In vitro translation of hypoxanthine/guanine phosphoribosyltransferase mRNA: characterization of a mouse neuroblastoma cell line that has elevated levels of hypoxanthine/guanine phosphoribosyltransferase protein

Prognostic significance and immunological role of HPRT1 in human cancers

Hypoxanthine phosphoribosyl transferase 1 (HPRT1), once considered a housekeeping gene, has been identified as playing an important role in several tumors. Its role in pan-cancer, however, has not been systematically studied. This study evaluates the relationship between HPRT1 and clinical parameters, survival prognosis, and tumor immunity based on multi omics data from the Cancer Genome Atlas (TCGA) and the Gene Expression Omnibus (GEO) databases. Drug sensitivity analysis screened 16 effective drugs against HPRT1, exploring the interactions with chemicals and genes. The significance of HPRT1 in tumor immunotherapy has also been investigated. Immunohistochemistry confirmed significant differences in the expression of HPRT1 between five tumor types (colon adenocarcinoma [COAD], head-neck squamous cell carcinoma [HNSC], lung adenocarcinoma [LUAD], thyroid carcinoma [THCA], and uterine corpus endometrial carcinoma [UCEC]) and adjacent normal tissues (P < 0.05). HPRT1 competitive endogenous RNA network was constructed in HNSC. Through cytological experiments, it was verified that HPRT1 plays a carcinogenic role in HNSC and is associated with tumor cell proliferation, migration, invasion, and apoptosis. In addition, there was a significant positive correlation between HPRT1 and programmed cell death-1 (PD-1) expression in HNSC (P < 0.05). These findings suggest that HPRT1 may be a potential biomarker for predicting and treating cancer.

Falling from grace: HPRT is not suitable as an endogenous control for cancer-related studies

HPRT is a housekeeping enzyme involved in recycling guanine and inosine in the purine salvage pathway. As a housekeeping gene, HPRT has been widely used as an endogenous control for molecular studies evaluating changes in gene expression. Yet, recent evidence has shown that HPRT exhibits high variability within malignant samples. We designed this study to determine whether this observed upregulation is consistently found, therefore rendering hprt an unsuitable normalization control in cancer. Utilizing protein and RNA-seq expression, we found that malignant and normal patient samples vary significantly both within the same tissue type and across organ sites. Upon staining for HPRT via immunohistochemistry, we found that expression is highly variable in malignant samples (Lung; 89.2-111.8, Breast; 66.7-98.3, Colon; 85.3-129.7, Prostate; 90.8-155.4, Pancreas; 74.1-132.1). Similarly, we observed high variability across cell lines via western blotting (p < 0.0001) which was further confirmed using RNA sequencing. Comparing normal and malignant patient samples, we observed consistent upregulation of HPRT expression within malignant samples relative to normal samples (p = 0.0001). These data indicate that HPRT is unsuitable as an endogenous control for cancer-related studies because its expression is highly variable and exceeds that of an appropriate control; therefore, we recommend its discontinued use as a normalization gene.

A review of HPRT and its emerging role in cancer

Hypoxanthine guanine phosphoribosyltransferase (HPRT) is a common salvage housekeeping gene with a historically important role in cancer as a mutational biomarker. As an established and well-known human reporter gene for the evaluation of mutational frequency corresponding to cancer development, HPRT is most commonly used to evaluate cancer risk within individuals and determine potential carcinogens. In addition to its use as a reporter gene, HPRT also has important functionality in the body in relation to purine regulation as demonstrated by Lesch-Nyhan patients whose lack of functional HPRT leads to significant purine overproduction and further neural complications. This regulatory role, in addition to an established connection between other salvage enzymes and cancer development, points to HPRT as an emerging influence in cancer. Recent work has shown that not only is the enzyme upregulated within malignant tumors, it also has significant surface localization within some cancer cells. With this is mind, HPRT has the potential to become a significant biomarker not only for the characterization of cancer, but also for its potential treatment.

