Drug selection with paclitaxel restores expression of linked IL-2 receptor gamma -chain and multidrug resistance (MDR1) transgenes in canine bone marrow

In vivo selection with lentiviral expression of Bcl2T69A/S70A/S87A mutant in hematopoietic stem cell-transplanted mice

Current in vivo selections for hematopoietic stem cell (HSC)-based gene therapy are drug dependent and not without risk of cytotoxicity or tumorigenesis. We developed a new in vivo selection system with the non-phosphorylatable Bcl2 mutant Bcl2^{T69A/S70A/S87A} (Bcl2^AAA), which makes in vivo selection drug independent and without risk of cytotoxicity or tumorigenesis. We demonstrated in HSC-transplanted mice that Bcl2^AAA facilitated efficient in vivo selection in the absence of any exogenously applied drugs under both myeloablative and non-myeloablative conditioning. In mice transplanted with retrovirally transduced sca-1-positive bone marrow cells, the marked cell level increased from 26.38% of input transduced cells to 92.61 ± 0.95% of peripheral blood cells for myeloablative transplantation or to 37.82 ± 6.35% for non-myeloablative transplantation 6 months after transplantation. Bcl2^AAA did not induce tumorigenesis and does not influence hematopoiesis and the function of the reconstituted blood system. However, the high-level constitutive expression of Bcl2^AAA mediated by retroviral vector induced exhaustion of the marked cells after tertiary transplantation. Fortunately, low-level constitutive expression of Bcl2^AAA driven by an internal promoter in lentiviral vector could both maintain the marked cell level (24.13 ± 5.27%, 27.17 ± 5.51%, 24.33 ± 5.08%, and 22.07 ± 4.44% for primary, secondary, tertiary, and quaternary recipients) and avoid the exhaustion of the marked cells even in quaternary recipients. Importantly, the low-level constitutive expression of Bcl2^AAA did not induce tumorigenesis. Thus, the in vivo selection employing the low-level constitutive expression of Bcl2^AAA provides a general platform which is relevant for widespread applications of gene therapy.

Towards in vivo amplification: Overcoming hurdles in the use of hematopoietic stem cells in transplantation and gene therapy

With the advent of safer and more efficient gene transfer methods, gene therapy has become a viable solution for many inherited and acquired disorders. Hematopoietic stem cells (HSCs) are a prime cell compartment for gene therapy aimed at correcting blood-based disorders, as well as those amenable to metabolic outcomes that can effect cross-correction. While some resounding clinical successes have recently been demonstrated, ample room remains to increase the therapeutic output from HSC-directed gene therapy. In vivo amplification of therapeutic cells is one avenue to achieve enhanced gene product delivery. To date, attempts have been made to provide HSCs with resistance to cytotoxic drugs, to include drug-inducible growth modules specific to HSCs, and to increase the engraftment potential of transduced HSCs. This review aims to summarize amplification strategies that have been developed and tested and to discuss their advantages along with barriers faced towards their clinical adaptation. In addition, next-generation strategies to circumvent current limitations of specific amplification schemas are discussed.

F2A sequence linking MGMT(P140K) and MDR1 in a bicistronic lentiviral vector enables efficient chemoprotection of haematopoietic stem cells

Chemoprotection of haematopoietic stem cells (HSCs) by gene therapeutic transfer of drug-resistance genes represents the encouraging approach to prevent myelosuppression, which is one of the most severe side effects in tumor therapy. Thus, we cloned and evaluated six different bicistronic lentiviral SIN vectors encoding two transgenes, MGMT(P140K) (an O(6)-benzylguanine-resistant mutant of methylguanine-DNA methyltransferase) and MDR1 (multidrug resistance 1), using various linker sequences (IRESEMCV, IRESFMDV and 2A-element of FMDV (F2A)). Expression of both transgenes in HL-60 and in K562 cells was assayed by quantitative real-time PCR. Combination therapy with ACNU plus paclitaxel in HL-60 cells and with carmustin (BCNU) plus doxorubicin in K562 cells resulted in the most significant survival advantage of cells transduced with the lentiviral vector HR'SIN-MGMT(P140K)-F2A-MDR1 compared with untransduced cells. In human HSCs, overexpression of both transgenes by this vector also caused significantly increased survival and enrichment of transduced cells after treatment with BCNU plus doxorubicin or temozolomide plus paclitaxel. In summary, we could show significant chemoprotection by overexpression of MDR1 and MGMT(P140K) with a lentiviral vector using the F2A linker element in two different haematopoietic cell lines and in human primary HSCs with various combination regimens. Consequently, we are convinced that these in vitro investigations will help to improve combination chemotherapy regimens by reducing myelotoxic side effects and increasing the therapeutic efficiency.

