Addition of the human interferon beta scaffold attachment region to retroviral vector backbones increases the level of in vivo transgene expression among progeny of engrafted human hematopoietic stem cells

Absence of durable high-level expression of transgenes from Moloney murine leukemia (Mo-MuLV) retroviral vectors has been a hurdle in bringing effective gene therapy to the clinic. In this study we have analyzed transgene expression among the progeny of mobilized hematopoietic stem cells (HSCs), comparing Mo-MuLV and mouse stem cell virus (MSCV) vectors, with or without addition of a scaffold attachment region (SAR) from the human interferon beta gene. Retroviral (RV) vector supernatant quality was assessed by comparing NGFR transgene expression by HEL cells, and transgene delivery and expression by CD34(+) cells 72 hr after transduction, using real-time PCR and FACS analysis. This is the first description of the effect of SAR within both Mo-MuLV and MSCV vector backbones on long-term RV transgene expression among in vivo HSC progeny in HSC repopulation assays (SCID-hu bone and NOD/SCID). After transduction of mobilized CD34(+) cells with MSCV-SAR vector, transgene expression was observed among a mean of 10% of donor HSC progeny in the SCID-hu bone (range, 0.6-43%). The predominant effect of SAR was to increase the mean fluorescence intensity (MFI) of transgene expression among HSC progeny in both in vivo bone repopulation models (three- to fourfold), and after long-term stromal cultures (twofold).

Real-time t(11;14) and t(14;18) PCR assays provide sensitive and quantitative assessments of minimal residual disease (MRD)

Non-Hodgkin's lymphoma (NHL) arises as a clonal transformation of normal B and T cell differentiation and is often characterized by a higher incidence of specific chromosomal translocations. We have developed real-time TaqMan PCR assays directed toward two of these tumor-associated DNA markers, the t(14;18)(q32;q21.3) at the major breakpoint region of the bcl-2 gene and the t(11;14)(q13;q32) at the bcl-1 major translocation cluster. During analysis of serial dilutions of t(14;18)-positive DNA, the t(14;18) real-time PCR was at least as sensitive as nested PCR and demonstrated enhanced quantitative potential. Moreover, in a blinded comparison of the t(14;18) real-time PCR and a clinically validated nested PCR protocol using 134 cell line and patient DNA samples, the real-time PCR detected the translocation in 30.0% more cases than nested PCR. Both the t(14;18) and t(11;14) real-time PCR assays were used to quantitate minimal residual disease (MRD) in an NHL clinical trial assessing the safety and efficacy of a tumor-purging protocol in autologous stem cell transplantation. The assays were also used to evaluate disease depletion in an ex vivo tumor spiking model in which normal peripheral blood was spiked with tumor cell lines and processed according to the clinical purging method. PCR data from both the clinical trial and the ex vivo model demonstrated a 4 to 6 log reduction in tumor cells during CD34+ and CD34+ Thy-1+ enrichment. Because the t(14;18) and t(11;14) real-time PCR assays are very sensitive, quantitative, rapid, and require no post-PCR manipulation, they may serve as practical alternatives to nested PCR.

Hematopoietic retroviral gene marking in patients with follicular non-Hodgkin's lymphoma

We conducted a double retroviral vector (RV) gene marking trial to test for the possible contribution to relapse of follicular non-Hodgkin's lymphoma (FNHL) cells present in bone marrow (BM) and peripheral blood (PB) grafts used for hematopoietic reconstitution of patients undergoing myelaoblative chemotherapy and autologous transplant. CD34 positive selection using the CellPro Ceprate CD34 column was performed on PB mononuclear cells obtained after cyclophosphamide/G-CSF mobilization. CD34 positive cells were exposed for 4-6 hours to the LNL6 or G1 Na RV in the absence of growth factors or stromal monolayers. One week later, BM mononuclear cells were similarly processed. Patients then received total body irradiation (TBI), cyclophosphamide, and etoposide followed by infusion of both PB and BM CD34 positive cells. Semiquantitative Southern blot analysis of DNA t(14;18) amplification products showed approximately a three log reduction in t(14;18) positive cells after CD34 positive selection. The first patient showed evidence of engraftment with RV positive BM and PB cells for 9 months. He relapsed one year after transplant. At relapse, one year after transplant, he had lost evidence of RV positive cells in ficolled mononuclear BM and PB cells as well as in CD19 positive cells. The second and third patients showed evidence of engraftment with RV positive cells up to 9 and 6 months post BMT respectively. The second and third patients are still in clinical remission. Our results demonstrate engraftment of RV transduced hematopoietic cells in the PB and BM for up to 9 months.