Ubiquitous and neuronal DNA-binding proteins interact with a negative regulatory element of the human hypoxanthine phosphoribosyltransferase gene

The hypoxanthine phosphoribosyltransferase (HPRT) gene is constitutively expressed at low levels in all tissues but at higher levels in the brain; the significance and mechanism of this differential expression are unknown. We previously identified a 182-bp element (hHPRT-NE) within the 5'-flanking region of the human HPRT (hHPRT) gene, which is involved not only in conferring neuronal specificity but also in repressing gene expression in nonneuronal tissues. Here we report that this element interacts with different nuclear proteins, some of which are present specifically in neuronal cells (complex I) and others of which are present in cells showing constitutive expression of the gene (complex II). In addition, we found that complex I factors are expressed in human NT2/D1 cells following induction of neuronal differentiation by retinoic acid. This finding correlates with an increase of HPRT gene transcription following neuronal differentiation. We also mapped the binding sites for both complexes to a 60-bp region (Ff; positions -510 to -451) which, when analyzed in transfection assays, functioned as a repressor element analogous to the full-length hHPRT-NE sequence. Methylation interference footprintings revealed a minimal unique DNA motif, 5'-GGAAGCC-3', as the binding site for nuclear proteins from both neuronal and nonneuronal sources. However, site-directed mutagenesis of the footprinted region indicated that different nucleotides are essential for the associations of these two complexes. Moreover, UV cross-linking experiments showed that both complexes are formed by the association of several different proteins. Taken together, these data suggest that differential interaction of DNA-binding factors with this regulatory element plays a crucial role in the brain-preferential expression of the gene, and they should lead to the isolation of transcriptional regulators important in neuronal expression of the HPRT gene.

Double replacement gene targeting for the production of a series of mouse strains with different prion protein gene alterations

We have developed a double replacement gene targeting strategy which enables the production of a series of mouse strains bearing different subtle alterations to endogenous genes. This is a two-step process in which a region of the gene of interest is first replaced with a selectable marker to produce an inactivated allele, which is then re-targeted with a second vector to reconstruct the inactivated allele, concomitantly introducing an engineered mutation. Five independent embryonic stem cell lines have been produced bearing different targeted alterations to the prion protein gene, including one which raises the level of expression. We have constructed mice bearing the codon 101 proline to leucine substitution linked to the human familial prion disease, Gerstmann-Straussler-Scheinker syndrome. We anticipate that this procedure will have applications to the study of human inherited diseases and the development of therapies.

5'-flanking sequences of the human HPRT gene direct neuronal expression in the brain of transgenic mice

Total deficiency of hypoxanthine phosphoribosyltransferase (HPRT) in humans causes the neurological disorder Lesch-Nyhan syndrome. The HPRT gene is expressed at basal levels in all tissues but at higher levels in the brain, the relevance and mechanism of which is unknown. To determine if cis-acting DNA elements play a role in the tissue-differential pattern of expression, we generated transgenic mice carrying different sequences of the human HPRT (hHPRT) promoter fused to the bacterial lacZ gene. We show that a 1.6 kb fragment of the hHPRT promoter contains essential information to direct beta-galactosidase expression preferentially to the basal ganglia, cerebral cortex, hippocampus, and several other areas of the forebrain. At least two elements within the 1.6 kb fragment appear to be required for neuronal expression. A 182 bp element (hHPRT-NE) represents one of these sequences and is involved not only in conferring neuronal specificity but also in repressing transgene expression in non-neuronal tissues. These studies provide molecular insight into the mechanism of increased HPRT expression in the brain.

Functional characterization of the human hypoxanthine phosphoribosyltransferase gene promoter: evidence for a negative regulatory element

The enzyme hypoxanthine phosphoribosyltransferase (HPRT) catalyzes the metabolic salvage of the purine bases hypoxanthine and guanine. We previously characterized the genomic structure of the human HPRT gene and described its promoter sequence. In this report, we identify cis-acting transcriptional control regions of the human HPRT gene by linking various 5'-flanking sequences to the bacterial chloramphenicol acetyltransferase gene. The sequence from positions -219 to -122 relative to the translation initiation site is required for maximal expression of this gene, and it functions equally in both normal and reverse orientations. In addition, a cis-acting negative element is present in the region spanning from positions -570 to -388. This negative element can also repress promoters of heterologous genes, such as those of adenosine deaminase and dihydrofolate reductase, which are structurally and functionally similar to the human HPRT promoter. Furthermore, this repressor element functions independently of its orientation but appears to be distance dependent. In vivo competition assays demonstrated that the trans-acting factor(s) that binds to this negative element specifically inhibits human HPRT promoter activity. Taken together, these data localize cis-acting sequences important in the regulation of human HPRT gene expression and should allow the study of protein-DNA interactions which modulate the transcription of this gene.

