A general high-efficiency procedure for production of microcell hybrids

Dynamic interplay between human alpha-satellite DNA structure and centromere functions

Maintenance of genome stability relies on functional centromeres for correct chromosome segregation and faithful inheritance of the genetic information. The human centromere is the primary constriction within mitotic chromosomes made up of repetitive alpha-satellite DNA hierarchically organized in megabase-long arrays of near-identical higher order repeats (HORs). Centromeres are epigenetically specified by the presence of the centromere-specific histone H3 variant, CENP-A, which enables the assembly of the kinetochore for microtubule attachment. Notably, centromeric DNA is faithfully inherited as intact haplotypes from the parents to the offspring without intervening recombination, yet, outside of meiosis, centromeres are akin to common fragile sites (CFSs), manifesting crossing-overs and ongoing sequence instability. Consequences of DNA changes within the centromere are just starting to emerge, with unclear effects on intra- and inter-generational inheritance driven by centromere's essential role in kinetochore assembly. Here, we review evidence of meiotic selection operating to mitigate centromere drive, as well as recent reports on centromere damage, recombination and repair during the mitotic cell division. We propose an antagonistic pleiotropy interpretation to reconcile centromere DNA instability as both driver of aneuploidy that underlies degenerative diseases, while also potentially necessary for the maintenance of homogenized HORs for centromere function. We attempt to provide a framework for this conceptual leap taking into consideration the structural interface of centromere-kinetochore interaction and present case scenarios for its malfunctioning. Finally, we offer an integrated working model to connect DNA instability, chromatin, and structural changes with functional consequences on chromosome integrity.

KaryoCreate: A CRISPR-based technology to study chromosome-specific aneuploidy by targeting human centromeres

Aneuploidy, the presence of chromosome gains or losses, is a hallmark of cancer. Here, we describe KaryoCreate (karyotype CRISPR-engineered aneuploidy technology), a system that enables the generation of chromosome-specific aneuploidies by co-expression of an sgRNA targeting chromosome-specific CENPA-binding ɑ-satellite repeats together with dCas9 fused to mutant KNL1. We design unique and highly specific sgRNAs for 19 of the 24 chromosomes. Expression of these constructs leads to missegregation and induction of gains or losses of the targeted chromosome in cellular progeny, with an average efficiency of 8% for gains and 12% for losses (up to 20%) validated across 10 chromosomes. Using KaryoCreate in colon epithelial cells, we show that chromosome 18q loss, frequent in gastrointestinal cancers, promotes resistance to TGF-β, likely due to synergistic hemizygous deletion of multiple genes. Altogether, we describe an innovative technology to create and study chromosome missegregation and aneuploidy in the context of cancer and beyond.

Understanding How Genetic Mutations Collaborate with Genomic Instability in Cancer

Chromosomal instability is the process of mis-segregation for ongoing chromosomes, which leads to cells with an abnormal number of chromosomes, also known as an aneuploid state. Induced aneuploidy is detrimental during development and in primary cells but aneuploidy is also a hallmark of cancer cells. It is therefore believed that premalignant cells need to overcome aneuploidy-imposed stresses to become tumorigenic. Over the past decade, some aneuploidy-tolerating pathways have been identified through small-scale screens, which suggest that aneuploidy tolerance pathways can potentially be therapeutically exploited. However, to better understand the processes that lead to aneuploidy tolerance in cancer cells, large-scale and unbiased genetic screens are needed, both in euploid and aneuploid cancer models. In this review, we describe some of the currently known aneuploidy-tolerating hits, how large-scale genome-wide screens can broaden our knowledge on aneuploidy specific cancer driver genes, and how we can exploit the outcomes of these screens to improve future cancer therapy.

Aneuploidy as a promoter and suppressor of malignant growth

Aneuploidy has been recognized as a hallmark of tumorigenesis for more than 100 years, but the connection between chromosomal errors and malignant growth has remained obscure. New evidence emerging from both basic and clinical research has illuminated a complicated relationship: despite its frequency in human tumours, aneuploidy is not a universal driver of cancer development and instead can exert substantial tumour-suppressive effects. The specific consequences of aneuploidy are highly context dependent and are influenced by a cell's genetic and environmental milieu. In this Review, we discuss the diverse facets of cancer biology that are shaped by aneuploidy, including metastasis, drug resistance and immune recognition, and we highlight aneuploidy's distinct roles as both a tumour promoter and an anticancer vulnerability.

Cybrid technology

The study of the mitochondrial DNA (mtDNA) has been hampered by the lack of methods to genetically manipulate the mitochondrial genome in living animal cells. This limitation has been partially alleviated by the ability to transfer mitochondria (and their mtDNAs) from one cell into another, as long as they are from the same species. This is done by isolating mtDNA-containing cytoplasts and fusing these to cells lacking mtDNA. This transmitochondrial cytoplasmic hybrid (cybrid) technology has helped the field understand the mechanism of several pathogenic mutations. In this chapter, we describe procedures to obtain transmitochondrial cybrids.

The diverse consequences of aneuploidy

Aneuploidy, or imbalanced chromosome number, has profound effects on eukaryotic cells. In humans, aneuploidy is associated with various pathologies, including cancer, which suggests that it mediates a proliferative advantage under these conditions. Here, we discuss physiological changes triggered by aneuploidy, such as altered cell growth, transcriptional changes, proteotoxic stress, genomic instability and response to interferons, and how cancer cells adapt to the changing aneuploid genome.