Methylguanine DNA methyltransferase-mediated drug resistance-based selective enrichment and engraftment of transplanted stem cells in skeletal muscle

Cell replacement therapy using stem cell transplantation holds much promise in the field of regenerative medicine. In the area of hematopoietic stem cell transplantation, O(6)-methylguanine-DNA methyltransferase MGMT (P140K) gene-mediated drug resistance-based in vivo enrichment strategy of donor stem cells has been shown to achieve up to 75%-100% donor cell engraftment in the host's hematopoietic stem cell compartment following repeated rounds of selection. This strategy, however, has not been applied in any other organ system. We tested the feasibility of using this MGMT (P140K)-mediated enrichment strategy for cell transplantation in skeletal muscles of mice. We demonstrate that muscle cells expressing an MGMT (P140K) drug resistance gene can be protected and selectively enriched in response to alkylating chemotherapy both in vitro and in vivo. Upon transplantation of MGMT (P140K)-expressing male CD34(+ve) donor stem cells isolated from regenerating skeletal muscle into injured female muscle treated with alkylating chemotherapy, donor cells showed enhanced engraftment in the recipient muscle 7 days following transplantation as examined by quantitative-polymerase chain reaction using Y-chromosome specific primers. Fluorescent in situ hybridization analysis using a Y-chromosome paint probe revealed donor-derived de novo muscle fiber formation in the recipient muscle 14 days following transplantation, with approximately 12.5% of total nuclei within the regenerated recipient muscle being of donor origin. Following engraftment, the chemo-protected donor CD34(+ve) cells induced substantial endogenous regeneration of the chemo-ablated host muscle that is otherwise unable to self-regenerate. We conclude that the MGMT (P140K)-mediated enrichment strategy can be successfully implemented in muscle.

In vivo selection of hematopoietic progenitor cells and temozolomide dose intensification in rhesus macaques through lentiviral transduction with a drug resistance gene

Major limitations to gene therapy using HSCs are low gene transfer efficiency and the inability of most therapeutic genes to confer a selective advantage on the gene-corrected cells. One approach to enrich for gene-modified cells in vivo is to include in the retroviral vector a drug resistance gene, such as the P140K mutant of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT*). We transplanted 5 rhesus macaques with CD34+ cells transduced with lentiviral vectors encoding MGMT* and a fluorescent marker, with or without homeobox B4 (HOXB4), a potent stem cell self-renewal gene. Transgene expression and common integration sites in lymphoid and myeloid lineages several months after transplantation confirmed transduction of long-term repopulating HSCs. However, all animals showed only a transient increase in gene-marked lymphoid and myeloid cells after O6-benzylguanine (BG) and temozolomide (TMZ) administration. In 1 animal, cells transduced with MGMT* lentiviral vectors were protected and expanded after multiple courses of BG/TMZ, providing a substantial increase in the maximum tolerated dose of TMZ. Additional cycles of chemotherapy using 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) resulted in similar increases in gene marking levels, but caused high levels of nonhematopoietic toxicity. Inclusion of HOXB4 in the MGMT* vectors resulted in no substantial increase in gene marking or HSC amplification after chemotherapy treatment. Our data therefore suggest that lentivirally mediated gene transfer in transplanted HSCs can provide in vivo chemoprotection of progenitor cells, although selection of long-term repopulating HSCs was not seen.

Gene therapy with drug resistance genes

A major side effect of cancer chemotherapy is myelosuppression. Expression of drug-resistance genes in hematopoietic stem cells (HSC) using gene transfer methodologies holds the promise of overcoming marrow toxicity in cancer chemotherapy. Adequate protection of marrow cells in cancer patients from myelotoxicity in this way would permit the use of escalating doses of chemotherapy for eradicating residual disease. A second use of drug-resistance genes is for coexpression with a therapeutic gene in HSCs to provide a selection advantage to gene-modified cells. In this review, we discuss several drug resistance genes, which are well suited for in vivo selection as well as other newer candidate genes with potential for use in this manner.

