Polymorphism of human Alpha class glutathione transferases

The recognition of the importance and utility of single nucleotide polymorphisms has generated an interest in the development of new strategies for their identification. Analysis of the Expressed Sequence Tag (EST) database can provide a rapid and efficient means of identifying polymorphisms. Screening of the Alpha class glutathione transferases (GSTs) in the EST database identified 10 putative polymorphisms in the coding region of the GSTA1 and GSTA2 genes, six of which were subsequently verified by sequence analysis. Polymerase chain reaction/restriction fragment length polymorphism analysis revealed the existence of three variants, a silent base substitution, K125K (G365A) in GSTA1, and T112S and E210A in GSTA2, in European Australian, African and Chinese populations. The variant isoforms of GSTA2 were expressed in Escherichia coli, purified, and enzymatically characterized. Modelling of the two GSTA2 polymorphisms into a three-dimensional structure of GSTA2, and characterization of their enzymatic properties, has shown that the structure and function of the wild-type GSTA2-2 isoenzyme is not significantly altered by these polymorphisms. This report demonstrates that analysis of the EST database provides a rapid and efficient means of identifying variant proteins.

The glutathione transferase structural family includes a nuclear chloride channel and a ryanodine receptor calcium release channel modulator

The ubiquitous glutathione transferases (GSTs) catalyze glutathione conjugation to many compounds and have other diverse functions that continue to be discovered. We noticed sequence similarities between Omega class GSTs and a nuclear chloride channel, NCC27 (CLIC1), and show here that NCC27 belongs to the GST structural family. The structural homology prompted us to investigate whether the human Omega class glutathione transferase GSTO1-1 forms or modulates ion channels. We find that GSTO1-1 modulates ryanodine receptors (RyR), which are calcium channels in the endoplasmic reticulum of various cells. Cardiac RyR2 activity was inhibited by GSTO1-1, whereas skeletal muscle RyR1 activity was potentiated. An enzymatically active conformation of GSTO1-1 was required for inhibition of RyR2, and mutation of the active site cysteine (Cys-32 --> Ala) abolished the inhibitory activity. We propose a novel role for GSTO1-1 in protecting cells containing RyR2 from apoptosis induced by Ca(2+) mobilization from intracellular stores.

Identification of a new mutation (Gly420Ser), distal to the active site, that leads to factor XIII deficiency

The molecular defects of the factor XIII A subunit gene were studied in a patient with factor XIII deficiency. Mutation analysis was performed on amplified DNA from each exon of this gene by single-strand conformation polymorphism (SSCP) and DNA sequencing techniques. A substitution of guanine by adenine at nucleotide 1258 in exon 10 of the coagulation factor XIII A subunit gene has been identified in the patient. The mutation results in the replacement of Gly420 by Ser in the core domain of the enzyme. Restriction enzyme analysis of amplified exon 10 DNA confirmed that the patient was homozygous for this mutation. A family study revealed that the mutation was inherited from both parents, who were first cousins. The potential effects of the mutation were predicted by molecular modeling of the amino acid substitution within the coordinates of the crystal structure. The substitution occurred within the core domain of the enzyme at a residue completely conserved among all known members of the transglutaminase family. The model of the mutant protein suggests that although the substitution of Gly420 by Ser causes only minor readjustment of the residues and does not appear to be particularly deleterious in terms of structure, the mutation is, however, likely to decrease the molecule's ability to undergo the conformational change that is thought to be required for full transglutaminase activity. Our data strongly support the previously published information about the functional significance of the residues surrounding, but not forming, the catalytic pocket in the A subunit of factor XIII.

Database analysis and gene discovery in pharmacogenetics

The global genome research effort has resulted in the creation of extensive DNA and protein sequence databases that are a valuable resource for the identification of new genes and polymorphic variants of enzymes of pharmacogenetic interest. Previously undescribed members of gene families with novel functions and substrate specificities can be identified by database searching and sequence alignment strategies. Since the expressed sequence tag (EST) database contains sequences from many individuals, it can be searched for evidence of polymorphisms that can significantly influence enzyme function. The different approaches to these forms of analysis are reviewed and illustrated with examples from the glutathione transferase gene family.

