X-linked dyskeratosis congenita is caused by mutations in a highly conserved gene with putative nucleolar functions.

X-linked recessive dyskeratosis congenita (DKC) is a rare bone-marrow failure disorder linked to Xq28. Hybridization screening with 28 candidate cDNAs resulted in the detection of a 3' deletion in one DKC patient with a cDNA probe (derived from XAP101). Five different missense mutations in five unrelated patients were subsequently identified in XAP101, indicating that it is the gene responsible for X-linked DKC (DKC1). DKC1 is highly conserved across species barriers and is the orthologue of rat NAP57 and Saccharomyces cerevisiae CBF5. The peptide dyskerin contains two TruB pseudouridine (psi) synthase motifs, multiple phosphorylation sites, and a carboxy-terminal lysine-rich repeat domain. By analogy to the function of the known dyskerin orthologues, involvement in the cell cycle and nucleolar function is predicted for the protein.

Dyskeratosis congenita: the diverse clinical presentation of mutations in the telomerase complex.

Dyskeratosis congenita is an inherited syndrome characterised by mucocutaneous features, bone marrow failure, an increased risk of malignancy and other somatic abnormalities. There is a considerable range of clinical severity and in its occult form the disease may present as idiopathic aplastic anaemia. Genes responsible for X-linked, autosomal dominant and autosomal recessive forms of the disease have been identified and been found to encode products involved in telomere maintenance. Premature shortening of telomeres could account for the pathology, affecting the tissues that turn over most rapidly. However, the protein that is mutated in the X-linked disease, dyskerin, also plays a fundamental role in ribosome biogenesis, directing the pseudouridylation of ribosomal RNA using H/ACA small nucleolar RNAs as guides. Heterozygous mutations in the RNA component of telomerase (TERC) cause the autosomal dominant form of the disease through haploinsufficiency. Disease anticipation described in these families is associated with progressive telomere shortening through the generations. Heterozygous mutations in the reverse transcriptase component of telomerase (TERT) have a more variable role, often displaying incomplete penetrance and diverse clinical presentation. The autosomal recessive form of the disease is genetically heterogeneous, although one sub-type has been described in which NOP10 is mutated. This small protein is also associated with the maturation of ribosomal RNA and the telomerase complex.

Cloning of the glucose 6-phosphate dehydrogenase gene from Plasmodium falciparum.

Glucose 6-phosphate dehydrogenase (G6PD) deficiency is one of the human genetic traits that confer relative resistance against malaria caused by Plasmodium falciparum. It has been previously shown that this organism, during its intraerythrocytic development, produces its own G6PD, which has properties different from those of human G6PD. In order to investigate the role of this enzyme in parasite-host cell interactions, we have isolated the G6PD gene from Plasmodium falciparum as a set of overlapping lambda gt11 clones. By sequence analysis we have found a single open reading frame, uninterrupted by introns, coding for a protein of 910 amino acids, almost twice as long as any previously sequenced G6PD molecule. The P. falciparum G6PD mRNA is 5.1 kb in size and has an exceptionally long 5' untranslated region of some 1000 nucleotides. We have mapped the G6PD gene to chromosome 14. The C-terminal portion of the predicted protein, from amino acid 310-910 (except for an 'insert' of 62 amino acids), has 39% homology to human G6PD, with a number of characteristic, fully conserved peptides. The N-terminal portion of the predicted protein has no homology to G6PD, but it contains a peptide in which 7 out of 12 amino acids are identical to the putative glutathione binding site of human glutathione S-transferase.

Identification and characterization of the novel FAD-binding lobe G75S mutation in cytochrome b(5) reductase: an aid to determine recessive congenital methemoglobinemia status in an infant.

