Isolation and characterization of interspecific heat-resistant hybrids between a temperature-sensitive chinese hamster cell asparaginyl-tRNA synthetase mutant and normal human leukocytes: assignment of human asnS gene to chromosome 18

Overview of somatic cell hybrid mapping

This unit provides an introduction to the various types of hybrid panels in use and reviews the status of commercially available panels.

Human chromosomal localization of genes encoding the gamma 1 and gamma 2 subunits of the gamma-aminobutyric acid receptor indicates that members of this gene family are often clustered in the genome

The gamma-aminobutyric acid (GABA) receptors are the major inhibitory neurotransmitter receptors in the brain and the site of action of a number of important pharmacological agents including barbiturates, benzodiazepines, and ethanol. The gamma 1 and gamma 2 subunits have been shown to be important in mediating responses to benzodiazepines, and a splicing variant of the gamma 2 subunit, gamma 2L, has been shown to be necessary for ethanol actions on the receptor, raising the possibility that the gamma 2 gene may be involved in human genetic predisposition to the development of alcoholism. We have assigned the human genes encoding the gamma 1 and gamma 2 subunits of the GABAA receptor to chromosomes 4 and 5, respectively, by PCR amplification of human-specific products from human-hamster somatic cell hybrid DNAs. Using panels of chromosome-specific natural deletion hybrids, we have further localized the gamma 1 gene (GABRG1) to 4p14-q21.1 and the gamma 2 gene (GABRG2) to 5q31.1-q33.2. These data indicate that the gamma 1 gene may be clustered together with the previously mapped alpha 2 and beta 1 genes on chromosome 4 and that the gamma 2 gene may be close to the previously localized alpha 1 gene on chromosome 5. To further examine the latter possibility the alpha 1 gene was mapped using the chromosome 5 deletion hybrids and shown to be within the same region as the gamma 2 gene, 5q31.1-q33.2. A PCR-based screening strategy was used to isolate a 450-kilobase human genomic yeast artificial chromosome clone containing both the alpha 1 and gamma 2 genes. Pulsed-field gel restriction mapping of the yeast artificial chromosome indicates that the two genes are within 200 kilobases of each other. The data presented here provide further evidence for the nonrandom organization of the human genome by demonstrating that members of the GABAA receptor gene family often occur in small gene clusters widely distributed in the genome.

Somatic cell hybrid deletion map of human chromosome 18

The creation of a physical map of chromosome 18 will be useful for the eventual identification of specific chromosomal regions that are critical in the occurrence of Edwards syndrome, the 18q- syndrome, and the 18p- syndrome. To begin the investigation of these syndromes, a physical map has been constructed to order random DNA fragments to specific portions of chromosome 18. A set of somatic cell hybrids that retain deletions or translocations involving chromosome 18 has been isolated and characterized. Over 200 lambda phage from a chromosome 18-specific library have been localized to 11 distinct regions of chromosome 18 using the chromosomal breakpoints present in the somatic cell hybrids.

Genomic organization, nucleotide sequence, and cellular distribution of a Shaw-related potassium channel gene, Kv3.3, and mapping of Kv3.3 and Kv3.4 to human chromosomes 19 and 1

Genomic and cDNA clones encoding a novel Shaw-related potassium channel gene have been isolated from mice and humans. The mouse-Kv3.3 gene encodes a protein of 679 amino acids. Unlike the vertebrate Shaker-related genes that have intronless coding regions, mouse Kv3.3 is encoded by at least two exons separated by 3 kb of intervening sequence. The amino-terminal 212 amino acids are encoded by a single exon, and the hydrophobic core of the protein beginning at the S1 transmembrane segment is contained in a separate exon. Multiple Kv3.3-hybridizing transcripts are visible in the mouse brain, liver, thymus, and heart. Using probes derived from a human genomic clone containing the 3' exon of human Kv3.3 (KCNC3), we have localized the gene to human chromosome 19. The related gene, human Kv3.4 (KCNC4), was localized to human chromosome 1.

Bone morphogenetic protein: chromosomal localization of human genes for BMP1, BMP2A, and BMP3

Bone morphogenetic protein (BMP) induces endochondral bone formation in vivo. The human genes have been cloned for a group of proteins containing BMP activity (BMP1, BMP2A, and BMP3). Two of the proteins are members of the transforming growth factor-beta supergene family (BMP2A and BMP3), while BMP1 is a novel regulatory protein. Using somatic cell hybrid lines, cDNA probes were used to map BMP1 to chromosome 8, BMP2A to chromosome 20, and BMP3 to the p14-q21 region of chromosome 4. This analysis reveals that the BMP2A and BMP3 genes map to conserved regions between mouse and human, while the BMP1 gene does not. The locations of the BMP genes were found to overlap with the loci for several disorders of cartilage and bone formation.

