The human HPRT gene on a yeast artificial chromosome is functional when transferred to mouse cells by cell fusion

Introduction of YACs containing a putative mammalian replication origin into mammalian cells can generate structures that replicate autonomously

Yeast artificial chromosomes (YACs) containing or lacking a biochemically defined DNA replication origin were transferred from yeast to mammalian cells in order to determine whether origin-dependent autonomous replication would occur. A specialized YAC vector was designed to enable selection for YACs in mammalian cells and for monitoring YAC abundance in individual mammalian cells. All of eight clones made with linear and circularized YACs lacking the origin and seven of nine clones made with linear and circularized YACs containing the origin region contained single copies of the transfected YAC, along with various amounts of yeast DNA, integrated into single but different chromosomal sites. By contrast, two transformants derived from circularized YACs containing the putative replication origin showed very heterogeneous YAC copy number and numerous integration sites when analyzed after many generations of in vitro propagation. Analysis of both clones at an early time after fusion revealed variously sized extrachromosomal YAC/yeast structures reminiscent of the extrachromosomal elements found in some cells harboring amplified genes. The data are consistent with the interpretation that YACs containing a biochemically defined origin of replication can initially replicate autonomously, followed by integration into multiple chromosomal locations, as has been reported to occur in many examples of gene amplification in mammalian cells.

Incorporation of copy-number control elements into yeast artificial chromosomes by targeted homologous recombination

We have developed a pair of vectors for exchanging yeast artificial chromosome (YAC) arms by targeted homologous recombination. These conversion vectors allow the introduction of copy-number control elements into YACs constructed with pYAC4 or related vectors. YACs modified in this way provide an enriched source of DNA for genetic or biochemical studies. A LYS2 gene on the conversion vector provides a genetic selection for the modified YACs after transformation with appropriately prepared vector. A background of Lys+ clones that do not contain modified YACs is also present. However, clones with converted YACs can be distinguished from this background by counter-screening for loss of the original p YAC4 TRP1 arm (Trp- phenotype). The elimination of yeast replication origins (ARS elements) from the conversion vectors increased the frequency of Lys+ Trp- clones, but resulted in weaker amplification. Several YACs have been converted with these vectors, and the fate of the transformed DNA and of the resident YAC DNA has been systematically investigated.

Large deletions are tolerated at the hprt locus of in vivo derived human T-lymphocytes

A cloning assay was used to recover hprt- T-lymphocytes from adult human males. Analysis of crude cellular extracts by polymerase chain reactions (PCRs) demonstrated that 7% (16/218) of the hprt mutations were due to total deletion of the hprt gene. 14 of the 16 mutants were examined by PCR for the presence of flanking DNA to determine the extent of the deletions. The deletion mutation in 13 mutants was at least 350 kb with 5 of these deletions being at least 700 kb. The largest deletions were greater than 15 times the size of the hprt gene. Therefore, large deletions are tolerated at the hprt locus of human T-lymphocytes.

A random STS strategy for construction of YAC contigs spanning defined chromosomal regions

Sequence tagged sites (STSs) that were generated via Alu-element-mediated polymerase chain reaction (Alu-PCR) and mapped to human Xq26 were used to isolate and overlap yeast artificial chromosomes (YACs). By collating the results of primary pool screening, the order of STSs and YACs was postulated directly. Subsequent isolation of 11 key YACs from 75 positive pools confirmed the proposed contig. Although only a small subset of the available Alu-PCR fragments was used, the STSs were generated at sufficient density to isolate all the YACs required and to identify all except one overlap directly. The results confirmed physical linkage of HPRT to DXS86 and DXS144E. Long-range continuity was determined purely by analysis of the 11 YAC colonies and required no end-rescue. This strategy is therefore an effective approach for the construction of YAC contigs spanning discrete chromosomal regions contained within somatic cell hybrids, with minimal prior knowledge of the region.

