Localization of loci for hypoxanthine phosphoribosyltransferase and glucose-6-phosphate dehydrogenase and biochemical evidence of nonrandom X chromosome expression from studies of a human X-autosome translocation

An Overall View of the Functional and Structural Characterization of Glucose-6-Phosphate Dehydrogenase Variants in the Mexican Population

Glucose-6-phosphate dehydrogenase (G6PD) deficiency, affecting an estimated 500 million people worldwide, is a genetic disorder that causes human enzymopathies. Biochemical and genetic studies have identified several variants that produce different ranges of phenotypes; thus, depending on its severity, this enzymopathy is classified from the mildest (Class IV) to the most severe (Class I). Therefore, understanding the correlation between the mutation sites of G6PD and the resulting phenotype greatly enhances the current knowledge of enzymopathies' phenotypic and genotypic heterogeneity, which will assist both clinical diagnoses and personalized treatments for patients with G6PD deficiency. In this review, we analyzed and compared the structural and functional data from 21 characterized G6PD variants found in the Mexican population that we previously characterized. In order to contribute to the knowledge regarding the function and structure of the variants associated with G6PD deficiency, this review aimed to determine the molecular basis of G6PD and identify how these mutations could impact the structure, stability, and function of the enzyme and its relation with the clinical manifestations of this disease.

COVID-19 in G6PD-deficient patients, oxidative stress, and neuropathology

Glucose-6-phosphate dehydrogenase (G6PD) is an enzyme that regulates energy metabolism mainly through the pentose phosphate pathway (PPP). It is well known that this enzyme participates in the antioxidant/oxidant balance via the synthesis of energy-rich molecules: nicotinamide adenine dinucleotide phosphate reduced (NADPH), the reduced form of flavin adenine dinucleotide (FADH) and glutathione (GSH), controlling reactive oxygen species generation. Coronavirus disease 19 (COVID-19), induced by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is considered a public health problem which has caused approximately 4.5 million deaths since December 2019. In relation to the role of G6PD in COVID-19 development, it is known from the existing literature that G6PD-deficient patients infected with SARS-CoV-2 are more susceptible to thrombosis and hemolysis, suggesting that G6PD deficiency facilitates infection by SARS-CoV-2. In relation to G6PD and neuropathology, it has been observed that deficiency of this enzyme is also present with an increase in oxidative markers. In relation to the role of G6PD and the neurological manifestations of COVID-19, it has been reported that the enzymatic deficiency in patients infected with SARS-CoV-2 exacerbates the disease, and, in some clinical reports, an increase in hemolysis and thrombosis was observed when patients were treated with hydroxychloroquine (OH-CQ), a drug with oxidative properties. In the present work, we summarize the evidence of the role of G6PD in COVID-19 and its possible role in the generation of oxidative stress and glucose metabolism deficits and inflammation present in this respiratory disease and its progression including neurological manifestations.

Glucose-6-phosphate dehydrogenase deficiency

Glucose 6-phosphate dehydrogenase (G6PD) deficiency is 1 of the commonest human enzymopathies, caused by inherited mutations of the X-linked gene G6PD. G6PD deficiency makes red cells highly vulnerable to oxidative damage, and therefore susceptible to hemolysis. Over 200 G6PD mutations are known: approximately one-half are polymorphic and therefore common in various populations. Some 500 million persons with any of these mutations are mostly asymptomatic throughout their lifetime; however, any of them may develop acute and sometimes very severe hemolytic anemia when triggered by ingestion of fava beans, by any of a number of drugs (for example, primaquine, rasburicase), or, more rarely, by infection. Approximately one-half of the G6PD mutations are instead sporadic: rare patients with these mutations present with chronic nonspherocytic hemolytic anemia. Almost all G6PD mutations are missense mutations, causing amino acid replacements that entail deficiency of G6PD enzyme activity: they compromise the stability of the protein, the catalytic activity is decreased, or a combination of both mechanisms occurs. Thus, genotype-phenotype correlations have been reasonably well clarified in many cases. G6PD deficiency correlates remarkably, in its geographic distribution, with past/present malaria endemicity: indeed, it is a unique example of an X-linked human polymorphism balanced through protection of heterozygotes from malaria mortality. Acute hemolytic anemia can be managed effectively provided it is promptly diagnosed. Reliable diagnostic procedures are available, with point-of-care tests becoming increasingly important where primaquine and its recently introduced analog tafenoquine are required for the elimination of malaria.

Mutagenicity monitoring in humans: Global versus specific origin of mutations

An underappreciated aspect of human mutagenicity biomonitoring is tissue specificity reflected in different assays, especially those that measure events that can only occur in developing bone marrow (BM) cells. Reviewed here are 9 currently-employed human mutagenicity biomonitoring assays. Several assays measure chromosome-level events in circulating T-lymphocytes (T-cells), i.e., traditional analyses of aberrations, translocation studies involving chromosome painting and fluorescence in situ hybridization (FISH) and determinations of micronuclei (MN). Other T-cell assays measure gene mutations. i.e., hypoxanthine-guanine phosphoriboslytransferase (HPRT) and phosphoribosylinositol glycan class A (PIGA). In addition to the T-cell assays, also reviewed are those assays that measure events in peripheral blood cells that necessarily arose in BM cells, i.e., MN in reticulocytes; glycophorin A (GPA) gene mutations in red blood cells (RBCs), and PIGA gene mutations in RBC or granulocytes. This review considers only cell culture- or cytometry-based assays to describe endpoints measured, methods, optimal sampling times, and sample summaries of typical quantitative and qualitative results. However, to achieve its intended focus on the target cells where events occur, kinetics of the cells of peripheral blood that derive at some point from precursor cells are reviewed to identify body sites and tissues where the genotoxic events originate. Kinetics indicate that in normal adults, measured events in T-cells afford global assessments of in vivo mutagenicity but are not specific for BM effects. Therefore, an agent's capacity for inducing mutations in BM cells cannot be reliably inferred from T-cell assays as the magnitude of effect in BM, if any, is unknown. By contrast, chromosome or gene level mutations measured in RBCs/reticulocytes or granulocytes must originate in BM cells, i.e. in RBC or granulocyte precursors, thereby making them specific indicators for effects in BM. Assays of mutations arising directly in BM cells may quantitatively reflect the mutagenicity of potential leukemogenic agents.

