Microcell-mediated transfer of murine chromosomes into mouse, Chinese hamster, and human somatic cells

Dendrimer mediated transfer of engineered chromosomes

Gene therapy encounters important problems such as insertional mutagenesis caused by the integration of viral vectors. These problems could be circumvented by the use of mammalian artificial chromosomes (MACs) that are unique and high capacity gene delivery tools. MACs were delivered into various target cell lines including stem cells by microcell-mediated chromosome transfer (MMCT), microinjection, and cationic lipid and dendrimer mediated transfers. MACs were also cleansed to more than 95% purity before transfer with an expensive technology. We present here a method by which MACs can be delivered into murine embryonic stem (ES) cells with a nonexpensive, less tedious, but still efficient way.

Mammalian artificial chromosomes and clinical applications for genetic modification of stem cells: an overview

Modifying multipotent, self-renewing human stem cells with mammalian artificial chromosomes (MACs), present a promising clinical strategy for numerous diseases, especially ex vivo cell therapies that can benefit from constitutive or overexpression of therapeutic gene(s). MACs are nonintegrating, autonomously replicating, with the capacity to carry large cDNA or genomic sequences, which in turn enable potentially prolonged, safe, and regulated therapeutic transgene expression, and render MACs as attractive genetic vectors for "gene replacement" or for controlling differentiation pathways in progenitor cells. The status quo is that the most versatile target cell would be one that was pluripotent and self-renewing to address multiple disease target cell types, thus making multilineage stem cells, such as adult derived early progenitor cells and embryonic stem cells, as attractive universal host cells. We will describe the progress of MAC technologies, the subsequent modifications of stem cells, and discuss the establishment of MAC platform stem cell lines to facilitate proof-of-principle studies and preclinical development.

Enhanced apoptosis during early neuronal differentiation in mouse ES cells with autosomal imbalance

Although particular chromosomal syndromes are phenotypically and clinically distinct, the majority of individuals with autosomal imbalance, such as aneuploidy, manifest mental retardation. A common abnormal phenotype of Down syndrome (DS), the most prevalent autosomal aneuploidy, shows a reduction in both the number and the density of neurons in the brain. As a DS model, we have recently created chimeric mice from ES cells containing a single human chromosome 21. The mice mimicked the characteristic phenotypic features of DS, and ES cells showed a higher incidence of apoptosis during early neuronal differentiation in vitro. In this study, we examined the induction of anomalous early neural development by aneuploidy in mouse ES cells by transferring various human chromosomes or additional mouse chromosomes. Results showed an elevated incidence of apoptosis in all autosome-aneuploid clones examined during early neuronal differentiation in vitro. Further, cDNA microarray analysis revealed a common cluster of down-regulated genes, of which eight known genes are related to cell proliferation, neurite outgrowth and differentiation. Importantly, targeting of these genes by siRNA knockdown in normal mouse ES cells led to enhanced apoptosis during early neuronal differentiation. These findings strongly suggest that autosomal imbalance is associated with general neuronal loss through a common molecular mechanism for apoptosis.

Hybrids of aneuploid human cancer cells permit complementation of simple and complex cancer defects

Causes for the complex phenotypes of cancers, such as altered differentiation, invasion and metastasis, are not known, and multigenic defects are likely. In contrast, well-defined deficiencies, such as those affecting DNA-repair mechanisms and enzymatic pathways, are simple, typically caused by one or a few gene mutations. Complementation by introducing defined genetic elements is used to study simple cancer phenotypes, while complementation by the fusion of whole cells is employed occasionally for complex ones. Hybrids formed solely from the common lines (aneuploid due to chromosomal instability, CIN) are rarely reported. We created stable hybrids of two CIN lines, producing a nearly complete genetic sum of the parental karyotypes. Complementation of a simple cancer phenotype, a Fanconi anemia pathway defective in both parental lines, occurred in all hybrids, restoring the normal drug-resistance phenotype. The grossly defective mitotic spindle checkpoint present in both parental lines was partially corrected in some hybrids, supporting a multigenic origin rather than a single gene defect. Using Affymetrix 100K SNP chips, we mapped chromosomal loci differing among the phenotypically distinct hybrid clones. Fusing CIN cell lines to form mapped hybrids offers new tools for positional cloning or classification of simple and complex cancer phenotypes, including mechanical defects and altered drug responses.

Construction of somatic cell hybrids

Somatic cell hybridization is the method of choice to separate a chromosome of interest from the full chromosome complement and obtain a permanent source of the chromosome. This unit begins with the choice of fusion techniques and selectable markers for hybrids containing a chromosome of interest. The first set of protocols outline the production of whole-cell hybrids by fusion of two cell lines: a monolayer (adherent) recipient and a donor that may be adherent or grown in suspension. The second set of protocols outline the production of micronuclei containing a limited number of chromosomes, and enucleation of the micronuclei to form microcells for fusion with recipient cells. Support protocols describe the preparation and use of cloning cylinders to isolate colonies in tissue culture, subcloning of whole-cell hybrid populations to isolate lines that have segregated additional chromosomes, purification of microcell preparations, and molecular and cytogenetic methods for characterizing.

Transfer of human artificial chromosome vectors into stem cells

Human chromosome fragments and human artificial chromosomes (HAC) represent feasible gene delivery vectors via microcell-mediated chromosome transfer. Strategies to construct HAC involve either 'build up' or 'top-down' approaches. For each approach, techniques for manipulating HAC in donor cells in order to deliver HAC to recipient cells are required. The combination of chromosome fragments or HAC with microcell-mediated chromosome transfer has facilitated human gene mapping and various genetic studies. The recent emergence of stem cell-based tissue engineering has opened up new avenues for gene and cell therapies. The task now is to develop safe and effective vectors that can deliver therapeutic genes into specific stem cells and maintain long-term regulated expression of these genes. Although the transfer-efficiency needs to be improved, HAC possess several characteristics that are required for gene therapy vectors, including stable episomal maintenance and the capacity for large gene insets. HAC can also carry genomic loci with regulatory elements, which allow for the expression of transgenes in a genetic environment similar to the natural chromosome. This review describes the lessons and prospects learned, mainly from recent studies in developing HAC and HAC-mediated gene expression in embryonic and adult stem cells, and in transgenic animals.

