Clinical potential of gene therapy: towards meeting the demand.

Since the discovery that new genetic material could be transferred into human cells resulting in induced expression of genes and proteins, clinicians and scientists have been working to harness the technology for clinical outcomes. This article provides a summary of the current status of developments within the broad discipline of clinical gene therapy. In pursuing the treatment of diverse clinical conditions, a wide variety of therapeutics, each tailor-made, may be required. Gene therapy offers the possibility of accurately and specifically targeting particular genetic abnormalities through gene correction, addition or replacement. It represents a compelling idea that adds a new dimension to our portfolio of credible therapeutic choices.

How we mobilize haemopoietic stem cells.

Mobilization and collection of haemopoietic stem and progenitor cells (HSPC) is the cornerstone of autologous and allogeneic stem cell transplantation for a wide variety of haematological and some non-haematological malignancies. Centres providing this service face the challenge of optimizing the likelihood of successful collection of transplantable doses of cells, while maximizing the efficiency of the apheresis unit and minimizing the risk of toxicity as well as mobilization failure. Recent developments in the understanding of the molecular mechanisms of mobilization have led to the emergence of novel strategies for HSPC mobilization, which may assist in meeting these imperatives. The task for clinicians is how to incorporate the use of these strategies into practice, in the light of emerging evidence for efficacy and safety of these agents. Herein, the literature is reviewed, and a proposed algorithm for HSPC mobilization is presented.

Loss of solute carriers in T cell-mediated rejection in mouse and human kidneys: an active epithelial injury-repair response.

T cell-mediated rejection of kidney allografts causes epithelial deterioration, manifested by tubulitis, but the mechanism remains unclear. We hypothesized that interstitial inflammation triggers a stereotyped epithelial response similar to that triggered by other types of injury such as ischemia-reperfusion. We identified solute carrier transcripts with decreased expression in mouse allografts, and compared their behavior in T cell-mediated rejection to native kidneys with ischemic acute tubular necrosis (ATN). Average loss of solute carrier expression was similar in ATN (77%) and T cell-mediated rejection (75%) with high correlation of individual transcripts. Immunostaining of SLC6A19 confirmed loss of proteins. Analysis of human kidney transplant biopsies confirmed that T cell-mediated rejection and ATN showed similar loss of solute carrier mRNAs. The loss of solute carrier expression was weakly correlated with interstitial inflammation, but kidneys with ATN showed decreased solute carriers despite minimal inflammation. Loss of renal function correlated better with decreased solute carrier expression than with histologic lesions (r = 0.396, p < 0.001). Thus the loss of epithelial transcripts in rejection is not a unique consequence of T cell-mediated rejection but an active injury-repair response of epithelium, triggered by rejection but also by other injury mechanisms.

A prospective randomized, controlled trial of intravenous versus oral iron for moderate iron deficiency anaemia of pregnancy.

BACKGROUND: Iron deficiency anaemia is the most common deficiency disorder in the world, affecting more than one billion people, with pregnant women at particular risk. OBJECTIVES AND DESIGN: We conducted a single site, prospective, nonblinded randomized-controlled trial to compare the efficacy, safety, tolerability and compliance of standard oral daily iron versus intravenous iron. SUBJECTS: We prospectively screened 2654 pregnant women between March 2007 and January 2009 with a full blood count and iron studies, of which 461 (18%) had moderate IDA. Two hundred women matched for haemoglobin concentration and serum ferritin level were recruited. INTERVENTIONS: Patients were randomized to daily oral ferrous sulphate 250 mg (elemental iron 80 mg) with or without a single intravenous iron polymaltose infusion. RESULTS: Prior to delivery, the intravenous plus oral iron arm was superior to the oral iron only arm as measured by the increase in haemoglobin level (mean of 19.5 g/L vs. 12 g/L; P < 0.001); the increase in mean serum ferritin level (222 microg/L vs. 18 ug/L; P < 0.001); and the percentage of mothers with ferritin levels below 30 microg/L (4.5% vs. 79%; P < 0.001). A single dose of intravenous iron polymaltose was well tolerated without significant side effects. CONCLUSIONS: Our data indicate that intravenous iron polymaltose is safe and leads to improved efficacy and iron stores compared to oral iron alone in pregnancy-related IDA.

ZNF265--a novel spliceosomal protein able to induce alternative splicing.