Molecular genotyping of G6PD mutations and Duffy blood group in Afro-descendant communities from Brazilian Amazon

Glucose-6-phosphate dehydrogenase deficiency (G6PDd) and Duffy-negative blood group are two red blood cells variants that confer protection against malaria. In this study, the distribution of the most common G6PD variants (G6PD*A-, GGPD*A and G6PD Mediterranean) and the major alleles of the Duffy blood group (FY*A, FY*B and FY*BES) were investigated in an Afro-descendant population from state of Pará, Brazilian Amazon. G6PD variants and Duffy blood group alleles were determined by TaqMan SNP genotyping assay. Overall, molecular genotyping revealed the presence of G6PD variants in 126 (24%) of the individuals studied (5% male and 19% female), and frequencies of the G6PD*A- and G6PD*A alleles were 0.061 and 0.104, respectively. Duffy blood group genotyping showed that 24.3% of people were Duffy-negative and 41.3% were heterozygous for FY*BES. The frequency of allele FY*BES was 41.0%. The results emphasize the need to monitor G6PD deficiency for the use of primaquine in the routine care of the Afro-descendant communities of the Trombetas, Erepecuru and Cumná rivers, evaluating the risks of hemolytic crisis in case of recurrence of malaria in the region. In addition, the possible greater protection against malaria conferred by these erythrocyte polymorphisms deserves to be better investigated and explored among these Afro-descendants.

Biochemical Analysis of Two Single Mutants that Give Rise to a Polymorphic G6PD A-Double Mutant

Glucose-6-phosphate dehydrogenase (G6PD) is a key regulatory enzyme that plays a crucial role in the regulation of cellular energy and redox balance. Mutations in the gene encoding G6PD cause the most common enzymopathy that drives hereditary nonspherocytic hemolytic anemia. To gain insights into the effects of mutations in G6PD enzyme efficiency, we have investigated the biochemical, kinetic, and structural changes of three clinical G6PD variants, the single mutations G6PD A+ (Asn126AspD) and G6PD Nefza (Leu323Pro), and the double mutant G6PD A− (Asn126Asp + Leu323Pro). The mutants showed lower residual activity (≤50% of WT G6PD) and displayed important kinetic changes. Although all Class III mutants were located in different regions of the three-dimensional structure of the enzyme and were not close to the active site, these mutants had a deleterious effect over catalytic activity and structural stability. The results indicated that the G6PD Nefza mutation was mainly responsible for the functional and structural alterations observed in the double mutant G6PD A−. Moreover, our study suggests that the G6PD Nefza and G6PD A− mutations affect enzyme functions in a similar fashion to those reported for Class I mutations.

The potential protective effect of Commelina nudiflora L. against carbon tetrachloride (CCl4)-induced hepatotoxicity in rats, mediated by suppression of oxidative stress and inflammation

BACKGROUND

This study aims to assess the hepatoprotective potential of Commelina nudiflora against CCl4-induced hepatic injury in rats.

METHOD

Antioxidant activities were determined. Phytochemical analysis was performed by gas chromatography mass spectrometry (GCMS). In the in vivo study, Sprague Dawley rats were pretreated with C. nudiflora (150, 300, and 450 mg kg body weight (b.wt.)) once daily for 14 days followed by two doses of CCl4 (1 ml/kg b.wt.). After 2 weeks, the rats were sacrificed and hepatoprotective analysis was performed.

RESULTS

In vitro studies have shown that the extract possessed strong antioxidant activity and has ability to scavenge 2,2-diphenyl-2-picrylhydrazyl-free radicals effectively. GCMS analysis of the C. nudiflora extract revealed the presence of various bioactive compounds. Administration of C. nudiflora significantly reduced the impact of CCl4 toxicity on serum markers of liver damage, serum aspartate transaminase (AST), and alanine transaminase (ALT). C. nudiflora also increased antioxidant levels of hepatic glutathione (GSH) and antioxidant enzymes and ameliorated the elevated hepatic formation of malondialdehyde (MDA) induced by CCl4 in rats. Histopathological examination indicated that C. nudiflora protect the liver from the toxic effect of CCl4 and healed lesions such as necrosis, fatty degeneration, and hepatocyte injury as irregular lamellar organization and dilations in the endoplasmic reticulum. The immunohistochemical studies revealed that pretreatment of C. nudiflora decreased the formation of 4-hydroxy-2-nonenal (HNE)-modified protein adducts and 8-hydroxy-2'-deoxyguanosine (8-OHdG). Furthermore, overexpression of the proinflammatory cytokines TNF-α, IL-6, and prostaglandin E2 is also reduced.

CONCLUSION

These findings exhibited the potential prospect of C. nudiflora as functional ingredients to prevent ROS-related liver damage.