Targeted integration of neomycin into yeast artificial chromosomes (YACs) for transfection into mammalian cells

Vectors have been constructed for the introduction of the neomycin resistance gene (neo) into the left arm, right arm or human insert DNA of yeast artificial chromosomes (YACs) by homologous recombination. These vectors contain a yeast selectable marker Lys-2, i.e. the alpha-aminoadipidate reductase gene, and a mammalian selection marker, neo, which confers G418 resistance. The vectors can be used to modify YACs in the most commonly used yeast strain for YAC library construction, AB1380. Specific targeting can be carried out by transfection of restriction endonuclease treated linear plasmids, with highly specific recombinogenic ends, into the YAC containing yeast cells. Analysis of targeted YACs confirmed that all three vectors can target correctly in yeast. Introduction of one of the targeted YACs into V79 (Chinese hamster fibroblast) cells showed complete and intact transfer of the YAC.

Transgenic mice generated by pronuclear injection of a yeast artificial chromosome

Transgenic mice have become invaluable for analysing gene function and regulation in vivo. However, the size of constructs injected has been limited by the cloning capacity of conventional vectors, a constraint that could be overcome with yeast artificial chromosomes (YACs). We investigated the feasibility of making transgenic mice with YACs by pronuclear injection of a small YAC carrying a gene encoding tyrosinase. Use of a vector with a conditional centromere allowed fifteenfold amplification of the YAC in yeast and its recovery in high yield. The albino phenotype of the recipient mice was rescued demonstrating the correct expression of the tyrosine gene from the construct. Furthermore, the telomeric sequences added by the yeast integrated into the mouse genome and did not reduce efficiency of integration. Using this technique future experiments with longer YACs will allow the expression of gene complexes such as Hox and the globin gene clusters to be analysed in transgenic animals.

Yeast artificial chromosomes (YACs) and the analysis of complex genomes

The development of yeast artificial chromosome (YAC) cloning vectors capable of carrying several hundred kilobase-pairs of DNA insert has greatly facilitated the study of complex genomes, and the cloning of large genes as single fragments. In addition, the ability to manipulate YAC sequences by homologous recombination makes this system extremely useful for the generation of disease models.

Rapid generation of chromosome-specific alphoid DNA probes using the polymerase chain reaction

Non-isotopic in situ hybridization of chromosome-specific alphoid DNA probes has become a potent tool in the study of numerical aberrations of specific human chromosomes at all stages of the cell cycle. In this paper, we describe approaches for the rapid generation of such probes using the polymerase chain reaction (PCR), and demonstrate their chromosome specificity by fluorescence in situ hybridization to normal human metaphase spreads and interphase nuclei. Oligonucleotide primers for conserved regions of the alpha satellite monomer were used to generate chromosome-specific DNA probes from somatic hybrid cells containing various human chromosomes, and from DNA libraries from sorted human chromosomes. Oligonucleotide primers for chromosome-specific regions of the alpha satellite monomer were used to generate specific DNA probes for the pericentromeric heterochromatin of human chromosomes 1, 6, 7, 17 and X directly from human genomic DNA.

Isolation and characterization of a yeast artificial chromosome (YAC) contig around the human steroid sulfatase gene

The region surrounding the steroid sulfatase (STS) locus on Xp22.3 is of particular interest since it represents a deletion hot spot, shares homology with the proximal long arm of the Y chromosome (Yq11.2), and contains genes for several well-described X-linked disorders. Here we describe yeast artificial chromosomes (YACs) covering 450 kb around the STS gene. Eight YAC clones were isolated from a human YAC library. Their STS exon content was determined and the overlap of the clones characterized. Two of the YAC clones were found to contain the entire STS gene. The most proximal and the most distal ends of the YAC contig were cloned but neither of them crossed the breakpoints in any of the previously described patients with entire STS gene deletions. This is consistent with deletions larger than 500 kb in all these patients. One of the YAC clones was found to contain sequences from the STS pseudogene on Yq11.2. Two anonymous DNA sequences, GMGXY19 and GMGXY3, previously mapped in the vicinity of the STS locus, were found within the YAC contig and their assignment with respect to the STS locus was thus possible. This contig is useful for the overlap cloning of the Xp22.3 region and for reverse genetic strategies for the isolation of disease genes in the region. Furthermore, it may provide insight into the molecular mechanisms of deletion and translocation events on Xp22.3 and in the evolution of sex chromosomes.