The formation of the active spliceosome, its recruitment to active areas of transcription, and its role in pre-mRNA splicing depends on the association of a number of multifunctional serine/arginine-rich (SR) proteins. ZNF265 is an arginine/serine-rich (RS) domain containing zinc finger protein with conserved pre-mRNA splicing protein motifs. Here we show that ZNF265 immunoprecipitates from splicing extracts in association with mRNA, and that it is able to alter splicing patterns of Tra2-beta1 transcripts in a dose-dependent manner in HEK 293 cells. Yeast two-hybrid analysis and immunoprecipitation indicated interaction of ZNF265 with the essential splicing factor proteins U1-70K and U2AF(35). Confocal microscopy demonstrated colocalization of ZNF265 with the motor neuron gene product SMN, the snRNP protein U1-70K, the SR protein SC35, and with the transcriptosomal components p300 and YY1. Transfection of HT-1080 cells with ZNF265-EGFP fusion constructs showed that nuclear localization of ZNF265 required the RS domain. Alignment with other RS domain-containing proteins revealed a high degree of SR dipeptide conservation. These data show that ZNF265 functions as a novel component of the mRNA processing machinery.

Activation of the mitogen-activated protein kinase pathway induces transcription of the PAC-1 phosphatase gene.

PAC-1, an early-response gene originally identified in activated T cells, encodes a dual-specificity mitogen-activated protein kinase phosphatase. Here we report on the regulation of PAC-1 expression in murine hemopoietic cells. PAC-1 mRNA levels rapidly increase in mitogen-stimulated lymphocytes, with the induced expression being transient in B cells but sustained in activated T cells. Transfection analysis of murine PAC-1 promoter-reporter constructs established that in T cells, sequences necessary for basal and induced transcription reside within a 200-bp region located immediately upstream of the transcription initiation sites. Basal transcription is regulated in part by an E-box element that binds a 53-kDa protein. PAC-1 transcription induced by phorbol myristate acetate stimulation and the expression of the v-ras or v-raf oncogene is mediated via the E-box motif and an AP-2-related site and coincides with increased binding activity of the constitutive 53-kDa E-box-binding protein and induced binding of AP-2. The ability of an interfering ERK-2 mutant to block phorbol myristate acetate and v-ras-dependent PAC-1 transcription indicates that mitogen-activated protein kinase activation is necessary for these stimuli to induce transcription of the PAC-1 gene in T cells.

Distribution of human endogenous retrovirus type W receptor in normal human villous placenta.

BACKGROUND: The fusion of trophoblast cells into the villous syncytiotrophoblast is crucial for appropriate placental function and fetal development. Fusion occurs following the interaction of syncytin-1, an envelope protein of the endogenous retrovirus HERV-W, and the RD114/mammalian type D retrovirus receptor (RDR/ASCT2) on adjacent cell membranes. This process must be tightly regulated in order to maintain the proliferative pool of cytotrophoblast cells as well as the function of the syncytia. AIM: We sought to investigate whether syncytial fusion of placental cytotrophoblast cells may be regulated via modulation of RDR/ASCT2 expression. METHODS: Expression of RDR/ASCT2 in term and first trimester villous placenta was assessed along with a number of molecular markers using immunofluorescent staining. In a complementary approach, Western blotting was used to investigate RDR/ASCT2 expression in a panel of choriocarcinoma cell lines before and after stimulation of fusion. RESULTS: Villous placental RDR/ASCT2 expression was found to be restricted to the cytotrophoblast compartment, being largely absent in the syncytiotrophoblast. Local variations in RDR/ASCT2 expression were not associated with the proliferative status of cytotrophoblast cells. RDR/ASCT2 expression was also shown to be down-regulated in BeWo choriocarcinoma cells after stimulation of syncytial fusion. CONCLUSION: This first report of the localisation and distribution of RDR/ASCT2 in human placental villi suggests that the fusion of placental trophoblast cells is not regulated by local or temporal variations of RDR/ASCT2 expression in villous cytotrophoblast cells.

Mobilization of bone marrow-derived progenitors.

