Local gene transfer to calcified tissue cells using prolonged infusion of a lentiviral vector.

Gene transfer using viral vectors offers the potential for the sustained expression of proteins in specific target tissues. However, in the case of calcified tissues, in vivo delivery remains problematic because of limited accessibility. The aim of this study was to test the efficiency of lentiviral vectors (LVs) on osteogenic cells in vitro, and determine the feasibility of directly transducing resident bone cells in vivo. LVs encoding for green fluorescent protein (GFP) and ameloblastin (AMBN), a protein associated with mineralization not reported in bone, were generated. The transduction efficiency of the LVs was evaluated using the MC3T3 cell line and primary calvaria-derived osteogenic cells. For in vivo delivery, the LVs were infused using osmotic minipumps through holes created in the bone of the rat hemimandible and tibia. The production of GFP and AMBN in vitro and in vivo was monitored using fluorescence microscopy. Both transgenes were expressed in MC3T3 and primary osteogenic cells. In vivo, GFP was detected at the infusion site and fibroblast-like cells, osteoblasts, osteocytes and osteoclasts expressed AMBN. Our data demonstrate, for the first time, that primary osteogenic cells are efficiently transduced with LVs and that their infusion is advantageous for locally delivering DNA to bone cells.

Correction of sickle cell disease in transgenic mouse models by gene therapy.

Sickle cell disease (SCD) is caused by a single point mutation in the human betaA globin gene that results in the formation of an abnormal hemoglobin [HbS (alpha2betaS2)]. We designed a betaA globin gene variant that prevents HbS polymerization and introduced it into a lentiviral vector we optimized for transfer to hematopoietic stem cells and gene expression in the adult red blood cell lineage. Long-term expression (up to 10 months) was achieved, without preselection, in all transplanted mice with erythroid-specific accumulation of the antisickling protein in up to 52% of total hemoglobin and 99% of circulating red blood cells. In two mouse SCD models, Berkeley and SAD, inhibition of red blood cell dehydration and sickling was achieved with correction of hematological parameters, splenomegaly, and prevention of the characteristic urine concentration defect.

Construction of a differentiated human hepatocyte cell line expressing the herpes simplex virus-thymidine kinase gene.

Transient support using a hybrid artificial liver (HAL) device is a promising treatment for the patients with acute liver failure. Primary human hepatocytes are an ideal source for HAL therapy; however, the number of human livers available for hepatocyte isolation is limited by competition for use in whole organ transplantation. To overcome this problem, we previously established a highly differentiated human fetal hepatocyte cell line OUMS-29. Considering the potential risk when using these genetically engineered cells in humans, additional safeguards should be added to make the cells more clinically useful. In this work, the herpes simplex virus thymidine kinase (HSVtk) gene was retrovirally introduced into OUMS-29 cells. One of the HSVtk-expressed clones, OUMS-29/thymidine kinase (TK), grew in chemically defined serum free medium and expressed the genes of albumin, asialoglycoprotein receptor, glutamine synthetase, glutathione-S-transferase pi, and blood coagulation factor X. In vitro sensitivity of the cells to ganciclovir was evaluated. Intrasplenic transplantation of 50 x 10(6) OUMS-29/TK cells prolonged the survival of 90% hepatectomized rats compared with medium injection alone (control). In the present study, we have established highly differentiated immortalized human hepatocytes with tight regulation. The cells may be clinically useful for HAL treatment.

Expansion of human hepatocyte populations by a retroviral gene transfer of simian virus 40 large T antigen.

A hybrid artificial liver (HAL) could be used to treat acute liver failure or to serve as a temporary support until orthotopic liver transplantation is available. Primary human hepatocytes are ideal as a source of hepatic function in a HAL device. However, the worldwide shortage of human livers available for hepatocyte isolation severely limits this form of therapy. A possible alternative is to use a tightly regulated cell line that can be economically grown in culture to have differentiated liver function. In this work, human hepatocytes were immortalized with a retroviral vector SSR#69 expressing the genes of simian virus 40 large T antigen and herpes simplex virus-thymidine kinase. One of the resulting clones, NKNT-3 , showed the gene expression of differentiated liver function and were sensitive to the antiviral agent ganciclovir. When transplanted into the spleen of rats subjected to 90% hepatectomy, NKNT-3 cells prolonged the survival of 90% hepatectomized rats. The cells provide the advantages of unlimited availability, sterility, uniformity, and freedom from pathogens. This work represents a potential novel strategy for resolving the organ shortage that currently limits the use of primary human hepatocytes to develop a HAL.

Successful retroviral gene transfer of simian virus 40 T antigen and herpes simplex virus-thymidine kinase into human hepatocytes.