CE-LIF coupled with flow cytometry for high-throughput quantitation of fluorophores in single intact cells

We report a method of coupled CE-LIF detection with flow cytometry for high-throughput determination and quantitation of fluorophores in single intact K562/S (KS) cells. The membrane properties of KS cell including fluophore transport rate and apparent permeability coefficient were further quantitatively characterized. The method has advantages for accurate quantitation and unique capacity of high-throughput analysis. The strategy will be useful for the quantitation of fluorophores in the intact cells, such as measurement of multidrug resistance, quantitation of specific protein expression, and quantitative characterization of protein and enzyme functions.

Hematopoietic stem cell transduction and amplification in large animal models

Progress in retroviral gene transfer to large animal hematopoietic stem cells (HSCs) has led to efficient, reproducible long-term marking in both canine and nonhuman primate models. Successes for HSC gene therapy have occurred in the severe combined immunodeficiency setting, in which transduced cells have a selective advantage. However, for most diseases, the therapeutic transgene does not confer a sufficient survival advantage, and increasing the percentage of gene-marked cells in vivo will be necessary to observe a therapeutic effect. In vivo amplification should expand the potential of HSC gene therapy, and progress in this area has benefited greatly from the use of large animal models where efficacy and toxicity have often not correlated with results in murine models. To date, the best results have been observed with O(6)-methylguanine-DNA methyltransferase (MGMT) selection, with which increases in gene-marked repopulating cells have been maintained long-term, likely because of the toxicity of 1,3-bis-(2-chloroethyl)-1-nitrosourea and temozolomide to quiescent HSCs. Using MGMT selection, long-term marking levels exceeding 50% can now be routinely attained with minimal toxicity. There is cause to be optimistic that HSC gene therapy with in vivo amplification will soon allow the treatment of several genetic and infectious diseases.

The molecular basis of multidrug resistance in cancer: the early years of P-glycoprotein research

The discovery and characterization of P-glycoprotein, an energy-dependent multidrug efflux pump, as a mechanism of multidrug resistance in cancer is generally accepted as a significant contribution to the ongoing effort to end death and suffering from this disease. The historical reflections of Victor Ling and Michael Gottesman concerning the early years of this research highlight the important contributions of the multidisciplinary teams involved in these studies, and illustrate how technological developments in biochemistry and molecular and cell biology enabled this discovery.

Hematopoietic stem cell gene therapy with drug resistance genes: an update

Transfer of drug resistance genes into hematopoietic stem cells (HSCs) has promise for the treatment of a variety of inherited, that is, X-linked severe combined immune deficiency, adenosine deaminase deficiency, thalassemia, and acquired disorders, that is, breast cancer, lymphomas, brain tumors, and testicular cancer. Drug resistance genes are transferred into HSCs either for providing myeloprotection against chemotherapy-induced myelosuppression or for selecting HSCs that are concomitantly transduced with another gene for correction of an inherited disorder. In this review, we describe ongoing experimental approaches, observations from clinical trials, and safety concerns related to the drug resistance gene transfer.

Survival of the fittest: in vivo selection and stem cell gene therapy

Stem cell gene therapy has long been limited by low gene transfer efficiency to hematopoietic stem cells. Recent years have witnessed clinical success in select diseases such as X-linked severe combined immunodeficiency (SCID) and ADA deficiency. Arguably, the single most important factor responsible for the increased efficacy of these recent protocols is the fact that the genetic correction provided a selective in vivo survival advantage. Since, for most diseases, there will be no selective advantage of gene-corrected cells, there has been a significant effort to arm vectors with a survival advantage. Two-gene vectors can be used to introduce the therapeutic gene and a selectable marker gene. Efficient in vivo selection strategies have been demonstrated in clinically relevant large-animal models. Mutant forms of the DNA repair-enzyme methylguanine methyltransferase in particular have allowed for efficient in vivo selection and have achieved sustained marking with virtually 100% gene-modified cells in large animals, and with clinically acceptable toxicity. Translation of these strategies to the clinical setting is imminent. Here, we review how in vivo selection strategies can be used to make stem cell gene therapy applicable to the treatment of a wider scope of genetic diseases and patients.