College women's awareness and consumption of folic acid for the prevention of neural tube defects

PURPOSE

Awareness and consumption of folic acid in the context of neural tube defect prevention among college women were assessed.

METHODS

Subjects documented folic acid awareness and multivitamin consumption. Beliefs about folic acid before and after an educational intervention were characterized using the Health Belief Model (HBM) and the Fetal Health Locus of Control Scale (FHLCS).

RESULTS

Awareness of folic acid was not associated with multivitamin consumption. Belief variables were not associated with awareness or consumption of folic acid.

CONCLUSION

This study does not support educational interventions based on the HBM or FHLCS to increase multivitamin consumption among college women.

Gene structure, expression and chromosomal localization of murine theta class glutathione transferase mGSTT1-1

We have isolated and characterized a cDNA and partial gene encoding a murine subfamily 1 Theta class glutathione transferase (GST). The cDNA derived from mouse GSTT1 has an open reading frame of 720 bp encoding a peptide of 240 amino acids with a calculated molecular mass of 27356 Da. The encoded protein shares only 51% deduced amino acid sequence identity with mouse GSTT2, but greater than 80% deduced amino acid sequence identity with rat GSTT1 and human GSTT1. Mouse GSTT1-1 was expressed in Escherichia coli as an N-terminal 6x histidine-tagged protein and purified using immobilized-metal affinity chromatography on nickel-agarose. The yield of the purified recombinant protein from E. coli cultures was approx. 14 mg/l. Recombinant mouse GSTT1-1 was catalytically active towards 1, 2-epoxy-3-(p-nitrophenoxy)propane, 4-nitrobenzyl chloride and dichloromethane. Low activity towards 1-menaphthyl sulphate and 1-chloro-2,4-dinitrobenzene was detected, whereas mouse GSTT1-1 was inactive towards ethacrynic acid. Recombinant mouse GSTT1-1 exhibited glutathione peroxidase activity towards cumene hydroperoxide and t-butyl hydroperoxide, but was inactive towards a range of secondary lipid-peroxidation products, such as the trans-alk-2-enals and trans,trans-alka-2,4-dienals. Mouse GSTT1 mRNA is most abundant in mouse liver and kidney, with some expression in intestinal mucosa. Mouse GSTT1 mRNA is induced in liver by phenobarbital, but not by butylated hydroxyanisole, beta-napthoflavone or isosafrole. The structure of mouse GSTT1 is conserved with that of the subfamily 2 Theta class GST genes mouse GSTT2 and rat GSTT2, comprising five exons interrupted by four introns. The mouse GSTT1 gene was found, by in situ hybridization, to be clustered with mouse GSTT2 on chromosome 10 at bands B5-C1. This region is syntenic with the location of the human Theta class GSTs clustered on chromosome 22q11.2. Similarity searches of a mouse-expressed sequence tag database suggest that there may be two additional members of the Theta class that share 70% and 88% protein sequence identity with mouse GSTT1, but less than 55% sequence identity with mouse GSTT2.

Polymorphism of phase II enzymes: identification of new enzymes and polymorphic variants by database analysis

The Phase II enzymes of xenobiotic metabolism are characterized by a high level of substrate diversity and genetic polymorphism. Genetic polymorphism of the Phase II enzymes can be of substantial clinical significance as some variants have differences in substrate specificity, stability and levels of expression. Variation in these factors can give rise to abnormal drug metabolism and susceptibility to carcinogens and toxins. A new approach to the discovery of additional members of Phase II enzyme families and the identification of polymorphic variants using searches of the EST databases has been investigated. The examples provided demonstrate that relatively simple search strategies can be highly productive.

Shifting substrate specificity of human glutathione transferase (from class Pi to class alpha) by a single point mutation

Substrate selectivity, among glutathione transferase (GST) isoenzymes, appears to be determined by a few residues. As part of study to determine which residues are class-specific determinants, Tyr 108 (an important residue of the class Pi) has been changed to a valine, the structural equivalent of a class Alpha enzyme. Using a panel of selected substrates, "diagnostic" for either class Pi or Alpha, it is shown here that this single mutation significantly alters the catalytic properties of the class Pi enzyme and shifts the substrate specificity of the enzyme toward that of the class Alpha enzyme.