Improved quantitation of minimal residual disease in multiple myeloma using real-time polymerase chain reaction and plasmid-DNA complementarity determining region III standards

The complementarity determining region III of the rearranged immunoglobulin heavy chain gene has been the target for tumor-specific PCR assays for the detection and follow-up of B-cell malignancies. Previously, these assays have relied on gel-based end point data collection methods (i.e., band densitometry) and, thus, have provided at best a semiquantitative assessment of tumor levels. We show the development of a novel, real-time TaqMan PCR assay to quantitate residual multiple myeloma cells in clinical samples after high-dose chemotherapy and autologous stem cell transplantation. We provide evidence that real-time PCR is reproducible, sensitive, and quantitative. In a 40-replicate PCR experiment targeting the beta-actin gene, the coefficient of variation for threshold cycle data was 1.6%, whereas it increased to 13.6% and 31%, respectively, for end point fluorescence and gel densitometry. Moreover, in an experiment directly comparing standard curves obtained from band densitometry and threshold cycle data, the standard curve constructed from threshold cycle data had a multiple R2 value of 1.00 and demonstrated a dynamic range >4 logs, compared with the 2-log linear range of gel densitometry. Finally, we show that when a complementarity determining region III-specific PCR primer is used in conjunction with a consensus primer for the immunoglobulin heavy chain joining gene, plasmid DNA can be used as a readily available and effective substitute for clonal plasma-cell genomic DNA when preparing standards. By applying real-time PCR to the analysis of clinical samples, we are able to quantitate levels of tumor involvement with unparalleled reproducibility and statistical confidence. Real-time PCR technology may well provide the accuracy and reliability necessary for minimal residual disease detection to have real prognostic significance.

Simultaneous genetic chemoprotection of normal marrow cells and genetic chemosensitization of breast cancer cells in a mouse cancer gene therapy model

Repeated exposures to high doses of chemotherapy are often required to eradicate solid tumors. The success of such high-dose therapy is often limited by the myelosuppressive and toxic effects of these drugs on bone marrow cells and by the intrinsic resistance of the cancer cells to chemotherapy. To test ways of using genetic modification of somatic cells to circumvent both of these problems, we first genetically modified normal bone marrow cells with multidrug resistance-1 (MDR-1) cDNA retroviral vectors to render these cells more resistant to p-glycoprotein-transported agents. Experiments conducted previously in a mouse model in our laboratory (E. G. Hanania et al., Cancer Gene Ther., 2: 251-261, 1995; E. G. Hanania and A. B. Deisseroth, Cancer Gene Ther., 1: 21-25, 1994), which involve transplantation of mouse marrow cells modified with the human MDR-1 cDNA, showed that the majority of the marrow cells of these animals were resistant to repetitive administration of myelotoxic doses of Taxol, a MDR-1-transported drug. Next, to test the effects of genetically modifying marrow cells to make them resistant to chemotherapy, and genetically modifying tumor cells to make them more sensitive to chemotherapy, a mouse breast cancer cell line was transfected with a plasmid expression vector that contained a wild-type p53 chemosensitization transcription unit. Others have shown that restoration of the p53 gene can lead to decreased proliferation, reduced tumorigenicity, and increased sensitivity to chemotherapy-induced apoptosis. In this animal model, the simultaneous use of both chemoprotection and chemosensitization vectors, which provided protection of the normal cells to the chemotherapy and at the same time sensitized the tumor cells to the toxic effects of the chemotherapy, resulted in levels of in vivo tumor reduction that were not possible when either genetic chemoprotection of marrow cells or chemosensitization of tumor cells was used alone. These data should be of interest to those who are studying ways of using genetic modification to improve the outcome of established chemotherapy treatment programs for solid tumors.