A genetic basis for tumour suppression

The technique of somatic cell hybridization has established the phenomenon of tumour suppression and provided evidence for a genetic basis for suppression. Further refinements aimed at eventually identifying 'tumour suppressor' genes include the use of monochromosome transfer via microcell hybridization. The application of this technique to the study of tumour suppression in tumorigenic HeLa cell x fibroblast hybrids, Wilms' tumour, retinoblastoma and osteosarcoma cells is described. The issue of whether tumour suppression involves a direct effect on expression of activated oncogenes is discussed. Transformation of normal human cells in culture by activated cellular oncogenes is an extremely rare event. This may be due to a relatively greater genomic stability of human cells compared to rodent cells. We describe the use of a spontaneously immortalized human keratinocyte cell line, HaCaT, for studies of the effects of introduction of activated c-Ha-ras oncogene into these cells, with particular reference to tumorigenic conversion.

Human artificial chromosome vectors meet stem cells: new prospects for gene delivery

The recent emergence of stem cell-based tissue engineering has now opened up new venues for gene therapy. The task now is to develop safe and effective vectors that can deliver therapeutic genes into specific stem cell lines and maintain long-term regulated expression of these genes. Human artificial chromosomes (HACs) possess several characteristics that require gene therapy vectors, including a stable episomal maintenance, and the capacity for large gene inserts. HACs can also carry genomic loci with regulatory elements, thus allowing for the expression of transgenes in a genetic environment similar to the chromosome. Currently, HACs are constructed by a two prone approaches. Using a top-down strategy, HACs can be generated from fragmenting endogenous chromosomes. By a bottom-up strategy, HACs can be created de novo from cloned chromosomal components using chromosome engineering. This review describes the current advances in developing HACs, with the main focus on their applications and potential value in gene delivery, such as HAC-mediated gene expression in embryonic, adult stem cells, and transgenic animals.

Artificial chromosome-based transgenes in the study of genome function

The transfer of large DNA fragments to the mouse genome in the form of bacterial, yeast or phage artificial chromosomes is an important process in the definition of transcription units, the modeling of inherited disease states, the dissection of candidate regions identified by linkage analysis and the construction of in vivo reporter genes. However, as with small recombinant transgenes, the transferred sequences are usually integrated randomly often with accompanying genomic alterations and variable expression of the introduced genes due to the site of integration and/or copy number. Therefore, alternative methods of integrating large genomic transgenes into the genome have been developed to avoid the variables associated with random integration. This review encourages the reader to imagine the large variety of applications where artificial chromosome transgenes can facilitate in vivo and ex vivo studies in the mouse and provides a context for making the necessary decisions regarding the specifics of experimental design.

Heat-regulated production and secretion of insulin from a human artificial chromosome vector

Human artificial chromosomes (HACs) behave as independent minichromosomes and are potentially useful as a way to achieve safe, long-term expression of a transgene. In this study, we sought to elucidate the potential of HAC vectors carrying the human proinsulin transgene for gene therapy of insulin-dependent diabetes mellitus (IDDM) using non-beta-cells as a host for the vector. To facilitate the production of mature insulin in non-beta-cells and to safely regulate the level of transgene expression, we introduced furin-cleavable sites into the proinsulin coding region and utilized the heat shock protein 70 (Hsp70) promoter. We used Cre-loxP-mediated recombination to introduce the gene cassettes onto 21DeltapqHAC, a HAC vector whose structure is completely defined, present in human fibrosarcoma HT1080 cells. We observed long-term expression and stable retention of the transgene without aberrant translocation of the HAC constructs. As expected, the Hsp70 promoter allowed us to regulate gene expression with temperature, and the production and secretion of intermediates of mature insulin were made possible by the furin-cleavable sites we had introduced into proinsulin. This study can be an initial step on the application of HAC vectors on the gene delivery to non-beta-cells, which might provide a direction for future treatment for diabetes.

The manipulation of chromosomes by mankind: the uses of microcell-mediated chromosome transfer

Microcell-mediated chromosome transfer (MMCT) was a technique originally developed in the 1970s to transfer exogenous chromosome material into host cells. Although, the methodology has not changed considerably since this time it is being used to great success in progressing several different fields in modern day biology. MMCT is being employed by groups all over the world to hunt for tumour suppressor genes associated with specific cancers, DNA repair genes, senescence-inducing genes and telomerase suppression genes. Some of these genomic discoveries are being investigated as potential treatments for cancer. Other fields have taken advantage of MMCT, and these include assessing genomic stability, genomic imprinting, chromatin modification and structure and spatial genome organisation. MMCT has also been a very useful method in construction and manipulation of artificial chromosomes for potential gene therapies. Indeed, MMCT is used to transfer mainly fragmented mini-chromosome between cell types and into embryonic stem cells for the construction of transgenic animals. This review briefly discusses these various uses and some of the consequences and advancements made by different fields utilising MMCT technology.

Construction of a novel expression system on a human artificial chromosome

Efficient regulation of transgene would greatly facilitate the analysis of gene function in biological systems for basic research and clinical applications. The tetracycline-regulatable system (TRS) has proven to be a promising tool for such purposes. Despite their widespread application, a number of challenges are still associated with the use of TRS, including clonal variability in the regulation and copy number. We have recently constructed a novel human artificial chromosome (HAC) called 21DeltaqHAC. By housing a TRS-based DNA-PKcs expression cassette in this HAC, we were able to circumvent the problems associated with conventional TRS-based vectors. We achieved tight control of DNA-PKcs expression and rescued the radiosensitive phenotype of DNA-PKcs-deficient CHO cells. The combined use of HAC and the TRS serves as a model for controllable and fixed copy number expression vectors. Our study also demonstrates the suitability of the HAC to accommodate multi-subunit constructs such as that of the TRS.

Construction of a novel human artificial chromosome vector for gene delivery

Potential problems of conventional transgenes include insertional disruption of the host genome and unpredictable, irreproducible expression of the transgene by random integration. Alternatively, human artificial chromosomes (HACs) can circumvent some of the problems. Although several HACs were generated and their mitotic stability was assessed, a practical way for introducing exogenous genes by the HACs has yet to be explored. In this study, we developed a novel HAC from sequence-ready human chromosome 21 by telomere-directed chromosome truncation and added a loxP sequence for site-specific insertion of circular DNA by the Cre/loxP system. This 21HAC vector, delivered to a human cell line HT1080 by microcell fusion, bound centromere proteins A, B, and C and was mitotically stable during long-term culture without selection. The EGFP gene inserted in the HAC vector expressed persistently. These results suggest that the HAC vector provides useful system for functional studies of genes in isogenic cell lines.