Antiproliferative Fate of the Tetraploid Formed after Mitotic Slippage and Its Promotion; A Novel Target for Cancer Therapy Based on Microtubule Poisons

Microtubule poisons inhibit spindle function, leading to activation of spindle assembly checkpoint (SAC) and mitotic arrest. Cell death occurring in prolonged mitosis is the first target of microtubule poisons in cancer therapies. However, even in the presence of microtubule poisons, SAC and mitotic arrest are not permanent, and the surviving cells exit the mitosis without cytokinesis (mitotic slippage), becoming tetraploid. Another target of microtubule poisons-based cancer therapy is antiproliferative fate after mitotic slippage. The ultimate goal of both the microtubule poisons-based cancer therapies involves the induction of a mechanism defined as mitotic catastrophe, which is a bona fide intrinsic oncosuppressive mechanism that senses mitotic failure and responds by driving a cell to an irreversible antiproliferative fate of death or senescence. This mechanism of antiproliferative fate after mitotic slippage is not as well understood. We provide an overview of mitotic catastrophe, and explain new insights underscoring a causal association between basal autophagy levels and antiproliferative fate after mitotic slippage, and propose possible improved strategies. Additionally, we discuss nuclear alterations characterizing the mitotic catastrophe (micronuclei, multinuclei) after mitotic slippage, and a possible new type of nuclear alteration (clustered micronuclei).

Retargeting of microcell fusion towards recipient cell-oriented transfer of human artificial chromosome

Background

Human artificial chromosome (HAC) vectors have some unique characteristics as compared with conventional vectors, carrying large transgenes without size limitation, showing persistent expression of transgenes, and existing independently from host genome in cells. With these features, HACs are expected to be promising vectors for modifications of a variety of cell types. However, the method of introduction of HACs into target cells is confined to microcell-mediated chromosome transfer (MMCT), which is less efficient than other methods of vector introduction. Application of Measles Virus (MV) fusogenic proteins to MMCT instead of polyethylene glycol (PEG) has partly solved this drawback, whereas the tropism of MV fusogenic proteins is restricted to human CD46- or SLAM-positive cells.

Results

Here, we show that retargeting of microcell fusion by adding anti-Transferrin receptor (TfR) single chain antibodies (scFvs) to the extracellular C-terminus of the MV-H protein improves the efficiency of MV-MMCT to human fibroblasts which originally barely express both native MV receptors, and are therefore resistant to MV-MMCT. Efficacy of chimeric fusogenic proteins was evaluated by the evidence that the HAC, tagged with a drug-resistant gene and an EGFP gene, was transferred from CHO donor cells into human fibroblasts. Furthermore, it was demonstrated that no perturbation of either the HAC status or the functions of transgenes was observed on account of retargeted MV-MMCT when another HAC carrying four reprogramming factors (iHAC) was transferred into human fibroblasts.

Conclusions

Retargeted MV-MMCT using chimeric H protein with scFvs succeeded in extending the cell spectrum for gene transfer via HAC vectors. Therefore, this technology could facilitate the systematic cell engineering by HACs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-015-0142-z) contains supplementary material, which is available to authorized users.

Global analysis of genome, transcriptome and proteome reveals the response to aneuploidy in human cells

Genomic, transcriptomic and proteomic profiles of human aneuploid cells reveal that mRNA levels increase with gene copy number, but protein levels are partially compensated. Aneuploid cells also exhibit common alterations in several pathways, including an activation of autophagy.

Comparative genomics, transcriptomics and proteomics of model human aneuploid cell lines reveal that whereas the mRNA levels increase proportionally to the chromosome copy numbers, the abundance of some proteins (e.g., subunits of complexes) is decreased to normal levels.

The pattern of up- and downregulated pathways was similar in all analyzed aneuploids, indicating that it might be possible to use aneuploidy as a cancer treatment target regardless of the exact chromosome composition of cancer cells.

Autophagy, in particular p62-dependent selective autophagy, is activated in aneuploid human cell lines.

Extra chromosome copies markedly alter the physiology of eukaryotic cells, but the underlying reasons are not well understood. We created human trisomic and tetrasomic cell lines and determined the quantitative changes in their transcriptome and proteome in comparison with their diploid counterparts. We found that whereas transcription levels reflect the chromosome copy number changes, the abundance of some proteins, such as subunits of protein complexes and protein kinases, is reduced toward diploid levels. Furthermore, using the quantitative data we investigated the changes of cellular pathways in response to aneuploidy. This analysis revealed specific and uniform alterations in pathway regulation in cells with extra chromosomes. For example, the DNA and RNA metabolism pathways were downregulated, whereas several pathways such as energy metabolism, membrane metabolism and lysosomal pathways were upregulated. In particular, we found that the p62-dependent selective autophagy is activated in the human trisomic and tetrasomic cells. Our data present the first broad proteomic analysis of human cells with abnormal karyotypes and suggest a uniform cellular response to the presence of an extra chromosome.

Advances in high-capacity extrachromosomal vector technology: episomal maintenance, vector delivery, and transgene expression

Recent developments in extrachromosomal vector technology have offered new ways of designing safer, physiologically regulated vectors for gene therapy. Extrachromosomal, or episomal, persistence in the nucleus of transduced cells offers a safer alternative to integrating vectors which have become the subject of safety concerns following serious adverse events in recent clinical trials. Extrachromosomal vectors do not cause physical disruption in the host genome, making these vectors safe and suitable tools for several gene therapy targets, including stem cells. Moreover, the high insert capacity of extrachromosomal vectors allows expression of a therapeutic transgene from the context of its genomic DNA sequence, providing an elegant way to express normal splice variants and achieve physiologically regulated levels of expression. Here, we describe past and recent advances in the development of several different extrachromosomal systems, discuss their retention mechanisms, and evaluate their use as expression vectors to deliver and express genomic DNA loci. We also discuss a variety of delivery systems, viral and nonviral, which have been used to deliver episomal vectors to target cells in vitro and in vivo. Finally, we explore the potential for the delivery and expression of extrachromosomal transgenes in stem cells. The long-term persistence of extrachromosomal vectors combined with the potential for stem cell proliferation and differentiation into a wide range of cell types offers an exciting prospect for therapeutic interventions.

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.

Chromosome transfer activates and delineates a locus control region for perforin

Perforin gene (PRF1) transcription regulates perforin expression in NK cells and CTL. Here we identified the locus-wide ensemble of cis-acting sequences that drives PRF1 transcription physiologically. By using chromosome transfer, we revealed that de novo activation of a silent PRF1 locus was controlled by a 150 kb domain comprised of 16 DNase I hypersensitive sites (DHSs). These cis-acting sequences included a locus control region (LCR) and conferred developmentally appropriate and lineage-specific expression of human perforin from BAC transgenes. The LCR included four distal DHSs that were required for perforin expression from its natural locus, and their engineered deletion from the PRF1 BAC transgene abolished LCR function and led to rapid gene silencing. Thus, LCR function is central for regulating the developmental and activation-specific PRF1 promoter activity characteristic of NK cells and CTL.

