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Query: UNIPROT:P00492 (hypoxanthine-guanine phosphoribosyltransferase)
 2,385 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

HeLA H23 cells are a mutant female human tumor cell line harboring defective hypoxanthine phosphoribosyltransferase (HPRT; IMP-pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) as a result of a mutation that alters the isoelectric point of the enzyme (G. Milman, E. Lee, G. S. Changas, J. R. McLaughlin, and J. George, Jr., Proc. Natl. Acad. Sci. USA 73:4589-4592, 1976). As shown by Milman et al. and confirmed by us here, rare HAT+ revertants arise spontaneously at 1.9 X 10(-8) frequency and express both mutant and wild-type polypeptides. Thus, the H23 mutant also carries a silent wild-type HPRT allele that is activated in revertants. To test whether the silent allele was activated via hypomethylation of genomic DNA, H23 cells were treated with inhibitors of DNA methylation, and revertants were scored by HAT or azaserine selection. At an optimal dose of 5 microM 5-azacytidine, the reversion frequency was increased about 50-fold when assayed by HAT selection and over 1,000-fold when assayed by azaserine selection. HAT+ and azaserine revertants were heterozygous for HPRT, expressing both wild-type and mutant HPRT polypeptides. Like spontaneous revertants, they contained active HPRT enzyme and were genetically unstable, reverting at about 10(-4) frequency. Similar results were found after treatment with N-methyl-N'-nitro-N-nitrosoguanidine, a DNA-alkylating agent and potent inhibitor of mammalian DNA methylation. By contrast, the DNA-ethylating agent, ethyl methanesulfonate (EMS), did not increase the HAT+ reversion frequency; it did, however, increase the frequency by which H23 revertants heterozygous for HPRT reverted to 6-thioguanine resistance. Of nine EMS revertants, seven lacked HPRT activity and had a substantially reduced expression of the wild-type polypeptide. These observations support the hypothesis that DNA methylation plays an important role in human X-chromosome inactivation and that EMS can inactivate gene expression by promoting enzymatic methylation of genomic DNA as found previously for the prolactin gene in GH3 rat pituitary tumor cells (R. D. Ivarie and J. A. Morris, Proc. Natl. Acad. Sci. USA 79:2967-2970, 1982; R. D. Ivarie, J. A. Morris, and J. A. Martial, Mol. Cell. Biol. 2:179-189, 1982).
Mol Cell Biol 1986 Jan
PMID:Activation of a nonexpressed hypoxanthine phosphoribosyltransferase allele in mutant H23 HeLa cells by agents that inhibit DNA methylation. 243 Dec 68

Three genes on the human inactive X chromosome retained in the Chinese hamster X human hybrid cell line X8/6T2 have been reactivated using the demethylating agent, 5-azacytidine (5-aza-CR). Pulse-labeling and histochemical methods permitted detection and measurement of reactivation rates of the hypoxanthine phosphoribosyltransferase (Hpt) and glucose-6-phosphate dehydrogenase (G6pd) genes within 48 h of treatment. About 50% of the cells became active for these genes, which represents a reactivation rate some 30-fold greater than previously reported in similar systems. The phosphoglycerate kinase (Pgk) gene was not reactivated as frequently as the Hpt or G6pd genes. Segregation analysis of progeny of treated cells showed that enzyme-positive and enzyme-negative cells were produced in proportions supporting the notion that 5-aza-CR causes demethylation by replicative loss and that demethylation leads to reactivation.
Somat Cell Mol Genet 1987 May
PMID:High-frequency reactivation of X-linked genes in Chinese hamster X human hybrid cells. 244 Jan 16

We have investigated the genetic activation of the hprt (hypoxanthine-guanine phosphoribosyltransferase) gene located on the inactive X chromosome in primary and transformed female diploid Chinese hamster cells after treatment with the DNA methylation inhibitor 5-azacytidine (5azaCR). Mutants deficient in HPRT were first selected by growth in 6-thioguanine from two primary fibroblast cell lines and from transformed lines derived from them. These HPRT- mutants were then treated with 5azaCR and plated in HAT (hypoxanthine-methotrexate-thymidine) medium to select for cells that had reexpressed the hprt gene on the inactive X chromosome. Contrary to previous results with primary human cells, 5azaCR was effective in activating the hprt gene in primary Chinese hamster fibroblasts at a low but reproducible frequency of 2 x 10(-6) to 7 x 10(-6). In comparison, the frequency in independently derived transformed lines varied from 1 x 10(-5) to 5 x 10(-3), consistently higher than in the nontransformed cells. This increase remained significant when the difference in growth rates between the primary and transformed lines was taken into account. Treatment with 5azaCR was also found to induce transformation in the primary cell lines but at a low frequency of 4 x 10(-7) to 8 x 10(-7), inconsistent with a two-step model of transformation followed by gene activation to explain the derepression of hprt in primary cells. Thus, these results indicate that upon transformation, the hprt gene on the inactive Chinese hamster X chromosome is rendered more susceptible to action by 5azaCR, consistent with a generalized DNA demethylation associated with the transformation event or with an increase in the instability of an underlying primary mechanism of X inactivation.
Mol Cell Biol 1989 Apr
PMID:Differential activation of the hprt gene on the inactive X chromosome in primary and transformed Chinese hamster cells. 247 Oct 66