Functional Analysis of Chromosome 18 in Pancreatic Cancer: Strong Evidence for New Tumour Suppressor Genes

BACKGROUND

In a previous work, we demonstrated that loss of heterozygosity of 18q is a frequent event significantly associated with poor prognosis in pancreatic cancer. We hypothesized that restoration of heterozygosity of chromosome 18 in pancreatic cancer cells would reduce their tumorigenicity. This study was intended to provide functional evidence for the existence of new tumour suppressor gene(s) located on chromosome 18.

METHOD

Restoration of heterozygosity was achieved by introducing a normal copy of chromosome 18 into pancreatic ductal carcinoma using a microcell-mediated chromosome transfer technique. The tumorigenicity and metastatic ability of both the parental cells and resulting hybrids were assessed in vitro and in vivo.

RESULTS

In vitro growth of hybrid clones was significantly delayed compared to parental cells. This was paralleled by a significantly lower rate of promoting invasive carcinoma in nude mice and a longer latency with hybrid cells compared with parental tumour cells. Hybrid clones showed significant suppression in the number of surface lung metastases when compared with parental cells.

CONCLUSION

These data represent strong functional evidence that chromosome 18q encodes strong tumour and metastasis suppressor activity that is able to switch human pancreatic cancer cells to a dormant phenotype.

Microcell-mediated chromosome transfer (MMCT): small cells with huge potential

Microcell-mediated chromosome transfer (MMCT) is a technique that has been in use since the 1970s for the fusion of microcells, containing single or a small number of chromosomes, with whole cells, and the subsequent selection of the hybrids. MMCT can be carried out with somatic cells, embryonic carcinoma (EC) or embryonic stem (ES) cell recipients, to study in vitro or in vivo effects of the transferred genetic material. These effects may be unpredictable--do the transferred genes function normally while in the regulatory milieu of the host cell? Will epigenetic effects become apparent, and how will these alter gene expression? What happens to the host cell phenotype? Here, we present a review of MMCT in which we argue that, although this is an old technique, its adaptability and efficiency make it an excellent method for the dissection of gene function and dysfunction in a very wide range of current systems.

Chromosome 17p13.2 transfer reverts transformation phenotypes and fas-mediated apoptosis in breast epithelial cells

Transformation of the human breast epithelial cells (HBEC) MCF-10F with the carcinogen benz(a)pyrene (BP) into BP1-E cells resulted in the loss of the chromosome 17 p13.2 locus (D17S796 marker) and formation of colonies in agar-methocel (colony efficiency (CE)), loss of ductulogenic capacity in collagen matrix, and resistance to anti-Fas monoclonal antibody (Mab)-induced apoptosis. For testing the role of that specific region of chromosome 17 in the expression of transformation phenotypes, we transferred chromosome 17 from mouse fibroblast donors to BP1-E cells. Chromosome 11 was used as negative control. After G418 selection, nine clones each were randomly selected from BP1-E-11neo and BP1-E-17neo hybrids, respectively, and tested for the presence of the donor chromosomes by fluorescent in situ hybridization and polymerase chain reaction-based restriction fragment length polymorphism (PCR-RFLP) analyses. Sensitivity to Fas Mab-induced apoptosis and evaluation of transformation phenotype expression were tested in MCF-10F, BP1-E, and nine BP1-E-11neo and BP1-E-17neo clones each. Six BP1-E-17neo clones exhibited a reversion of transformation phenotypes and a dose dependent sensitivity to Fas Mab-induced apoptosis, behaving similarly to MCF-10F cells. All BP1-E-11neo, and three BP1-E-17neo cell clones, like BP1-E cells, retained a high CE, loss of ductulogenic capacity, and were resistant to all Fas Mab doses tested. Genomic analysis revealed that those six BP1-E-17neo clones that were Fas-sensitive and reverted their transformed phenotypes had retained the 17p13.2 (D17S796 marker) region, whereas it was absent in all resistant clones, indicating that the expression of transformation phenotypes and the sensitivity of the cells to Fas-mediated apoptosis were under the control of genes located in this region.

Expression of Smad4 in the FaDu cell line partially restores TGF-beta growth inhibition but is not sufficient to regulate fibronectin expression or suppress tumorigenicity

Mutations of the Smad4 gene, a member of a group of TGF-beta signal transduction components, occur in several types of cancer suggesting that its inactivation significantly affects TGF-beta responsiveness in these tumors. To further investigate the role of Smad4 with respect to TGF-beta signaling and carcinogenesis, we re-expressed the Smad4 gene in the Smad4-deficient cancer cell line FaDu by microcell-mediated chromosome transfer (MMCT) and retroviral infection to closely approximate physiological protein levels. The Smad4-expressing FaDu clones were then evaluated for TGF-beta responsiveness to assess the role of Smad4 in TGF-beta-induced growth inhibition and target gene regulation. We found that the re-expression of the Smad4 gene by either method partially restored TGF-beta responsiveness in FaDu cells with respect to both growth inhibition and expression of p21WAF1/CIP1 and p15INK4B. However, only the microcell hybrids showed growth retardation in organotypic raft culture and an enhanced ability to upregulate fibronectin. In contrast, the re-expression of Smad4 by either method failed to suppress tumorigenicity. These results suggest that in addition to a homozygous deletion of Smad4, FaDu cells contain additional defects within the TGF-beta signaling pathway, thereby limiting the extent of TGF-beta responsiveness upon Smad4 re-expression and perhaps accounting for the inability to induce p15INK4B to a high level. They also demonstrate the advantages of providing a physiological extracellular environment, when assessing TGFbeta responsiveness.