Glucose-6-Phosphate Dehydrogenase: Update and Analysis of New Mutations around the World

Glucose-6-phosphate dehydrogenase (G6PD) is a key regulatory enzyme in the pentose phosphate pathway which produces nicotinamide adenine dinucleotide phosphate (NADPH) to maintain an adequate reducing environment in the cells and is especially important in red blood cells (RBC). Given its central role in the regulation of redox state, it is understandable that mutations in the gene encoding G6PD can cause deficiency of the protein activity leading to clinical manifestations such as neonatal jaundice and acute hemolytic anemia. Recently, an extensive review has been published about variants in the g6pd gene; recognizing 186 mutations. In this work, we review the state of the art in G6PD deficiency, describing 217 mutations in the g6pd gene; we also compile information about 31 new mutations, 16 that were not recognized and 15 more that have recently been reported. In order to get a better picture of the effects of new described mutations in g6pd gene, we locate the point mutations in the solved three-dimensional structure of the human G6PD protein. We found that class I mutations have the most deleterious effects on the structure and stability of the protein.

Functional and Biochemical Characterization of Three Recombinant Human Glucose-6-Phosphate Dehydrogenase Mutants: Zacatecas, Vanua-Lava and Viangchan

Glucose-6-phosphate dehydrogenase (G6PD) deficiency in humans causes severe disease, varying from mostly asymptomatic individuals to patients showing neonatal jaundice, acute hemolysis episodes or chronic nonspherocytic hemolytic anemia. In order to understand the effect of the mutations in G6PD gene function and its relation with G6PD deficiency severity, we report the construction, cloning and expression as well as the detailed kinetic and stability characterization of three purified clinical variants of G6PD that present in the Mexican population: G6PD Zacatecas (Class I), Vanua-Lava (Class II) and Viangchan (Class II). For all the G6PD mutants, we obtained low purification yield and altered kinetic parameters compared with Wild Type (WT). Our results show that the mutations, regardless of the distance from the active site where they are located, affect the catalytic properties and structural parameters and that these changes could be associated with the clinical presentation of the deficiency. Specifically, the structural characterization of the G6PD Zacatecas mutant suggests that the R257L mutation have a strong effect on the global stability of G6PD favoring an unstable active site. Using computational analysis, we offer a molecular explanation of the effects of these mutations on the active site.

Mutations of Glucose-6-Phosphate Dehydrogenase Durham, Santa-Maria and A+ Variants Are Associated with Loss Functional and Structural Stability of the Protein

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common enzymopathy in the world. More than 160 mutations causing the disease have been identified, but only 10% of these variants have been studied at biochemical and biophysical levels. In this study we report on the functional and structural characterization of three naturally occurring variants corresponding to different classes of disease severity: Class I G6PD Durham, Class II G6PD Santa Maria, and Class III G6PD A+. The results showed that the G6PD Durham (severe deficiency), and the G6PD Santa Maria and A+ (less severe deficiency) (Class I, II and III, respectively) affect the catalytic efficiency of these enzymes, are more sensitive to temperature denaturing, and affect the stability of the overall protein when compared to the wild type WT-G6PD. In the variants, the exposure of more and buried hydrophobic pockets was induced and monitored with 8-Anilinonaphthalene-1-sulfonic acid (ANS) fluorescence, directly affecting the compaction of structure at different levels and probably reducing the stability of the protein. The degree of functional and structural perturbation by each variant correlates with the clinical severity reported in different patients.

A modified intraoral resin mouthguard to prevent self-mutilations in lesch-nyhan patients

Lesch-Nyhan syndrome, described in 1964 by Lesch and Nyhan, is a X-linked recessive disorder, occurring in 1 : 100000 to 1 : 380000 live births. LNS is characterized by a decrease in activity of hypoxanthine guanine phosphoribosyl transferase, an enzyme involved in purine metabolism, resulting in overproduction of uric acid. Hyperuricemia and neurological features including choreoathetoid spasticity, self-mutilation, and mental retardation clinically characterize this syndrome. In LNS patients the typical feature is loss of tissue from biting themselves with partial or complete amputation of fingers, lips, and tongue. The self-mutilation compares with the eruption of the deciduous teeth. Several drugs trials have been administered to improve self-destructive behavior and invasive treatment approaches, such as extractions of teeth and orthognathic surgery, have been suggested with variable effectiveness. Nowadays prevention is, therefore, the standard of care. The role of dentistry is essential in the management of the self-mutilating behavior, because the teeth represent the main self-injury instrument. This report presents a revision of various therapeutic approaches to manage self-destruction, highlighting the effectiveness of a preventive treatment. It describes a new technique: a resin mouthguard, realized at Gaslini Hospital, to obtain immediate healing of the oral lesions, confirmed in the follow-up period.

Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency in Greek newborns: the Mediterranean C563T mutation screening

Glucose-6-Phosphate Dehydrogenase (G6PD) gene is located at the X-chromosome at Xq28 and the disease is recessively inherited predominantly in males. More than 400 variants have been proposed based on clinical and enzymatic studies. The aim of the current study was to identify C563T mutation in G6PD-deficient newborns and to correlate the enzyme residual activity with the presence of the mutation. Some 1189 full-term neonates aged 3-5 days old were tested for G6PD activity in dried blood spots from Guthrie cards using a commercial kit. DNA extraction from Guthrie cards and mutation identification among the deficient samples were performed with current techniques. A total of 92 (7.7%) newborns were G6PD-deficient. In 46 (50%), the mutation C563T was identified. The residual activity in C563T hemizygote males (n = 28) was statistically significantly lower (1.23 ± 0.93 U/g Hb) than that in non-C563T G6PD-deficient males (n = 25) (4.01 ± 1.20 U/g Hb, p < 0.0001) and in controls (13.6 ± 2.9 U/g Hb, p < 0.0001). In C563T heterozygote females, the estimated enzyme activity was lower than that determined in non-C563T females. Male C563T hemizygotes suffer from G6PD deficiency and severe neonatal jaundice. G6PD activity showed statistically significant correlation with total bilirubin blood levels.