Functional characterization of the human hypoxanthine phosphoribosyltransferase gene promoter: evidence for a negative regulatory element

The enzyme hypoxanthine phosphoribosyltransferase (HPRT) catalyzes the metabolic salvage of the purine bases hypoxanthine and guanine. We previously characterized the genomic structure of the human HPRT gene and described its promoter sequence. In this report, we identify cis-acting transcriptional control regions of the human HPRT gene by linking various 5'-flanking sequences to the bacterial chloramphenicol acetyltransferase gene. The sequence from positions -219 to -122 relative to the translation initiation site is required for maximal expression of this gene, and it functions equally in both normal and reverse orientations. In addition, a cis-acting negative element is present in the region spanning from positions -570 to -388. This negative element can also repress promoters of heterologous genes, such as those of adenosine deaminase and dihydrofolate reductase, which are structurally and functionally similar to the human HPRT promoter. Furthermore, this repressor element functions independently of its orientation but appears to be distance dependent. In vivo competition assays demonstrated that the trans-acting factor(s) that binds to this negative element specifically inhibits human HPRT promoter activity. Taken together, these data localize cis-acting sequences important in the regulation of human HPRT gene expression and should allow the study of protein-DNA interactions which modulate the transcription of this gene.

A regulatory element is characterized by purine-mediated and cell-type-specific gene transcription

Purines and purine nucleotides were found to affect transcription of the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene in whole nuclei isolated from intestinal mucosa of adult rats fed a purine- and purine nucleotide-free diet. Nuclear run-on transcription assays, performed on whole nuclei from different tissues and cell types, identified an intestine-specific decrease in the overall incorporation of [alpha-32P]UTP in HPRT transcripts from intestinal epithelial cell nuclei when exogenous purines or purine nucleotides were omitted from either the diet or culture medium. Using a 990-base-pair genomic fragment that contains the 5'-flanking region from the HPRT gene, we generated plasmid constructs with deletions, transfected the DNA into various cell types, and assayed for chloramphenicol acetyltransferase (CAT) reporter activity in vitro. We determined that an element upstream from the putative transcriptional start site is necessary to maintain the regulatory response to purine and nucleotide levels in cultured intestinal epithelial cells. These results were tissue and cell type specific and suggest that in the absence of exogenous purines, the presence of specific factors influences transcriptional initiation of HPRT. This information provides evidence for a mechanism by which the intestinal epithelium, which has been reported to lack constitutive levels of de novo purine nucleotide biosynthetic activity, could maintain and regulate the salvage of purines and nucleotides necessary for its high rate of cell and protein turnover during fluctuating nutritional and physiological conditions. Furthermore, this information may provide more insight into regulation of the broad class of genes recognized by their lack of TATA and CCAAT box consensus sequences within the region proximal to the promoter.

A regulatory element is characterized by purine-mediated and cell-type-specific gene transcription

Purines and purine nucleotides were found to affect transcription of the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene in whole nuclei isolated from intestinal mucosa of adult rats fed a purine- and purine nucleotide-free diet. Nuclear run-on transcription assays, performed on whole nuclei from different tissues and cell types, identified an intestine-specific decrease in the overall incorporation of [alpha-32P]UTP in HPRT transcripts from intestinal epithelial cell nuclei when exogenous purines or purine nucleotides were omitted from either the diet or culture medium. Using a 990-base-pair genomic fragment that contains the 5'-flanking region from the HPRT gene, we generated plasmid constructs with deletions, transfected the DNA into various cell types, and assayed for chloramphenicol acetyltransferase (CAT) reporter activity in vitro. We determined that an element upstream from the putative transcriptional start site is necessary to maintain the regulatory response to purine and nucleotide levels in cultured intestinal epithelial cells. These results were tissue and cell type specific and suggest that in the absence of exogenous purines, the presence of specific factors influences transcriptional initiation of HPRT. This information provides evidence for a mechanism by which the intestinal epithelium, which has been reported to lack constitutive levels of de novo purine nucleotide biosynthetic activity, could maintain and regulate the salvage of purines and nucleotides necessary for its high rate of cell and protein turnover during fluctuating nutritional and physiological conditions. Furthermore, this information may provide more insight into regulation of the broad class of genes recognized by their lack of TATA and CCAAT box consensus sequences within the region proximal to the promoter.

Antisense RNA inhibition of HPRT synthesis

The abundant production of antisense hypoxanthine phosphoribosyltransferase (HPRT) RNA in NIH-3T3, COS, or HeLa cells leads to an inhibition of HPRT synthesis. HPRT enzyme levels in cells transfected with mouse HPRT antisense RNA expression vectors are reduced to less than 1% of parental cell activity, resulting in resistance to 6-thioguanine (6TG). The expression of antisense HPRT RNA leads to a marked reduction in the steady-state levels of endogenous HPRT mRNA. Furthermore, we demonstrate that intron-specific antisense RNA, complementary to sequences adjacent to splice donor or acceptor sites of the first intron of the mouse HPRT gene, are effective in depressing endogenous HPRT levels. These studies suggest that antisense RNA can inhibit gene expression in the nucleus, possibly by perturbing nuclear RNA processing.