Chromosome 18 suppresses tumorigenic properties of human prostate cancer cells

Although prostate cancer is still the most diagnosed cancer in men, most genes implicated in its progression are yet to be identified. Chromosome abnormalities have been detected in human prostate tumors, many of them associated with prostate cancer progression. Indeed, alterations (including deletions or amplifications) of more than 15 human chromosomes have been reported in prostate cancer. We hypothesized that transferring normal human chromosomes into human prostate cancer cells would interfere with their tumorigenic and/or metastatic properties. We used microcell-mediated chromosome transfer to introduce human chromosomes 10, 12, 17, and 18 into highly tumorigenic (PC-3M-Pro4) and highly metastatic (PC-3M-LN4) PC-3-derived cell lines. We tested the in vitro and in vivo properties of these hybrids. Introducing chromosome 18 into the PC-3M-LN4 prostate cancer cell line greatly reduced its tumorigenic phenotype. We observed retarded growth in soft agar, decreased invasiveness through Matrigel, and delayed tumor growth into nude mice, both subcutaneously and orthotopically. This phenotype is associated with a marker in the 18q21 region. Combined with the loss of human chromosome 18 regions often seen in patients with advanced prostate cancer, our results show that chromosome 18 encodes one or more tumor-suppressor genes whose inactivation contributes to prostate cancer progression.

Mutant mitochondrial elongation factor G1 and combined oxidative phosphorylation deficiency

Although most components of the mitochondrial translation apparatus are encoded by nuclear genes, all known molecular defects associated with impaired mitochondrial translation are due to mutations in mitochondrial DNA. We investigated two siblings with a severe defect in mitochondrial translation, reduced levels of oxidative phosphorylation complexes containing mitochondrial DNA (mtDNA)-encoded subunits, and progressive hepatoencephalopathy. We mapped the defective gene to a region on chromosome 3q containing elongation factor G1 (EFG1), which encodes a mitochondrial translation factor. Sequencing of EFG1 revealed a mutation affecting a conserved residue of the guanosine triphosphate (GTP)-binding domain. These results define a new class of gene defects underlying disorders of oxidative phosphorylation.

Human artificial chromosomes containing chromosome 17 alphoid DNA maintain an active centromere in murine cells but are not stable

Human artificial chromosomes (HACs) are autonomous molecules that can function and segregate as normal chromosomes in human cells. De novo HACs have successfully been used as gene expression vectors to complement genetic deficiencies in human cultured cells. HACs now offer the possibility of studying the regulation and expression of large genes in a variety of cell types from different tissues and correcting gene deficiencies caused by human inherited diseases. Complementary gene expression studies in mice, especially in mouse models of human genetic diseases, are also important in determining if large human transgenes can be expressed appropriately from artificial chromosomes. Toward this aim we are establishing artificial chromosomes in murine cells as novel gene expression vectors. Initially we transferred HAC vectors into murine cells, but were unable to generate de novo HACs at a reasonable frequency. We then transferred HACs previously established in human HT1080 cells to three different murine cell types by microcell fusion, followed by positive selection. We observed that the HACs in murine cells bound centromere protein C (CENP-C), a marker of active centromeres, and were detected under selection but rapidly lost when selection was removed. These results suggest that the HACs maintain at least a partially functional centromere complex in murine cells, but other factors are required for stability and segregation. Artificial chromosomes containing mouse centromeric sequences may be required for better stability and maintenance in murine cells.

The microcell mediated transfer of human chromosome 8 into highly metastatic rat liver cancer cell line C5F

AIM: Our previous research on the surgical samples of primary liver cancer with CGH showed that the loss of human chromosome 8p had correlation with the metastatic phenotype of liver cancer. In order to seek the functional evidence that there could be a metastatsis suppressor gene (s) for liver cancer on human chromosome 8, we tried to transfer normal human chromosome 8 into rat liver cancer cell line C5F, which had high metastatic potential to lung.

METHODS: Human chromosome 8 randomly marked with neo gene was introduced into C5F cell line by MMCT and positive microcell hybrids were screened by double selections of G418 and HAT. Single cell isolation cloning was applied to clone microcell hybrids. Finally, STS-PCR and WCP-FISH were used to confirm the introduction.

RESULTS: Microcell hybrids resistant to HAT and G418 were obtained and 15 clones were obtained by single-cell isolation cloning. STS-PCR and WCP-FISH proved that human chromosome 8 had been successfully introduced into rat liver cancer cell line C5F. STS-PCR detected a random loss in the chromosome introduced and WCP-FISH found a consistent recombination of the introduced human chromosome with the rat chromosome.

CONCLUSION: The successful introduction of human chromosome 8 into highly metastatic rat liver cancer cell line builds the basis for seeking functional evidence of a metastasis suppressor gene for liver cancer harboring on human chromosome 8 and its subsequent cloning.

Delayed replication timing leads to delayed mitotic chromosome condensation and chromosomal instability of chromosome translocations

Chromosomal rearrangements are found in virtually all types of human cancers. We show that certain chromosome translocations display a delay in mitotic chromosome condensation that is associated with a delay in the mitosis-specific phosphorylation of histone H3. This delay in mitotic condensation is preceded by a delay in both the initiation as well as the completion of chromosome replication. In addition, chromosomes with this phenotype participate in numerous secondary translocations and rearrangements. Chromosomes with this phenotype were detected in five of seven tumor-derived cell lines and in five of thirteen primary tumor samples. These data suggest that certain chromosomal rearrangements found in tumor cells cause a significant delay in replication timing of the entire chromosome that subsequently results in delayed mitotic chromosome condensation and ultimately in chromosomal instability.