Bone marrow (BM) is a source of various stem and progenitor cells in the adult, and it is able to regenerate a variety of tissues following transplantation. In the 1970s the first BM stem cells identified were hematopoietic stem cells (HSCs). HSCs have the potential to differentiate into all myeloid (including erythroid) and lymphoid cell lineages in vitro and reconstitute the entire hematopoietic and immune systems following transplantation in vivo. More recently, nonhematopoietic stem and progenitor cells have been identified that can differentiate into other cell types such as endothelial progenitor cells (EPCs), contributing to the neovascularization of tumors as well as ischemic tissues, and mesenchymal stem cells (MSCs), which are able to differentiate into many cells of ectodermal, endodermal, and mesodermal origins in vitro as well as in vivo. Following adequate stimulation, stem and progenitor cells can be forced out of the BM to circulate into the peripheral blood, a phenomenon called "mobilization." This chapter reviews the molecular mechanisms behind mobilization and how these have led to the various strategies employed to mobilize BM-derived stem and progenitor cells in experimental and clinical settings. Mobilization of HSCs will be reviewed first, as it has been best-explored--being used extensively in clinics to transplant large numbers of HSCs to rescue cancer patients requiring hematopoietic reconstitution--and provides a paradigm that can be generalized to the mobilization of other types of BM-derived stem and progenitor cells in order to repair other tissues.

ASCT2 regulates glutamine uptake and cell growth in endometrial carcinoma.

Glutamine commonly becomes a conditionally essential amino acid in cancer. Glutamine is supplied to the cell by transporters such as ASCT2 (SLC1A5), which is frequently upregulated in multiple cancers. Here we investigated the expression of ASCT2 in endometrial carcinoma, and evaluated the contribution of ASCT2 to glutamine uptake and endometrial cancer cell growth. Analysis of human gene expression data showed that ASCT2 was significantly upregulated in both endometrioid and serous subtypes of endometrial carcinoma, compared to normal, age-matched endometrium. Furthermore, immunohistochemical staining of primary human endometrioid adenocarcinomas showed that tumours stain positive for ASCT2 in either a uniform or mosaic expression pattern, while normal adjacent glands appeared predominantly negative for ASCT2 staining. Chemical inhibition of glutamine transport by benzylserine or GPNA led to a significant decrease in endometrial cancer cell growth and spheroid cross-sectional area. ASCT2 knockdown recapitulated the decrease of cell growth and spheroid cross-sectional area in HEC1A cells, suggesting a reliance on ASCT2-mediated glutamine uptake. ASCT2 knockdown in Ishikawa cells led to lower glutamine uptake and cell growth, but did not affect spheroid area. Ishikawa cells express higher levels of the glutamine transporter SNAT1 compared to HEC1A cells, suggesting these cells may rely on both ASCT2 and SNAT1 for glutamine uptake. Since SNAT1 is also significantly upregulated in the endometrioid and serous subtypes, these data indicate that ASCT2 and SNAT1 could be used as markers of malignancy, and/or potential therapeutic targets in patients with endometrial carcinoma.

CTCF genetic alterations in endometrial carcinoma are pro-tumorigenic.

CTCF is a haploinsufficient tumour suppressor gene with diverse normal functions in genome structure and gene regulation. However the mechanism by which CTCF haploinsufficiency contributes to cancer development is not well understood. CTCF is frequently mutated in endometrial cancer. Here we show that most CTCF mutations effectively result in CTCF haploinsufficiency through nonsense-mediated decay of mutant transcripts, or loss-of-function missense mutation. Conversely, we identified a recurrent CTCF mutation K365T, which alters a DNA binding residue, and acts as a gain-of-function mutation enhancing cell survival. CTCF genetic deletion occurs predominantly in poor prognosis serous subtype tumours, and this genetic deletion is associated with poor overall survival. In addition, we have shown that CTCF haploinsufficiency also occurs in poor prognosis endometrial clear cell carcinomas and has some association with endometrial cancer relapse and metastasis. Using shRNA targeting CTCF to recapitulate CTCF haploinsufficiency, we have identified a novel role for CTCF in the regulation of cellular polarity of endometrial glandular epithelium. Overall, we have identified two novel pro-tumorigenic roles (promoting cell survival and altering cell polarity) for genetic alterations of CTCF in endometrial cancer.

Induced dystrophin exon skipping in human muscle explants.

