An electrophoretic method for detecting hypoxanthine-guanine phosphoribosyl transferase variants

An electrophoretic method that detects hypoxanthine-guanine phosphoribosyl transferase variants is described.Aided by National Institutes of Health Grants HD 00486, HD 00004, GM 14155, and AM 10813.     

Reversion in expression of hypoxanthine-guanine phosphoribosyl transferase following cell hybridization.

Hybridization of mutant cell lines deficient in hypoxanthine-guanine phosphoribosyl transferase (HGPRT; E.C.: 2.4.2.8) from a variety of established rodent sources with HGPRT plus human cells yielded progeny cells which grew in selective medium containing hypoxanthine, aminopterin and thymidine (HAT). The same result was obtained when the human cell used was an HGPRT minus transformed line derived from a patient with the Lesch-Nyhan syndrome. Electrophoretic analysis indicated that all HAT-resistant progeny clones contained an active HGPRT enzyme which was indistinguishable from the wild type enzyme of the corresponding normal rodent cells. In contrast, no HAT-resistant cells have been obtained when the same HGPRT minus rodent cells were subjected to fusion processes in the absence of human cells or when they fused with similarly derived HGPRT minus mutant cells of other rodents. Reversion in expression of the rodent gene for HGPRT was detected in clones which retained one or more human chromosomes and in clones which contained no detectable human chromosomal material. The observed re-expression of rodent HGPRT in HAT-resistant clones suggests that HGPRT plus as well as HGPRT minus human cells contributed a factor which determined the expression of respective rodent structural genes for HGPRT. In contrast, HGPRT minus rodent cells were unable to induce the synthesis or normal HGPRT in the cells derived from the patient with the Lesch-Nyhan syndrome.     

Azaguanine resistant hamster cell lines not deficient in hypoxanthine-guanine phosphoribosyl transferase

Using a serial selection technique in which Chinese hamster cells were treated first with 8-azaguanine and then subsequently with HAT medium it was found that approximately 15% of azaguanine resistant clones were also resistant to HAT. Several such clones were subcultured and found to be stably resistant to azaguanine, in some cases at a higher level than the usual azaguanine resistant mutants which are HAT sensitive. Measurements of hypoxanthine-guanine phosphoribosyl transferase levels were in some cases lower than the parental line but in three of the clonal lines were higher than the parental strain. The fact that azaguanine resistant lines constitute a biochemically heterogeneous population underscores the importance of careful characterization of mammalian cell culture variants.     

Human mitochondrial NADP-dependent isocitrate dehydrogenase in man-mouse somatic cell hybrids.

Human mitochondrial NADP-dependent isocitrate dehydrogenase (IDH-2) is expressed in man-mouse somatic cell hybrids as a dimeric molecule. The gene specifing this enzyme was observed to be syntenic with the mannose phosphate isomerase locus in the 56 primary man-mouse clones in this series. The human IDH-2 locus, therefore, may be assigned to chromosome 15.     

Increase in hypoxanthine-guanine phosphoribosyl transferase gene mutations by exposure to electric field

Previously, we reported that exposure to extremely low frequency magnetic field (400 mT) increased in hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene mutations. However, it is unclear these mutations were induced by magnetic field (MF), electric field (EF), or both. To explore this question, a new exposure apparatus for EF was manufactured. We observed an increase in HPRT gene mutations in Chinese hamster ovary (CHO) cells after exposure to EF (10 V/m, 60 Hz) for 10 h. The mutant frequency by EF-exposure was an approximate 2-fold of that by sham-exposure. Our data suggest that the mutations induced by exposure of cells to the variable magnetic field at 400 mT may be, in part, due to the induced EF.     

Quantitative analysis of human chromosome segregation in man-mouse somatic cell hybrids.

The presumed random and independent process of human chromosome segregation in man-mouse somatic cell hybrids was studied. The results of chromosome analysis on 196 cells from 15 related hybrid strains have provided the first convincing evidence that segregation of human chromosomes can be nonindependent and often concordant. Different human chromosomes were not retained with equal frequency in these hybrid clones. Some were present in 80% of all the cells, whereas others appeared in less than 10% of the same cells. Linear regression analysis was used to test for correlation of the frequencies of all pair-wise combinations of human chromosomes present in these hybrid clones. Twenty-two of 136 possible correlations were statistically significant, indicating that concordant segregation of particular pairs of human chromosomes is a rather frequent occurrence.     