Expression and chromosomal localization of a lymphocyte K+ channel gene

We recently isolated a family of three closely related mouse K+ channel genes (MK1, MK2, and MK3) with coding regions contained in single uninterrupted exons. Here we have used patch-clamp recordings from Xenopus oocytes injected with mRNA to show that MK3 encodes a channel with biophysical and pharmacological properties indistinguishable from those of voltage-gated type n K+ channels in T cells. In addition, we used the polymerase chain reaction to demonstrate the presence of MK3 mRNA in T cells. These data suggest that MK3 may encode the T-cell voltage-gated type n K+ channel. We also show that MK3 and MK2 are localized on human chromosomes 13 and 12, respectively.

Threonyl-tRNA synthetase gene maps close to leucyl-tRNA synthetase gene on human chromosome 5

The structural gene for threonyl-tRNA synthetase was mapped to human chromosome 5 by an analysis of the isoelectric focusing patterns of this enzyme from human X Chinese hamster interspecific somatic cell hybrids. The threonyl-tRNA synthetase gene is the fourth of seven aminoacyl-tRNA synthetase genes mapped in humans to be assigned to this chromosome. Regional mapping studies showed that the threonyl-tRNA synthetase gene is on the short arm of chromosome 5, p13-cen, and is close to, but separable from, the gene for leucyl-tRNA synthetase which maps to 5cen-5q11.

Chromosomal localization of human gene for histidyl-tRNA synthetase: clustering of genes encoding aminoacyl-tRNA synthetases on human chromosome 5

The gene encoding histidyl-tRNA synthetase was localized to human chromosome 5 using human-Chinese hamster ovary cell hybrids. Seven of the functionally related genes encoding the 20 different aminoacyl-tRNA synthetases have not been mapped in humans and four are located on a single chromosome, number 5, which represents less than 7% of the total genome. Thus, there appears to be significant clustering of this group of genes which may reflect their evolutionary relatedness or common factors involved in regulating their expression.

Assignment of human gene encoding thymidylate synthase to chromosome 18 using interspecific cell hybrids between thymidylate synthase-negative mouse mutant cells and human diploid fibroblasts

We have constructed interspecific somatic cell hybrids between a thymidine-auxotrophic mutant cell line of mouse FM3A cells that lacks thymidylate synthase and human diploid fibroblasts derived from a male patient with fragile X-linked mental retardation. Twenty primary hybrid clones were isolated independently, all of which exhibited the thymidine-prototrophic phenotype. Segregation of the hybrid cells in nonselective culture conditions gave rise to thymidine-auxotrophic hybrid clones. Both electrophoretic assay of thymidylate synthase activity and karyotype analysis of the segregants revealed a strong correlation between the expression of the human form of the enzyme and the presence of human chromosome 18. Thus, it is concluded that the functional gene for human thymidylate synthase, designated TS, is located on this chromosome.

Isolation of Chinese hamster ovary cells that overproduce asparaginyl-tRNA synthetase

Temperature-resistant revertants, derived from the temperature-sensitive CHO asparaginyl-tRNA synthetase mutant, Asn-5, were isolated and characterized. Several lines of evidence indicate that the temperature-resistant phenotype of the revertants is due to their overproducing the same altered enzyme present in the Asn-5 parent.

Assignment of the human MARS gene, encoding methioninyl-tRNA synthetase, to chromosome 12 using human X Chinese hamster cell hybrids

We have isolated interspecific somatic cell hybrids between a temperature-sensitive Chinese hamster ovary (CHO) cell methioninyl -tRNA synthetase mutant and human peripheral leukocytes. The hybrids were selected at 39 degrees C which requires the retention and expression of the human gene, MARS , which complements the defective CHO gene. In vitro heat-inactivation experiments on the methioninyl -tRNA synthetase activity in cell-free extracts from heat-resistant hybrids indicate that the human form of this enzyme and, therefore, the human MARS gene is present in hybrid cells. Cytogenetic analysis of three independent temperature-resistant hybrids revealed the presence of a single human chromosome, number 12. Two other independent hybrids examined contained human chromosome 12 as well as a second human chromosome. Electrophoretic analysis of extracts from hybrid cell lines for a human chromosome 12 marker isozyme, LDH-B, showed a pattern of heterotetrameric bands consistent with the presence of the human form of this enzyme in these cells. The correlation between the presence of the human form of methioninyl -tRNA synthetase and human chromosome 12 in temperature-resistant hybrids indicates that the human MARS locus is located on this chromosome.