Chromosomal destabilization during gene amplification

Acentric extrachromosomal elements, such as submicroscopic autonomously replicating circular molecules (episomes) and double minute chromosomes, are common early, and in some cases initial, intermediates of gene amplification in many drug-resistant and tumor cell lines. In order to gain a more complete understanding of the amplification process, we investigated the molecular mechanisms by which such extrachromosomal elements are generated and we traced the fate of these amplification intermediates over time. The model system consists of a Chinese hamster cell line (L46) created by gene transfer in which the initial amplification product was shown previously to be an unstable extrachromosomal element containing an inverted duplication spanning more than 160 kilobases (J. C. Ruiz and G. M. Wahl, Mol. Cell. Biol. 8:4302-4313, 1988). In this study, we show that these molecules were formed by a process involving chromosomal deletion. Fluorescence in situ hybridization was performed at multiple time points on cells with amplified sequences. These studies reveal that the extrachromosomal molecules rapidly integrate into chromosomes, often near or at telomeres, and once integrated, the amplified sequences are themselves unstable. These data provide a molecular and cytogenetic chronology for gene amplification in this model system; an early event involves deletion to generate extrachromosomal elements, and subsequent integration of these elements precipitates a cascade of chromosome instability.

Chromosomal destabilization during gene amplification

Acentric extrachromosomal elements, such as submicroscopic autonomously replicating circular molecules (episomes) and double minute chromosomes, are common early, and in some cases initial, intermediates of gene amplification in many drug-resistant and tumor cell lines. In order to gain a more complete understanding of the amplification process, we investigated the molecular mechanisms by which such extrachromosomal elements are generated and we traced the fate of these amplification intermediates over time. The model system consists of a Chinese hamster cell line (L46) created by gene transfer in which the initial amplification product was shown previously to be an unstable extrachromosomal element containing an inverted duplication spanning more than 160 kilobases (J. C. Ruiz and G. M. Wahl, Mol. Cell. Biol. 8:4302-4313, 1988). In this study, we show that these molecules were formed by a process involving chromosomal deletion. Fluorescence in situ hybridization was performed at multiple time points on cells with amplified sequences. These studies reveal that the extrachromosomal molecules rapidly integrate into chromosomes, often near or at telomeres, and once integrated, the amplified sequences are themselves unstable. These data provide a molecular and cytogenetic chronology for gene amplification in this model system; an early event involves deletion to generate extrachromosomal elements, and subsequent integration of these elements precipitates a cascade of chromosome instability.

Cytogenetic manifestations associated with the reversion, by gene amplification, at the HGPRT locus in V79 Chinese hamster cells

Some HGPRT spontaneous revertants were isolated from a mutant line (E2) of V79 Chinese hamster cells and phenotypically characterized. Dot-Blot hybridization with a 32P-labelled HGPRT probe revealed an increase in the number of HGPRT sequences in some of these revertants, suggesting the occurrence of gene amplification. Cytogenetic analysis performed in three of these revertants showed a characteristic abnormally banding region (ABR) on the elongated p arm of the X chromosome. In situ hybridization in one revertant (RHE2) showed that the amplified sequences reside on the p+ arm of the X chromosome in two different localizations. Because of the very probably clonal origin of the revertant, these features indicate that the amplified sequences might rearrange after their integration into the chromosome.

Targeted mutation of the Hprt gene in mouse embryonic stem cells

The hypoxanthine-guanine phosphoribosyltransferase (Hprt) gene has been mutated in mouse blastocyst-derived embryonic stem cells by site-directed homologous recombination. Embryonic stem cells were electroporated in the presence of a targeting DNA fragment containing two specific features: (i) The targeting DNA contained a promoterless neomycin phosphotransferase (neo) gene that, when located within the endogenous Hprt locus, could be transcribed from the promoter of the target locus. (ii) The targeting fragment had two short regions of homology with the endogenous Hprt gene: one, 132 base pairs long and the other, 1.2 kilobase pairs long. Targeted cells in which the designed homologous recombination event occurred were isolated either by selection with G418 followed by 6-thioguanine or by selection with 6-thioguanine alone. Even though less than 2 kilobases of homology existed between the exogenous and target DNAs, an average of 2.6 embryonic stem cells were successfully targeted for every 10(5) colonies surviving electroporation. Six of the Hprt- cell lines showed homologous recombination. These six lines were further analyzed by nucleotide sequencing a fragment that spans one crossover point after amplification by the polymerase chain reaction. Four lines had the expected sequence, whereas two lines had small deletions abutting the 132-base-pair region of homology.