Tolerance by Selective In Vivo Expansion of Foreign Major Histocompatibility Complex-Transduced Autologous Bone Marrow1

BACKGROUND

Application of gene therapy to induce antigen-specific immune tolerance could be important for transplantation or treatment of autoimmune diseases. Hematopoietic stem cell-based gene therapy has been hampered by relatively weak gene expression in vivo and loss of transduced cells over time. Selective expansion of transduced hematopoietic stem cells has been accomplished by incorporating the dihydrofolate reductase (DHFR) gene into the gene transfer vector.

METHODS

To assess whether this strategy could be applied to transplantation, we constructed a retroviral vector plasmid (KA274) containing the cDNA encoding human leukocyte antigen (HLA)-A2.1 and a tyr22 mutant DHFR and generated vesicular stomatitis virus-G-pseudotyped recombinant retrovirus by transfection into 293GPG cells. Bone marrow cells from C57BL/6 mice were infected with KA274 at a multiplicity of infection of 100, and transplanted into lethally irradiated syngeneic mice.

RESULTS

After transplantation with transduced bone marrow, the proportion of peripheral blood cells expressing HLA-A2 ranged from 3.2% to 38% and increased 2- to 4.9-fold after selection for DHFR-expressing cells using trimetrexate and nitrobenzylmercaptpurine riboside 5' monophosphate. HLA-A2 expression remained above pretreatment levels throughout the study. Cytotoxic spleen cells from reconstituted mice lysed third-party HLA-B7-expressing targets but were unable to lyse HLA-A2-expressing targets. All KA274 reconstituted C57BL/6 mice accepted skin grafts from HLA-A2.1 transgenic mice for more than 245 days but rejected third-party Balb/c skin grafts in 12 days.

CONCLUSION

Long-term transgene expression and immunologic tolerance to retrovirus-encoded HLA-A2, equivalent to that obtained by donor bone marrow transplantation, was accomplished, and selective expansion of transduced bone marrow cells was induced using DHFR as a selectable marker.

Polyclonal chemoprotection against temozolomide in a large-animal model of drug resistance gene therapy

Incorporation of drug resistance genes into gene vectors has 2 important roles in stem cell gene therapy: increasing the proportion of gene-corrected cells in vivo (ie, in vivo selection) and marrow protection to permit higher or more tightly spaced doses of chemotherapy in the treatment of malignant diseases. We studied in a clinically relevant canine model of gene therapy the P140K mutant of the drug resistance gene methylguanine methyltransferase (MGMT), which encodes a DNA-repair enzyme that confers resistance to the combination of the MGMT inhibitor O(6)-benzylguanine (O(6)BG) and nitrosourea drugs such as carmustine and methylating agents such as temozolomide. Two dogs received MGMT(P140K)-transduced autologous CD34(+)-selected cells. After stable engraftment, gene marking in granulocytes was between 3% and 16% in the 2 animals, respectively. Repeated administration of O(6)BG and temozolomide resulted in a multilineage increase in gene-modified repopulating cells with marking levels of greater than 98% in granulocytes. MGMT(P140K) overexpression prevented the substantial myelosuppression normally associated with this drug combination. Importantly, hematopoiesis remained polyclonal throughout the course of the study. Extrahematopoietic toxicity was minimal, and no signs of myelodysplasia or leukemia were detected. These large-animal data support the evaluation of MGMT(P140K) in conjunction with O(6)BG and temozolomide in clinical trials.

Topical Colchicine Selection of Keratinocytes Transduced with the Multidrug Resistance Gene (MDR1) Can Sustain and Enhance Transgene Expression in vivo

