radiotherapy. Gene therapy--the transfer of genetic material for therapeutic purposes--might have therapeutic potential for patients with progressive metastatic MTC that is incurable by conventional treatments. To date, a number of gene-therapy strategies have been explored, primarily those that use replication-deficient adenovirus vectors to transfer therapeutic genes to tumor cells. Tissue-specific expression of the promoter for calcitonin and calcitonin-related polypeptide alpha has allowed therapeutic genes to be specifically expressed in calcitonin-secreting cells and in the MTC tumors derived from them; such tissue-specific expression contributes to improved safety of gene therapies and has the potential to increase their therapeutic index. In addition, the identification of an MTC-specific peptide ligand raises the possibility of developing an MTC-selective vector. In this article, we have described the exciting area of gene therapy in the management of MTC with a focus on preclinical in vitro and in vivo MTC models.

Gene therapy for thyroid cancer

Although most thyroid cancers may be cured by surgery and ¹³¹I therapy, approximately 10-20% of patients die from advanced differentiated and anaplastic tumors that are unresponsive to conventional treatments. Thus, alternative approaches such as gene therapy are of interest, especially using targeted therapeutic gene delivery. Several strategies have been designed specifically for thyroid cancer and some have proven to be feasible in preclinical studies. In particular, it is suggested that combined gene therapy approaches, as well as multimodality therapeutic regimens, including gene therapy and conventional treatments, should be pursued to achieve clinically significant results. The recent discovery of new markers of thyroid cancer should improve the efficacy of gene therapy.

Gene therapy for thyroid cancer

Thyroid carcinomas are suitable targets for gene therapy because they can be highly lethal on one hand, while being susceptible to specific tumour targeting on the other hand. Several gene therapy modalities have been evaluated so far in experimental models of thyroid cancer, including tumour suppressor gene replacement, oncogene inhibition, suicide gene therapy, immunotherapy, antiangiogenesis, and viral oncolysis. All of these strategies have shown promising results, but clinical studies are lacking. Based on the clinical experience achieved in a pilot study in patients with advanced thyroid cancer and on clinical results in other types of solid cancer, it is suggested that combined gene therapy approaches, as well as multimodality therapeutic regimens, including gene therapy and conventional treatments, should be pursued to achieve clinically significant results.

MIA (melanoma inhibitory activity) promoter mediated tissue-specific suicide gene therapy of malignant melanoma

Suicide gene therapy of malignant melanoma essentially requires efficient gene transfer and highly selective therapeutic gene expression. To achieve this, recombinant adeno-associated virus (rAAV) particles were constructed containing the tissue-specific promoter of the human melanoma inhibitory activity (hMIA) gene combined with four copies of the enhancer element of the murine tyrosinase gene. Three melanoma and one cervix carcinoma cell line were infected with rAAV particles carrying a reporter gene under control of the enhancer/hMIA promoter in order to determine transcriptional activity and specificity of this system. Viral particles containing the enhancer/hMIA promoter mediated reporter gene activity only in melanoma cells, whereas infection with a cytomegalovirus (CMV)-based promoter construct induced unspecific gene expression. Correspondingly, transient transduction with viral particles bearing the HSVtk gene under the control of the enhancer/MIA promoter elements followed by treatment with ganciclovir (GCV) resulted in growth inhibition only in melanoma cells, whereas the CMV promoter-based construct induced unspecific cytotoxicity. In vivo experiments in nude mice demonstrated that tumors originating from human melanoma cells disappeared after stable, but not transient transduction with vectors bearing the HSVtk gene under the control of the enhancer/hMIA promoter in response to GCV application. In face of higher transduction efficiency, these rAAV particles might therefore be a useful tool for suicide gene therapy of malignant melanoma.

Gene therapy platform for bone regeneration using an exogenously regulated, AAV-2-based gene expression system

Viral delivery of the therapeutic gene bone morphogenetic protein-2 (BMP-2) is a promising approach for bone regeneration. The human parvovirus adeno-associated virus (AAV) type 2 is considered one of the most encouraging viral vector systems because of its high transduction rates and biosafety ratings. Bone morphogenetic protein-2 is a highly potent osteoinductive protein, which induces bone formation in vivo and osteogenic differentiation in vitro. The exogenous regulation of BMP-2 expression in bone-regenerating sites is required to control BMP-2 protein secretion, thus promoting safe and controlled bone formation and regeneration. We have therefore constructed a dual-construct vector for the recombinant AAV (rAAV)-based recombinant human BMP-2 (rhBMP-2) gene delivery system, which is regulated by the tetracycline-sensitive promoter (TetON). Each vector was encapsidated separately, yielding two recombinant viruses. We evaluated the efficiency of rAAV-hBMP-2 to induce bone formation in ectopic and orthotopic sites. Doxycycline (Dox), an analogue of tetracycline, was orally administered to mice via their drinking water to induce rhBMP-2 expression. Bone formation was measured using quantitative imaging-microcomputerized tomography and cooled charge-coupled device imaging-to detect osteogenic activity at the cellular level, detecting osteocalcin expression. The rAAV-hBMP-2-treated mice that were given Dox demonstrated bone formation in both in vivo models compared to none in mice prevented from receiving Dox. Thus, the Tet-regulated rAAV-hBMP-2 vector is an effective means of induction and regulation of bone regeneration and repair.