Functional cloning of a tumor suppressor gene, TSLC1, in human non-small cell lung cancer

The identification of a tumor suppressor gene in non-small cell lung cancer (NSCLC) is one of the most important issues to elucidate the molecular mechanisms of this type of refractory cancer and to establish a novel strategy against it. Since NSCLC, like most other human cancers, develops as a sporadic disease, linkage analysis is not available for gene cloning. This review describes the functional cloning approaches to a tumor suppressor gene in sporadic cancers. Suppression of the malignant phenotype of cancer cells by fusion with a normal fibroblast was the first demonstration of the recessive phenotype of cancer cells in 1969. Evidence of tumor suppressor genes on the specific chromosomes was later provided by functional complementation of the cancer phenotype through microcell-mediated chromosome transfer. Further introduction of more restricted DNA fragments by YAC transfer provides a potent tool to localize the gene to a small segment, appropriate for the subsequent gene cloning. TSLC1, a novel tumor suppressor gene in NSCLC, was identified on chromosome 11q23.2 through a series of functional complementation of A549 cells in tumorigenicity. Two-hit inactivation of the TSLC1 by promoter methylation and gene deletion was observed in 40% of primary NSCLC tumors. The strong tumor suppressor activity of TSLC1, and its possible involvement in cell adhesion, suggest that the functional cloning approach could cast a new light on a group of genes that have not yet been characterized, but are important for general human carcinogenesis as well as tumor suppression.

Generation of human artificial chromosomes expressing naturally controlled guanosine triphosphate cyclohydrolase I gene

BACKGROUND

Human artificial chromosomes (HACs) are generated from the precursor DNA constructs containing alpha-satellite DNA with CENP-B boxes, and the process could be used for the incorporation of large genes in the HACs. Guanosine triphosphate cyclohydrolase I (GCH1) is the first and rate-limiting enzyme for the biosynthesis of tetrahydrobiopterin, the essential co-factor of aromatic amino acid hydroxylases and nitric oxide synthase.

RESULTS

We constructed HACs carrying a 180 kb genome segment encoding the human GCH1 gene and its control region from the bacterial artificial chromosome (BAC) with the GCH1 segment by co-transfection with the alpha-satellite DNA-containing BAC to a human fibroblast cell line. Two cell lines carrying a HAC with GCH1 genes were obtained. Both HACs were composed of multiple copies of precursor BACs and were maintained stably in human and mouse cell lines. The GCH1 activities of the HAC-carrying human fibroblast cell lines were elevated but still highly sensitive to IFN-gamma induction, mimicking the response of the gene expression from the authentic chromosomal genes.

CONCLUSION

These HACs will provide a useful system for analysis of the complex regulatory circuit of the GCH1 gene in vivo and also function as a tool for gene delivery in animal models or in therapeutic trials.

Expression of transferrin receptor 2 in normal and neoplastic hematopoietic cells

Iron is essential for cell proliferation, heme synthesis, and a variety of cellular metabolic processes. In most cells, transferrin receptor-mediated endocytosis is a major pathway for cellular iron uptake. Recently, transferrin receptor 2 (TfR2), another receptor for transferrin, was cloned. High levels of expression of TfR2 messenger RNA (mRNA) occur in the liver, as well as in HepG2 (a hepatoma cell line) and K562 (an erythroid leukemia cell line). In this study, TfR2 mRNA expression was analyzed in hematological cell lines, normal erythroid cells at various stages of differentiation, and leukemia and preleukemia cells. High levels of TfR2 expression occurred in all of the erythroid cell lines that were examined. Erythroid-specific expression of TfR2 protein in bone marrow cells was confirmed by immunohistochemical staining. Expression of TfR2 mRNA was high in normal CD34(+) erythroid precursor cells, and levels decreased during erythroid differentiation in vitro. Levels of expression of TfR2-alpha mRNA were significantly higher in erythroleukemia (M6) marrow samples than in nonmalignant control marrow samples. In addition, relatively higher levels of TfR2-alpha mRNA expression occurred in some samples of myelodysplastic syndrome that had erythroid hyperplasia in bone marrow, acute myelogenous leukemia M1, M2, and chronic myelogenous leukemia. Expression profiles of normal members of the erythroid lineage suggest that TfR2-alpha may be a useful marker of early erythroid precursor cells. The clinical significance of TfR2-alpha expression in leukemia cells remains to be determined.

Dual control of replication timing. Stochastic onset but programmed completion of mammalian chromosome duplication

In mammalian cells, DNA replication proceeds according to a precise temporal order during the S phase, but how this program is controlled remains poorly understood. We analyzed the replication-dependent bromodeoxyuridine banding of chromosomes in Chinese hamster cells treated with the spindle poison nocodazole. In these cells, nocodazole induces a transient mitotic arrest, followed by DNA re-replication without intervening cell division. Nuclear fragmentation is often observed in tetraploid derivatives, and previous studies suggest that replication timing of chromosomes could be affected when they are segregated into different micronuclei. Here we show that the onset of replication is frequently asynchronous on individual chromosomes during the re-replication process. Moreover, fluorescence in situ hybridization analysis revealed that replication synchrony is equally altered in fragmented and non-fragmented nuclei, indicating that asynchronous onset of replication is not dependent on physical separation of the chromosomes into isolated compartments. We also show that the ordered program of replication is always preserved along individual chromosomes. Our results demonstrate that the onset of replication of individual chromosomes in the same nuclear compartment can be uncoupled from the time of S-phase entry and from the programmed replication of chromosome sub-domains, revealing that multi-level controls contribute to establish replication timing in mammalian cells.

Stability of transferred human chromosome fragments in cultured cells and in mice

Chromosome fragments represent feasible gene delivery vectors with the use of microcell-mediated chromosome transfer. To test a prerequisite for a gene delivery vector, we examined the stability of human chromosome fragments (hCFs) in cultured cells and in trans-chromosomic (Tc) mice. Fragments of human chromosomes 2 (hCF(2-W23)), 11 (hCF-11) and 14 (hCF(SC20)) tagged with neo were introduced into the TT2F mouse ES cells, and retention of the hCFs was examined by FISH during long-term culture without selection. In contrast to the gradual loss of hCF(2-W23) and hCF-11, hCF(SC20) remained stable over 70 population doublings in the ES cells. The hCF(SC20) was also stable in cultured human tumor cells and chicken DT40 cells. We have previously generated chimeric mice using the ES cells harboring the hCF(2-W23) or hCF(SC20), followed by production of Tc mice. Although both the hCF(2-W23) and hCF(SC20) persisted in cells of Tc mice as an additional chromosome and were transmitted to offspring, the hCF(SC20) was more stable than the hCF(2-W23) in F1 and F2 mice. The present study shows that the stability of hCFs in Tc mice differs with tissue types and with genetic background used for successive breedings. Thus, the hCF(SC20), which was relatively stable in both mouse and human cells, may be a promising candidate for development as a gene delivery vector.