Molecular characterization of glucose-6-phosphate dehydrogenase deficiency in Pakistani population

INTRODUCTION

Glucose-6-phosphate dehydrogenase (G6PD; E.C. 1.1.1.49) deficiency is the commonest inborn error of metabolism with more than 140 genetic variants. The incidence of G6PD deficiency is 2-9% in Pakistan, but G6PD variants were never studied comprehensively. We therefore designed this study to describe the frequency of G6PD variants and their associated enzyme activities in Pakistan.

METHODS

Patients diagnosed with G6PD deficiency were enrolled. RFLP-PCR was utilized to identify common mutations previously reported from Asian countries. Where mutational analysis failed, amplification of 9-12 exons with subsequent gene sequencing was performed. G6PD enzyme activity was assessed through the quantitative enzyme assay.

RESULTS

Two hundred and seventy-six G6PD-deficient subjects (237 male and 39 women) were investigated. G6PD Mediterranean (563C-T) was the most common genetic variant (n=216 or 78%). G6PD Chatham (1003A-G) and G6PD Orissa (131C-G) were observed in 14 (5%) and two (0.7%) subjects respectively. A novel mutation 973 G-A with a predicated amino acid change of asp325asn was identified in exon 9. This was named G6PD Karachi after the place of origin of proband. Polymorphism in position 1311C/T was uniformly observed with all variants. Forty-three or 17% of DNA samples remained uncharacterized. Very low levels of G6PD enzyme activity was observed with 563C-T mutation.

CONCLUSION

We concluded that 563C-T was the commonest G6PD variant, while 1003A-G and 131C-G were less-frequent genetic variants of G6PD in Pakistani population. A novel genetic variant 973G-A was also identified. Very low levels of G6PD enzyme activity was seen with G6PD 563C-T. Mutational analysis failed in a significant proportion of samples warranting further work.

Population study of 1311 C/T polymorphism of Glucose 6 Phosphate Dehydrogenase gene in Pakistan - an analysis of 715 X-chromosomes

Background

Nucleotide 1311 polymorphism at exon 11 of G6PD gene is widely prevalent in various populations of the world. The aim of the study was to evaluate 1311 polymorphism in subjects carrying G6PD Mediterranean gene and in general population living in Pakistan.

Results

Patients already known to be G6PD deficient were tested for 563C-T (G6PD Mediterranean) and 1311 C-T mutation through RFLP based PCR and gene sequencing. A control group not known to be G6PD deficient was tested for 1311C/T only.

C-T transition at nt 1311 was detected in 60/234 X-chromosomes with 563 C-T mutation (gene frequency of 0.26) while in 130 of normal 402 X-chromosomes (gene frequency of 0.32).

Conclusion

We conclude that 1311 T is a frequent polymorphism both in general populations and in subjects with G6PD Mediterranean gene in Pakistan. The prevalence is higher compared to most of the populations of the world. The present study will help in understanding genetic basis of G6PD deficiency in Pakistani population and in developing ancestral links of its various ethnic groups.

Genomic analysis of cancer tissue reveals that somatic mutations commonly occur in a specific motif

Somatic mutations are hallmarks of cancer progression. We sequenced 26 matched human prostate tumor and constitutional DNA samples for somatic alterations in the SRD5A2, HPRT, and HSD3B2 genes, and identified 71 nucleotide substitutions. Of these substitutions, 79% (56/71) occur within a WKVnRRRnVWK sequence (a novel motif we call THEMIS [from the ancient Greek goddess of prophecy]: W=A/T, K=G/T, V=G/A/C, R=purine (A/G), and n=any nucleotide), with one mismatch allowed. Literature searches identified this motif with one mismatch allowed in 66% (37/56) of the somatic prostate cancer mutations and in 74% (90/122) of the somatic breast cancer mutations found in all human genes analyzed. We also found the THEMIS motif with one allowed mismatch in 88% (23/26) of the ras1 gene somatic mutations formed in the sensitive to skin carcinogenesis (SENCAR) mouse model, after induction of error-prone DNA repair following mutagenic treatment. The high prevalence of the motif in each of the above mentioned cases cannot be explained by chance (P<0.046). We further identified 27 somatic mutations in the error-prone DNA polymerase genes pol eta, pol kappa, and pol beta in these prostate cancer patients. The data suggest that most somatic nucleotide substitutions in human cancer may occur in sites that conform to the THEMIS motif. These mutations may be caused by "mutator" mutations in error-prone DNA polymerase genes.

Three-dimensional modeling of glucose-6-phosphate dehydrogenase-deficient variants from German ancestry

BACKGROUND

Loss of function of dimeric glucose-6-phosphate dehydrogenase (G6PD) represents the most common inborn error of metabolism throughout the world affecting an estimated 400 million people. In Germany, this enzymopathy is very rare.