Structure and organization of the human theta-class glutathione S-transferase and D-dopachrome tautomerase gene complex

The structure and organization of the human Theta-class glutathione S-transferase (GST) genes have been determined. GSTT1 and GSTT2 are separated by approx. 50 kb. They have a similar structure, being composed of five exons with identical exon/intron boundaries. GSTT1 is 8.1 kb in length, while GSTT2 is only 3.7 kb. The GSTT2 gene lies head-to-head with a gene encoding d-dopachrome tautomerase (DDCT), which extends over 8.5 kb and contains four exons. The sequence between GSTT2 and DDCT may contain a bidirectional promoter. The GSTT2 and DDCT genes have been duplicated in an inverted repeat. Sequence analysis of the duplicated GSTT2 gene has identified an exon 2/intron 2 splice site abnormality and a premature translation stop signal at codon 196. These changes suggest that the duplicate gene is a pseudogene, and it has been named GSTT2P.

Genetic heterogeneity of the structure and function of GSTT2 and GSTP1

In this study new methods for the detection of two polymorphic sites in the GSTP1 coding region have been developed. Both sites are polymorphic in several racial groups and there are significant differences between groups, in the gene frequency at each site. Although previous studies of recombinant GSTP1-1 have suggested that there are significant differences in the specific activity and stability of the I105 or V105 isoforms, no differences in the distribution of GSTP1-1 activities in normal blood donors with different GSTP1 genotypes were detected in this study. These data were obtained with CDNB as a substrate and greater differences may be apparent with different substrates. The structure and organization of the GSTT2 gene was also investigated and a pseudogene that occurs at a polymorphic frequency in European Australians was discovered. This pseudogene can be detected by PCR/RFLP analysis.

Missense mutations in the human glutathione synthetase gene result in severe metabolic acidosis, 5-oxoprolinuria, hemolytic anemia and neurological dysfunction

Severe glutathione synthetase (GS) deficiency is a rare genetic disorder with neonatal onset. The enzymatic block of the gamma-glutamyl cycle leads to a generalized glutathione deficiency. Clinically affected patients present with severe metabolic acidosis, 5-oxoprolinuria, increased rate of hemolysis and defective function of the central nervous system. The disorder is inherited in an autosomal recessive mode and, until recently, the molecular basis has remained unknown. We have sequenced 18 GS alleles associated with enzyme deficiency and we detected missense mutations by direct sequencing of cDNAs and genomic DNA. In total, 13 different mutations were identified. Four patients were found to be compound heterozygotes and two individuals were apparently homozygous. Reduced enzymatic activities were demonstrated in recombinant protein expressed from cDNAs in four cases with different missense mutations. The results from biochemical analysis of patient specimens, supported by the properties of the expressed mutant proteins, indicate that a residual activity is present in affected individuals. Our results suggest that complete loss of function of both GS alleles is probably lethal. It is postulated that missense mutations will account for the phenotype in the majority of patients with severe GS deficiency.

Glucose-6-phosphate dehydrogenase deficiency mutations in Papua New Guinea

We characterize the molecular basis of two G6PD deficiency variants from Papua New Guinea (PNG) and use the information to examine the extent of molecular heterogeneity underlying G6PD variation in the country. The Wosera G6PD- enzyme was found in a male from the East Sepik Province; it had unique biochemical characteristics compared with other previously described G6PD variants from PNG. The Wosera mutation occurred in exon 12 and led to an Arg463-->His substitution. The Kalo mutation, which led to G6PD deficiency in a male from the Central Province along the south coast of PNG, was found in exon 11 and resulted in an Arg454-->Cys substitution. Although screening for the two molecular mutations using sequence-specific oligonucleotide (SSO) hybridization revealed a wide distribution for the Kalo variant along the coastal belt, no additional copy of the Wosera variant was found in a range of samples. Moreover, 28 G6PD- genes from various parts of PNG failed to hybridize with oligonucleotides encoding either the Kalo or Wosera variants. This suggests that more than the two G6PD mutations exist in PNG, confirming the previously noted biochemical heterogeneity of G6PD deficiency in the Melanesian populations of this region.