Demethylation, reactivation, and destabilization of human fragile X full-mutation alleles in mouse embryocarcinoma cells

The major causes of fragile X syndrome are mutational expansion of the CGG repeat in the FMR1 gene, hypermethylation, and transcriptional silencing. Most fragile X embryos develop somatic mosaicism of disease-causing "full" expansions of different lengths. Homogeneity of the mosaic patterns among multiple tissues in the same individual indicates that these previously unstable expansions acquire mitotic stability early in fetal life. Since mitotic stability is found strictly associated with hypermethylation in adult tissues, current theory has fixed the time of instability to developmental stages when fully expanded CGG repeats exist in an unmethylated state. We used murine embryocarcinoma (EC) cells (PC13) as a model system of pluripotent embryonic cells. Hypermethylated and unmethylated full expansions on human fragile X chromosomes were transferred from murine A9 hybrids into EC cells, by means of microcell fusion. As demonstrated in the present study for the first time, even full expansion alleles that were fully methylated and stable in the donors' fibroblasts and in A9 became demethylated, reactivated, and destabilized in undifferentiated EC hybrids. When destabilized expansions were reintroduced from EC cells into A9, instability was reversed to stability. Our results strongly support the idea that fully expanded alleles are initially unstable and unmethylated in the human embryo and gain stability upon genetic or epigenetic change of the embryonic cells.

Suppression of tumorigenicity in the human prostate cancer cell line M12 via microcell-mediated restoration of chromosome 19

Previously we immortalized human, nontransformed prostate epithelial cells with SV40 large T-antigen (SV40TAg) and derived increasingly aggressive sublines from the immortalized line. The progression of the tumorigenic sublines to metastatic capacity was accompanied by the formation of an unbalanced translocation between chromosomes 16 and 19, resulting in loss of 19p and proximal 19q. To test whether the tumorigenic and/or metastatic phenotype was causally related to this genetic alteration, we restored a neo-tagged human chromosome 19 to M12 cells by microcell-mediated transfer and assessed their growth. In vitro, the resultant hybrids grew more slowly in monolayer culture and showed a significant reduction in anchorage-independent growth as compared to M12neo controls. In vivo, all mice (13/13) injected subcutaneously (SC) with control M12neo cells developed tumors after 9-15 days. In contrast, 9/15 mice injected SC with microcell-transferred chromosome 19 hybrid cells failed to form tumors, with 6/15 producing very small tumors after 120 days. Analysis of three of these six tumors showed consistent, new chromosomal changes. Furthermore, in one of the tumors, loss of a chromosome 19 was noted in 40% of the cells. After intraprostatic injections of the hybrid cells, only 2/7 mice developed microscopic tumors, with no metastases. These data suggest the presence of a gene or genes on chromosome 19 that function to suppress growth.

Chromosome 18 suppresses the tumorigenicity of prostate cancer cells

Microcell-mediated chromosome transfer allows for the introduction of normal chromosomes into tumor cells in an effort to identify putative tumor suppressor genes. We have used this approach to introduce an intact copy of chromosome 18 into the prostate cancer cell line DU145, and independently to introduce human chromosomes 8 and 18 into the prostate cancer cell line TSU-PR1. Introduction of an extra copy of human chromosome 8 had no effect on the growth properties in vitro or the tumorigenicity in vivo of TSU-PR1 cells. However, microcell hybrids containing an introduced copy of human chromosome 18 exhibited a longer population doubling time, retarded growth in soft agar, and slowed tumor growth in athymic nude mice. These experiments provide functional evidence for the presence of one or more tumor suppressor genes on human chromosome 18 that are involved in prostate cancer.

A novel syndrome affecting multiple mitochondrial functions, located by microcell-mediated transfer to chromosome 2p14-2p13

We have studied cultured skin fibroblasts from three siblings and one unrelated individual, all of whom had fatal mitochondrial disease manifesting soon after birth. After incubation with 1 mM glucose, these four cell strains exhibited lactate/pyruvate ratios that were six times greater than those of controls. On further analysis, enzymatic activities of the pyruvate dehydrogenase complex, the 2-oxoglutarate dehydrogenase complex, NADH cytochrome c reductase, succinate dehydrogenase, and succinate cytochrome c reductase were severely deficient. In two of the siblings the enzymatic activity of cytochrome oxidase was mildly decreased (by approximately 50%). Metabolite analysis performed on urine samples taken from these patients revealed high levels of glycine, leucine, valine, and isoleucine, indicating abnormalities of both the glycine-cleavage system and branched-chain alpha-ketoacid dehydrogenase. In contrast, the activities of fibroblast pyruvate carboxylase, mitochondrial aconitase, and citrate synthase were normal. Immunoblot analysis of selected complex III subunits (core 1, cyt c(1), and iron-sulfur protein) and of the pyruvate dehydrogenase complex subunits revealed no visible changes in the levels of all examined proteins, decreasing the possibility that an import and/or assembly factor is involved. To elucidate the underlying molecular defect, analysis of microcell-mediated chromosome-fusion was performed between the present study's fibroblasts (recipients) and a panel of A9 mouse:human hybrids (donors) developed by Cuthbert et al. (1995). Complementation was observed between the recipient cells from both families and the mouse:human hybrid clone carrying human chromosome 2. These results indicate that the underlying defect in our patients is under the control of a nuclear gene, the locus of which is on chromosome 2. A 5-cM interval has been identified as potentially containing the critical region for the unknown gene. This interval maps to region 2p14-2p13.

Functional evidence for the presence of tumor suppressor gene on chromosome 10p15 in human prostate cancers

Loss of heterozygosity on chromosome 10p was observed frequently in human prostate cancers. Studies have demonstrated that the introduction of the short arm of human chromosome 10 into a human prostate cancer cell line, PPC-1, by microcell-mediated chromosome transfer (MMCT), suppressed the malignant phenotype, suggesting the presence of a prostate tumor suppressor gene(s) within a region of 17 cM at distal 10p. To narrow down the candidate region harboring the tumor suppressor gene, a series of 10p fragments were transferred into PPC-1 cells by MMCT using a panel of hamster-human hybrid cells containing various portions of 10p. Four of the six hybrid cells obtained showed decreased tumorigenicity when injected subcutaneously into athymic nude mice. Tumors developed only at six of 40 injection sites for these four hybrid cells. In contrast, the other two hybrid cells, as well as parental PPC-1 cells, were judged to be fully tumorigenic because tumors appeared at a total 26 of 32 sites for the two hybrid cells and 15 of 16 sites for PPC-1. Allelotyping of 10p combined with fluorescence in situ hybridization in these hybrid cells suggested that a prostate tumor suppressor gene was located within a fragment of approximately 1.2 Mb flanked by D10S1172 and D10S226 on 10p15.1.