Characterization, evolutionary relationships, and chromosome location of processed mouse HPRT pseudogene

Studies on a cell line with amplified copies of the mouse hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene and HPRT gene transfer experiments revealed the existence of a nonfunctional HPRT-related sequence in the mouse genome. This sequence was isolated and found to be a processed HPRT pseudogene. With the exception of a small internal deletion, the pseudogene is believed to comprise a complete reverse transcript of HPRT mRNA, although the 3' end of the pseudogene was lost in the cloning process. A probe from a region flanking the mouse pseudogene was used to investigate the evolutionary relationships of mammalian HPRT pseudogenes. The pseudogenes in mouse and Chinese hamster appear to have a common origin, but no homology to any of the four known human HPRT pseudogenes was detected. A pseudogene-linked restriction fragment length polymorphism was used to map the pseudogene to the distal end of mouse chromosome 17.

Characterisation of cDNA clones for hypoxanthine-guanine phosphoribosyltransferase from the human malarial parasite, Plasmodium falciparum: comparisons to the mammalian gene and protein

The isolation of cDNA clones for hypoxanthine-guanine phosphoribosyltransferase (HPRT) from the human malarial parasite, Plasmodium falciparum, is described. Northern analysis indicates that P. falciparum HPRT mRNA is the same size as that coding for mammalian HPRT. The predicted amino acid sequence of the P. falciparum HPRT protein shows extensive homology to the mammalian enzyme. Homology between the two proteins occurs in distinct blocks and a putative catalytic binding domain in the centre of the protein is also conserved. Five out of the seven characterised mammalian HPRT missense mutations map to regions which are conserved in the P. falciparum protein.

Induction of adenine salvage in mouse cell lines deficient in adenine phosphoribosyltransferase

Adenine phosphoribosyltransferase (APRT) (EC 2.4.2.7) pseudorevertant cell lines were isolated under selective conditions requiring adenine salvage for survival; yet they were found to be deficient in measurable APRT activity and resistant to the purine analog 2'6'-diaminopurine (DAP) (M.S. Turker, J. A. Tischfield, P. Rabinovitch, P.J. Stambrook, J.J. Trill, A.C. Smith, C.E. Ogburn, and G.M. Martin, manuscript in preparation). Adenine salvage was examined in two APRT pseudorevertant cell lines, their two APRT homozygous deficient parental cell lines, and a genotypic APRT revertant cell line (i.e., one with measurable APRT activity and DAP sensitivity). Adenine accumulation was observed in both revertant phenotypes and was demonstrated by high-performance liquid chromatography to be linked with adenine metabolism. The ability to salvage adenine declined substantially in the pseudorevertant cell lines when they were removed from selective media containing inhibitors of de novo 5'-AMP synthesis (alanosine and azaserine); for one pseudorevertant cell line this decline was accelerated by the addition of DAP to the medium. The readdition of alanosine or azaserine to the growth medium of the pseudorevertant lines induced adenine salvage to its previous levels. An APRT-like cross-reacting material was found in the pseudorevertant cell lines, although its relationship to adenine salvage is unknown. A low level of constitutive adenine salvage was found in the parental APRT-deficient lines, and it was also possible to induce adenine salvage in these cell lines. These findings suggest a novel regulatory mechanism for adenine salvage.

Induction of adenine salvage in mouse cell lines deficient in adenine phosphoribosyltransferase

Adenine phosphoribosyltransferase (APRT) (EC 2.4.2.7) pseudorevertant cell lines were isolated under selective conditions requiring adenine salvage for survival; yet they were found to be deficient in measurable APRT activity and resistant to the purine analog 2'6'-diaminopurine (DAP) (M.S. Turker, J. A. Tischfield, P. Rabinovitch, P.J. Stambrook, J.J. Trill, A.C. Smith, C.E. Ogburn, and G.M. Martin, manuscript in preparation). Adenine salvage was examined in two APRT pseudorevertant cell lines, their two APRT homozygous deficient parental cell lines, and a genotypic APRT revertant cell line (i.e., one with measurable APRT activity and DAP sensitivity). Adenine accumulation was observed in both revertant phenotypes and was demonstrated by high-performance liquid chromatography to be linked with adenine metabolism. The ability to salvage adenine declined substantially in the pseudorevertant cell lines when they were removed from selective media containing inhibitors of de novo 5'-AMP synthesis (alanosine and azaserine); for one pseudorevertant cell line this decline was accelerated by the addition of DAP to the medium. The readdition of alanosine or azaserine to the growth medium of the pseudorevertant lines induced adenine salvage to its previous levels. An APRT-like cross-reacting material was found in the pseudorevertant cell lines, although its relationship to adenine salvage is unknown. A low level of constitutive adenine salvage was found in the parental APRT-deficient lines, and it was also possible to induce adenine salvage in these cell lines. These findings suggest a novel regulatory mechanism for adenine salvage.