In gene therapy, a clinically relevant therapeutic effect requires long-term expression of the desired gene at a level sufficient to correct or at least alleviate the underlying gene defect. One approach to achieve persistent as well as high-level transgene expression in a significant percentage of target cells would be to select cells expressing both the desired transgene and a linked selectable gene--such as the human multi-drug resistance (MDR1) gene--in a bicistronic vector. Because of its accessibility, the skin is a very attractive target tissue to select genetically modified cells, allowing topical application of a selecting agent, thus minimizing potential toxic side effects. Among the potential selecting drugs, agents that block cell division, such as colchicine, are of particular interest because the use of anti-mitotic drugs takes advantage of the rapid keratinocyte (KC) turnover in the epidermis and the need for continued proliferation to substitute the KC lost due to selection. Before assessing the therapeutic benefit of such an approach, several key questions need to be answered in preclinical models: (1) Does topical colchicine application achieve the desired in vivo effect by blocking KC mitosis without eliciting unwanted toxic side effects? (2) Are MDR-transduced (MDR+) human KC still able to proliferate and differentiate when treated with colchicine? (3) Can MDR+ KC be enriched by topical selection? (4) Does topical selection result in persistent transgene expression by selecting KC stem cells expressing MDR? To answer these questions and to test the feasibility of such an approach both an in vitro skin equivalent and an in vivo human skin graft model were developed in which MDR+ KC were treated with different dosages of colchicine. Quantitative and qualitative analyses of MDR expression in human KC showed that topical colchicine treatment selects high-level transgene expression in a high percentage of KC. Moreover, determination of transgene copy numbers demonstrated that MDR+ KC progenitor cells were enriched by topical selection resulting in long-term expression of the transgene in the skin. Thus, in summary, these models demonstrate that topical selection of MDR+ KC is a safe approach to efficiently enhance long-term gene expression in the skin and holds future promise for clinical gene therapy applications.

Modulating erythrocyte chimerism in a mouse model of pyruvate kinase deficiency

In vivo selection may provide a means to increase the relative number of cells of donor origin in recipients with hemopoietic chimerism. We have tested whether in vivo selection using chemical inducers of dimerization (CIDs) can direct the expansion of transduced normal donor erythrocytes in recipients with chimerism using a mouse model of pyruvate kinase deficiency. Marrow cells from normal CBA/N mice were transduced with a vector (F36Vmpl(GFP)) that promotes cell growth in the presence of CIDs. Transduced cells were then transplanted into minimally conditioned, pyruvate kinase-deficient recipients (CBA-Pk-1(slc)/Pk-1(slc)) to establish stable chimerism. CID administration resulted in expansion of normal donor erythrocytes and improvement of the anemia. The preferential expansion of normal erythrocytes also resulted in a decrease in erythropoietin levels, reducing the drive for production of pyruvate kinase-deficient red blood cells. CID-mediated expansion of genetically modified erythrocytes could prove a useful adjunct to transplantation methods that achieve erythroid chimerism.

Transient in vivo selection of transduced peripheral blood cells using antifolate drug selection in rhesus macaques that received transplants with hematopoietic stem cells expressing dihydrofolate reductase vectors

One of the main obstacles for effective human gene therapy for hematopoietic disorders remains the achievement of an adequate number of genetically corrected blood cells. One approach to this goal is to incorporate drug resistance genes into vectors to enable in vivo selection of hematopoietic stem cells (HSCs). Although a number of drug resistance vectors enable HSC selection in murine systems, little is known about these systems in large animal models. To address this issue, we transplanted cells transduced with dihydrofolate resistance vectors into 6 rhesus macaques and studied whether selection of vector-expressing cells occurred following drug treatment with trimetrexate and nitrobenzylmercaptopurineriboside-phosphate. In some of the 10 administered drug treatment courses, substantial increases in the levels of transduced peripheral blood cells were noted; however, numbers returned to baseline levels within 17 days. Attempts to induce stem cell cycling with stem cell factor and granulocyte-colony stimulating factor prior to drug treatment did not lead to sustained enrichment for transduced cells. These data highlight an important species-specific difference between murine and nonhuman primate models for assessing in vivo HSC selection strategies and emphasize the importance of using drugs capable of inducing selective pressure at the level of HSCs.

Animal models for mucopolysaccharidoses and their clinical relevance

The mucopolysaccharidoses (MPS) are characterized by the accumulation of glycosaminoglycans (GAG) and result from the impaired function of one of 11 enzymes required for normal GAG degradation. MPS II was the first MPS to be defined clinically in humans and is caused by deficient activity of the enzyme iduronate-2-sulphatase. MPS VI was the first MPS recognized in an animal; since then, all but MPS IIIC and IX have been described as naturally occurring in animals or made by knock-out technology. As in humans, all are inherited as autosomal recessive traits, except for MPS II, which is X-linked. Most animal colonies have been established from single related heterozygous animals, making the affected offspring homozygous for the same mutant allele. Importantly, these models have disease pathology that is similar to that seen in humans, making the animals extremely valuable for the investigation of disease pathogenesis and the testing of therapies. Large animal homologues are similar to humans in natural genetic diversity, approaches to therapy and care, and the possibility of evaluating long-term effects of treatment. Therapeutic strategies for MPS include enzyme replacement therapy, heterologous bone marrow transplantation, and somatic cell gene transfer, all of which have been tested in animals with some success.