METHODOLOGY/PRINCIPAL FINDINGS

On the basis of G6PD crystal structures, we have analyzed six G6PD variants of German ancestry by three-dimensional modeling. All mutations present in the German population are either close to one of the three G6P or NADP(+) units or to the interface of the two monomers. Two of the three mutated amino acids of G6PD Vancouver are closer to the binding site of NADP(+). The G6PD Aachen mutation is also closer to the second NADP(+) unit. The G6PD Wayne mutation is closer to the G6P binding region. These mutations may affect the binding of G6P and NADP(+) units. Three mutations, i.e. G6PD Munich, G6PD Riverside and G6PD Gastonia, lie closer to the interface of the two monomers. These may also affect the interface of two monomers.

CONCLUSION

None of these G6PD variants share mutations with the common G6PD variants known from the Mediterranean, Near East, or Africa indicating that they have developed independently. The G6PD variants have been compared with mutants from other populations and the implications for survival of G6PD variants from natural selection have been discussed.

A new, X-linked endothelial corneal dystrophy

PURPOSE

To describe the clinical spectrum, the histopathologic findings obtained from one corneal button, and the genetic mapping of an X-linked endothelial corneal dystrophy (XECD).

DESIGN

Observational case series and experimental study.

METHODS

We examined a total of 60 members of a family with this dystrophy at the slit-lamp. Light and electron microscopic findings of the corneal button were recorded following one male patient's penetrating keratoplasty. A panel of 25 microsatellite markers covering the X chromosome was typed in genomic DNA from 50 family members. The data were analyzed using the ALLEGRO program to obtain two-point and multipoint likelihood of the odds (LOD) scores and to generate haplotypes.

RESULTS

A total of 35 trait carriers were identified in four generations of the family. Nine male patients demonstrated severe corneal opacifications: two congenital corneal cloudings in form of ground glass, milky appearance and seven subepithelial band keratopathies combined with endothelial changes resembling moon craters. Twenty-two female and four male patients disclosed only endothelial alterations resembling moon craters. No instance of male-to-male transmission of the disease was encountered in the family. Light and electron microscopy disclosed focal discontinuities and degeneration of the endothelial cell layer and marked thickening of Descemet's membrane. Multipoint analysis showed linkage with a maximum LOD score of 10.90 between markers DXS8057 and DXS1047.

CONCLUSIONS

To the best of our knowledge, this represents the first fully documented report of X-linked inheritance of an endothelial corneal dystrophy. Late subepithelial band keratopathy is a landmark of XECD. A locus for this corneal dystrophy maps to Xq25.

Reliability of quantitative and qualitative tests to identify heterozygotes carrying severe or mild G6PD deficiency

OBJECTIVES

To determine if the qualitative test (cytochemical) is more reliable than the quantitative test (differential pH-metry) in identifying heterozygous G6PD- subjects.

DESIGN AND METHODS

Identification of heterozygous G6PD- subjects was done by the measurement of G6PD activity and by a cytochemical test.

RESULTS

A cytochemical test is more sensitive than differential pH-metry to identify heterozygous G6PD- subjects.

CONCLUSIONS

A cytochemical test is a reliable method for mass screening for G6PD deficiency to identify G6PD- heterozygotes.

Lesch-Nyhan disease in a female with a clinically normal monozygotic twin

Lesch-Nyhan disease (LND) is an inborn error of purine metabolism caused by defective activity of the enzyme hypoxanthine guanine phosphoribosyl transferase (HPRT, EC 2.4.2.8), resulting from mutation in the corresponding gene on the long arm of the X chromosome (Xq26). The classic phenotype occurs almost exclusively in males and is characterized by hyperuricemia, mental retardation, severe dystonia, and self-injurious behavior. Heterozygous carrier females are usually clinically normal. However, a small number of clinically affected females have been described. In all previous cases there was a mutation in one HPRT allele and non-random inactivation of the X chromosome carrying the normal HPRT gene. We have analyzed a female MZ twin pair discordant for Lesch-Nyhan disease. The mother and both twins are heterozygous carriers of a HPRT splicing mutation (IVS8 + 4A > G; c.609 + 4A > G) and all three express the mutant allele at similar frequencies in peripheral blood T cells. The mother and one sister are clinically normal. In the affected twin, the clinical phenotype is classical for Lesch-Nyhan disease, despite the fact that HPRT activity in the blood was also normal. X inactivation analysis showed a skewed pattern in the fibroblasts of the affected twin sister, with the X chromosome carrying the normal HPRT allele preferentially inactivated. As in many other reported cases of X-linked diseases, the discordant phenotype of the two monozygous twin sisters suggests that the process responsible for monozygotic twinning can trigger skewed X inactivation.

Distinct mechanisms lead to HPRT gene mutations in leukemic cells

Leukemias are considered malignant clonal disorders arising from the accumulation of mutations in hematopoietic cells; the majority of these mutations are thought to be acquired somatically. Measurement of mutation frequency (Mf) at the hypoxanthine phosphoribosyltransferase (HPRT) locus has been developed as a method for estimating genomic instability. We investigated the Mf in 16 leukemic cell lines to determine whether these cell lines showed evidence of genomic instability. Although some leukemic cell lines had markedly elevated Mfs, the Mfs at the HPRT locus in leukemic cell lines were not always higher than those of B-lymphoblastoid cell lines and T lymphocytes from normal individuals. We were able to identify the HPRT mutation for 159 of 160 individual HPRT mutants. The HPRT mutations were characterized at a molecular level and classified as either gross chromosomal rearrangements (GCRs) or point mutations, such as single-nucleotide substitutions, insertions, or deletions. With rare exceptions, individual leukemic cell lines showed either point mutations or GCR, but not both. Of note, all the cell lines that primarily showed point mutations are known to be defective in mismatch repair machinery.