Novel tumor suppressor locus in human chromosome region 3p14.2

BACKGROUND

Alterations of chromosome region 3p14 are observed in numerous human malignancies. Because the pattern of allelic losses suggests the existence of at least one tumor suppressor gene within this region, we established a library of yeast artificial chromosomes (YACs) containing contiguous human 3p14 sequences to permit a search for tumor suppressor loci within the 3p14 region by use of functional complementation.

METHODS

YACs specific for human chromosome region 3p14 were transduced by spheroplast fusion into cells of the human nonpapillary renal carcinoma cell line RCC-1, which shows a cytogenetically detectable 3p deletion and is tumorigenic in nude mice.

RESULTS

We identified a 3p14.2-specific YAC clone, located in the vicinity of the fragile histidine triad (FHIT) gene (but toward the telomere), that is capable of inducing sustained suppression of tumorigenicity in nude mice and of activating cellular senescence in vitro. Among 23 mice given injections of RCC-1 cells containing this YAC, 16 (70%) remained tumor free for at least 6 months, whereas tumor formation occurred after a median of 6 weeks in control mice given injections of either RCC-1 parental cells or a revertant cell line (in which the YAC had lost all human sequences) or RCC-1 parental cells containing other, unrelated YACs. Similar results were obtained following microcell-mediated transfer of the entire human chromosome 3.

CONCLUSION

These data provide strong evidence for the existence of a novel tumor suppressor locus adjacent to the previously identified candidate tumor suppressor gene, FHIT, in 3p14.2. Positional cloning of the novel suppressor element within the 3p14.2-specific YAC and the sequence's molecular and functional characterization should add to the understanding of the pathogenesis of renal cell carcinoma and other human tumors that exhibit 3p14 aberrations.

Mouse autosomal trisomy: two's company, three's a crowd

Autosomal trisomy causes a large proportion of all human pregnancy loss and so is a significant source of lethality in the human population. The autosomal trisomy syndromes each have a different phenotype and are probably caused by the effects of specific genes that are present in three copies, rather than the normal two. Identifying these genes will require the application of classical genetic and new genome-manipulation approaches. Recent advances in chromosome engineering are now allowing us to create precisely defined autosomal trisomies in the mouse, and so provide new routes to identifying the critical, dosage-sensitive genes that are responsible for these highly deleterious, yet very common, syndromes.

Transchromosomal mouse embryonic stem cell lines and chimeric mice that contain freely segregating segments of human chromosome 21

At least 8% of all human conceptions have major chromosome abnormalities and the frequency of chromosomal syndromes in newborns is >0.5%. Despite these disorders making a large contribution to human morbidity and mortality, we have little understanding of their aetiology and little molecular data on the importance of gene dosage to mammalian cells. Trisomy 21, which results in Down syndrome (DS), is the most frequent aneuploidy in humans (1 in 600 live births, up to 1 in 150 pregnancies world-wide) and is the most common known genetic cause of mental retardation. To investigate the molecular genetics of DS, we report here the creation of mice that carry different human chromosome 21 (Hsa21) fragments as a freely segregating extra chromosome. To produce these 'transchromosomal' animals, we placed a selectable marker into Hsa21 and transferred the chromosome from a human somatic cell line into mouse embryonic stem (ES) cells using irradiation microcell-mediated chromosome transfer (XMMCT). 'Transchromosomal' ES cells containing different Hsa21 regions ranging in size from approximately 50 to approximately 0.2 Mb have been used to create chimeric mice. These mice maintain Hsa21 sequences and express Hsa21 genes in multiple tissues. This novel use of the XMMCT protocol is applicable to investigations requiring the transfer of large chromosomal regions into ES or other cells and, in particular, the modelling of DS and other human aneuploidy syndromes.

Simultaneous transfer of mitochondrial DNA and single chromosomes in somatic cells: a novel approach for the study of defects in nuclear-mitochondrial communication

The assembly and function of respiratory-competent mitochondria in eukaryotic cells depends on collaboration between the nuclear and mitochondrial genomes, but the molecular mechanisms underlying such cross-talk are poorly understood. Microcell-mediated chromosome transfer has been used to transfer intact chromosomes from one mammalian cell to another, helping to map loci implicated in different diseases and in the senescence process. In the present work, we show that microcells have a significant number of mitochondria which can be transferred to another cell simultaneously with a limited number of chromosomes. By fusing microcells from a colon carcinoma cell line with a mitochondrial DNA (mtDNA)-less osteosarcoma cell line, we were able to isolate transmitochondrial hybrids containing only one of three selectable chromosomes and mtDNA from the donor cell. The proportion of transmitochondrial hybrids containing one chromosomal marker with respect to the total transmitochondrial hybrids and cybrids was approximately 1% and no hybrids were isolated containing more than one nuclear marker. The genetic data correlated well with the composition and structure of the microcell preparations, which showed the presence of cytoplast-like structures and microcells containing mitochondria surrounding the micronuclei. Microcell-mediated mtDNA and chromosome transfer can be used to identify nuclear factors implicated in mtDNA maintenance and gene expression, as well as to investigate nuclear factors which modulate clinical phenotypes in mitochondrial disorders.

Evidence for uniparental, paternal expression of the human GABAA receptor subunit genes, using microcell-mediated chromosome transfer

We have constructed mouse A9 hybrids containing a single normal human chromosome 15, via microcell-mediated chromosome transfer. Cytogenetic and DNA-polymorphic analyses identified mouse A9 hybrids that contained either a paternal or maternal human chromosome 15. Paternal specific expression of the known imprinted genes SNRPN (small nuclear ribonucleoprotein-associated polypeptide N gene) and IPW (imprinted gene in the Prader-Willi syndrome region) was maintained in the A9 hybrids. Using this system, we first demonstrated that human GABAAreceptor subunit genes, GABRB3 , GABRA5 and GABRG3 , were expressed exclusively from the paternal allele and that E6-AP (E6-associated protein or UBE3A ) was biallelically expressed. Moreover, the 5' portion of the GABRB3 gene was found to be hypermethylated on the paternal allele. Our data imply that GABAAreceptor subunit genes are imprinted and are possible candidates for Prader-Willi syndrome, and that this human monochromosomal hybrid system enables the efficient analysis of imprinted loci.