Isolation and characterization of a Chinese hamster lung cell tryptophanyl-tRNA synthetase mutant

A novel type of temperature-sensitive protein synthetic mutant was isolated from V-79 Chinese hamster lung cells using an amino acid analog suicide selection. The expression of the temperature sensitive phenotype of the mutant is greatly affected by the concentration of tryptophan in the culture medium. In addition, the activity of tryptophanyl-tRNA synthetase is undetectable in cell-free extracts prepared from the mutant cells. The results suggest that the mutant has an alteration in the structural gene encoding tryptophanyl-tRNA synthetase.

Assignment of structural gene for asparagine synthetase to human chromosome 7

Somatic cell hybrids obtained from the fusion of human B lymphocytes and an asparagine synthetase-deficient Chinese hamster ovary cell line were isolated after growth in asparagine-free medium. The human and hamster forms of asparagine synthetase differ significantly in their rate of inactivation at 47.5 degrees C. The asparagine synthetase activity expressed in the hybrids was inactivated at 47.5 degrees C at the same rate as the human form of the enzyme. Karyotypic analysis and analysis for chromosome-specific enzyme markers showed that the structural gene for asparagine synthetase is located on chromosome 7 in humans. The heat-inactivation profile for asparagine synthetase in extracts of hybrids formed between human peripheral leukocytes and a hamster cell line expressing asparagine synthetase activity was intermediate between the two parental types when human chromosome 7 was present, but was identical to the hamster parent when chromosome 7 was absent.

Efficient procedure for transferring specific human genes into Chinese hamster cell mutants: interspecific transfer of the human genes encoding leucyl- and asparaginyl-tRNA synthetases

We have developed a simple and efficient procedure for transferring specific human genes into mutant Chinese hamster ovary cell recipients that does not rely on using calcium phosphate-precipitated high-molecular-weight DNA. Interspecific cell hybrids between human leukocytes and temperature-sensitive Chinese hamster cell mutants with either a thermolabile leucyl-tRNA synthetase or a thermolabile asparaginyl-tRNA synthetase were used as the starting material in these experiments. These hybrids contain only one or a few human chromosomes and require expression of the appropriate human aminoacyl-tRNA synthetase gene to grow at 39 degrees C. Hybrids were exposed to very high doses of gamma-irradiation to extensively fragment the chromosomes and re-fused immediately to the original temperature-sensitive Chinese hamster mutant, and secondary hybrids were isolated at 39 degrees C. Secondary hybrids, which had retained small fragments of the human genome containing the selected gene, were subjected to another round of irradiation, refusion, and selection at 39 degrees C to reduce the amount of human DNA even further. Using this procedure, we have constructed Chinese hamster cell lines that express the human genes encoding either asparaginyl- or leucyl-tRNA synthetase, yet less than 0.1% of their DNA is derived from the human genome, as quantitated by a sensitive dot-blot nucleic acid hybridization procedure. Analysis of these cell lines with Southern blots confirmed the presence of a small number of restriction endonuclease fragments containing human DNA specifically. These cell lines represent a convenient and simple means to clone the human genomic sequences of interest.

Efficient procedure for transferring specific human genes into Chinese hamster cell mutants: interspecific transfer of the human genes encoding leucyl- and asparaginyl-tRNA synthetases

We have developed a simple and efficient procedure for transferring specific human genes into mutant Chinese hamster ovary cell recipients that does not rely on using calcium phosphate-precipitated high-molecular-weight DNA. Interspecific cell hybrids between human leukocytes and temperature-sensitive Chinese hamster cell mutants with either a thermolabile leucyl-tRNA synthetase or a thermolabile asparaginyl-tRNA synthetase were used as the starting material in these experiments. These hybrids contain only one or a few human chromosomes and require expression of the appropriate human aminoacyl-tRNA synthetase gene to grow at 39 degrees C. Hybrids were exposed to very high doses of gamma-irradiation to extensively fragment the chromosomes and re-fused immediately to the original temperature-sensitive Chinese hamster mutant, and secondary hybrids were isolated at 39 degrees C. Secondary hybrids, which had retained small fragments of the human genome containing the selected gene, were subjected to another round of irradiation, refusion, and selection at 39 degrees C to reduce the amount of human DNA even further. Using this procedure, we have constructed Chinese hamster cell lines that express the human genes encoding either asparaginyl- or leucyl-tRNA synthetase, yet less than 0.1% of their DNA is derived from the human genome, as quantitated by a sensitive dot-blot nucleic acid hybridization procedure. Analysis of these cell lines with Southern blots confirmed the presence of a small number of restriction endonuclease fragments containing human DNA specifically. These cell lines represent a convenient and simple means to clone the human genomic sequences of interest.