Results of MDR-1 vector modification trial indicate that granulocyte/macrophage colony-forming unit cells do not contribute to posttransplant hematopoietic recovery following intensive systemic therapy

To formally test the hypothesis that the granulocyte/macrophage colony-forming unit (GM-CFU) cells can contribute to early hematopoietic reconstitution immediately after transplant, the frequency of genetically modified GM-CFU after retroviral vector transduction was measured by a quantitative in situ polymerase chain reaction (PCR), which is specific for the multidrug resistance-1 (MDR-1) vector, and by a quantitative GM-CFU methylcellulose plating assay. The results of this analysis showed no difference between the transduction frequency in the products of two different transduction protocols: "suspension transduction" and "stromal growth factor transduction." However, when an analysis of the frequency of cells positive for the retroviral MDR-1 vector posttransplantation was carried out, 0 of 10 patients transplanted with cells transduced by the suspension method were positive for the vector MDR-1 posttransplant, whereas 5 of 8 patients transplanted with the cells transduced by the stromal growth factor method were positive for the MDR-1 vector transcription unit by in situ or in solution PCR assay (a difference that is significant at the P = 0.0065 level by the Fisher exact test). These data suggest that only very small subsets of the GM-CFU fraction of myeloid cells, if any, contribute to the repopulation of the hematopoietic tissues that occurs following intensive systemic therapy and transplantation of autologous hematopoietic cells.

Regulation of retinoblastoma gene expression in a mouse mammary tumor model

We initiated studies to investigate the involvement of the murine retinoblastoma (RB) gene in mammary carcinogenesis using cell lines derived from mammary glands of irradiated mice. We found that the RB mRNA levels as well as the amounts of the nuclear phosphoprotein were significantly reduced as the cells progressed in vitro from non-tumorigenic to tumorigenic stages. To further investigate RB gene expression with cellular development and tumorigenicity, we transfected malignant cells with expression vectors containing the murine RB cDNA driven by either the SV40 or the mouse metallothionein promoter sequences. The neomycin resistant gene was included in both vectors and was used as a selective marker for the transfected cells. Cells with reduced levels of endogenous RB mRNA were stably transfected and showed increased expression of RB. In addition, the morphology of these cells were altered and their growth rates in culture were reduced. Injection of the transfected cells into host mice resulted in a delayed onset of tumors compared with nontransfected parental cells. Our studies provide experimental data to confirm that loss of RB gene activity is involved in neoplastic transformation of cells and support the multistep theory of carcinogenesis.

Recent advances in the application of gene therapy to human disease

PURPOSE

To review the recent advances in the application of genetic modification strategies to the therapy of human diseases for which a molecular defect is known.

METHODS

A computerized data bank search, the minutes of the National Institutes of Health (NIH) Recombinant DNA Advisory Committee published in the Federal Record, and reports of human clinical trials were used as data sources for this review. Clinical trials included in this review were published in the literature or approved by the NIH Recombinant DNA Advisory Committee.

STUDY SELECTION

Evaluations of the efficacy of genetic modification strategies in clinical trials in human and in animal models are summarized. The design and outcome of the genetic modification strategies employed are reviewed for 16 marking trials, 16 gene replacement trials for molecular deficiency diseases, 3 chemoprotection and 4 chemotherapy sensitization trials, 11 cancer vaccine trials, 2 antisense oligonucleotide trials, and 3 molecular immunotherapy trials.

DATA SYNTHESIS

The marking trials have shown that residual leukemia cells in the infused autologous marrow can contribute to relapse following autologous bone marrow transplants. The use of genetic modification for the replacement of missing or deficient genes in severe combined immunodeficiency, familial hypercholesterolemia, and cystic fibrosis has been associated with encouraging results so far. Clinical genetic therapy trials involving cancer vaccines, antisense oligonucleotides, adoptive immunotherapy with genetically modified T cells, delivery vectors containing interleukin-1 receptor inhibitor for arthritis, replacement strategies for storage diseases, and genetic suppression of human immunodeficiency viral replication are just commencing.

CONCLUSIONS

The clinical application of genetic modification techniques has thus far been successful in the beginning phases of this field. These early results suggest that continuation of gene therapy trials designed to correct the molecular changes that lead to disease states in humans is warranted. Evaluation of such clinical trials in the future may be based on the analysis of assays for short-term surrogate endpoints, as well as on the therapeutic outcomes of the trial, such as survival or remission.