Chromosomal localization of the gene for the human theta class glutathione transferase (GSTT1)

Two loci encoding Theta class glutathione transferases (GSTs) have been identified in humans. In situ hybridization studies have localized the GSTT1 gene to 22q11.2. This is the same band to which we previously localized the GSTT2 gene. This finding confirms the trend for human GST genes to be found in class-specific clusters.

BCR gene recombines with genomically distinct sites on band 11Q13 in complex BCR-ABL translocations of chronic myeloid leukemia

We have analysed a cloned 11q13/3'BCR junction fragment, one recombination product of a complex t(9;11;22) translocation in a patient with chronic myeloid leukemia. 3'M-Bcr recombined with chromosome band 11q13 at a specific point between two Alu elements lying in opposite orientation. We present new molecular data comparing the genomic location of the 11q13 breakpoint in our patient with that of one other recently reported to lie within the GSTP1 gene. This is the first time that specific breakpoint sites within a chromosomal region highly involved in complex Ph translocations have been relatively mapped. These early results argue against a precise site in 11q13 with which M-Bcr preferentially recombines and favour instead a larger recombination-prone domain. Both of the 11q13 breakpoint regions show Alu repeat elements in close proximity to the site of recombination.

Glutathione S-transferase M1 and T1 polymorphisms: susceptibility to colon cancer and age of onset

The M1 member of the Mu subclass of glutathione S-transferase (GSTM1) is only expressed in about 50% of individuals. In contrast, GSTT1, a member of the theta class which has been recently shown to be polymorphic, is expressed in 85% of Australian individuals. Previous studies have shown a significant excess of homozygous null GSTM1 genotypes among individuals with colorectal cancer, particularly those with proximal tumours. This suggests that GSTM1 plays a role in susceptibility to this neoplasm. In this study of 132 individuals with colorectal cancer and 200 controls, no significant excess of GSTM1 homozygous null genotypes was found among colorectal cancer patients with either a proximal or distal tumour. This suggests that the association between GSTM1 homozygous null genotypes and colorectal cancer is of smaller effect than has been reported previously using larger sample sizes. We have also examined the frequency of homozygous null GSTT1 genotypes in patients with colorectal cancer. Although the frequency was not significantly different in cases compared to control individuals, GSTT1 null homozygotes were significantly more common in patients who were diagnosed before the age of 70 years than in those who were diagnosed at an older age. This suggests that the GSTT1 genotype, and perhaps also the GSTM1 genotype for which a similar, but non-significant effect was seen, might influence the age of onset of colorectal cancer.

Mutations causing coagulation factor XIII subunit A deficiency: characterization of the mutant proteins after expression in yeast

We identified the mutations causing factor XIII A subunit deficiency in two families. Two distinct mutations were identified in the S family: the nonsense mutation Tyr 441-->stop in exon 11, inherited through the paternal line, and the missense mutation Asn 60-->Lys in exon 3, inherited through the maternal line. Two members of the J family were heterozygous for the previously described type 3 A subunit. The substitution giving rise to the type 3 variant was found to be Gly 501-->Arg in exon 12. The Asn 60-->Lys and Gly 501-->Arg mutations were constructed in cDNA clones and expressed in yeast (Saccharomyces cerevisiae AH22). Although mRNA could be detected, protein containing the Asn 60-->Lys substitution could not be detected, suggesting extreme instability or susceptibility to proteolysis. A subunits containing the Gly 501-->Arg substitution were expressed and found to be enzymatically active in fresh yeast lysates. This variant has thermal instability and lost activity during storage or purification. Gel filtration studies suggested that the type 3 variant assembled as a dimer, as do normal A subunits. The data suggest that the Gly 501-->Arg (Type 3 variant) would cause severe factor XIII deficiency if inherited in the homozygous form or as a compound heterozygote with another deleterious mutation.