Tumor-specific gene expression using regulatory elements of the glucose transporter isoform 1 gene

In order to achieve tumor-specific targeting of adeno-associated virus (AAV)-mediated gene expression, the promoter of the glucose transporter isoform 1 (GLUT1) gene was cloned upstream of the enhanced green fluorescence protein (EGFP) and the herpes simplex virus thymidine kinase (HSVtk) gene. FACS analysis performed at 48 h after transient infection with rAAV/cytomegalovirus (CMV)egfp viral particles revealed an increase of fluorescence in all the cell lines tested. However, EGFP expression under control of the GLUT1 promoter element (rAAV/GTI-1.3egfp) was limited to the tumor cells and oncogene-transformed cells. Evidence for phosphorylation of the HSVtk substrates ganciclovir (GCV) and 125I-deoxycytidine was found in all transfected tumor cell lines compared to noninfected controls (HCT116: 111%; MH3924A: 130%; HaCaT-RT3: 257% increase), but not in HaCaT and HUVEC cells. Furthermore, tumor cells and the oncogene-transformed (ras) cell line HaCaT-RT3 showed a GCV-induced reduction in cell number (HCT116: -71%; MH3924A: -43% and HaCaT-RT3: -31%). No statistically relevant cytotoxic effect was observed in HaCaT (6% decrease) and HUVEC cells (2% decrease). Furthermore, a reduction of 3H-thymidine incorporation into the DNA was seen after treatment with GCV (HCT116: 38%; MH3924A: 33% and HaCaT-RT3: 37% decrease). In a therapy study of HSVtk-expressing tumors with GCV, we achieved total tumor remission.

Gene therapy for thyroid cancer: current status and future prospects

Despite multimodality treatment for thyroid cancer, including surgical resection, radioiodine therapy, thyrotropin (TSH)-suppressive thyroxine treatment, and chemotherapy/radiotherapy, survival rates have not improved over the last decades. Therefore, development and evaluation of novel treatment strategies, including gene therapy, are urgently needed. A variety of gene therapy approaches have been evaluated for the treatment of follicular cell-derived and medullary thyroid cancer, including corrective gene therapy (p53 restoration, expression of a dominant negative RET mutant), cytoreductive gene therapy (suicide gene/prodrug strategy herpes simplex virus-thymidine kinase [HSV-tk]/ganciclovir, antiangiogenic therapy with endostatin) and immunomodulatory gene therapy (expression of interleukin (IL)-2 and IL-12). Furthermore, cloning of the sodium iodide symporter (NIS) gene has paved the way for the development of a novel cytoreductive gene therapy strategy based on NIS gene transfer followed by the application of radioiodine therapy ((131)I). NIS gene delivery into medullary and follicular cell-derived thyroid cancer cells has been shown to be capable of establishing or restoring radioiodine accumulation and might therefore represent an effective therapy for medullary and dedifferentiated thyroid tumors that lack iodide accumulating activity. The data summarized in this review article clearly demonstrate that the currently available strategies represent potentially curative novel therapeutic approaches for future gene therapy of thyroid cancer. The combination of different therapeutic genes has been demonstrated to be very useful to enhance therapeutic efficacy and seems to have a promising role at least as part of a multimodality approach for advanced thyroid cancer.