Analysis of human hypoxanthine-guanine phosphoribosyl transferase isozymes by isoelectric focusing in polyacrylamide gel

The method of isoelectric focusing in polyacrylamide gel was used to separate HGPRT isoenzymes in crude hemolysates of human and rat erythrocytes. HGPRT from erythrocytes of a normal human male donor consistently revealed three peaks of activity. Their mean isoelectric points, using pH 5–7 range ampholytes, were, peak I, pI 6.00; peak II, pI 5.8³; and peak III, pI 5.71. Peak III was wide and tailed. It always had a shoulder with a mean pI of 5.62. HGPRT from rat erythrocytes revealed two peaks of activity, corresponding to isoelectric points of 5.90 and 5.80. The method of isoelectric focusing in polyacrylamide gel is presented as a new way of detecting isoenzyme patterns.This study was supported by Grant No. 38-5768 from the Lebanese National Council for Scientific Research and Grant No. 18-5240 from the Medical Research Committee, American University of Beirut.     

X-linked hypoxanthine-guanine phosphoribosyl transferase deficiency: Detection of heterozygotes by selective medium

Skin fibroblasts from five unrelated males with X-linked hypoxanthine-guanine phosphoribosyl transferase deficiency and from their families have been exposed to medium containing 6-thioguanine. This purine analogue selects against cells with normal hypoxanthine-guanine phosphoribosyl transferase activity and therefore permits detection of mutant cells in heterozygous populations. The results of these studies are compared to those obtained by autoradiography of single-cell clones of skin fibroblasts from the same subjects. In each case, the results of the selective method are similar to those obtained by clonal analysis. The use of selective medium therefore provides a sensitive means to detect heterozygosity at this locus and may provide a general method to select cells with X-linked markers from heterozygous populations.This work was supported by grants from the U.S.P.H.S. (#HD 00486), The Joseph P. Kennedy, Jr., Memorial Fluid Research Fund, and the National Foundation for Neuromuscular Diseases, Inc.     

Electrophoretic properties of hypoxanthine-guanine phosphoribosyl transferase in erythrocytes of subjects with Lesch-Nyhan syndrome

Hypoxanthine-guanine phosphoribosyl transferase (HGPRT; E.C. 2.4.2.8) has been studied in erythrocytes of patients with Lesch-Nyhan syndrome by polyacrylamide gel electrophoresis. The location of this enzyme in gel was determined by radiochemical assay. Inosine monophosphate (the reaction product of HGPRT with radioactive hypoxanthine and 5-phosphorylribose-1-pyrophosphate) was precipitated in the gel at the site of its formation with lanthanum chloride. The zone containing radioactive inosine monophosphate was located by continuous monitoring of mechanically fractionated gels in a scintillation spectrometer. The sensitivity of this method has permitted the detection of the very low HGPRT activity in the electropherograms of hemolysates of patients with Lesch-Nyhan syndrome. Among six patients, four had a mutant enzyme which migrated 15% faster than the normal; the other two had a mutant enzyme with about 12% faster migration. These mutants were designated HGPRT-LN and HGPRT-LN slow, respectively. These observations indicate that the mutant gene on the X chromosome codes for a protein of altered structure.Aided by U.S. Public Health Service grants Nos. HD 04608 and HD 03015 from the National Institute of Child Health and Human Development and GM 17702 from the National Institute of General Medical Sciences, National Institutes of Health.Presented in part at the 1971 meeting of the Western Society for Pediatric Research, Carmel, California.     

Cytogenetics of somatic cell hybrids. I. Progression of stemlines in continuous uncloned cultures of man-mouse cell hybrids.

Karyotypes of hybrid cells were studied in continuous uncloned cultures by Q- and C-bandings. Cultures were initiated by virus-mediated or spontaneous cell fusions from normal human diploid fibroblasts and mouse heteroploid RAG cells. Heterokaryons containing complete genomes of both parental cells randomly lost chromosomes from both species. The majority of cells in early growth stages, however, still possessed a nearly complete human genome. The rate of human chromosome loss in subsequent growth periods was not uniform, being gradual in some and rapid in others. The initially predominant 2n human-1s mouse (1h:1m) type was soon replaced by a less frequent 2n human-2s mouse (1h:2m) type. Over an increased period of time in mass culture, the number of stemlines decreased. One stemline, often a (1h:2m) type with a greatly reduced human complement, outgrew the others and occupied the entire culture. Therefore, the usual process of clonal isolation may confer a negative selection bias against cell hybrids retaining a large number of human chromosomes. Hybrid stemlines with stable karyotypes were established in the present HAT-agar selection system before 36 days after fusion had elapsed.     

Increased phosphoribosylpyrophosphate synthetase activity in fibroblasts of hypoxanthine-guanine phosphoribosyl transferase deficient patients.

An increased activity of phosphoribosylpyrophosphate synthetase at physiological levels of inorganic phosphate is demonstrated in extracts of skin fibroblast cultures derived from a patient with Lesch-Nyhan syndrome. This eccessive response of the phosphoribosylpyrophosphate synthetase at physiological levels of inorganic phosphate results in increased levels of phosphoribosylpyrophosphate and thus contributes to purine overproduction characteristic of this disorder. The level of enzyme response in skin fibroblast extracts from the carrier mother was between activity of the patient and normals, further suggesting the x-linkage of human phosphoribosylpyrophosphate synthetase.     