Localization of the Fanconi anemia complementation group D gene to a 200-kb region on chromosome 3p25.3

Fanconi anemia (FA) is a rare autosomal recessive disease manifested by bone-marrow failure and an elevated incidence of cancer. Cells taken from patients exhibit spontaneous chromosomal breaks and rearrangements. These breaks and rearrangements are greatly elevated by treatment of FA cells with the use of DNA cross-linking agents. The FA complementation group D gene (FANCD) has previously been localized to chromosome 3p22-26, by use of microcell-mediated chromosome transfer. Here we describe the use of noncomplemented microcell hybrids to identify small overlapping deletions that narrow the FANCD critical region. A 1.2-Mb bacterial-artificial-chromosome (BAC)/P1 contig was constructed, bounded by the marker D3S3691 distally and by the gene ATP2B2 proximally. The contig contains at least 36 genes, including the oxytocin receptor (OXTR), hOGG1, the von Hippel-Lindau tumor-suppressor gene (VHL), and IRAK-2. Both hOGG1 and IRAK-2 were excluded as candidates for FANCD. BACs were then used as probes for FISH analyses, to map the extent of the deletions in four of the noncomplemented microcell hybrid cell lines. A narrow region of common overlapping deletions limits the FANCD critical region to approximately 200 kb. The three candidate genes in this region are TIGR-A004X28, SGC34603, and AA609512.

Hypomethylation of an expanded FMR1 allele is not associated with a global DNA methylation defect

The vast majority of fragile-X full mutations are heavily methylated throughout the expanded CGG repeat and the surrounding CpG island. Hypermethylation initiates and/or stabilizes transcriptional inactivation of the FMR1 gene, which causes the fragile X-syndrome phenotype characterized, primarily, by mental retardation. The relation between repeat expansion and hypermethylation is not well understood nor is it absolute, as demonstrated by the identification of nonretarded males who carry hypomethylated full mutations. To better characterize the methylation pattern in a patient who carries a hypomethylated full mutation of approximately 60-700 repeats, we have evaluated methylation with the McrBC endonuclease, which allows analysis of numerous sites in the FMR1 CpG island, including those located within the CGG repeat. We report that the expanded-repeat region is completely free of methylation in this full-mutation male. Significantly, this lack of methylation appears to be specific to the expanded FMR1 CGG-repeat region, because various linked and unlinked repetitive-element loci are methylated normally. This finding demonstrates that the lack of methylation in the expanded CGG-repeat region is not associated with a global defect in methylation of highly repeated DNA sequences. We also report that de novo methylation of the expanded CGG-repeat region does not occur when it is moved via microcell-mediated chromosome transfer into a de novo methylation-competent mouse embryonal carcinoma cell line.

SURF1, encoding a factor involved in the biogenesis of cytochrome c oxidase, is mutated in Leigh syndrome

Leigh Syndrome (LS) is a severe neurological disorder characterized by bilaterally symmetrical necrotic lesions in subcortical brain regions that is commonly associated with systemic cytochrome c oxidase (COX) deficiency. COX deficiency is an autosomal recessive trait and most patients belong to a single genetic complementation group. DNA sequence analysis of the genes encoding the structural subunits of the COX complex has failed to identify a pathogenic mutation. Using microcell-mediated chromosome transfer, we mapped the gene defect in this disorder to chromosome 9q34 by complementation of the respiratory chain deficiency in patient fibroblasts. Analysis of a candidate gene (SURF1) of unknown function revealed several mutations, all of which predict a truncated protein. These data suggest a role for SURF1 in the biogenesis of the COX complex and define a new class of gene defects causing human neurodegenerative disease.

Mutations of SURF-1 in Leigh disease associated with cytochrome c oxidase deficiency

Leigh disease associated with cytochrome c oxidase deficiency (LD[COX-]) is one of the most common disorders of the mitochondrial respiratory chain, in infancy and childhood. No mutations in any of the genes encoding the COX-protein subunits have been identified in LD(COX-) patients. Using complementation assays based on the fusion of LD(COX-) cell lines with several rodent/human rho0 hybrids, we demonstrated that the COX phenotype was rescued by the presence of a normal human chromosome 9. Linkage analysis restricted the disease locus to the subtelomeric region of chromosome 9q, within the 7-cM interval between markers D9S1847 and D9S1826. Candidate genes within this region include SURF-1, the yeast homologue (SHY-1) of which encodes a mitochondrial protein necessary for the maintenance of COX activity and respiration. Sequence analysis of SURF-1 revealed mutations in numerous DNA samples from LD(COX-) patients, indicating that this gene is responsible for the major complementation group in this important mitochondrial disorder.

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.

Duplication of ATR inhibits MyoD, induces aneuploidy and eliminates radiation-induced G1 arrest

Chromosome 3q alterations occur frequently in many types of tumours. In a genetic screen for loci present in rhabdomyosarcomas, we identified an isochromosome 3q [i(3q)], which inhibits muscle differentiation when transferred into myoblasts. The i(3q) inhibits MyoD function, resulting in a non-differentiating phenotype. Furthermore, the i(3q) induces a 'cut' phenotype, abnormal centrosome amplification, aneuploidy and loss of G1 arrest following gamma-irradiation. Testing candidate genes within this region reveals that forced expression of ataxia-telangiectasia and rad3-related (ATR) results in a phenocopy of the i(3q). Thus, genetic alteration of ATR leads to loss of differentiation as well as cell-cycle abnormalities.