Transduction of MDR1 into human and mouse haemopoietic progenitor cells: use of rhodamine (Rh123) to determine transduction frequency and in vivo selection

The MDR1 gene product P-glycoprotein (P-gp) extrudes several anticancer drugs including taxol and fluorescent dyes such as rhodamine (Rh123). Modulation of the level of P-gp expression has the potential of overcoming multidrug resistance. One possible approach is the retroviral transfer of the human MDR1 gene into murine and human bone marrow (BM) progenitor cells. The rationale for this approach is increased chemoprotection, which allows chemotherapy of a greater level of intensity to be delivered. In this study, flow cytometric measurement of Rh123 extrusion was used to test P-gp function in human and mouse haemopoietic progenitor cells, which had been transduced with a virus containing the human MDR1 transcription unit. Human CD34+ selected cells were analysed immediately following transduction. In two successive experiments MDR1 cDNA transduction resulted in a 7% and 11% increase of P-gp expressing Rh123 dull cells. To monitor transduction efficiency over time as well as the possibility of in vivo selection of drug-resistant BM cells in mice treated with increasing numbers of taxol cycles, the assay was also successfully applied to peripheral blood lymphocytes of mice transplanted with MDR1 transduced BM cells, demonstrating increased Rh123 efflux in transduced cells. Analysis of another fluorescence assay using fluorescein di-beta galactopyranoside as a substrate for beta-galactosidase in cells transduced with a MDR1: beta-gal activity. We conclude that the Rh123 efflux assay is a sensitive method to monitor P-gp function in MDR1 cDNA transduced cells, and may be used to enrich transduced cells via flow cytometric cell sorting for Rh123 dull cells.

Resistance to taxol chemotherapy produced in mouse marrow cells by safety-modified retroviruses containing a human MDR-1 transcription unit

We used an animal model system to transplant lethally-irradiated mice with one million marrow cells which had been: (1) collected from 5-fluorouracil (5-FU) treated mice; and (2) transduced with retroviruses containing a multiple drug resistance-1 (MDR-1) gene transcription unit. Following recovery from the transplant, we exposed these mice to doses of taxol ranging from 7 mg/kg to 30 mg/kg, which corresponds to doses of 68 to 268 mg/m2 in man. The median white blood cell count by 5 days after taxol (expressed as the percentage of the white blood cell count before taxol) was 83% (range 46-100%) in 11 courses of taxol in mice transplanted once with MDR-1 transduced marrow immediately after transplant, whereas the median white blood cell count by 5 days after taxol in mice not transplanted with MDR-1 marrow was 41% in nine courses of taxol (range 11-66%). This difference is statistically different at the P < 0.001 level (Wilcoxon test). One million marrow cells from the MDR-1 transplanted mice were harvested and serially transplanted through five additional cohorts of mice, and tested with taxol after each cohort. The white blood cell count (expressed as the percentage of pre-taxol white blood cell count) after each cohort ranged from 94-146% in the 29 mice transplanted with the transduced MDR-1 marrow, which had been through more than one transplant. This is statistically different from the median white blood cell count recovery after taxol in mice which have no human MDR-1 modified marrow (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

PML/RARalpha, a fusion protein in acute promyelocytic leukemia, prevents growth factor withdrawal-induced apoptosis in TF-1 cells

A unique mRNA produced by the t(15;17) (q22-24;q11-21) translocation in the leukemic cells of acute promyelocytic leukemia patients encodes a chimeric protein, PML/RARalpha, which is formed by the fusion of the retinoic acid receptor alpha (RARalpha) and the promyelocytic locus gene (PML). This translocation is often the only visible karyotypic aberration present which is detected in almost 100% of acute promyelocytic leukemia patients. As an initial step to study the role of PML/RARalpha in leukemogenesis, we attempted to express the fusion protein in hematopoietic cells through retrovirus-mediated gene transfer of the retroviral vector, pGPRCHT, which contains the PML/RARalpha cDNA. Transduction of the PML/RARalpha cDNA fragment used in this vector, which extends from the position 31 bp to the position 2638 bp in a transcription unit driven by the Moloney murine sarcoma virus LTR, was found to abrogate the growth factor dependence of TF-1 cells. In addition, introduction of PML/RARalpha into TF-1 cells can protect these cells from the apoptosis usually induced in TF-1 cells by growth factor withdrawal, as measured by three assays for apoptosis: morphology, DNA ladder formation, and end labeling of nicked DNA with fluorescent-conjugated nucleotide precursors followed by a fluorescence-activated cell sorting assay. These data suggest that the PML/RARalpha fusion protein may inhibit programmed cell death in myeloid cells.