Partial hypoxanthine-guanine phosphoribosyl transferase deficiency with full expression of the Lesch-Nyhan syndrome

Summary A patient with the full clinical expression of the classical Lesch-Nyhan syndrome is presented with a residual hypoxanthine-guanine phosphoribosyl transferase (HGPRT) activity of 5–10% in erythrocyte lysate and about 30% in fibroblast lysate. The activities of other erythrocyte enzymes of purine metabolism were typical for a classical Lesch-Nyhan patient. The effects of allopurinol therapy on the excretion of urinary purine metabolites were studied by a newly developed isotachophoretic technique.The unusually high residual activity of HGPRT in erythrodytes and fibroblasts of the patient enabled the enzymologic characterization of the mutant enzyme: in fibroblasts the affinities for the substrates hypoxanthine and guanine were normal. However, there was an increased apparent K _m for phosphoribosylpyrophosphate (PRPP), a complete absence of product inhibition by IMP and GMP, and a decreased heat stability. Addition of PRPP did not stabilize the mutant enzyme. In addition to the altered properties of the fibroblast enzyme, the K _m of the erythrocyte enzyme for hypoxanthine was also increased.Immunoprecipitation experiments revealed the presence of an approximately normal amount of material cross-reacting with anti-human HGPRT antiserum. However, it appeared that this cross-reacting material had a decreased stability. When intact erythrocytes were incubated with radiolabeled purine bases, no formation of IMP or GMP could be detected, despite the relatively high residual activity of HGPRT in the hemolysate. The results fit the following hypothesis: as a consequence of a structural mutation affecting the PRPP-site of the enzyme and a decreased heat stability, the activity of the mutant enzyme under in vivo conditions is virtually zero.In the erythrocytes of the patient's mother a normal HGPRT-activity was found. However, the activity in her fibroblasts was lower than normal, while a decreased heat stability and an intermediate behavior towards IMP could be shown.Hair root analysis of several members of the patient's family confirmed the heterozygosity of the mother, whereas no other heterozygotes could be detected. The family anamnesis did not show other cases of Lesch-Nyhan syndrome. These findings were taken as evidence that the patient described in this paper might represent a mutation orginating from the gametes in either of the maternal grandparents.     

Induction of mutations by chemical agents at the hypoxanthine-guanine phosphoribosyl transferase locus in human epithelial teratoma cells

Induction of 6-thioguanine (TG) resistance by chemical mutagens was examined in a line of cells derived from a human epithelial teratocarcinoma cell clone. The cells, designated as P3 cells, have a stable diploid karyotype with 46(XX) chromosomes, including a translocation between chromosomes 15 and 20. Efficient recovery of TG-resistant mutants induced by the direct-acting mutagens: N-methyl-N′-nitro-N-nitrosoguanidine (MNNG); 7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE); and benzo[a]pyrene (B[a]P); activated in a cell-mediated assay, required an expression time of 7 days and a saturation density of 2 × 10⁴ cells/60-mm petri dish. The TG-resistant mutant cells induced by MNNG and BPDE maintained their resistant phenotype 4–6 weeks after isolation. This mutant phenotype was associated with a more than 10-fold reduction in hypoxanthine-guanine phosphoribosyl transferase (HGPRT) activity relative to that of the parental P3 cell line, which was shown to catalyze the formation of 4.6 pmoles inosine-5′-monophosphate (IMP)/min/μg protein. Induction of TG resistance was also observed in P3 cells cocultivated in a cell-mediated assay with human breast carcinoma cells, which are capable of polyclinic aromatic hydrocarbon (PAH) metabolism, after treatment with the carcinogenic PAHs: B[a]P, chrysene, 7,12-dimethylbenz[a]anthracene (DMBA), and 3-methylcholanthrene (MCA). The degree of mutant induction in this assay was related to the carcinogenic potency of these PAHs in experimental animals. The most potent mutagen was DMBA, followed in decreasing order by MCA, B[a]P, and chrysene. DMBA, at 0.4 μM, increased the frequency of mutants for TG resistance from 2 for the control to about 200 TG-resistant mutants/10⁶ colony-forming cells (CFC). Benzo[e]pyrene (B[e]P) and pyrene, which are not carcinogenic, were not effective in the assay. None of the PAHs was mutagenic in the P3 cells cultivated in the absence of the PAH-metabolizing cells. These results indicate that the P3 cells can be useful for the study of mutagenesis at the HGPRT locus by direct-acting chemical mutagens, as well as by chemicals activated in a cell-mediated assay.