Effects of oxidative stress and inhibitors of the pentose phosphate pathway on sexually dimorphic production of IFN-? by bovine blastocysts

Bovine interferon-tau (IFN-tau), the anti-luteolytic factor secreted by conceptuses of pecoran ruminants, is a product of autosomal genes, yet in vitro produced (IVP) female expanded blastocysts (EB) secrete about twice as much IFN-tau as males. Two possible explanations have been tested here. One is that embryos of one sex are differentially susceptible to oxidative stress. The second is that female EB produce more IFN-tau because pentose-phosphate pathway (PPP) activity is elevated as a result of delayed X-chromosome inactivation. IVP bovine zygotes were cultured to the 8-cell stage and placed under conditions designed either to promote oxidative stress (+/-H2O2; 20 vs. 5% O2), or to inhibit glucose 6-phosphate dehydrogenase (G6PDH) activity (addition of dehydroepiandrosterone, DHEA or 6-aminonicotinamide, 6-AN to the medium). At day 8, blastocysts were cultured individually for a further 48 hr to assess IFN-tau production, and embryo sex determined retrospectively. Blastocyst numbers were reduced (P < 0.05) and their continued development impaired (P < 0.05) in presence of H2O2 (200 microM) and 20% O2, but neither IFN-tau production nor sexually dimorphic expression of IFN-tau were affected. IFN-tau production was reduced, particularly in females (P < 0.05), and sexual dimorphic differences in production were lost in the presence of both DHEA (100 microM) and 6-AN (1 microM). In the case of 6-AN, these effects were achieved without a significant decline in blastocyst developmental progression, quality, or cell number. The data suggest that the higher production of IFN-tau by female EB is an indirect outcome of the increased activity of the oxidative arm of the PPP pathway.

Introduction of single base substitutions at homologous chromosomal sequences by adeno-associated virus vectors

Adeno-associated virus (AAV) vectors can modify homologous chromosomal sequences at high rates. This gene targeting transduction pathway is distinct from the integrating and episomal pathways used in gene addition approaches. In previous studies, AAV vectors were used to introduce small insertion and deletion mutations at homologous chromosomal loci. Here we show that AAV-mediated gene targeting can also be used to introduce all possible types of single base substitution mutations at the endogenous single-copy hypoxanthine phosphoribosyl transferase locus. Southern blot and sequence analysis showed that the point mutations were introduced with high fidelity. We also show that AAV vectors can repair chromosomal alkaline phosphatase genes containing point mutations. Our results suggest that AAV vectors can be used to introduce single base substitutions at high frequencies in normal human cells, including the correction of point mutations responsible for genetic diseases.

Genetic disorders and urolithiasis

A recent analysis of the McKusick's On-Line Mendelian Inheritance in Man (OMIM) database revealed over 30 genetic or putatively genetic conditions in which urolithiasis contributes to the disease pathology at least to some extent. There is wide clinical, biochemical, and genetic heterogeneity in many of these conditions.

Several mutations including two novel mutations of the glucose-6-phosphate dehydrogenase gene in Polish G6PD deficient subjects with chronic nonspherocytic hemolytic anemia, acute hemolytic anemia, and favism

DNA sequencing revealed seven different glucose-6-phosphate dehydrogenase (G6PD) mutations in G6PD deficient subjects from 10 Polish families. Among them we found two novel mutations: 679C-->T (G6PD Radlowo, class 2) and a 1006A-->G (G6PD Torun, class 1). Variant G6PD Radlowo was characterized biochemically. Both novel mutations were analyzed using a model of the tertiary structure of the human enzyme. The main chain of G6PD Torun is different from the wild-type G6PD. The remaining mutations identified by us in deficient Polish patients were: 542A-->T (G6PD Malaga), 1160G-->A (G6PD Beverly Hills), 1178G-->A (G6PD Nashville), 1192G-->A (G6PD Puerto Limon), and 1246G-->A (G6PD Tokyo). Variant Tokyo was found in four families. In one of them favism was the first clinical sign of G6PD deficiency and chronic nonspherocytic hemolytic anemia (CNSHA) was diagnosed later. Variants G6PD Nashville and G6PD Puerto Limon were accompanied by the silent mutation 1311C-->T of the G6PD gene.

High-resolution landmark framework for the sequence-ready mapping of Xq23-q26.1

We have established a landmark framework map over 20-25 Mb of the long arm of the human X chromosome using yeast artificial chromosome (YAC) clones. The map has approximately one landmark per 45 kb of DNA and stretches from DXS7531 in proximal Xq23 to DXS895 in proximal Xq26, connecting to published framework maps on its proximal and distal sides. There are three gaps in the framework map resulting from the failure to obtain clone coverage from the YAC resources available. Estimates of the maximum sizes of these gaps have been obtained. The four YAC contigs have been positioned and oriented using somatic-cell hybrids and fluorescence in situ hybridization, and the largest is estimated to cover approximately 15 Mb of DNA. The framework map is being used to assemble a sequence-ready map in large-insert bacterial clones, as part of an international effort to complete the sequence of the X chromosome. PAC and BAC contigs currently cover 18 Mb of the region, and from these, 12 Mb of finished sequence is available.