Chemotherapy resistance to taxol in clonogenic progenitor cells following transduction of CD34 selected marrow and peripheral blood cells with a retrovirus that contains the MDR-1 chemotherapy resistance gene

A retrovirus containing the multiple drug resistance (MDR-1) cDNA, was used to transduce cultures of CD34 selected human marrow cells, on stromal monolayers in the presence of hematopoietic growth factors IL-3 and IL-6, following collection from patients recently recovered from chemotherapy-induced myelosuppression. In one experiment, these CD34 selected cells were grown in Dexter cultures for 35 days or more following MDR-1 transduction, and then plated in methylcellulose. Polymerase chain reaction (PCR) analysis of colonies picked after 10-14 days of methylcellulose culture, using a set of primers that are specific for the endogenous or the retrovirally transduced MDR-1, showed that the long-term culture initiating cells (LTCICs) were transduced by the MDR-1 virus. Analysis of the colonies from the CD34 selected MDR-1 transduced cells, with a reverse transcription (RT) PCR assay that could distinguish viral MDR-1 mRNA from endogenous MDR-1 mRNA, showed that the viral MDR-1 mRNA levels were much higher than that of the MDR-1 mRNA from the endogenous MDR-1 gene in the transduced CD34 selected cells. Fluorescence activated cell sorting (FACS) analysis of the CD34 selected transduced marrow cells within 48 h after the transduction, using the C219 and UIC2 monoclonal antibodies for p-glycoprotein, showed that the transduction frequency under these conditions varied from 7 to 20%. Rhodamine efflux studies showed that this additional p-glycoprotein was functional and that the frequency of cells with high p-glycoprotein levels was higher in the transduced cells than in the non-transduced cells. The resistance to taxol of the CD34 selected transduced cells, as judged by the plating efficiency of clonogenic progenitor cells derived from these cells by growth in methylcellulose supplemented with taxol was much higher in the transduced cells than in untransduced cells. In order to test the reproducibility of the transduction frequency of the retroviral supernatants from PA317 MDR-1 viral producer cells on CD34 selected cells, the virus produced from 12 different lots of supernatants from the PA317 MDR-1 producer cell line was used to transduce CD34 selected marrow cells from four different patients, and to transduce the peripheral blood cells of two additional patients collected following chemotherapy-induced myelosuppression. The supernatant lots used for these transduction experiments were tested by Microbiological Associates (Rockville, MD, USA), by the Mus dunni co-cultivation and amplification tests in the S+L-assay and found to be negative for replication-competent retrovirus, and later approved for human use by the Food and Drug Administration.(ABSTRACT TRUNCATED AT 400 WORDS)

Suppression of tumorigenesis by transcription units expressing the antisense E6 and E7 messenger RNA (mRNA) for the transforming proteins of the human papilloma virus and the sense mRNA for the retinoblastoma gene in cervical carcinoma cells

Human cervical carcinoma cell lines that harbor human papilloma virus (HPV) have been reported to express HPV E6 and E7 proteins at least in the beginning stages if not at all stages of the disease. The HPV E6 and E7 proteins bind to and inactivate the products of the p53 and retinoblastoma (Rb) tumor suppressor genes, which thereby allow the cervical carcinoma cells to circumvent the action of these tumor suppressor genes. We observed that the introduction of the antisense HPV 18 E6 and E7 sequences, as well as a sense cDNA for the human wild-type Rb gene into a human cervical carcinoma cell line (HeLa), which is positive for the HPV 18 provirus, decreased the in vitro and in vivo growth rate of the transfected cells if both antisense transcripts for the HPV 18 E6 and E7 and sense transcripts for human Rb were expressed. In addition, overexpression of a complementary DNA (cDNA) for the Rb messenger RNA was sufficient to slow the proliferation of HeLa cells, and the level of Rb cDNA expression was correlated with the degree to which the rate of growth of the tumor was slowed. The results of our experiments show that the presence of HPV E6 and E7 proteins and the resultant inactivation of Rb in cervical carcinoma cells contributes to the neoplastic phenotype even in highly evolved cervical carcinoma cell lines such as HeLa, which have been derived from a cervical carcinoma patient at an advanced stage of the disease process. These data suggest that the HPV proteins play a role not only at the beginning of cervical cancer, but also at advanced stages of this disease. These experiments may lead to genetic approaches to the control of this disease that involve antisense sequences that downregulate the E6 and E7 genes or lead to expression of the Rb gene.