Drug-selected co-expression of P-glycoprotein and gp91 in vivo from an MDR1-bicistronic retrovirus vector Ha-MDR-IRES-gp91

BACKGROUND

Retroviral transduction of human hematopoietic stem cells is an attractive strategy in gene therapy; however, transduction efficiency and duration of transgene expression may not be satisfactory in current protocols. Co-expression of a human multidrug resistance gene (MDR1) with a therapeutic gene affords selectable growth advantage to genetically modified cells.

METHODS

A bicistronic retrovirus vector, Ha-MDR-IRES-gp91, was constructed for the co-expression of MDR1 and gp91, a gene responsible for X-linked chronic granulomatous disease (X-CGD). Drug-selected co-expression of P-glycoprotein and gp91 was evaluated in transduced cells.

RESULTS

Epstein-Barr virus-transformed B cells from X-CGD patients transduced with Ha-MDR-IRES-gp91 co-expressed human P-glycoprotein and gp91, and acquired superoxide-generating activity. Human CD34-positive cells from an X-CGD patient were transduced with Ha-MDR-IRES-gp91 and subsequently treated with 2 ng/ml vincristine. After 13 days, 20% of Ha-MDR-IRES-gp91-transduced cells were P-glycoprotein- and gp91-positive by FACS analysis. The superoxide-generating activity of the transduced population was 27% of that of normal cells. Mice transplanted with Ha-MDR-IRES-gp91-transduced bone marrow cells showed co-expression of P-glycoprotein and gp91 in peripheral blood mononuclear cells. By administering paclitaxel, the proportions of P-glycoprotein- and gp91-positive cells were increased in all the four mice examined. When mice transplanted with Ha-MDR-IRES-gp91-transduced cells were repeatedly administered paclitaxel, the ratios of P-glycoprotein- and gp91-positive cells were maintained for over 1 year.

CONCLUSIONS

These results suggest that MDR1-bicistronic vectors may be useful to select the transduced hematopoietic cells in vivo. This may lead to the sustained expression of transgenes in the blood cells of patients treated with stem cell gene therapy.

Side effects of retroviral gene transfer into hematopoietic stem cells

Recent conceptual and technical improvements have resulted in clinically meaningful levels of gene transfer into repopulating hematopoietic stem cells. At the same time, evidence is accumulating that gene therapy may induce several kinds of unexpected side effects, based on preclinical and clinical data. To assess the therapeutic potential of genetic interventions in hematopoietic cells, it will be important to derive a classification of side effects, to obtain insights into their underlying mechanisms, and to use rigorous statistical approaches in comparing data. We here review side effects related to target cell manipulation; vector production; transgene insertion and expression; selection procedures for transgenic cells; and immune surveillance. We also address some inherent differences between hematopoiesis in the most commonly used animal model, the laboratory mouse, and in humans. It is our intention to emphasize the need for a critical and hypothesis-driven analysis of "transgene toxicology," in order to improve safety, efficiency, and prognosis for the yet small but expanding group of patients that could benefit from gene therapy.

Topical colchicine selection of keratinocytes transduced with the multidrug resistance gene (MDR1) can sustain and enhance transgene expression in vivo

For skin gene therapy, achieving prolonged high-level gene expression in a significant percentage of keratinocytes (KC) is difficult because we cannot selectively target KC stem cells. We now demonstrate that topical colchicine treatment can be used to select, in vivo, KC progenitor cells transduced with the multidrug resistance gene (MDR1). When human skin equivalents containing MDR1-transduced KC were grafted onto immunocompromised mice, topical colchicine treatments significantly increased (7-fold) the percentage of KC expressing MDR1, compared to vehicle-treated controls, for up to 24 wk. Topical colchicine treatment also significantly enhanced the amount of MDR1 protein expressed in individual KC. Furthermore, quantitative real-time PCR analysis of MDR1 transgene copy number demonstrates that topical colchicine treatment selects and enriches for KC progenitor cells in the skin that contain and express MDR1. For clinical skin gene therapy applications, this in vivo selection approach promises to enhance both the duration and expression level of a desired therapeutic gene in KC, by linking its expression to the MDR1 selectable marker gene.