Immortalized fibroblasts from a male patient with xeroderma pigmentosum from complementation group D (XP-D) were treated with either ethyl methane sulfonate (EMS) or bleomycin (BLM) to obtain mutations in hypoxanthine phosphoribosyltransferase (HPRT) activity. The aneuploid parental cell line, MH3-XPD, was found to have a single copy of the HPRT gene, indicating that this cell line remained physically hemizygous for this locus during the transformation process. Subcloning of 6-thioguanine-resistant (6TG') isolates resulted in clones without detectable HPRT activity. Continued maintenance in elevated concentrations of 6TG (30-60 muM) produced cell populations with negligible growth in counterselection medium. No HPRT-deficient clones arose from unmutagenized cell cultures. Molecular analysis of the HPRT mutations in five clones with undetectable HPRT activity showed that four had large deletions. Two bleomycin-generated isolates were both found to have an approximately 28-kb intragenic deletion beginning with the first intron near exon 1 and ending within the fourth intron near exon 4. Messenger RNA from these clones was truncated by approximately 370 nucleotides. Our findings indicate that these two clones originated from the same mutational event within a founder cell. The three EMS-induced mutants fell into two classes: a putative point mutation or small deletion and two complete gene deletions.
Somat Cell Mol Genet 1989 Jul
PMID:Ethyl methane sulfonate- and bleomycin-generated deletion mutations at HPRT locus in xeroderma pigmentosum complementation group D fibroblasts. 247 61

The C86 line of female embryonal carcinoma cells contains one active and one inactive X chromosome. Following methylnitrosourea mutagenesis, a clone called C86AGM2 was isolated that carries a mutated hprt gene on the active X chromosome. This hprtm allele encodes an HPRT enzyme that has less than 1% normal enzyme activity, is thermolabile, and has an altered isoelectric point. Following treatment with drugs that demethylate DNA, the hprt+ gene from the inactive X chromosome in C86AGM2 cells became active as determined by the appearance of HPRT activity with the thermodenaturation and electrofocusing characteristics of the normal enzyme. No expression of this hprt+ gene occurred if C86AGM2 cells were induced to differentiate prior to DNA demethylation. Stable lines of C86AGM2 cells expressing both the hprtm and hprt+ genes did not inactivate either gene following differentiation.
Somat Cell Mol Genet 1989 Sep
PMID:Reactivation of hprt on the inactive X chromosome with DNA demethylating agents. 247 61

We have selected mutations in genes encoding components of the signaling pathway for alpha interferon (IFN-alpha) by using a specially constructed cell line. The upstream region of the IFN-regulated human gene 6-16 was fused to the Escherichia coli guanine phosphoribosyltransferase (gpt) gene and transfected into hypoxanthine-guanine phosphoribosyltransferase-negative human cells. These cells express gpt only in the presence of IFN-alpha. They grow in medium containing hypoxanthine, aminopterin, and thymidine plus IFN and are killed by 6-thioguanine plus IFN. Two different types of mutants were obtained after treating the cells with mutagens. A recessive mutant, selected in 6-thioguanine plus IFN, was completely resistant to IFN-alpha but responded normally to IFN-gamma and, unexpectedly, partially to IFN-beta. A constitutive mutant, selected in hypoxanthine-aminopterin-thymidine alone, was abnormal in expressing endogenous genes in the absence of IFN. Both types revert infrequently, allowing selection for complementation of the defects by transfection.
Mol Cell Biol 1989 Nov
PMID:Use of a selectable marker regulated by alpha interferon to obtain mutations in the signaling pathway. 251 75