Yeast artificial chromosomes spanning 8 megabases and 10-15 centimorgans of human cytogenetic band Xq26

A successful test is reported to generate long-range contiguous coverage of DNA from a human cytogenetic band in overlapping yeast artificial chromosomes (YACs). Seed YACs in band Xq26 were recovered from a targeted library of clones from Xq24-q28 with 14 probes, including probes for the hypoxanthine guanine phosphoribosyltransferase- and coagulation factor IX-encoding genes and nine probes used in linkage mapping. Neighboring YACs were then identified by 25 "walking" steps with end-clones, and the content of 71 probes in cognate YACs was verified by further hybridization analyses. The resultant contig extends across 8 million base pairs, including most of band Xq26, with an order of markers consistent with linkage data. YAC-based mapping, thus, permits steps toward a fully integrated physical and genetic map and is probably adequate to sustain most of the human genome project.

Yeast artificial chromosome-based genome mapping: some lessons from Xq24-q28

Yeast artificial chromosomes (YACs) have recently provided a potential route to long-range coverage of complex genomes in contiguous cloned DNA. In a pilot project for 50 Mb (1.5% of the human genome), a variety of techniques have been applied to assemble Xq24-q28 YAC contigs up to 8 Mb in length and assess their quality. The results indicate the relative strength of several approaches and support the adequacy of YAC-based methods for mapping the human genome.

Transfer of the human HPRT and GART genes from yeast to mammalian cells by microinjection of YAC DNA

DNA of two yeast artificial chromosomes (YACs) containing selectable human genes was transferred by microinjection to rodent cells in tissue culture. The human hypoxanthine phosphoribosyltransferase (HPRT) gene, spanning 45 kb, is contained on the 660-kb YAC yHPRT as described elsewhere. The human phosphoribosylglycinamide formyltransferase (GART) gene, spanning approximately 40 kb, is contained on the 590-kb YAC yGART2 as described previously. YAC DNA was isolated from pulsed-field gels and microinjected into mammalian cells in which the human HPRT and GART genes can be selected. The cell lines that were selected contain the entire human genes. Some of the cell lines contain multiple copies of the genes integrated at the same chromosomal position. The YAC yGART2 could not be purified away from natural yeast chromosomes of similar size, and the cell lines into which the human GART gene was introduced contain variable amounts of yeast DNA in addition to the human DNA.

Detection and characterization of chimeric yeast artificial-chromosome clones

Methods for the construction of yeast artificial-chromosome (YAC) clones have been designed to isolate single, large (100-1000 kb) segments of chromosomal DNA. It is apparent from early experience with this cloning system that the major artifact in YAC clones involves the formation of YACs that contain two or more unrelated pieces of DNA. Such "chimeric" YACs are not easily recognized, particularly in libraries constructed from the total DNA of an organism. In some libraries, they have been found to constitute a major fraction of the clones. Here we discuss some of our experiences with chimeric YACs, with particular emphasis on the approaches that we have employed to detect such aberrant clones. In addition, we describe the detailed characterization of one chimeric YAC isolated from a library prepared from total human DNA. The organization of this clone indicates that it formed by in vivo recombination, presumably in yeast, between two Alu sequences located on unrelated segments of human DNA.

Sequence-tagged site (STS) content mapping of human chromosomes: theoretical considerations and early experiences

The magnitude of the effort required to complete the human genome project will require constant refinements of the tools available for the large-scale study of DNA. Such improvements must include both the development of more powerful technologies and the reformulation of the theoretical strategies that account for the changing experimental capabilities. The two technological advances described here, PCR and YAC cloning, have rapidly become incorporated into the standard armamentarium of genome analysis and represent key examples of how technological developments continue to drive experimental strategies in molecular biology. Because of its high sensitivity, specificity, and potential for automation, PCR is transforming many aspects of DNA mapping. Similarly, by providing the means to isolate and study larger pieces of DNA, YAC cloning has made practical the achievement of megabase-level continuity in physical maps. Taken together, these two technologies can be envisioned as providing a powerful strategy for constructing physical maps of whole chromosomes. Undoubtedly, future technological developments will promote even more effective mapping strategies. Nonetheless, the theoretical projections and practical experience described here suggest that constructing YAC-based STS-content maps of whole human chromosomes is now possible. Random STSs can be efficiently generated and used to screen collections of YAC clones, and contiguous YAC coverage of regions exceeding 2 Mb can be readily obtained. While the predicted laboratory effort required for mapping whole human chromosomes remains daunting, it is clearly feasible.