Glutathione S-conjugate transport by cultured human cells

Elimination of the products of xenobiotic metabolism is an important step in cellular detoxification and involves a specific transport system or "export pump". ATP-dependent transport of glutathione S-conjugates has previously been demonstrated in a variety of tissues, mainly from rat. However, the characteristics of this pump have not been fully explored in human cells. This study investigated transport of a glutathione S-conjugate, 2,4-dinitrophenyl glutathione (GS-DNP), by a variety of cultured human cell lines. GS-DNP was generated intracellularly after treatment of cells with 1-chloro-2,4-dinitrobenzene and subsequent transport of the conjugate into the extracellular medium was measured spectrophotometrically at 340 nm. Calculation of the initial transport rates at 37 degrees C revealed considerable variation in GS-DNP secretion between cell lines which was statistically significant in some cases. A 2-fold increase in GS-DNP efflux was observed between Jurkat and HL-60 cells (11.360 +/- 3.893 vs. 5.662 +/- 2.263 nmol/10(6) cells/h, P < 0.007). The highest rate of transport was found in HepG2 cells (14.171 +/- 4.790 nmol/10(6) cells/h) whereas the 5637 cell line had the lowest level with a transport rate of 1.475 +/- 0.631 nmol/10(6) cells/h. For each cell line, transport of GS-DNP was almost totally inhibited or markedly reduced on ice. Pre-incubation of cells at 42 degrees C also lowered the initial transport rates compared with cells maintained at 37 degrees C but these were not significantly different except in the case of HeLa cells. ATP levels ranged from 30.5 to 89.3 nmol/mg protein and there was variation in the glutathione content and glutathione S-transferase activities of the cells. This report demonstrates firstly that transport of glutathione conjugates is a feature of many cell types in vitro and secondly that the basal levels of GS-DNP secretion vary significantly between human cells.

Purification, molecular cloning and heterologous expression of a glutathione S-transferase from the Australian sheep blowfly (Lucilia cuprina)

Three glutathione S-transferases from Lucilia cuprina (Australian sheep blowfly) pupae were purified by affinity chromatography and anion-exchange chromatography. One isoenzyme was composed of M(r)-24,800 subunits, and two isoenzymes had subunits of M(r) 23,900. The M(r)-23,900 subunits showed immunological identity and were immunologically distinct from the M(r)-24,800 subunits. All three enzymes were active with the substrate 1-chloro-2,4-dinitrobenzene and had low activity with 1,2-dichloro-4-nitrobenzene. A cDNA clone encoding a M(r)-23,900 subunit (LuGST1) was isolated and sequenced. The sequence has close similarities (> 81%) to that of GSTs from the fruitfly Drosophila melanogaster and Musca domestica (housefly). The deduced amino acid sequence of the Lu GST1 subunit showed no significant similarity to that of the mammalian GSTs to the Alpha, Mu and Pi classes, but shows some similarity (33%) over the first 100 residues with the rat subunit 12 Theta-class GST. Southern blots of genomic DNA hybridized with the LuGST1 cDNA identified many hybridizing fragments. Taken together, these data indicated that the L. cuprina genome contains multiple glutathione S-transferase genes.

Evidence against a relationship between fatty acid ethyl ester synthase and the Pi class glutathione S-transferase in humans

Recently, Bora et al. (Bora, P. S., Bora, N. S., Wu, X., and Lange, L. G. (1991) J. Biol. Chem. 266, 16774-16777) reported the cloning and expression of a human fatty acid ethyl ester synthase III (FAEES-III) cDNA that has only four amino acid substitutions compared with human glutathione S-transferase (GST) GSTP1-1, and, when expressed in MCF-7 cells, the protein has both FAEES and GST activities. By site-directed mutagenesis of a GSTP1 cDNA, we have constructed a clone that encodes the FAEES-III protein described by Bora et al. (1991). The recombinant FAEES-III protein was expressed in Escherichia coli and has been shown to be devoid of FAEES and GST activities. The recombinant FAEES-III protein does not bind to a glutathione agarose affinity matrix, presumably because two of the substituted amino acids, Trp-39-->Cys and Gln-52-->Glu, are thought to contribute to the GST glutathione binding site. One of the base substitutions in the FAEES-III cDNA encodes an extra SacI site not found in the GSTPI cDNA. Polymerase chain reaction amplification of human genomic DNA has identified the GSTPI gene, but no DNA from the proposed FAEES gene with a diagnostic SacI site has been detected. Evaluation of the hybridization pattern of HindIII genomic restriction fragments has identified fragments that contain the GSTPI gene and a pseudogene (Board et al. 1992), and there do not appear to be any hybridizing fragments that could contain the FAEES-III gene. Our results do not provide any evidence in support of a relationship between FAEES-III and GST, and the cDNA reported by Bora et al. (1991) may have resulted from a cloning artifact.