Current clinical reports have indicated that hepatocyte transplantation (HTX) could be used in patients with liver failure and in children with liver-based metabolic diseases. One of the major limiting factors of HTX is a serious shortage of donor livers for hepatocyte isolation. To address this issue, we immortalized adult human hepatocytes with a retroviral vector SSR#69 expressing the genes of simian virus 40 large T antigen and herpes simplex virus-thymidine kinase simultaneously. One of the resulting clones, NKNT-3, grew steadily in chemically defined serum-free medium without any obvious crisis and showed the gene expression of differentiated liver functions. Under the administration of 5 microM ganciclovir, NKNT-3 cells stopped proliferation and died in in vitro experiments. We have established a tightly regulated immortal human hepatocyte cell line. The cells could allow the need for immediate availability of consistent and functionally uniform cells in sufficient quantity and adequate quality.

Cre/loxP-based reversible immortalization of human hepatocytes.

An ideal alternative to the primary human hepatocytes for hepatocyte transplantation would be to use a clonal cell line that grows economically in culture and exhibits the characteristics of differentiated, nontransformed hepatocytes following transplantation. The purpose of the present studies was to establish a reversibly immortalized human hepatocyte cell line. Human hepatocytes were immortalized with a retroviral vector SSR#69 expressing simian virus 40 large T antigen (SV40Tag) gene flanked by a pair of loxP recombination targets. One of the resulting clones, NKNT-3, showed morphological characteristics of liver parenchymal cells and expressed the genes of differentiated liver functions. NKNT-3 cells offered unlimited availability. After an adenoviral delivery of Cre recombinase and subsequent differential selection, efficient removal of SV40Tag from NKNT-3 cells was performed. Here we represent that elimination of the retrovirally transferred SV40Tag gene can be excised by adenovirus-mediated site-specific recombination.

Construction of a non-tumorigenic rat hepatocyte cell line for transplantation: reversal of hepatocyte immortalization by site-specific excision of the SV40 T antigen.

BACKGROUND/AIMS: Hepatocytes immortalized with a temperature-sensitive SV40 large T antigen (SV40Tag) function as well as primary hepatocytes following transplantation to reverse hepatic encephalopathy and improve survival in rodents with liver failure. The continued presence of SV40Tag in the conditionally immortalized hepatocytes may increase the risk of malignant tumor growth in transplant recipients. METHODS: We immortalized hepatocytes using a recombinant retrovirus containing the gene encoding SV40Tag flanked by loxP recombination target sites. Excision of SV40Tag from immortalized cells could then be accomplished by site-specific recombination with Cre-recombinase. RESULTS: Cells immortalized with this recombinant virus expressed SV40Tag and doubled in number every 48 h. After excision of the gene encoding SV40Tag with Cre-recombinase, cells stopped growing, DNA synthesis fell by 90%, and production of liver-specific mRNAs was either increased or became newly detectable. In addition, the morphology and epithelial cell polarity of the cells became more characteristic of differentiated hepatocytes. To determine their malignant potential, immortalized hepatocytes were transfected to express a second oncogene, activated H-ras. SV40Tag+/H-ras+-immortalized cells were capable of anchorage-independent growth and developed into tumors when injected in severe combined immunodeficiency mice. While Cre-recombinase delivery by recombinant adenovirus infection was not 100% efficient, when SV40Tag excision occurred anchorage-independent growth stopped and tumor formation in immunodeficient mice was abolished. Immortalized hepatocytes also contained the gene encoding herpes simplex virus thymidine kinase and treatment with ganciclovir produced complete regression of established tumors in mice. CONCLUSIONS: These studies extend previous work that indicates that a transplantable hepatocyte cell line could be developed for clinical use.

Prevention of acute liver failure in rats with reversibly immortalized human hepatocytes.

Because of a critical shortage in suitable organs, many patients with terminal liver disease die each year before liver transplantation can be performed. Transplantation of isolated hepatocytes has been proposed for the temporary metabolic support of patients awaiting liver transplantation or spontaneous reversion of their liver disease. A major limitation of this form of therapy is the present inability to isolate an adequate number of transplantable hepatocytes. A highly differentiated cell line, NKNT-3, was generated by retroviral transfer in normal primary adult human hepatocytes of an immortalizing gene that can be subsequently and completely excised by Cre/Lox site-specific recombination. When transplanted into the spleen of rats under transient immunosuppression, reversibly immortalized NKNT-3 cells provided life-saving metabolic support during acute liver failure induced by 90% hepatectomy.

Efficient Cre/loxP site-specific recombination in a HepG2 human liver cell line.