Functional expression and germline transmission of a human chromosome fragment in chimaeric mice

Human chromosomes or chromosome fragments derived from normal fibroblasts were introduced into mouse embryonic stem (ES) cells via microcell-mediated chromosome transfer (MMCT) and viable chimaeric mice were produced from them. Transferred chromosomes were stably retained, and human genes, including immunoglobulin (Ig) kappa, heavy, lambda genes, were expressed in proper tissue-specific manner in adult chimaeric tissues. In the case of a human chromosome (hChr.) 2-derived fragment, it was found to be transmitted to the offspring through the germline. Our study demonstrates that MMCT allows for introduction of very large amounts of foreign genetic material into mice. This novel procedure will facilitate the functional analyses of human genomes in vivo.

Induction of erythroid gene expression by microcell fusion

The molecular events which underlie lineage commitment and differentiation in hematopoietic cells are still incompletely understood. Microcell fusion is a versatile technique which has been utilized in characterizing and mapping genes involved in tumor suppression, cell senescence, and certain aspects of differentiation. Microcell fusion has the potential to contribute to the understanding of hematopoietic differentiation; however, application of this technique is limited by the need to use adherent cells as microcell donors, by the need to tag candidate chromosomes with a selectable marker, and by the need for prolonged selection of fused cells prior to characterization of their phenotype. We developed a modified technique of microcell fusion using square wave electroporation, which allows higher efficiency fusion than polyethylene glycol fusion. By using cross-species fusion and species-specific PCR primers, we were able to detect new gene induction events 48 h after microcell fusion. To study erythroid gene expression, we fused microcells from human erythroid K562 cells to murine B-lymphoid SP-2 cells. We found that microcell fusion induced the nonerythroid recipient cells to express alpha-globin mRNA in a dose-dependent manner. They also expressed RNA for beta-globin, GATA-1, and NF-E2. In contrast, there was no expression of heart- or liver-specific genes. We conclude that microcells from erythroid cells contain all the information necessary to induce expression of multiple erythroid genes. Analysis of the components of the microcells responsible for this new gene induction may allow the characterization of cellular factors responsible for erythroid-specific gene expression.

Isolation of monochromosomal hybrids for mouse chromosomes 3, 6, 10, 12, 14, and 18

Mouse/human somatic cell hybrids constitute a valuable resource for both genetic and physical mapping. In this report, we describe the production and characterization of a series of six monochromosomal hybrids generated by fusion of murine micro-cells with intact human recipient cells. The presence of each mouse chromosome was characterized by PCR analysis and the integrity of the mouse chromosome retained in the hybrids confirmed by fluorescence in situ hybridization (FISH) analysis.

Deletions of the short arm of chromosome 3 in solid tumors and the search for suppressor genes

The concept that cells can become malignant upon the elimination of parts of chromosomes inhibiting cell division dates back to Boveri in 1914. Deletions occurring in tumor cells are therefore considered a first indication of possible locations of tumor suppressor gene. Approaches used to localize and identify the paradigm of tumor suppressors, RB1, have also been applied to localize tumor suppressor genes on 3p, the short arm of chromosome 3. This review discusses the methodological advantages and limitations of the various approaches. From a review of the literature on losses of 3p in different types of solid tumors it appears that some tumor types show involvement of the same region, while between others the regions involved clearly differ. Also discussed are results of functional assays of tumor suppression by transfer of part of chromosome 3 into tumor cell lines. The likelihood that a common region of deletions would contain a tumor suppressor is strongly enhanced by coincidence of that region with a chromosome fragment suppressing tumorigenicity upon introduction in tumor cells. Such a situation exists for a region in 3p21.3 as well as for one or more in 3p12-p14. The former region is considered the location of a lung cancer suppressor. The same gene or a different one in the same region may also play a role in the development of other cancers including renal cell cancer. In the latter cancer, there may be additional roles of the VHL region and/or a 3p12-p14 region. The breakpoint region of a t(3;8) originally found to be constitutively present in a family with hereditary renal cell cancer now seems to be excluded from such a role. Specific genes on 3p have been suggested to act as suppressor genes based on either their location in a common deletion region, a markedly reduced expression or presence of aberrant transcripts, their capacity to suppress tumorigenicity upon transfection in to tumor cells, the presumed function of the gene product, or a combination of several of these criteria. A number of genes are evaluated for their possible role as a tumor suppressor according to these criteria. General agreement on such a role seems to exist only for VHL. Though hMLH1 plays an obvious role in the development of specific mismatch repair-deficient cancers, it cannot revert the tumor phenotype and therefore cannot be considered a proper tumor suppressor. The involvement of VHL and MLH1 also in some specific hereditary cancers allowed to successfully apply linkage analysis for their localization. TGFBR2 might well have a tumor suppressor function. It does reduce tumorigenicity upon transfection. Other 3p genes coding for receptor proteins THRB and RARB, are unlikely candidates for tumor suppression. Present observations on a possible association of FHIT with tumor development leave a number of questions unanswered, so that provisionally it cannot be considered a tumor suppressor. Regions that have been identified as crucial in solid tumor development appear to be at the edge of synteny blocks that have been rearranged through the chromosome evolution which led to the formation of human chromosome 3. Although this may merely represent a chance occurrence, it might also reflect areas of genomic instability.

Loss of oncogenic ras expression does not correlate with loss of tumorigenicity in human cells

ras oncogenes are mutated in at variety of human tumors, which suggests that they play an important role in human carcinogenesis. To determine whether continued oncogenic ras expression is necessary to maintain the malignant phenotype, we studied the human fibrosarcoma cell line, HT1080, which contains one mutated and one wild-type N-ras allele. We isolated a variant of this cell line that no longer contained the mutated copy of the N-ras gene. Loss of mutant N-ras resulted in cells that displayed a less transformed phenotype characterized by a flat morphology, decreased growth rate, organized actin stress fibers, and loss of anchorage-independent growth. The transformed phenotype was restored following reintroduction of mutant N-ras. Although loss of the oncogenic N-ras drastically affected in vitro growth parameters, the variant remained tumorigenic in nude mice indicating that mutated N-ras expression is not necessary for maintenance of the tumorigenic phenotype. We confirmed this latter observation in colon carcinoma cell lines that have lost activated K-ras expression via targeted knockout of the mutant K-ras gene.