Construction of a panel of canine-rodent hybrid cell lines for use in partitioning of the canine genome

We have constructed a collection of canine-rodent microcell hybrid cell lines by fusion of canine fibroblast microcell donors with immortalized rodent recipient cells. Characterization of the hybrid cell lines using a combination of fluorescence in situ hybridization and PCR analysis of canine microsatellite repeat sequences allowed selection of a panel of hybrids in which most canine chromosomes are represented. Approximately 90% of genetic markers and genes that were tested could be assigned to 1 of 31 anonymous canine chromosome groups, based on common patterns of retention in the hybrid set. Many of these putative chromosome groups have now been validated by linkage analysis. This panel of cell lines provides a tool for development of genetic, physical, and comparative maps of the canine genome.

Selective loss of the hepatic phenotype due to the absence of a transcriptional activation pathway

Liver-enriched trans-acting factors hepatocyte nuclear factor-1 alpha (HNF1 alpha) and -4 (HNF4) are components of a transcriptional activation pathway that is thought to play a major role in hepatic gene activation. We previously described the isolation and characterization of distinct classes of hepatoma variants which lack the HNF4-->HNF1 alpha pathway (1). In order to determine the influence of the HNF4-->HNF1 alpha pathway on hepatic gene expression, genetic rescue experiments were done using hepatoma variant line H11 as a model system. Results suggest that this pathway is required for basal expression of a number of endogenous hepatocyte-specific genes. Complementation groups were established by fusion of H11 cells with other variant lines. Lastly, introduction of human chromosome 20 (containing the HNF4 locus) or randomly-marked human chromosomes into H11 cells failed to rescue the hepatic phenotype, suggesting that what appears to be a 'simple' defect may involve multiple genetic loci.

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.

Amplification of MDM2 inhibits MyoD-mediated myogenesis

One obvious phenotype of tumor cells is the lack of terminal differentiation. We previously classified rhabdomyosarcoma cell lines as having either a recessive or a dominant nondifferentiating phenotype. To study the genetic basis of the dominant nondifferentiating phenotype, we utilized microcell fusion to transfer chromosomes from rhabdomyosarcoma cells into C2C12 myoblasts. Transfer of a derivative chromosome 14 inhibits differentiation. The derivative chromosome 14 contains a DNA amplification. MDM2 is amplified and overexpressed in these nondifferentiating hybrids and in the parental rhabdomyosarcoma. Forced expression of MDM2 inhibits MyoD-dependent transcription. Expression of antisense MDM2 restores MyoD-dependent transcriptional activity. We conclude that amplification and overexpression of MDM2 inhibit MyoD function, resulting in a dominant nondifferentiating phenotype.

Microcell mediated chromosome transfer maps the Fanconi anaemia group D gene to chromosome 3p

Fanconi anaemia (FA) is an autosomal recessive disorder characterized by progressive pancytopenia, short stature, radial ray defects, skin hyperpigmentation and a predisposition to cancer. Cells from FA patients are hypersensitive to cell killing and chromosome breakage induced by DNA cross-linking agents such as mitomycin C (MMC) and diepoxybutane (DEB). Consequently, the defect in FA is thought to be in DNA crosslink repair. Additional cellular phenotypes of FA include oxygen sensitivity, poor cell growth and a G2 cell cycle delay. At least 5 complementation groups for Fanconi anaemia exist, termed A through E. One of the five FA genes, FA(C), has been identified by cDNA complementation, but no other FA genes have been mapped or cloned until now. The strategy of cDNA complementation, which was successful for identifying the FA(C) gene has not yet been successful for cloning additional FA genes. The alternative approach of linkage analysis, followed by positional cloning, is hindered in FA by genetic heterogeneity and the lack of a simple assay for determining complementation groups. In contrast to genetic linkage studies, microcell mediated chromosome transfer utilizes functional complementation to identify the disease bearing chromosome. Here we report the successful use of this technique to map the gene for the rare FA complementation group D (FA(D)).

A human glial hybrid cell line differentially expressing genes subserving oligodendrocyte and astrocyte phenotype

We have developed a series of immortal human-human hybrid cell lines that express phenotypic characteristics of primary oligodendrocytes, by fusing a 6-thioguanine-resistant mutant of the human rhabdomyosarcoma RD with adult human oligodendrocytes by a lectin-enhanced polyethylene glycol procedure. Hybrids were selected in an aminopterin-containing media. In contrast to the tumor parent cells, a hybrid clone M03.13 expressed surface immunoreactivity for galactosyl cerebroside and intracellular immunoreactivity for myelin basic protein (MBP), proteolipid protein (PLP), and glial fibrillary acidic protein (GFAP). Serum deprivation or chronic treatment with a protein kinase C activator 4-beta-phorbol 12-myristate 13-acetate (PMA), but not dibutyl cyclic adenosine monophosphate induced coordinate up-regulation or de novo induction of oligodendrocyte phenotypic markers with concomitant down-regulation of GFAP expression. Consistent with immunohistochemical studies, northern blot analysis demonstrated that both MBP and PLP mRNA were up-regulated in MO3.13 cells by PMA treatment. M03.13 cells provide an immortalized clonal model system suitable for study of gene expression subserving oligodendrocyte and astrocyte phenotypes.

A tumor suppressor locus within 3p14-p12 mediates rapid cell death of renal cell carcinoma in vivo

High frequency loss of alleles and cytogenetic aberrations on the short arm of chromosome 3 have been documented in renal cell carcinoma (RCC). Potentially, three distinct regions on 3p could encode tumor suppressor genes involved in the genesis of this cancer. We report that the introduction of a centric fragment of 3p, encompassing 3p14-q11, into a highly malignant RCC cell line resulted in a dramatic suppression of tumor growth in athymic nude mice. Another defined deletion hybrid contained the region 3p12-q24 of the introduced human chromosome and failed to suppress tumorigenicity. These data functionally define a tumor suppressor locus, nonpapillary renal carcinoma-1 (NRC-1), within 3p14-p12, the most proximal region of high frequency allele loss in sporadic RCC as well as the region containing the translocation breakpoint in familial RCC. Furthermore, we provide functional evidence that NRC-1 controls the growth of RCC cells by inducing rapid cell death in vivo.