A worldwide shortage of donor livers is a limiting factor of the clinical application of hepatocyte transplantation (HTX). To resolve this issue, we focused on a reversible immortalization system that allows temporary expansion of primary hepatocyte populations by transfer of an oncogene that can be subsequently excised. As a preliminary test toward this goal, we examined the efficacy of Cre/loxP site-specific recombination in a transformed human liver cell line, HepG2. The present study utilized retroviral transfer of a prototypical immortalizing gene, simian virus 40 large T antigen (SV40Tag), flanked by a pair of loxP recombination targets and adenovirus-mediated Cre/loxP recombination. Here we report that complete elimination of the retroviral transferred oncogene was achieved by site-specific recombination using a replication-deficient recombinant adenovirus vector producing Cre recombinase (Ad-Cre).

A new approach to develop a biohybrid artificial liver using a tightly regulated human hepatocyte cell line.

Currently patients with liver failure have been treated with a various liver support systems including a whole liver perfusion, a non-biological artificial liver, and a biohybrid artificial liver. In a hepatocyte-based bioreactor, porcine hepatocytes or transformed human liver tumor cells have been utilized because of the ease of preparation. According to the clinical data reported as of now, satisfactory results have not been obtained from the use of currently available liver support devices. One of the problems is limited availability of primary human liver cells for developing live support systems because of the shortage of human liver. To resolve this issue, human hepatocytes were immortalized with a retroviral vector SSR#69 which contained the genes of simian virus 40 large T antigen (SV40Tag) and herpes simplex virus-thymidine kinase (HSV-TK). One of the immortal cell lines, NKNT-3, showed the gene expression of differentiated liver functions, grew steadily in chemically defined serum-free CS-C medium, and doubled in number in about 48 hours. Essentially unlimited availability of NKNT-3 cells supports their clinical use for liver support devices. To realize the high density culture of NKNT-3 cells in a bioartificial liver device, we have developed cellulose microspheres (CMS) which contain cell adhesive GRGDS (Gly-Arg-Gly-Asp-Ser) peptides. Within 24 hours after starting a stirring suspension culture, GRGDS-CMS efficiently immobilized NKNT-3 cells. An electron microscopic examination demonstrated that NKNT-3 cells attached on GRGDS-CMS had well-developed mitochondria, rough reticulums, and villous extensions. In this article, we review the history of extracorporeal liver support systems and describe an attractive strategy for developing a novel extracorporeal liver assist device using NKNT-3 cells and GRGDS-coated cellulose microspheres.

Site-specific chromosomal integration in mammalian cells: highly efficient CRE recombinase-mediated cassette exchange.

Expression of experimental constructs in mammalian cells or transgenic animals is difficult to control because it is markedly influenced by position effects. This has limited both the analysis of cis -DNA regulatory elements for transcription and replication, and the physiological analysis of proteins expressed from transgenes. We report here two new methods based on the concept of recombinase-mediated cassette exchange (RMCE) to perform site-specific chromosomal integration. The first method permits the exchange of a negative selectable marker pre-localized on the chromosome with a transgene via a CRE-mediated double recombination between inverted Lox sites. Integration efficiency is close to 100 % of negatively selected mouse erythroleukemia cells and ranges from 10 to 50 % in embryonic stem cells. The second method allows RMCE with no selection at all except for cells that have taken up plasmid transiently. While less efficient, this technique permits novel experimental approaches. We find that integration of a transgene at a given genomic site leads to reproducible expression. RMCE should be useful to develop artificial genetic loci that impart specific and reproducible regulation of transgenes in higher eukaryotes. This should facilitate the analysis of cis -regulatory DNA elements governing expression and position effects, improve our control over the physiological effects of transgenes, and accelerate the development of animal models for complex human diseases.

Reversible immortalization of mammalian cells mediated by retroviral transfer and site-specific recombination.

A procedure of reversible immortalization of primary cells was devised by retrovirus-mediated transfer of an oncogene that could be subsequently excised by site-specific recombination. This study focused on the early stages of immortalization: global induction of proliferation and life span extension of cell populations. Comparative analysis of Cre/LoxP and FLP/FRT recombination in this system indicated that only Cre/LoxP operates efficiently in primary cells. Pure populations of cells in which the oncogene is permanently excised were obtained, following differential selection of the cells. Cells reverted to their preimmortalized state, as indicated by changes in growth characteristics and p53 levels, and their fate conformed to the telomere hypothesis of replicative cell senescence. By permitting temporary and controlled expansion of primary cell populations without retaining the transferred oncogene, this strategy may facilitate gene therapy manipulations of cells unresponsive to exogenous growth factors and make practical gene targeting by homologous recombination in somatic cells. The combination of retroviral transfer and site-specific recombination should also extend gene expression studies to situations previously inaccessible to experimentation.