Rapid determination of nucleotide content and its application to the study of genome structure

We have developed a sensitive, reliable and accurate procedure for estimating the base composition of small samples of DNAs. This method has been applied to the analysis of genomic DNAs from several sources including large regions of human DNA cloned as yeast artificial chromosomes. To determine whether the human genome is compartmentalized into large segments of homogeneous base composition, we examined the GC content of a 1.2 megabase contig spanning the cystic fibrosis gene.

Transfer of yeast artificial chromosomes from yeast to mammalian cells

Human DNA can be cloned as yeast artificial chromosomes (YACs), each of which contains several hundred kilobases of human DNA. This DNA can be manipulated in the yeast host using homologous recombination and yeast selectable markers. In relatively few steps it is possible to make virtually any change in the cloned human DNA from single base pair changes to deletions and insertions. In order to study the function of the cloned DNA and the effects of the changes made in the yeast, the human DNA must be transferred back into mammalian cells. Recent experiments indicate that large genes can be transferred from the yeast host to mammalian cells in tissue culture and that the genes are transferred intact and are expressed. Using the same methods it may soon be possible to transfer YAC DNA into the mouse germ line so that the expression and function of genes cloned in YACs can be studied in developing and adult mammalian animals.

Rescue of end fragments of yeast artificial chromosomes by homologous recombination in yeast

Yeast artificial chromosomes (YACs) provide a powerful tool for the isolation and mapping of large regions of mammalian chromosomes. We developed a rapid and efficient method for the isolation of DNA fragments representing the extreme ends of YAC clones by the insertion of a rescue plasmid into the YAC vector by homologous recombination. Two rescue vectors were constructed containing a yeast LYS2 selectable gene, a bacterial origin of replication, an antibiotic resistance gene, a polylinker containing multiple restriction sites, and a fragment homologous to one arm of the pYAC4 vector. The 'end-cloning' procedure involves transformation of the rescue vector into yeast cells carrying a YAC clone, followed by preparation of yeast DNA and transformation into bacterial cells. The resulting plasmids carry end-specific DNA fragments up to 20 kb in length, which are suitable for use as hybridization probes, as templates for direct DNA sequencing, and as probes for mapping by fluorescence in situ hybridization. These vectors are suitable for the rescue of end-clones from any YAC constructed using a pYAC-derived vector. We demonstrate the utility of these plasmids by rescuing YAC-end fragments from a human YAC library.

Vectors for inserting selectable markers in vector arms and human DNA inserts of yeast artificial chromosomes (YACs)

To facilitate studies of gene expression and homologous recombination, plasmids have been developed which permit the insertion of neomycin resistance-encoding gene (NmR) into either the human DNA insert or the vector arm of a yeast artificial chromosome (YAC). To integrate into the YAC arm, the plasmid pRV1 contains a LYS2 (encoding alpha-aminoadipate reductase) gene for selection in the yeast host, and a NmR gene for subsequent selection after transfection of mammalian cells. These two sequences are bracketed by fragments of the URA3 gene (encoding orotidine-5'-phosphate decarboxylase) that can disrupt the URA3 gene in the YAC arm by homologous recombination in yeast. To integrate a selectable marker into the insert, the plasmid pRV2 contains a NmR gene and an intact copy of the URA3 gene, bracketed by segments of an L1 (LINEs) repetitive element. In this case, the vector has been designed for use with YACs that have already been fitted in the vector arm with a different marker (i.e., TK) that has disrupted the URA3 gene in the vector arm. Selection is for the restoration of URA3 gene activity attendant on recombination into an L1 element in the YAC insert. Use of the vectors is illustrated with a YAC clone containing ribosomal DNA.