Direct formation of microcells from mitotic cells for use in chromosome transfer

Microcells, cytoplasmic fragments that contain micronuclei composed of one or a few chromosomes, can be generated directly from mitotic cells. Cytochalasin B, which causes nuclear extrusion in interphase cells, has a similar effect on the chromosomes of colcemid-blocked mitotic cells. The forces generated during centrifugation in a Percoll gradient are sufficient to separate the extruded microcells from the parent cell. The chromosomes contained in an extruded microcell form micronuclei during the process, and in all respects are comparable to microcells generated from micronucleated cells except that they are uniformly in the G1 phase of the cell replication cycle. The procedure is probably applicable to all mammalian cells that grow in culture and can be employed to make microcells for the transfer of both intact and fragmented chromosomes.

Neuroblastoma x spinal cord (NSC) hybrid cell lines resemble developing motor neurons

We have developed a series of mouse-mouse neural hybrid cell lines by fusing the aminopterin-sensitive neuroblastoma N18TG2 with motor neuron-enriched embryonic day 12-14 spinal cord cells. Of 30 neuroblastoma-spinal cord (NSC) hybrids displaying a multipolar neuron-like phenotype, 10 express choline acetyltransferase, and 4 induce twitching in cocultured mouse myotubules. NSC-19, NSC-34, and their subclones express additional properties expected of motor neurons, including generation of action potentials, expression of neurofilament triplet proteins, and acetylcholine synthesis, storage, and release. In addition, NSC-34 cells induce acetylcholine receptor clusters on cocultured myotubes, and undergo a vimentin-neurofilament switch with maturation in culture, similar to that occurring in neuronal development. NSC cell lines appear to model selected aspects of motor neuron development in an immortalized clonal system.

Partitioning of the chicken genome by microcell hybridization

Chicken fibroblast cells isolated from 7-day-old White Leghorn S-line chicken embryos were used for microcell hybridization. Cells transfected with a pSV2-neo plasmid, which carries the Neor gene and confers resistance to geneticin (G418), were selected and maintained in medium containing G418 (350 micrograms/mL). Cells were micronucleated by colcemid arrest and hypotonic treatment. Enucleation was carried out by centrifugation in a Percoll gradient in the presence of cytochalasin B. Purified microcells and HeLa S3 cells were mixed and agglutinated by addition of phytohemagglutinin P, followed by polyethylene glycol fusion to generate microcell hybrids. Chicken by human microcell hybrids were selected in RPMI-1640 medium containing 1.4 mg/mL G418. Cloned hybrid cell lines were maintained in the same medium containing .7 mg/mL of G418. The presence of chicken chromosomes in hybrid cells was demonstrated by cytogenetic analysis and high-resolution nonisotopic chromosomal in situ hybridization. Three out of 28 hybrid cell lines analyzed retained single chicken chromosomes.

Isolation and characterization of microprotoplasts from APM-treated suspension cells ofNicotiana plumbaginifolia

Subprotoplasts with a DNA content of less than the G1 level (microprotoplasts) were isolated from micronucleated cells of transformedNicotiana plumbaginifolia ('Doba' line resistant to kanamycin) and characterized cytologically as well as by flow cytometry and Feulgen microdensitometry. Micronuclei were induced upon treatment of the suspension cells with the anti-microtubule drug amiprophos-methyl (APM). Protoplasts were fractionated on a continuous iso-osmotic gradient of Percoll; this resulted in several visible bands. Flow cytometric analysis of fluorescein and nuclear DNA contents after staining with fluorescein and DAPI respectively showed that the main band contained mostly evacuolated, intact (sub)protoplasts. Microprotoplasts contained one or a few micronuclei surrounded by a thin rim of cytoplasm and an intact plasma membrane. A maximum of 40% of the microprotoplasts in the fraction just below the main band had a DNA content less than the G1 level, in other fractions this maximum was 20%. Some of these contained an amount equivalent to that of one or a few chromosomes. The application of microprotoplasts for chromosome-mediated gene transfer in plants is indicated.

Monochromosomal mouse microcell hybrids containing inserted selectable neo genes

Normal mouse fibroblasts at early passage levels were used as a starting material to construct mouse-hamster microcell hybrids (MCH). The neor gene, carried on the pSV2neo and pZIP-NeoSV(X)1 plasmids, was introduced into the mouse fibroblasts by gene transfection and retroviral infection, respectively, prior to microcell hybridization into the E36 Chinese hamster cell line. In total about 180 MCH clones were isolated and their amount of mouse DNA was estimated by dot-blot analysis. About 50% of the transfection based hybrids (T-hybrids) showed signals indicating one mouse chromosome, less than 10% more than one mouse chromosome, and the remaining clones contained only subchromosomal amounts of mouse DNA. In the infection-based hybrid series (I-hybrids) more than 95% showed only subchromosomal mouse DNA content. Chromosomal integration analysis verified the presence of neor insertions in all 42 hybrid clones analyzed. C-banding analysis verified 14 of 15 hybrids scored as monochromosomals on dot blots. Chromosome fragmentation in T-type MCH was found to be (1) nonrandom, preferentially occurring in MCH derived from certain transfectants, (2) late in clonal establishment, and (3) essentially not related to prolonged cultivation in vitro. Once established, most T-type MCH clones including mono- and subchromosomal hybrids were essentially stable during prolonged cultivation. In contrast MCH initially containing several mouse chromosomes tend to lose the nonselectable ones during prolonged cultivation. In total we estimate the number of independent monochromosomal MCH derived in this study to more than 30.