Metastasis suppressed, but tumorigenicity and local invasiveness unaffected, in the human melanoma cell line MelJuSo after introduction of human chromosomes 1 or 6

Progression of human melanoma toward increasing malignant behavior is associated with several nonrandom chromosomal aberrations, most commonly involving chromosomes 1, 6, 7, 9, and 10. We previously showed that introduction of human chromosome 6 into the highly metastatic human malignant melanoma cell line C8161 completely suppressed metastasis without altering tumorigenicity (Welch DR, Chen P, Miele ME, et al., Oncogene 9:255-262, 1994). Alterations of chromosome 1 are the most frequent chromosome abnormality observed in melanomas, and they frequently arise late in tumor progression. The purpose of the study presented here was to compare the effects of chromosomes 1 and 6 on malignant melanoma metastasis. By using microcell-mediated chromosome transfer, single copies of neo-tagged human chromosomes 1 or 6 were introduced into the human melanoma cell line MelJuSo. The presence of the added chromosome was verified by G banding of karyotypes, fluorescence in situ hybridization, and screening for polymorphic markers on each chromosome. The incidence and number of metastases per lung after intravenous or intradermal injection of parental MelJuSo cells was significantly (P<0.01) greater than those of hybrids containing either chromosome 1 or chromosome 6, although chromosome 1 was a less potent inhibitor of metastasis than chromosome 6. Cultures established from primary tumors and metastases remained neomycin resistant, suggesting that portions of the added chromosomes were retained. These results strengthen the evidence for the presence of a melanoma metastasis suppressor gene on chromosome 6. neo6/MelJuSo hybrids expressed 2.4- to 3.4-fold more of the melanoma differentiation-associated gene mda-6 (previously shown to be identical to WAF1/CIP1/Sdi1/CAP20) than parental metastatic cells. mda-6/WAF1 is among the candidate genes on chromosome 6. These results also demonstrate, for the first time, the existence of metastasis suppressor genes on human chromosome 1, although these genes appear to be less potent than the one encoded on chromosome 6.

Tumor suppression and apoptosis of human prostate carcinoma mediated by a genetic locus within human chromosome 10pter-q11

Prostate cancer is the second leading cause of male cancer deaths in the United States. Yet, despite a large international effort, little is known about the molecular mechanisms that underlie this devastating disease. Prostate secretory epithelial cells and androgen-dependent prostate carcinomas undergo apoptosis in response to androgen deprivation and, furthermore, most prostate carcinomas become androgen independent and refractory to further therapeutic manipulations during disease progression. Definition of the genetic events that trigger apoptosis in the prostate could provide important insights into critical pathways in normal development as well as elucidate the perturbations of those key pathways in neoplastic transformation. We report the functional definition of a novel genetic locus within human chromosome 10pter-q11 that mediates both in vivo tumor suppression and in vitro apoptosis of prostatic adenocarcinoma cells. A defined fragment of human chromosome 10 was transferred via microcell fusion into a prostate adenocarcinoma cell line. Microcell hybrids containing only the region 10pter-q11 were suppressed for tumorigenicity following injection of microcell hybrids into nude mice. Furthermore, the complemented hybrids undergo programmed cell death in vitro via a mechanism that does not require nuclear localization of p53. These data functionally define a novel genetic locus, designated PAC1, for prostate adenocarcinoma 1, involved in tumor suppression of human prostate carcinoma and furthermore strongly suggest that the cell death pathway can be functionally restored in prostatic adenocarcinoma.

Localization of a tumor suppressor gene in 11p15.5 using the G401 Wilms' tumor assay

Multiple studies have underscored the importance of loss of tumor suppressor genes in the development of human cancer. To identify these genes, we used somatic cell hybrids in a functional assay for tumor suppression in vivo. A tumor suppressor gene in 11p15.5 was detected by transferring single human chromosomes into the G401 Wilms' tumor cell line. In order to better map this gene, we created a series of radiation-reduced t(X;11) chromosomes and characterized them at 24 loci between H-RAS and beta-globin. Interestingly, three of the chromosomes were indistinguishable as determined by genomic and cytogenetic analyses. Each contains an interstitial deletion with one breakpoint in 11p14.1 and the other breakpoint between the D11S601 and D11S648 loci in 11p15.5. PFGE analysis localized the 11p15.5 breakpoints to a 175 kb MluI fragment that hybridized to D11S601 and D11S648 probes. Genomic fragments from this 175 kb region were hybridized to DNA from mouse hybrid lines containing the delta t(X;11) chromosomes. This analysis detected the identical 11p15.5 breakpoint which disrupts a 7.8 kb EcoRI fragment in all three of the delta t(X;11) chromosomes, suggesting they are subclones of the same parent colony. Upon transfer into G401 cells, one of the chromosomes suppressed tumor formation in nude mice, while the other two chromosomes lacked this ability. Thus, our mapping data indicate that the gene in 11p15.5 which suppresses tumor formation in G401 cells must lie telomeric to the D11S601 locus. Koi et al. (Science 260: 361-364, 1993) have used a similar functional assay to localize a growth suppressor gene for the RD cell line centromeric to the D11S724 locus. The combination of functional studies by our lab and theirs significantly narrows the location of the tumor suppressor gene in 11p15.5 to the approximately 500 kb region between D11S601 and D11S724.

Human chromosome 11 suppresses the tumorigenicity of adenovirus transformed baby rat kidney cells: involvement of the Wilms' tumor 1 gene

Human chromosome 11 was introduced into adenovirus-transformed baby rat kidney (BRK) cells by microcell-mediated chromosome transfer. The resulting microcell hybrids (MCHs) showed a reduced ability to form tumors upon s.c. injection into athymic mice. Further analysis, with the use of defined deletion chromosomes of 11p, indicated that the presence of region 11p13-p12 is necessary for the suppression of tumorigenicity. In contrast, the presence of region 11p15-14.1 appeared to increase the rate of tumor growth. Expression studies on the human Wilms' tumor I (WTI) and the insulin-like growth factor II (IGF-II) genes, which lie in regions 11p13 and 11p15, respectively, suggested the involvement of both genes in determining the degree of suppression of tumorigenicity. Finally, stable expression of a murine WTI protein in the adenovirus-transformed cells resulted in almost complete suppression of tumorigenicity, establishing the WTI protein as a tumor suppressor in this cell system.