Structure, expression, and mutation of the hypoxanthine phosphoribosyltransferase gene

The wild-type mouse hypoxanthine phosphoribosyltransferase (HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) gene has been isolated from genomic libraries and its structure has been determined. This X chromosome-linked gene is greater than 33 kilobases long and is split into nine exons. All the exon sequences have been determined, and a single-base substitution in the HPRT cDNA coding sequence from a mouse neuroblastoma cell line that overproduces a mutant HPRT protein has been identified. The 5' end of the gene has been defined, both by nuclease S1 protection and primer extension studies and by a functional assay in which an HPRT minigene, capable of expression in cultured cells, was created by ligating the 5' end of the gene onto wild-type human HPRT cDNA. Sequences normally associated with eukaryotic promoters are not present in the immediate 5'-flanking region of the HPRT gene, which is instead highly G+C rich. This observation is discussed in relation to the possible link between DNA methylation and X-chromosome inactivation.

Cloning of cDNA sequences of human adenosine deaminase

Cloned cDNA sequences of human adenosine deaminase (ADA; adenosine aminohydrolase, EC 3.5.4.4) have been isolated from a cDNA library constructed in bacteriophage lambda gt10. The cDNA for the library was prepared from poly(A)+ RNA isolated from a human T-lymphoblast cell line, CCRF-CEM. The library was initially screened by differential plaque hybridization to labeled cDNA prepared from human T- and B-lymphoblast cell lines with a 21-fold difference in levels of translatable ADA mRNA. Two recombinants containing cloned cDNA sequences for ADA were identified by hybridization-selected translation. Both recombinants contained approximately 1,600 base pairs of inserted human DNA. Restriction maps of the two inserts were not identical. One contained approximately 40 base pairs of additional DNA toward the center of the cDNA. The cloned cDNA specifically hybridized to five fragments generated by HindIII digestion of human genomic DNA. It also hybridized to human lymphoblast RNA species 1.6 and 5.8 kilobases in length. The cDNA was used as a probe to estimate ADA mRNA levels in human lymphoblast cell lines. ADA mRNA levels correlate closely with levels of ADA catalytic activity and ADA protein in cell lines containing structurally normal ADA. A leukemic T-lymphoblast line produced 6 to 9 times as much ADA protein and ADA mRNA as transformed B-lymphoblast lines. Two mutant B-lymphoblast lines from patients with hereditary ADA deficiency contained unstable ADA protein but had 3 to 4 times the normal level of ADA mRNA.

Deletion and amplification of the HGPRT locus in Chinese hamster cells

Somatic cell selective techniques and hybridization analyses with a cloned cDNA probe were used to isolate and identify Chinese hamster cell lines in which the X-linked gene for hypoxanthine-guanine phosphoribosyltransferase (HGPRT) has been altered. Two of 19 HGPRT-deficient mutants selected were found to have major DNA deletions affecting the HGPRT locus. Cytogenetic studies revealed that the X chromosome of each deletion mutant had undergone a translocation event, whereas those from the remaining 17 mutants were normal. Phenotypic revertants of the thermosensitive HGPRT mutant RJK526 were isolated, and amplification of the mutant allele was shown to be the predominant mechanism of reversion. Comparisons of restriction enzyme fragments of DNA from deletion versus amplification strains identified two regions of the Chinese hamster genome that contained homology to the cDNA probe. One was shown to be much larger than the 1,600-nucleotide mRNA for HGPRT and to be comprised of linked fragments that contained the functional HGPRT gene. The second was neither transcribed nor tightly linked to the functional gene. These initial studies of HGPRT alterations at the level of DNA thus identified molecular mechanisms of phenotypic variation.

Occurrence and evolution of homogeneously staining regions may be due to breakage-fusion-bridge cycles following telomere loss

Chromosomes with homogeneously staining regions (HSR) were analysed in a subclone of the H4 rat hepatoma cell line, where they represent amplification of the ribosomal RNA (rRNA) genes. Detailed G-band analysis of the subclone revealed that an HSR on the short arm of chromosome 3 became unstable and changed its position within the chromosome. The evolution of this marker chromosome was associated with the terminal deletion of the normal long arm of the HSR-bearing chromosome 3 and may have involved ring formation as a result of fusion between the HSR on the short arm and the broken end of the long arm. Evidence was obtained for breakage at different sites within the ring, producing chromosomes with HSRs located terminally on either the long arms or both arms. The terminally located HSR underwent elongation in some cells presumably as a result of a breakage-fusion-bridge cycle characteristic of instability due to telomeric loss. It is suggested that terminally located HSRs may generally occur this way.