Regulation of chimeric phosphoenolpyruvate carboxykinase genes by the trans-dominant locus TSE1

Extinction of phosphoenolpyruvate carboxykinase (PCK) gene expression in hepatoma x fibroblast hybrids is mediated by a trans-acting genetic locus designated tissue-specific extinguisher 1 (TSE1). To identify PCK gene sequences required for extinction, hepatoma transfectants expressing PCK-thymidine kinase (TK) chimeric genes were fused with TK- fibroblasts and PCK-TK expression in the resulting hybrids was monitored. Expression of a PCK-TK chimera containing PCK sequences between base pairs -548 and +73 was extinguished in four of five hepatoma transfectants tested, although hybrids derived from one transfectant clone failed to extinguish PCK-TK expression. In contrast, crosses between hepatoma transfectants expressing the herpesvirus TK gene from its own promoter and TK- fibroblasts produced TK+ hybrids; extinction of the transfected TK gene was not observed. Thus, rat PCK gene sequences between base pairs -548 and +73 are sufficient for tissue-specific extinction in hybrid cells. Extinction of PCK-TK gene expression in transfectant microcell hybrids mapped specifically to human chromosome 17, the site of human TSE1.

Regulation of chimeric phosphoenolpyruvate carboxykinase genes by the trans-dominant locus TSE1

Extinction of phosphoenolpyruvate carboxykinase (PCK) gene expression in hepatoma x fibroblast hybrids is mediated by a trans-acting genetic locus designated tissue-specific extinguisher 1 (TSE1). To identify PCK gene sequences required for extinction, hepatoma transfectants expressing PCK-thymidine kinase (TK) chimeric genes were fused with TK- fibroblasts and PCK-TK expression in the resulting hybrids was monitored. Expression of a PCK-TK chimera containing PCK sequences between base pairs -548 and +73 was extinguished in four of five hepatoma transfectants tested, although hybrids derived from one transfectant clone failed to extinguish PCK-TK expression. In contrast, crosses between hepatoma transfectants expressing the herpesvirus TK gene from its own promoter and TK- fibroblasts produced TK+ hybrids; extinction of the transfected TK gene was not observed. Thus, rat PCK gene sequences between base pairs -548 and +73 are sufficient for tissue-specific extinction in hybrid cells. Extinction of PCK-TK gene expression in transfectant microcell hybrids mapped specifically to human chromosome 17, the site of human TSE1.

Tumorigenicity in human melanoma cell lines controlled by introduction of human chromosome 6

Chromosome banding analysis of human malignant melanoma has documented the nonrandom alteration of chromosome 6. To determine the relevance of chromosome 6 abnormalities in melanoma, a normal chromosome 6 was directly introduced into melanoma cell lines. The resulting (+6) microcell hybrids were significantly altered in their phenotypic properties in culture and lost their ability to form tumors in nude mice. The loss of the chromosome 6 from melanoma microcell hybrids resulted in the reversion to tumorigenicity of these cells in mice. The introduction of the selectable marker (psv2neo) alone into melanoma cell lines had no effect on tumorigenicity. These results support the idea that one or more genes on chromosome 6 may control the malignant expression of human melanoma.

Construction of microcell hybrid panel containing different neo gene insertions in mouse chromosome 17 used for chromosome-mediated gene transfer

A panel of four microcell hybrids representing different sites of insertion of the exogenous neo gene into mouse chromosome 17 has been constructed. These constructions were based on a cotransfer of mouse chromosome 17 and neomycin resistance generated in a stepwise procedure involving (1) random insertion of the neo gene into a primary cell hybrid containing mouse chromosome 17 in a hamster cell background, (2) microcell-mediated chromosome transfer (MMCT) to segregate mouse and hamster chromosomes, and (3) identification of the mouse chromosome containing cells using a novel cell dotting procedure for mass screening at the cell colony level by molecular hybridization. Using this panel of four microcell hybrids for chromosome mediated gene transfer (CMGT), we obtained one transformant containing a chromosome fragment derived from the t-complex region located on mouse chromosome 17. It is concluded that the specific chromosome based procedure used here to generate CMGT transfectants may provide a general means to produce large numbers of transfectants containing megabase fragments covering, in principle, all regions of a given chromosome.

Isolation of microcell hybrid clones containing retroviral vector insertions into specific human chromosomes

We sought an efficient means to introduce specific human chromosomes into stable interspecific hybrid cells for applications in gene mapping and studies of gene regulation. A defective amphotropic retrovirus was used to insert the gene conferring G418 resistance (neo), a dominant selectable marker, into the chromosomes of diploid human fibroblasts, and the marked chromosomes were transferred to mouse recipient cells by microcell fusion. We recovered five microcell hybrid clones containing one or two intact human chromosomes which were identified by karyotype and marker analysis. Integration of the neo gene into a specific human chromosome in four hybrid clones was confirmed by segregation analysis or by in situ hybridization. We recovered four different human chromosomes into which the G418 resistance gene had integrated: human chromosomes 11, 14, 20, and 21. The high efficiency of retroviral vector transformation makes it possible to insert selectable markers into any mammalian chromosomes of interest.

Efficient induction by amiprophos-methyl and flow-cytometric sorting of micronuclei in Nicotiana plumbaginifolia

Amiprophos-methyl (APM) is a potential herbicide which acts at the level of microtubules. By exposure of suspension cells of Nicotiana plumbaginifolia to this agent, a high degree of metaphase arrest was observed and single as well as groups of chromosomes were scattered throughout the cell, offering good prospects for application in cytology and chromosome isolation. After prolonged exposure to the drug, the chromosomes decondensed and micronuclei were formed. Based on their DNA content, the micronuclei were sorted by flow cytometry. Prospects for application of isolated micronuclei for partial genome transfer and gene mapping are discussed.

Isolation of microcell hybrid clones containing retroviral vector insertions into specific human chromosomes

We sought an efficient means to introduce specific human chromosomes into stable interspecific hybrid cells for applications in gene mapping and studies of gene regulation. A defective amphotropic retrovirus was used to insert the gene conferring G418 resistance (neo), a dominant selectable marker, into the chromosomes of diploid human fibroblasts, and the marked chromosomes were transferred to mouse recipient cells by microcell fusion. We recovered five microcell hybrid clones containing one or two intact human chromosomes which were identified by karyotype and marker analysis. Integration of the neo gene into a specific human chromosome in four hybrid clones was confirmed by segregation analysis or by in situ hybridization. We recovered four different human chromosomes into which the G418 resistance gene had integrated: human chromosomes 11, 14, 20, and 21. The high efficiency of retroviral vector transformation makes it possible to insert selectable markers into any mammalian chromosomes of interest.