Impact of functional genomics and proteomics on radionuclide imaging

The assessment of gene function following the completion of human genome sequencing may be performed using radionuclide imaging procedures. These procedures are needed for the evaluation of genetically manipulated animals or newly designed biomolecules, which requires a thorough understanding of physiology, biochemistry, and pharmacology. The experimental approaches will involve many new technologies, including in vivo imaging with single photon emission computed tomography and positron emission tomography. Nuclear medicine procedures may be applied for the determination of gene function and regulation using established and new tracers, or using in vivo reporter genes, such as genes encoding enzymes, receptors, antigens, or transporters. Visualization of in vivo reporter gene expression can be performed using radiolabeled substrates, antibodies, or ligands. Combinations of specific promoters and in vivo reporter genes may deliver information about the regulation of the corresponding genes. Furthermore, protein-protein interactions and activation of signal transduction pathways may be visualized noninvasively. The role of radiolabeled antisense molecules for the analysis of messenger ribonucleic acid (RNA) content has to be investigated. However, possible applications are therapeutic intervention using triplex oligonucleotides with therapeutic isotopes, which can be brought near to specific deoxyribonucleic acid sequences to induce deoxyribonucleic acid strand breaks at selected loci. Imaging of labeled siRNA makes sense if these are used for therapeutic purposes to assess the delivery of these new drugs to their target tissue. Pharmacogenomics will identify new surrogate markers for therapy monitoring, which may represent potential new tracers for imaging. Drug distribution studies for new therapeutic biomolecules are needed at least during preclinical stages of drug development. New treatment modalities, such as gene therapy with suicide genes, will need procedures for therapy planning and monitoring. Finally, new biomolecules will be developed by bioengineering methods, which may be used for the isotope-based diagnosis and treatment of disease.

Endocrine aspects of cancer gene therapy

The field of cancer gene therapy is in continuous expansion, and technology is quickly moving ahead as far as gene targeting and regulation of gene expression are concerned. This review focuses on the endocrine aspects of gene therapy, including the possibility to exploit hormone and hormone receptor functions for regulating therapeutic gene expression, the use of endocrine-specific genes as new therapeutic tools, the effects of viral vector delivery and transgene expression on the endocrine system, and the endocrine response to viral vector delivery. Present ethical concerns of gene therapy and the risk of germ cell transduction are also discussed, along with potential lines of innovation to improve cell and gene targeting.

Noninvasive imaging of protein-protein interactions in living organisms

Genomic research is expected to generate new types of complex observational data, changing the types of experiments as well as our understanding of biological processes. The investigation and definition of relationships among proteins is essential for understanding the function of each gene and the mechanisms of biological processes that specific genes are involved in. Recently, a study by Paulmurugan et al. demonstrated a tool for in vivo noninvasive imaging of protein-protein interactions and intracellular networks.

Functional genomics and radioisotope-based imaging procedures

After the sequencing of the human genome has been completed, non-invasive imaging studies are needed to assess the function of new genes in living organisms. The evaluation of genetically manipulated animals or new designed biomolecules will require a thorough understanding of physiology, biochemistry and pharmacology, and the experimental approaches will involve many new technologies including in vivo imaging with single photon emission computed tomography (SPECT) and positron emission tomography (PET). Nuclear medicine procedures may be applied for the determination of gene function and regulation using established and new tracers or using in vivo reporter genes such as enzymes, receptors, antigens or transporters. Pharmacogenomics will identify new surrogate markers for therapy monitoring which may represent potential new tracers for imaging. Also, drug distribution studies for new therapeutic biomolecules are needed at least during preclinical stages of drug development. Clinical gene therapy needs non-invasive tools to evaluate the efficiency of gene transfer. These informations can be used for therapy planning, follow-up studies in treated tumors and as an indicator of prognosis. Therapy planning is performed by the assessment of gene expression for example using radio-labeled specific substrates to determine the activity of suicide enzymes such as the Herpes Simplex Virus thymidine kinase. Follow-up studies with single photon emission tomography or positron emission tomography may be done to evaluate early or late effects of gene therapy on tumor metabolism or proliferation. Finally, new biomolecules will be developed by bioengineering methods which may be used for isotope-based diagnosis and treatment of disease.

Cardiomyocyte-specific gene expression following recombinant adeno-associated viral vector transduction

Recombinant adeno-associated viral (rAAV) vectors hold promise for delivering genes for heart diseases, but cardiac-specific expression by the use of rAAV has not been demonstrated. To achieve this goal rAAV vectors were generated expressing marker or potentially therapeutic genes under the control of the cardiac muscle-specific alpha myosin heavy chain (MHC) gene promoter. The rAAV-MHC vectors expressed in primary cardiomyocytes with similar kinetics to rAAV-CMV; however, expression by the rAAV-MHC vectors was restricted to cardiomyocytes. rAAV vectors have low cytotoxicity, and it is demonstrated here that rAAV fails to induce apoptosis in cardiomyocytes compared with a recombinant adenoviral vector. rAAV-MHC or rAAV-CMV vectors were administered to mice to determine the specificity of expression in vivo. The rAAV-MHC vectors expressed specifically in cardiomyocytes, whereas the control rAAV-CMV vector expressed in heart, skeletal muscle, and brain. rAAV-MHC transduction resulted in long term (16 weeks) expression of human growth hormone following intracardiac, yet not intramuscular, injection. Finally, we defined the minimal MHC enhancer/promoter sequences required for specific and robust in vivo expression in the context of a rAAV vector. For the first time we describe a panel of rAAV vectors capable of long term cardiac specific expression of intracellular and secreted proteins.