Gene expression in response to retinoic acid in novel human chromosome 21 monochromosomal cell hybrids

To access a wide a variety of expressed sequence from human chromosome 21 we have placed this chromosome into undifferentiated P19 mouse embryonic carcinoma cells. Cell lines resulting from these experiments have a range of morphologies and a wide variety of karyotypes. We have studied the retinoic acid response of five cell lines, compared to P19 cells, by observing three markers of retinoic acid induced P19 differentiation--cell morphology, RAR alpha and Wnt1 transcription. We see an 'early' retinoic acid response effect, however this response breaks down by the time the 'late' gene. Wnt1 would be transcribed in P19 cells. A highly responsive cell line will be useful for cloning expressed sequences from human chromosome 21 which are produced by early genes in retinoic acid inducible pathways, such as those involved in neurogenesis.

Factors affecting formation of micronucleus-like structures after colchicine treatment of Trichoderma reesei

Micronucleus-like structures were produced in Trichoderma reesei only when 0.1% colchicine treatment was used to enhance nuclear division. The average DNA content of these 'small nuclei' was 30% that of the normal nuclei, indicating that they were aneuploid nuclei. Such small nuclei may be useful in transferring small amounts of DNA into protoplasts.

Simple purification of human chromosomes to homogeneity using muntjac hybrid cells

Chromosome sorting from hybrid cells offers enormous advantages for gene mapping and cloning, but purification of most chromosomes has been largely hindered by their similarity in size to other chromosomes. We have developed a novel cell line and strategy that allows simple, mass purification of mammalian chromosomes, permitting significant target genome enrichment. This strategy takes advantage of the small number of giant chromosomes (1,2,X) of the female Indian muntjac, a barking deer, avoiding the problem of size similarity. We introduced human chromosomes into a cell line derived from a muntjac and purified them to homogeneity using a relatively simple technique. This strategy should facilitate the isolation of chromosomes from species other than human for which hybrid cells are not available currently.

Tissue-specific extinguisher loci in the murine genome: a screening study based on a rat/mouse microcell hybrid panel

Extinction of tissue-specific traits in intertypic somatic cell hybrids is a well-known phenomenon. In the past few years, microcell hybrids have been used in attempts to dissect this phenotype genetically, and tissue-specific extinguisher loci have been mapped to two different mouse chromosomes. When transferred from fibroblast into hepatoma cells by microcell fusion, these loci down-regulate expression of specific liver genes in trans. However, other liver genes that are extinguished in genotypically complete hybrids seem not to be extinguished in monochromosomal hybrids. To assess the generality of monochromosomal extinction phenotypes, we assembled a collection of rat hepatoma/mouse fibroblast microcell hybrids that represent most of the mouse chromosome complement, and we screened them for expression of a large number of liver-specific genes. Phosphoenolpyruvate carboxykinase gene expression was down-regulated in hybrids containing mouse chromosome 7 or mouse chromosome 11, but other extinction phenotypes were not readily apparent. These results indicate that extinction of many liver genes may be a polygenic trait.

Tissue-specific extinguisher loci in the human genome: a screening study based on random marking and transfer of human chromosomes

Expression of many liver-specific genes is extinguished when cultured hepatoma cells are fused with fibroblasts, but liver genes can be reexpressed in hybrid segregants that have lost fibroblast chromosomes. To map extinguisher loci involved in this process, hepatoma microcell hybrids retaining single fibroblast chromosomes have been employed. Two different, transdominant loci that affect liver gene expression have been defined in this way. To determine whether other monochromosomal extinction phenotypes could be observed, we inserted a selectable marker into many human chromosomal sites and transferred the marked human chromosomes into rat hepatoma recipient cells by microcell fusion. Nearly 200 microcell hybrid clones were isolated and screened for expression of liver-specific mRNAs. Most liver transcripts continued to be expressed. However, PEPCK mRNA was extinguished in 12 hybrid clones. Some of these hybrids contained human TSE1, the previously characterized extinguisher locus on chromosome 17, but others contained a novel extinguishing function that mapped to human chromosome 14. The implications of these findings are discussed.

Evidence that wild-type TP53, and not genes on either chromosome 1 or 11, controls the tumorigenic phenotype of the human fibrosarcoma HT1080

The specific transfer of normal chromosomes via microcell fusion has been instrumental in identifying putative tumor suppressor gene loci in a variety of human cancers. Using this same technique it has been proposed that the tumorigenicity of the human fibrosarcoma cell line HT1080 is controlled by functionally distinct tumor suppressor genes on human chromosomes I and II. To address these results and perhaps further localize the suppressive effect to particular regions on these two chromosomes, we transferred into HT1080 seven different fibroblast-derived human chromosomes containing either intact or discrete portions of chromosome I or II. Interestingly, we found no evidence of genes on these chromosomes that could alter the growth of HT1080 either in vitro or in vivo. Based on these results we were left with the possibility that a gene, or genes, residing on an entirely different chromosome(s) was involved in the tumorigenesis of HT1080. Since TP53 mutation has been documented in a variety of human tumor types, and we found both copies of TP53 to be mutated in HT1080, we were prompted to examine its role by both cDNA transfection and chromosome transfer. Although by cDNA transfection we found that expression of exogenous wild-type TP53 was incompatible with continued proliferation of HT1080 cells in vitro, chromosome 17 transfer studies revealed that a more physiologic expression of exogenous wild-type TP53 could be tolerated in vitro while being completely incompatible with growth in vivo. These studies demonstrate a differential effect of TP53 growth inhibition and clearly show that TP53 tumor suppressing function can be independent from its potent growth suppressing effect in vitro.

An early history of gene transfer and therapy

The term "gene therapy" was coined to distinguish it from the Orwellian connotations of "human genetic engineering," which, in turn, was derived from the term "genetic engineering." Genetic engineering was first used at the Sixth International Congress of Genetics held in 1932 and was taken to mean "the application of genetic principles to animal and plant breeding." Once the basics of molecular genetics and gene transfer in bacteria were established in the 1960s, gene transfer into animals and humans using either viral vectors and/or genetically modified cultured cells became inevitable. Despite the early exposition of the concept of gene therapy, progress awaited the advent of recombinant DNA technology. The lack of trustworthy techniques did not stop many researchers from attempting to transfer genes into cells in culture, animals, and humans. Viral genomes were used for the development of the first relatively efficient methods for gene transfer into mammalian cells in culture. In the late 1970s, early transfection techniques were combined with selection systems for cultured cells and recombinant DNA technology. With the development of retroviral vectors in the early 1980s, the possibility of efficient gene transfer into mammalian cells for the purpose of gene therapy became widely accepted.