Evolution of chromosomal regions containing transfected and amplified dihydrofolate reductase sequences

A modular dihydrofolate reductase gene has been introduced into Chinese hamster ovary cells lacking dihydrofolate reductase. Clones capable of growth in the absence of added nucleosides contain one to five copies of the plasmid DNA integrated into the host genome. Upon stepwise selection to increasing methotrexate concentrations, cells are obtained which have amplified the transforming DNA over several hundredfold. A detailed analysis of the chromosomes in three clones indicated the appearance of cytologically distinct chromosomal regions containing the amplified plasmid DNA which differ in surrounding sequence composition, structure, and location. Two of the clones examined have extensive, homogeneously staining regions. The DNA in these homogeneously staining regions replicates in the early part of the S phase. The amplified plasmid DNA is found associated at or near the ends of chromosomes or on dicentric chromosomes. We propose that integration of DNA may disrupt telomeric structures and facilitate the formation of dicentric chromosomes, which may then undergo bridge breakage-fusion cycles. These phenomena are discussed in relation to DNA transfer experiments and modes of gene amplification and chromosome rearrangement.

A three-allele restriction-fragment-length polymorphism at the hypoxanthine phosphoribosyltransferase locus in man

Using cloned cDNA sequences of murine and human hypoxanthine phosphoribosyltransferase (HPRT: IMP: pyrophosphate phosphoribosyltransferase, EC 2.4.2.8), we have identified and characterized a three-allele restriction-fragment-length polymorphism for the restriction endonuclease BamHI at the human HPRT locus. The alleles are expressed phenotypically on Southern blots as three distinct pairs of fragments that hybridize to HPRT cDNA: (i) a 22-kilobase (kb)/25-kb pair, (ii) a 12-kb/25-kb pair, and (iii) a 22-kb/18-kb pair. In addition to fragments from the HPRT locus, sequences recognized by both HPRT cDNA probes are also present on at least two autosomes in the human genome. Allele frequencies in an unselected Caucasian population are 0.77 for the 22-kb/25-kb allele. 0.16 for the 12-kb/25-kb allele, and 0.07 for the 22-kb/18-kb allele, resulting in an average heterozygosity of 38% in females in this population. This polymorphism should facilitate gene mapping by linkage in this region of the human X chromosome.

Adenosine deaminase messenger RNAs in lymphoblast cell lines derived from leukemic patients and patients with hereditary adenosine deaminase deficiency

Hereditary deficiency of adenosine deaminase (ADA) usually causes profound lymphopenia with severe combined immunodeficiency disease. Cells from patients with ADA deficiency contain less than normal, and sometimes undetectable, amounts of ADA catalytic activity and ADA protein. The molecular defects responsible for hereditary ADA deficiency are poorly understood. ADA messenger RNAs and their translation products have been characterized in seven human lymphoblast cell lines derived as follows: GM-130, GM-131, and GM-2184 from normal adults; GM-3043 from a partially ADA deficient, immunocompetent !Kung tribesman; GM-2606 from an ADA deficient, immunodeficient child; CCRF-CEM and HPB-ALL from leukemic children. ADA messenger (m)RNA was present in all lines and was polyadenylated. The ADA synthesized by in vitro translation of mRNA from each line reacted with antisera to normal human ADA and was of normal molecular size. There was no evidence that posttranslational processing of ADA occurred in normal, leukemic, or mutant lymphoblast lines. Relative levels of specific translatable mRNA paralleled levels of ADA protein in extracts of the three normal and two leukemic lines. However, unexpectedly high levels of ADA specific, translatable mRNA were found in the mutant GM-2606 and GM-3043 lines, amounting to three to four times those of the three normal lines. Differences in the amounts of ADA mRNA and rates of ADA synthesis appear to be of primary importance in maintaining the differences in ADA levels among lymphoblast lines with structurally normal ADA. ADA deficiency in at least two mutant cell lines is not caused by deficient levels of translatable mRNA, and unless there is some translational control of this mRNA, the characteristic cellular ADA deficiency is most likely secondary to synthesis and rapid degradation of a defective ADA protein.