High-Resolution Landmark Framework for the Sequence-Ready Mapping of Xq23–q26.1

We have established a landmark framework map over 20–25 Mb of the long arm of the human X chromosome using yeast artificial chromosome (YAC) clones. The map has approximately one landmark per 45 kb of DNA and stretches from DXS7531 in proximal Xq23 to DXS895 in proximal Xq26, connecting to published framework maps on its proximal and distal sides. There are three gaps in the framework map resulting from the failure to obtain clone coverage from the YAC resources available. Estimates of the maximum sizes of these gaps have been obtained. The four YAC contigs have been positioned and oriented using somatic-cell hybrids and fluorescence in situ hybridization, and the largest is estimated to cover ∼15 Mb of DNA. The framework map is being used to assemble a sequence-ready map in large-insert bacterial clones, as part of an international effort to complete the sequence of the X chromosome. PAC and BAC contigs currently cover 18 Mb of the region, and from these, 12 Mb of finished sequence is available.

CD40 ligand expression deficiency in a female carrier of the X-linked hyper-IgM syndrome as a result of X chromosome lyonization

We report on the case of a girl with an immune deficiency characterized by recurrent infections of the upper and lower respiratory tract, low IgG and IgA serum levels as well as deficiency of the in vivo antibody response. Since this patient is the sister of a boy affected with a hyper-IgM syndrome due to a defect in CD40 ligand (CD40L) expression, the involvement of CD40L in this phenotypic expression was investigated. A very low fraction of activated T cells (5%) in this female patient expressed CD40L. This resulted from the presence of a heterozygous CD40L nonsense mutation associated with a skewed pattern of X chromosome inactivation as determined by methylation pattern analysis. Although carriers of X-linked hyper-IgM are considered to be asymptomatic, this study indicates that extreme lyonization of the normal X can lead to a mild expression of the hyper-IgM syndrome which is similar to common variable immune deficiency (CVID). Therefore, it is possible that some cases of CVID in females represent partial deficiency of CD40L expression in carriers of the CD40L mutation.

The molecular basis of haemophilia B

Over the past 15 years, our knowledge of the molecular basis of haemophilia B has increased dramatically. Following the cloning and characterization of the factor IX gene in 1982, major advances have been made in documenting the molecular pathology that underlies this condition. This review will summarize the current state of information in this area, and the reader is referred to the Haemophilia B Mutation Database World Wide Web site at http://www.umds.ac.uk/molgen/haemBdatabase for a complete current listing of the mutations that cause this phenotype. In addition, other recent reviews have discussed complementary issues relating to this topic.

Nature of mutation in the human hprt gene following in vivo exposure to ionizing radiation of cesium-137

The current study comprises the analysis of mutations in 10 individuals accidentally exposed to cesium-137 during the 1987 radiological accident in Goiânia, Brazil. Their exposures were among the highest experienced, ranging from 1 to 7 Gy. Peripheral T-lymphocyte samples were obtained 3.3 years after the original exposure and mutation was studied at the hprt locus using the 6-thioguanine-resistance selection assay. The mutational spectrum for the exposed population is comprised of 90 independent mutants. Based on T-cell receptor analysis, only 5% (5/95) were clonally related. Mutants were initially studied using RT-PCR and directly sequenced using an automated laser fluorescent DNA sequencer. Mutants that repeatedly failed to produce cDNAs were studied using a multiplex PCR assay with genomic DNA. Missense mutations were the most frequent event recovered, comprising 40% (23/57) of the spectral sample. An excess of events involving A:T base pairs was observed, exhibiting a significant difference (chi 2 = 12.7, P = 0.0004) when compared to the spontaneous spectrum. This finding may reflect the effect of ionizing radiation-induced damage, suggesting a potential similarity to radiation effects in prokaryotes. At the genomic level, 36.7% (33/90) of the mutants exhibited gross structural alterations, as detected by multiplex PCR. Deletion events were over-represented in our spectral sample, displaying a twofold increase when compared to the frequency observed in the spontaneous mutation database.

Novel nonsense mutation in the hypoxanthine guanine phosphoribosyltransferase gene and nonrandom X-inactivation causing Lesch-Nyhan syndrome in a female patient

Lesch-Nyhan (LN) disease is a severe X-linked recessive neurological disorder associated with a loss of hypoxanthine guanine phosphoribosyltransferase activity (HPRT, EC 2.4.2.8). We have studied the second example of a female patient with LN disease. The molecular basis of HPRT deficiency in this patient was a previously undescribed nucleotide substitution in exon 6. In this gene, designated HPRT PARIS, a single nucleotide substitution from T to G at base position 558 changed a tyrosine (TAT) to a codon STOP (TAG) (Y153X). Analysis of the mother revealed a normal sequence of the HPRT cDNA and demonstrated that this mutation arose through a de novo gametic event. Allele-specific amplification of exon 6 from the patient's genomic DNA confirmed the single base substitution and showed that the patient was heterozygous for this mutation. Investigation of X-chromosomal inactivation by comparison of methylation patterns of patient's DNA isolated from fibroblasts, T lymphocytes, and polymorphonuclear cells digested with PstI and BstXI, with or without HpaII, and hybridized with M27 beta probe indicated a nonrandom pattern of X-chromosomal inactivation in which there was preferential inactivation of the maternal allele. The data indicate that nonrandom X-inactivation leading to selective inactivation of the maternal gene and a de novo point mutation in the paternal gene were responsible for the lack of HPRT activity in this patient.

Muscle expression of glucose-6-phosphate dehydrogenase deficiency in different variants

Muscle expression of G6PD deficiency has been investigated in Mediterranean, Seattle-like and A-variants. G6PD activity was detected in samples obtained from biopsies on the quadriceps muscle of seven males and one female. The type of genetic variant was determined by molecular analysis of DNA, extracted from blood samples. All variants showed the enzyme defect in muscle. A statistically significant relationship was found in the activity of G6PD between erythrocytes and muscle of the male subjects (r = 0.968; p = 0.00008). The equation for the best fit line was: Y = 0.390X + 0.198. The results suggest that, for a given variant, the extent of the enzyme defect in muscle may be determined, using this equation, from the G6PD activity of erythrocytes.