Antisense oligonucleotide (AO) manipulation of pre-mRNA splicing of the dystrophin gene is showing promise in overcoming Duchenne muscular dystrophy (DMD)-causing mutations. To date, this approach has been limited to studies using animal models or cultured human muscle cells, and evidence that AOs can induce exon skipping in human muscle has yet to be shown. In this study, we used different AO analogues to induce exon skipping in muscle explants derived from normal and DMD human tissue. We propose that inducing exon skipping in human muscle explants is closer to in vivo conditions than cells in monolayer cultures, and may minimize the numbers of participants in Phase I clinical studies to demonstrate proof of principle of exon skipping in human muscle.

Cell growth inhibition by the multifunctional multivalent zinc-finger factor CTCF.

The 11-zinc finger protein CCTC-binding factor (CTCF) employs different sets of zinc fingers to form distinct complexes with varying CTCF- target sequences (CTSs) that mediate the repression or activation of gene expression and the creation of hormone-responsive gene silencers and of diverse vertebrate enhancer-blocking elements (chromatin insulators). To determine how these varying effects would integrate in vivo, we engineered a variety of expression systems to study effects of CTCF on cell growth. Here we show that ectopic expression of CTCF in many cell types inhibits cell clonogenicity by causing profound growth retardation without apoptosis. In asynchronous cultures, the cell-cycle profile of CTCF-expressing cells remained unaltered, which suggested that progression through the cycle was slowed at multiple points. Although conditionally induced CTCF caused the S-phase block, CTCF can also arrest cell division. Viable CTCF-expressing cells could be maintained without dividing for several days. While MYC is the well-characterized CTCF target, the inhibitory effects of CTCF on cell growth could not be ascribed solely to repression of MYC, suggesting that additional CTS-driven genes involved in growth-regulatory circuits, such as p19ARF, are likely to contribute to CTCF-induced growth arrest. These findings indicate that CTCF may regulate cell-cycle progression at multiple steps within the cycle, and add to the growing evidence for the function of CTCF as a tumor suppressor gene.

Experimental approaches to studying the nature and impact of splicing variation in zebrafish.

From a fixed number of genes carried in all cells, organisms create considerable diversity in cellular phenotype through differential regulation of gene expression. One prevalent source of transcriptome diversity is alternative pre-mRNA splicing, which is manifested in many different forms. Zebrafish models of splicing dysfunction due to mutated spliceosome components provide opportunity to link biochemical analyses of spliceosome structure and function with whole organism phenotypic outcomes. Drawing from experience with two zebrafish mutants: cephalophonus (a prpf8 mutant, isolated for defects in granulopoiesis) and caliban (a rnpc3 mutant, isolated for defects in digestive organ development), we describe the use of glycerol gradient sedimentation and native gel electrophoresis to resolve components of aberrant splicing complexes. We also describe how RNAseq can be employed to examine relatively rare alternative splicing events including intron retention. Such experimental approaches in zebrafish can promote understanding of how splicing variation and dysfunction contribute to phenotypic diversity and disease pathogenesis.

ASCT2/SLC1A5 controls glutamine uptake and tumour growth in triple-negative basal-like breast cancer.

Alanine, serine, cysteine-preferring transporter 2 (ASCT2; SLC1A5) mediates uptake of glutamine, a conditionally essential amino acid in rapidly proliferating tumour cells. Uptake of glutamine and subsequent glutaminolysis is critical for activation of the mTORC1 nutrient-sensing pathway, which regulates cell growth and protein translation in cancer cells. This is of particular interest in breast cancer, as glutamine dependence is increased in high-risk breast cancer subtypes. Pharmacological inhibitors of ASCT2-mediated transport significantly reduced glutamine uptake in human breast cancer cell lines, leading to the suppression of mTORC1 signalling, cell growth and cell cycle progression. Notably, these effects were subtype-dependent, with ASCT2 transport critical only for triple-negative (TN) basal-like breast cancer cell growth compared with minimal effects in luminal breast cancer cells. Both stable and inducible shRNA-mediated ASCT2 knockdown confirmed that inhibiting ASCT2 function was sufficient to prevent cellular proliferation and induce rapid cell death in TN basal-like breast cancer cells, but not in luminal cells. Using a bioluminescent orthotopic xenograft mouse model, ASCT2 expression was then shown to be necessary for both successful engraftment and growth of HCC1806 TN breast cancer cells in vivo. Lower tumoral expression of ASCT2 conferred a significant survival advantage in xenografted mice. These responses remained intact in primary breast cancers, where gene expression analysis showed high expression of ASCT2 and glutamine metabolism-related genes, including GLUL and GLS, in a cohort of 90 TN breast cancer patients, as well as correlations with the transcriptional regulators, MYC and ATF4. This study provides preclinical evidence for the feasibility of novel therapies exploiting ASCT2 transporter activity in breast cancer, particularly in the high-risk basal-like subgroup of TN breast cancer where there is not only high expression of ASCT2, but also a marked reliance on its activity for sustained cellular proliferation.

scl, a gene frequently activated in human T cell leukaemia, does not induce lymphomas in transgenic mice.