Deletion and amplification of the HGPRT locus in Chinese hamster cells

Somatic cell selective techniques and hybridization analyses with a cloned cDNA probe were used to isolate and identify Chinese hamster cell lines in which the X-linked gene for hypoxanthine-guanine phosphoribosyltransferase (HGPRT) has been altered. Two of 19 HGPRT-deficient mutants selected were found to have major DNA deletions affecting the HGPRT locus. Cytogenetic studies revealed that the X chromosome of each deletion mutant had undergone a translocation event, whereas those from the remaining 17 mutants were normal. Phenotypic revertants of the thermosensitive HGPRT mutant RJK526 were isolated, and amplification of the mutant allele was shown to be the predominant mechanism of reversion. Comparisons of restriction enzyme fragments of DNA from deletion versus amplification strains identified two regions of the Chinese hamster genome that contained homology to the cDNA probe. One was shown to be much larger than the 1,600-nucleotide mRNA for HGPRT and to be comprised of linked fragments that contained the functional HGPRT gene. The second was neither transcribed nor tightly linked to the functional gene. These initial studies of HGPRT alterations at the level of DNA thus identified molecular mechanisms of phenotypic variation.

Evolution of chromosomal regions containing transfected and amplified dihydrofolate reductase sequences

A modular dihydrofolate reductase gene has been introduced into Chinese hamster ovary cells lacking dihydrofolate reductase. Clones capable of growth in the absence of added nucleosides contain one to five copies of the plasmid DNA integrated into the host genome. Upon stepwise selection to increasing methotrexate concentrations, cells are obtained which have amplified the transforming DNA over several hundredfold. A detailed analysis of the chromosomes in three clones indicated the appearance of cytologically distinct chromosomal regions containing the amplified plasmid DNA which differ in surrounding sequence composition, structure, and location. Two of the clones examined have extensive, homogeneously staining regions. The DNA in these homogeneously staining regions replicates in the early part of the S phase. The amplified plasmid DNA is found associated at or near the ends of chromosomes or on dicentric chromosomes. We propose that integration of DNA may disrupt telomeric structures and facilitate the formation of dicentric chromosomes, which may then undergo bridge breakage-fusion cycles. These phenomena are discussed in relation to DNA transfer experiments and modes of gene amplification and chromosome rearrangement.

Isolation and characterization of a full-length expressible cDNA for human hypoxanthine phosphoribosyl transferase

We have cloned a full-length 1.6-kilobase cDNA of a human mRNA coding for hypoxanthine phosphoribosyltransferase (HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) into a simian virus 40-based expression vector and have determined its full nucleotide sequence. The inferred amino acid sequence agrees with a partial amino acid sequence determined for authentic human HPRT protein. Transfection of HPRT-deficient mouse LA9 cells with the purified plasmid leads to the expression of human HPRT enzyme activity in cells stably transfected and selected for enzyme activity in hypoxanthine/aminopterin/thymidine medium.

Hypoxanthine-guanine phosphoribosyltransferase genes of mouse and Chinese hamster: construction and sequence analysis of cDNA recombinants

Recombinant plasmids containing DNA inserts complementary to mRNA coding for hypoxanthine-guanine phosphoribosyltransferase (HPRT) from mouse and Chinese hamster cell lines have been isolated from cDNA libraries and characterized by DNA sequence analysis. A total of 1292 nucleotides of the mouse cDNA sequence and 1301 nucleotides of the Chinese hamster cDNA sequence has been determined. Each of these sequences includes an open reading frame of 654 nucleotides (218 amino acids) corresponding to the HPRT protein coding region. The deduced amino acid sequences for the mouse and Chinese hamster enzymes are presented and compared to that of human HPRT. At least 95% of the amino acids are conserved in the three species. In addition, we present evidence that two species of HPRT mRNA, which differ in the site of polyadenylation that is utilized during processing of the RNA transcripts, exist in Chinese hamster cells.

Cloned cDNA sequences of the hypoxanthine/guanine phosphoribosyltransferase gene from a mouse neuroblastoma cell line found to have amplified genomic sequences

Cloned cDNA sequences of the murine hypoxanthine/guanine phosphoribosyltransferase (HPRT; EC 2.4.2.8) gene have been isolated by using a mouse neuroblastoma cell line containing increased levels of a variant HPRT protein. We have used these sequences as probes to demonstrate that protein overproduction in this cell line is a consequence of at least a 20-fold increase in HPRT mRNA levels resulting from approximately 50-fold amplification of HPRT genomic sequences. The largest cDNA insert so far characterized represents about 70% of the HPRT mRNA sequence. This cDNA is shown to possess regions of homology with mRNA and DNA from Chinese hamster, baboon, and human, thus facilitating detailed analysis of this locus in these four species.