Molecular characterization of a deleted X chromosome (Xq13.3-Xq21.31) exhibiting random X inactivation

As a result of selection following random X chromosome inactivation in human females, X chromosomes with visible deletions are usually inactive in every somatic cell. We have studied a female with mental retardation and dysmorphic features whose karyotype includes an X chromosome with a visible interstitial deletion in the proximal long arm. Based on cytogenetic analysis, the proximal breakpoint appeared to be in band Xq13.1, and the distal one in band q21.3. However, molecular analyses show that less of the q13 band is missing than cytogenetic studies indicated, as the deletion includes only loci from the region Xq13.3 to Xq21.31. Unexpectedly, studies of chromosome replication show that the pattern of X inactivation is random. Whereas the deleted X chromosome is late replicating in some cells from all tissues studied, it is early replicating in the majority of blood lymphocytes and skin fibroblasts, and is the active X chromosome in many of the hybrids derived from skin fibroblasts. As this chromosome is able to inactivate, it must include those DNA sequences from the X-inactivation center (XIC) that are essential for cis X inactivation. Molecular studies show that the XIC region, at Xq13.2, is present, so it is unlikely that the lack of consistent inactivation of this chromosome is attributable to close proximity of the breakpoint to the XIC. Supporting this conclusion is the similarity of the breakpoints to those of the other chromosomes we studied, whose deletions clearly do not interfere with the ability to inactivate. Our results show that deletions distal to DXS441 in Xq13.2 do not interfere with cis X inactivation. We attribute the random pattern of X inactivation reported here to the fact that in the tissues studied, cells with this interstitial deletion are not at a selective disadvantage.

Characterization of spontaneous and induced mutations in SV40-transformed normal and ataxia telangiectasia cell lines

Spontaneous and induced mutations at the HPRT locus were analyzed in one normal (MRC5CV1) and one ataxia telangiectasia (AT5BIVA) SV40-transformed cell line derived from male donors. Multiplex PCR and Southern analyses revealed a high frequency of spontaneous deletion mutations that may be a consequence of the SV40 transformation. Four mutagens (ethyl methanesulfonate, bromodeoxyuridine, bleomycin, adriamycin), which differ in their types of primary DNA lesions, caused specific patterns of mutations. By using fluorescence in situ hybridization (FISH) techniques, we were able to show that more than 90% of the AT5BIVA cells contained two X chromosomes with HPRT alleles, while in more than 90% of the MRC5CV1 cells genomic hemizygosity for the HPRT gene was found. Taking into account these findings we found that the AT5BIVA cell line possesses spontaneous hypermutability as well as hypersensitivity and hypermutability to bleomycin (BLM) and adriamycin (AM). Both mutagens induced deletion mutations in both cell lines, but more complex mutations and larger deletions were found in AT5BIVA cells.

Variants of glucose-6-phosphate dehydrogenase are due to missense mutations spread throughout the coding region of the gene

Glucose-6-phosphate dehydrogenase (G6PD) is remarkable for its genetic diversity in humans. Many variants of G6PD have been described with wide ranging levels of enzyme activity and associated clinical symptoms. Fifty-eight different mutations have now been identified and these account for 97 named G6PD variants. The mutations are almost exclusively missense mutations, causing single amino acid substitutions. They are spread throughout the coding region of the gene, although there appears to be a cluster of mutations that cause a more severe clinical phenotype towards the 3' end of the gene. The absence of large deletions, frameshift mutations and nonsense mutations is consistent with the notion that a total lack of G6PD activity would be lethal.

Screening for molecular pathologies in Lesch-Nyhan syndrome

Heteroduplex detection by hydrolink gel electrophoresis was performed to screen for small mutations in 12 Lesch-Nyhan syndrome families with characterised molecular pathology which included nine point mutations, two small deletions, and a 1-bp insertion. This modified protocol for heteroduplex detection by hydrolink gel electrophoresis detected all 12 of these mutations and was utilised to rapidly determine the carrier status of females from affected families. On the basis of these results this approach appears to be a rapid and reliable screening method for point mutations in addition to small length mutations and for carrier detection in Lesch-Nyhan syndrome.

A review of the molecular basis of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency

Hypoxanthine-guanine phosphoribosyltransferase (HPRT, EC 2.4.2.8) is a purine salvage enzyme that catalyses the conversion of hypoxanthine and guanine to their respective mononucleotides. Partial deficiency of this enzyme can result in the overproduction of uric acid leading to a severe form of gout, whilst a virtual absence of HPRT activity causes the Lesch-Nyhan syndrome which is characterised by hyperuricaemia, mental retardation, choreoathetosis and compulsive self-mutilation. The HPRT-encoding gene is located on the X chromosome in the region q26-q27 and consists of nine exons and eight introns totalling 57 kb. This gene is transcribed to produce an mRNA of 1.6 kb, which contains a protein encoding region of 654 nucleotides. With the advent of increasingly refined techniques of molecular biology, it has been possible to study the HPRT gene of individuals with a deficiency in HPRT activity to determine the genetic basis of the enzyme deficiency. Many different mutations throughout the coding region have been described, but in the absence of precise information on the three-dimensional structure of the HPRT protein, it remains difficult to determine any consistent correlation between the structure and function of the enzyme.