Identification of a point mutation in factor XIII A subunit deficiency

Oligonucleotide primers have been designed for the amplification of all 15 exons of the human coagulation factor XIII A subunit gene. Each exon and its intron flanking regions has been amplified and sequenced from a patient with severe A subunit deficiency. A single G to A transition in the last base of exon 14 has been identified in the homozygous proband and in his heterozygous parents. The mutation would result in the substitution 681 Arg to His in the mature protein product. However, because the mutation is at a splice junction, the deficiency may result from a defect in pre-messenger RNA splicing.

Erythrocyte membrane transport of glutathione conjugates and oxidized glutathione in the Dubin-Johnson syndrome and in rats with hereditary hyperbilirubinemia

The Dubin-Johnson syndrome is manifested by conjugated hyperbilirubinemia and pigment accumulation in hepatocellular lysosomes. The TR-rat model is a phenotypic model of the Dubin-Johnson syndrome and is characterized by defective ATP-dependent transport of a group of nonbile acid organic anions, including glutathione-S-conjugates and oxidized glutathione, across the bile canaliculus. Similar ATP-dependent transport mechanisms have been described in erythrocytes. Intact erythrocytes and inverted erythrocyte membrane vesicles from Dubin-Johnson patients, TR-rats and appropriate controls were studied with regard to ATP-dependent transport of dinitrophenyl glutathione and oxidized glutathione. No significant differences were observed, indicating that the erythrocyte and canalicular ATP-dependent transporters for these substrates are functionally and potentially genetically distinct.

Genetic heterogeneity of the human glutathione transferases: a complex of gene families

The glutathione transferases (GSTs) are involved in the metabolism of a wide range of compounds of both exogenous and endogenous origin. There is evidence that deficiency of GST may increase sensitivity to certain environmentally derived carcinogens. In contrast, elevated expression has been implicated in resistance to therapeutic drugs. The GSTs are the products of several gene families. This review summarizes the present knowledge of the genetic interrelationships between the various isoenzymes, their deficiencies and the physical locations of their genes.

HLA A1, B8, DR3 extended haplotypes in autoimmune chronic hepatitis

Genetic determinants of the autoimmune type of chronic active hepatitis include the major histocompatibility complex alleles HLA-B8 and HLA-DR3, which are usually present as the haplotype A1, B8, DR3. In certain other autoimmune diseases, an extended haplotype including complement alleles confers a greater relative risk than does B8, DR3. Hence, extended haplotypes were ascertained in autoimmune chronic active hepatitis by typing for HLA, complement alleles C4A, C4B, and Bf, and glyoxalase type 1 or 2. Eight of the 10 B8, DR3 haplotypes were A1, B8, DR3. Of the 8, 7 had the extended haplotype A1, B8, C4AQ0, C4B1, BfS, DR3, but this haplotype occurred in four instances with glyoxalase 2 and in three with glyoxalase 1. Thus, we find that in autoimmune chronic active hepatitis there is a high frequency of null alleles for complement but an extended haplotype does not cause any greater risk for disease than B8, DR3 alone.

A new storage medium for canine blood

A solution consisting of ascorbate phosphate, citric acid, sodium citrate, sodium phosphate, and dextrose was developed to extend the shelf life of canine blood stored for transfusion. The 24-hour poststorage viability remained above 70% for 6 weeks of storage at 4 C. The concentration of 2,3 diphosphoglycerate remained constant for 3 weeks, then declined slowly. After 6 weeks of storage, the 2,3 diphosphoglycerate content was still sufficiently high to allow adequate dissociation of oxygen from oxyhemoglobin in vivo. It was concluded that blood stored up to 6 weeks in this solution would be safe to use for transfusion.