The scl gene is implicated in human T cell acute lymphoblastic leukaemia (T-ALL) through its involvement in the t(1;14)(p32;q11) chromosomal translocation and, more frequently, as a result of a tumour-specific interstitial deletion on chromosome 1. The consequence of both these chromosomal alterations is overexpression of scl in the leukaemic cells. Despite the strong inference of a role in human T-ALL, scl has not yet been demonstrated to be causally involved in neoplastic transformation. We attempted to do this by generating transgenic mice in which scl expression was directed to the T cell lineage using the CD2 enhancer and the strong SR alpha viral promoter (CD2-scl mice). Three transgenic lines, all of which expressed the scl transgene at a high level, were bred and analysed. No alterations in T cell development were seen in the mice. Unexpectedly CD2-scl mice did not develop tumours, nor did the transgene enhance tumourigenesis by Moloney murine leukaemia virus. These findings throw into question the mechanism by which aberrant scl expression contributes to T cell leukaemogenesis.

Leukemic transformation of immortalized FDC-P1 cells engrafted in GM-CSF transgenic mice.

Injection of 10(6) immortalized, but non-leukemic, granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent FDC-P1 cells into GM-CSF transgenic hybrid mice with elevated GM-CSF levels led to death within three months with elevated blast cell numbers in the blood, massive organ infiltration by blast cells, and associated anemia and thrombocytopenia. No disease developed within this period in littermate mice injected with 10(6) FDC-P1 cells. All moribund transgenic recipients contained transformed FDC-P1 cells able to produce rapidly-growing transplanted leukemias in syngeneic normal DBA/2 recipients. The leukemias appeared to arise in the primary recipients by independent transformation events. The transformed cells from different mice differed in their in vitro growth characteristics, their ability to produce GM-CSF or multipotential CSF, and in the nature of the transplanted tumors derived from the primary cells. While all primary recipients at death contained fully transformed leukemic cells, the bulk of the large population of FDC-P1 cells appeared either to be untransformed or to have altered characteristics not yet representing full transformation. If the FDC-P1 engrafted model has some validity for myelodysplasia, the results suggest that sustained CSF administration to myelodysplastic patients possessing abnormal, potentially preleukemic, granulocyte-macrophage populations may increase the risk of death either from accumulated pretransformed or from fully transformed leukemic cells.

Gene deletion explains both in vivo and in vitro generated chromosome 2 aberrations associated with murine myeloid leukemia.

Ninety-five percent of radiation-induced murine myeloid leukemias contain chromosome 2 aberrations. A dominant molecular defect has not yet been identified: both deletions and breakpoint-specific events have been postulated. We have generated a model in which chromosome 2 lesions have been generated in vitro in a clonal tumor cell line. In this study cytogenetic and molecular comparisons are made between two of these in vitro generated lesions and eight derived in vivo: seven by the conventional radiation protocol, and one by infection with Moloney leukemia virus. All 10 lines consistently exhibited hemizygous loss of an 18 cM region between Hoxd-4 and II-1 alpha, with variable breakpoints at both ends. These results are consistent with deletion of a gene in common rather than breakpoint-specific events, for lesions resulting from all three protocols. This will allow a novel approach to the identification of a putative tumor suppressor gene, ie to describe the biological effect of the in vitro generated deletion, and to clone the gene by complementation. In preparation for this approach, we have further narrowed the region to approximately 6.5 cM by microsatellite mapping of 22 radiation-induced F1 tumors. In addition, we have eliminated the possibility that imprinting ablates expression from the remaining undeleted chromosome.

Establishment of multipotential and antigen presenting cell lines derived from myeloid leukemias in GM-CSF transgenic mice.