Genetic marking shows that Ph+ cells present in autologous transplants of chronic myelogenous leukemia (CML) contribute to relapse after autologous bone marrow in CML

Relapse after autologous bone marrow transplantation for chronic myelogenous leukemia (CML) can be due either to the persistence of leukemia cells in systemic tissues following preparative therapy, or due to the persistence of leukemia cells in the autologous marrow used to restore marrow function after intensive therapy. To help distinguish between these two possible causes of relapse, we used safety-modified retroviruses, which contain the bacterial resistance gene NEO, to mark autologous marrow cells that had been collected from patients early in the phase of hematopoietic recovery after in vivo chemotherapy. The cells were then subjected to ex vivo CD34 selection following collection and 30% of the bone marrow were exposed to a safety-modified virus. This marrow was infused after delivery of systemic therapy, which consisted of total body irradiation (1,020 cGy), cyclophosphamide (120 mg/kg), and VP-16 (750 mg/m2). RT PCR assays specific for the bacterial NEO mRNA, which was coded for by the virus, and the bcr-abl mRNA showed that in two evaluable CML patients transplanted with marked cells, sufficient numbers of leukemia cells remained in the infused marrow to contribute to systemic relapse. In addition, both normal and leukemic cells positive for the retroviral transgenome persisted in the systemic circulation of the patients for at least 280 days posttransplant showing that the infused marrow was responsible for the return of hematopoiesis following the preparative therapy. This observation shows that it is possible to use a replication-incompetent safety-modified retrovirus in order to introduce DNA sequences into the hematopoietic cells of patients undergoing autologous bone marrow transplantation. Moreover, this data suggested that additional fractionation procedures will be necessary to reduce the probability of relapse after bone marrow transplantation in at least the advanced stages of the disease in CML patients undergoing autologous bone marrow transplantation procedures.

Serial transplantation shows that early hematopoietic precursor cells are transduced by MDR-1 retroviral vector in a mouse gene therapy model

The administration of high and repeated doses of chemotherapy has been hampered by the bone marrow toxicity imposed by these drugs. This obstacle can be circumvented by the introduction of chemotherapy resistance genes into the normal marrow cells, which are then transplanted back into the patient. To show that this approach can improve our ability to safely deliver high doses of chemotherapy, we used an animal model system to transplant bone marrow cells which have been transduced with a safety-modified retrovirus containing human multiple-drug resistance (MDR-1) cDNA into lethally irradiated mice. These studies produced mice whose bone marrow and peripheral blood displayed an increased level of MDR-1 expression and resistance to the myelotoxic side effects of Taxol. To determine whether sufficient numbers of early hematopoietic precursor cells were transduced with the MDR-1 retrovirus so that durable Taxol-resistant hematopoiesis would result, we serially transplanted the modified bone marrow cells into each of six successive cohorts of BALB/c mice. Taxol-resistant hematopoiesis with little or no myelosuppression was observed in all six of the cohorts. These data suggest that very early precursor cells were transduced by the vector. This animal model may be of use in the development of genetic therapy programs which use bone marrow to introduce therapeutic molecules into the systemic circulation, since it permits in vivo selection of genetically modified hematopoietic progenitor cells. Furthermore, the retroviral vector system we have used could have an immediate impact in the clinical setting, where it can protect patients from the myelosuppressive side effects of Taxol in advanced stages of human epithelial cancers.

Molecular approaches to the diagnosis and treatment of cancer

We have used autologous bone marrow transplantation (ABMT) as a setting to develop the genetic therapy of cancer in hematopoietic neoplasms based on the use of the bone marrow as a conduit through which to introduce regulatory molecules into the patient. This has involved three developmental phases: 1) learning how to develop genetic modification techniques; 2) learning how to develop in vivo selection techniques for the genetically modified cells; and 3) developing molecular vectors for modification of hematopoietic cells for therapy of cancer. These programs will be summarized in terms of their progress at the conference.

Genetic therapy of human neoplastic disease

Molecular biology has provided clinical investigators and basic scientists with the tools to identify those changes present within neoplastic hematopoietic and epithelial cells that lead to the evolution of unregulated patterns of cell growth. This information has made possible the development of therapy that involves genetic modification of either the normal hematopoietic cells (for chemoprotection), or the tumor cells themselves to suppress the growth of these cells. This article will summarize the clinical and laboratory data that is evolving in this area.