Human DNA was used to transform adenosine kinase (AK)-deficient BHK cells followed by selection of AK+ cells in medium containing alanosine, adenosine, and uridine (AAU medium). Twenty AAUr isolates were analyzed, and none of them contained AK activity. Several purine salvage enzymes were, however, found to be affected in these cells. The levels of hypoxanthine-guanine phosphoribosyltransferase and adenylosuccinate synthetase activities were elevated, while the adenylosuccinase activity was reduced. AAU-resistance may be explained by elevated activity of adenylosuccinate synthetase to overcome the alanosine block; thus AAUr cells were able to convert exogenous adenosine----inosine----hypoxanthine----IMP----AMPS----AMP. Moreover, these AAUr cells required exogenous purines for growth. HPLC analyses of endogenous nucleotide pools of AAUr cells showed that the levels of adenine nucleotides have diminished to less than 10% of the parental levels. These results suggest that the AAU-resistant mutation, which elicits pleiotropic phenotypes in BHK cells, affects an important component in the regulation of adenine nucleotide synthesis. By including erthyro-9-(2-hydroxy-3-nonyl)adenine in the AAU medium (renamed as AAUE medium) to block deamination of adenosine, AK+ BHK cells were isolated.
Somat Cell Mol Genet 1989 Mar
PMID:Imbalance of purine nucleotides in alanosine-resistant baby hamster kidney cells. 253 26

Complete deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) results in a devastating neurological disease, the Lesch-Nyhan syndrome. This disorder has been identified as a candidate for initial attempts at somatic cell gene therapy. We have previously reported the construction of a recombinant herpes simplex virus type 1 (HSV-1) vector containing human hprt cDNA sequences under the regulatory control of the viral thymidine kinase gene (tk) [Palella et al., Mol. Cell. Biol. 8 (1988) 457-460]. Infection of HPRT- cultured rat neuronal cells with these vectors resulted in transient expression of human hprt. In this paper, we report the expression of human hprt mRNA transcripts in the brains of mice infected in vivo with this vector by direct intracranial inoculation. Human hprt transcripts were distinguished from endogenous mouse transcripts by RNase A mapping using riboprobes transcribed from human hprt cDNA. These initial studies demonstrate the transfer and transcription of a human gene in brain cells by direct in vivo infection with recombinant HSV-1 vectors.
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PMID:Expression of human HPRT mRNA in brains of mice infected with a recombinant herpes simplex virus-1 vector. 255 79

In order to characterize in vivo gene mutations that occur during fetal development, molecular analyses were undertaken of mutant 6-thioguanine resistant T-lymphocytes isolated from placental cord blood samples of 13 normal male newborns. These mutant T-cells were studied to define hypoxanthine-guanine phosphoribosyltransferase (hprt) gene structural alterations and to determine T-cell receptor (TCR) gene rearrangement patterns. Structural hprt alterations, as shown by Southern blot analyses, occurred in 85% of these mutant clones. These alterations consisted mostly of deletion of exons 2 and 3. These findings contrast with the 10-20% of gross structural alterations (i.e., those visible on Southern blots) occurring randomly across the entire gene previously reported for T-cell mutants isolated from normal young adults. Iterative analyses of hprt structural alterations and TCR gene rearrangement patterns show that approximately one-third of the newborn derived mutants may have originated as pre- or intrathymic hprt mutations. This too contrasts with previous findings in adults where the background in vivo hprt mutations appeared to originate in postthymic T-lymphocytes.
Environ Mol Mutagen 1989
PMID:Molecular analyses of in vivo hprt mutations in human T-lymphocytes: IV. Studies in newborns. 258 30

Altered sequences were determined of 52 independent spontaneous mutations occurring in a cDNA of the human hypoxanthine phosphoribosyltransferase (hprt) gene, which was integrated into chromosomal DNA of the mouse cell as a part of the retroviral shuttle vector. Spontaneous mutations comprised a variety of events: base substitutions, frameshifts, deletions, duplications, and complex mutational events, and were distributed randomly over the coding region of the gene. Frameshifts were the most frequent mutational event (38%), and base substitutions were the next most frequent (25%), followed by deletions (19%). Frameshift and deletion mutations commonly occurred preferentially at sites flanked by short direct repeats. Short inverted repeats were frequently found to be associated with duplication and complex mutational events. Analysis of the sequence alterations in the mutant genes suggests that misalignment mutagenesis represents an important molecular mechanism for the generation of spontaneous mutations in eukaryotic cells.
Mol Gen Genet 1989 Nov
PMID:Spectrum of spontaneous mutations in a cDNA of the human hprt gene integrated in chromosomal DNA. 262 50


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