In this study neonatal mice expressing a GM-CSF transgene (GMT mice), and their normal littermate controls, were infected with Moloney murine leukemia virus (MoMLV) to examine in vivo tumorigenesis. By 200 days, all of the GMT mice had died whereas median survival had not been reached in the littermates (P < 0.0003). Thymomas developed in 32% of GMT mice and were more frequently CD4+CD8+ (83%) compared to the CD4+CD8- phenotype seen in 90% of thymomas developing in MoMLV-infected littermate mice. A primitive myeloid leukemia was induced in 21% of GMT mice, but none of the littermates. To characterize further the nature of the leukemic cells, a factor-dependent cell line (DGM36) was derived. DGM36 cells were tumorigenic, capable of differentiation to neutrophils, macrophages and eosinophils, and contained a partial deletion of chromosome 2. A subline arose spontaneously that was factor-independent and produced GM-CSF in an autocrine manner (IGM36 cells). Stimulation of the IGM36 cells with TNF alpha and IFNgamma resulted in increased expression of B7-1, class I MHC and class II MHC and consequent presentation of antigen in allogeneic MLRs. IGM36 cells thereby satisfy many of the criteria of dendritic cells and consequently may be used to examine antigen presentation by leukemic cells. This is the first report of primary myeloid leukemias arising in GMT mice and documents the derivation of a multipotential, autocrine leukemic cell line with dendritic cell characteristics.

The flt3/flk-2 ligand: receptor distribution and action on murine haemopoietic cell survival and proliferation.

In this study the distribution and quantitation of the flt3/flk-2 receptor was examined on bone marrow cells and defined haemopoietic subpopulations. Undifferentiated cells expressed the greatest numbers of flt3/flk-2 receptors: 19% of primitive lin-kit+sca-1+ bone marrow cells and 16% of fetal liver lin-aa4.1+ cells exhibited over 15 000 receptors per cell as determined by binding of the radiolabeled cognate ligand (flt3/flk-2 ligand, FL). Moderate binding was demonstrated on early B lymphocyte subsets (4400 receptors per cell) and very low levels were detected on monocytes. Binding was not detected on promyelocytes, myelocytes, promonocytes, metamyelocytes, polymorphonuclear cells, eosinophils or nucleated erythroid cells. FL enhanced the survival of primitive lin kit+sca-1+ cells with an efficacy s with an efficacy equivalent to stem cell factor (SCF). FL stimulated predominantly blast and granulocyte-macrophage colony formation in cultures of bone marrow cells by both direct and indirect mechanisms. Marked synergistic effects of FL with combinations of colony stimulating factors (CSFs) or interleukin-6 occurred in the proliferation of primitive lin-kit+sca-1+ cells, but not lin-kit+sca-1- progenitor cells. Surprisingly, recloning experiments revealed that FL plus IL-3 increased the generation of progenitor cells by lin-kit a-1- cells compared with SCF plus IL-3. Thus FL functions as a factor with both direct and indirect stimulatory activities directed to the expansion, maintenance of clonogenic potential, and possibly limited self-renewal, of early haemopoietic cells.

The cytokine receptor repertoire specifies autocrine growth factor production in factor-dependent cells.

Mechanisms of helper virus-induced growth factor-independence were examined in FDC-P1 cells and FDC-P1 cells expressing the erythropoietin receptor (FDER cells). Retroviral mutagenesis of FDC-P1 cells led to factor-independent (FI) colonies from which cell lines could readily be established; whereas control cells exhibited at least 20 to 40-fold lower rates of factor-independence. From 44 independent experiments using either FDC-P1 or FDER cells, 205 autonomous cell lines were obtained. Sixteen colonies displayed a novel ("satellite-inducing") appearance in agar and produced up to 4.1 x 10(5) U/mL granulocyte-macrophage colony-stimulating factor (GM-CSF) (some with altered GM-CSF transcript sizes) and/or interleukin-3 (IL-3). Retroviral mutagenesis of FDER cells increased the repertoire of autocrine growth factors now responsible for stimulating autocrine proliferation: 3% of FI cell lines produced erythropoietin (Epo) (0.5 U/mL). Unexpectedly, in every autonomous FDC-P1 cell line, reverse transcriptase-PCR demonstrated expression of a growth factor normally required for proliferation. Thus, a profound selection for cells able to produce growth factors as the mechanism for achieving autonomous proliferation was documented. The ectopic expression of a receptor lacking a cognate ligand ("orphan") followed by retroviral mutagenesis and selection for autocrine mutants may offer an effective method for identifying new ligands.