Functional hemizygosity for the MDH2 locus in Chinese hamster ovary cells

Newly identified CHO ERCC3/XPB mutations and phenotype characterization

Nucleotide excision repair (NER) is a complex multistage process involving many interacting gene products to repair a wide range of DNA lesions. Genetic defects in NER cause human hereditary diseases including xeroderma pigmentosum (XP), Cockayne syndrome (CS), trichothiodystrophy and a combined XP/CS overlapping symptom. One key gene product associated with all these disorders is the excision repair cross-complementing 3/xeroderma pigmentosum B (ERCC3/XPB) DNA helicase, a subunit of the transcription factor IIH complex. ERCC3 is involved in initiation of basal transcription and global genome repair as well as in transcription-coupled repair (TCR). The hamster ERCC3 gene shows high degree of homology with the human ERCC3/XPB gene. We identified new mutations in the Chinese hamster ovary cell ERCC3 gene and characterized the role of hamster ERCC3 protein in DNA repair of ultraviolet (UV)-induced and oxidative DNA damage. All but one newly described mutations are located in the protein C-terminal region around the last intron-exon boundary. Due to protein truncations or frameshifts, they lack amino acid Ser751, phosphorylation of which prevents the 5' incision of the UV-induced lesion during NER. Thus, despite the various locations of the mutations, their phenotypes are similar. All ercc3 mutants are extremely sensitive to UV-C light and lack recovery of RNA synthesis (RRS), confirming a defect in TCR of UV-induced damage. Their limited global genome NER capacity averages approximately 8%. We detected modest sensitivity of ercc3 mutants to the photosensitizer Ro19-8022, which primarily introduces 8-oxoguanine lesions into DNA. Ro19-8022-induced damage interfered with RRS, and some of the ercc3 mutants had delayed kinetics. All ercc3 mutants showed efficient base excision repair (BER). Thus, the positions of the mutations have no effect on the sensitivity to, and repair of, Ro19-8022-induced DNA damage, suggesting that the ERCC3 protein is not involved in BER.

Retroviral insertional mutagenesis identifies a small protein required for synthesis of diphthamide, the target of bacterial ADP-ribosylating toxins

Retroviral insertional mutagenesis was used to produce a mutant Chinese hamster ovary cell line that is completely resistant to several different bacterial ADP-ribosylating toxins. The gene responsible for toxin resistance, termed diphtheria toxin (DT) and Pseudomonas exotoxin A (ETA) sensitivity required gene 1 (DESR1), encodes two small protein isoforms of 82 and 57 residues. DESR1 is evolutionally conserved and ubiquitously expressed. Only the longer isoform is functional because the mutant cell line can be complemented by transfection with the long but not the short isoform. We demonstrate that DESR1 is required for the first step in the posttranslational modification of elongation factor-2 at His(715) that yields diphthamide, the target site for ADP ribosylation by DT and ETA. KTI11, the analog of DESR1 in yeast, which was originally identified as a gene regulating the sensitivity of yeast to zymocin, is also required for diphthamide biosynthesis, implicating DESR1/KTI11 in multiple biological processes.

Clostridium septicum alpha toxin uses glycosylphosphatidylinositol-anchored protein receptors

The alpha toxin produced by Clostridium septicum is a channel-forming protein that is an important contributor to the virulence of the organism. Chinese hamster ovary (CHO) cells are sensitive to low concentrations of the toxin, indicating that they contain toxin receptors. Using retroviral mutagenesis, a mutant CHO line (BAG15) was generated that is resistant to alpha toxin. FACS analysis showed that the mutant cells have lost the ability to bind the toxin, indicating that they lack an alpha toxin receptor. The mutant cells are also resistant to aerolysin, a channel-forming protein secreted by Aeromonas spp., which is structurally and functionally related to alpha toxin and which is known to bind to glycosylphosphatidylinositol (GPI)-anchored proteins, such as Thy-1. We obtained evidence that the BAG15 cells lack N-acetylglucosaminyl-phosphatidylinositol deacetylase-L, needed for the second step in GPI anchor biosynthesis. Several lymphocyte cell lines lacking GPI-anchored proteins were also shown to be less sensitive to alpha toxin. On the other hand, the sensitivity of CHO cells to alpha toxin was increased when the cells were transfected with the GPI-anchored folate receptor. We conclude that alpha toxin, like aerolysin, binds to GPI-anchored protein receptors. Evidence is also presented that the two toxins bind to different subsets of GPI-anchored proteins.

Spontaneous CHO APRT heterozygotes reflect high-frequency, allele-specific deletion of the chromosome Z4 APRT gene

In Chinese hamster ovary (CHO) cells, heterozygotes for the adenine phosphoribosyltransferase (APRT) locus arise spontaneously at high frequencies. Paradoxically, such heterozygotes yield APRT mutants only at much lower spontaneous rates, suggesting that the high-frequency event may occur at only one of the two APRT genes. In an attempt to understand the genetic basis for the apparent refractivity of one of the APRT alleles to the high-frequency genetic event and to determine whether differences in the genomic environments of the two CHO APRT alleles specifically render one gene more susceptible to high-frequency spontaneous deletion or inactivation, we have mapped the wild-type APRT allele in 16 independently derived spontaneous APRT heterozygotes. In 15 of these 16 heterozygotes, the functional, wild-type APRT gene was found to reside on the Z7 chromosome, indicating that the high-frequency event is indeed highly specific for the Z4 APRT allele. All but one of these heterozygotes were hemizygous for the APRT locus, suggesting that the high-frequency event generally involves deletion rather than spontaneous inactivation or mutation of the Z4 APRT allele.

Preferential loss or inactivation of chromosome Z4 APRT allele in CHO cells

In CHO cells, heterozygotes for the adenine phosphoribosyltransferase (APRT) locus arise spontaneously at high frequencies. However, such heterozygotes always yield APRT- mutants at low spontaneous rates. In an attempt to determine whether differences in the genomic environments of the two CHO APRT alleles might render one gene more susceptible to high-frequency spontaneous inactivation or deletion, we have mapped the functional APRT allele in four different spontaneous APRT heterozygotes. In each case, the functional APRT gene was found to reside on the Z7 chromosome; it was always the Z4 APRT allele that had been lost or inactivated. Two of these heterozygotes were shown to be physically hemizygous while the other two retained two copies of the APRT gene, indicating that the high-frequency event can involve either spontaneous deletion or inactivation.