NADH-cytochrome b(5) reductase deficiency results clinically in either type I or type II recessive congenital methemoglobinemia. The more severe type II form is associated with a global deficiency of cytochrome b(5) reductase and is characterized by cyanosis with neurological dysfunction. In contrast, the only symptom for type I is cyanosis. We have identified a novel G to A mutation at position 15,635 in the DIAI gene of a 4-month-old baby that results in a glycine to serine substitution at codon 75 in the cytochrome b(5) reductase protein. The G75S mutation, located in the FAD-binding lobe of cytochrome b(5) reductase, was found in association with the previously described V252M variant. The V252M mutation is present in the NADH-binding domain and associated with both types I and II recessive congenital methemoglobinemia. Since the G75S and V252M mutations represent radical changes in differing regions of cytochrome b(5) reductase, generating and characterizing these variants singly and in combination using a rat heterologous expression system would provide insight into the differences between types I and II disease at the molecular level. Although all three variants were found to retain stoichiometric levels of FAD with spectroscopic and thermodynamic properties comparable to those of native cytochrome b(5) reductase, all exhibited decreased catalytic efficiency and reduced protein stability reflecting the position of the mutations in the primary structure. The G75S variant retained only 11% of the catalytic efficiency of the wild-type enzyme. Thus, cytochrome b(5) reductase deficient patients who are heterozygous for either FAD- or NADH-binding lobe mutations can exhibit the clinically less severe type I phenotype.

Glucose-6-phosphate dehydrogenase deficiency.

Glucose-6-phosphate dehydrogenase (G6PD) is expressed in all tissues, where it catalyses the first step in the pentose phosphate pathway. G6PD deficiency is prevalent throughout tropical and subtropical regions of the world because of the protection it affords during malaria infection. Although most affected individuals are asymptomatic, there is a risk of neonatal jaundice and acute haemolytic anaemia, triggered by infection and the ingestion of certain drugs and broad beans (favism). A rare but more severe form of G6PD deficiency is found throughout the world and is associated with chronic non-spherocytic haemolytic anaemia. Many deficient variants of G6PD have been described. DNA sequence analysis has shown that the vast majority of these are caused by single amino acid substitutions. The three-dimensional structure of G6PD shows a classical dinucleotide binding domain and a novel beta + alpha domain involved in dimerization.

Severe hemolytic anemia associated with the homozygous state for an unstable hemoglobin variant (Hb Bushwick).

We have investigated a 13-year-old girl from first cousin parents who presented with severe hemolytic anemia. Hematologic studies showed unstable hemoglobin (Hb) disease (chronic Heinz body anemia), and DNA analysis showed that the patient was homozygous for the previously reported abnormal Hb called Hb Bushwick (beta 74E18 gly-->val). Hb Bushwick is unstable in vitro and in vivo. In addition, using globin chain biosynthetic studies, we show that the beta (Bushwick) chains are unstable. Six members of the patient's family were heterozygous for Hb Bushwick and had a compensated hemolytic disorder. By contrast, the homozygous patient had chronic anemia caused by a combination of hemolysis and ineffective erythropoiesis that was subject to severe exacerbation concomitant with infection. Thus, although unstable Hb disease is correctly regarded as dominant, we clearly see a dosage effect in its expression, whereby the homozygous state is still compatible with life although the red blood cells contain nearly 100% unstable Hb.

Molecular characterization of G6PD deficiency in Oman.

Screening of unselected university students in the Sultanate of Oman revealed an overall frequency of glucose-6-phosphate dehydrogenase (G6PD) deficiency of 26% in males. Samples from 23 G6PD-deficient individuals (a random sub-sample of the student population), were characterised biochemically and at the molecular level. Of 20 deficient men, 15 had G6PD Mediterranean, 2 had G6PD Chatham, 1 had G6PD A- and in 2 the mutation is not yet known. Of the 3 G6PD-deficient woman, 2 were homozygous for the G6PD Mediterranean mutation and 1 was a genetic compound, G6PD Mediterranean/G6PD A- (the first report of this genotype). Our findings establish that the G6PD Mediterranean mutation accounts for most cases of G6PD deficiency in Oman. The presence of G6PD A- at a polymorphic frequency can be regarded as evidence of significant gene flow from Africa.

Very short telomeres in the peripheral blood of patients with X-linked and autosomal dyskeratosis congenita.

Dyskeratosis congenita (DC) is an inherited bone marrow failure syndrome in which patients undergo premature ageing and have a predisposition to malignancy. X-linked and autosomal (dominant and recessive) forms of the disease are recognized. The gene responsible for X-linked DC (DKC1) encodes a highly conserved protein called dyskerin that is believed to be essential in ribosome biogenesis and may also be involved in telomerase RNP assembly. Here we show that in X-linked DC, peripheral blood cells have dramatically reduced telomere lengths but normal levels of telomerase activity. We also find that subjects with autosomal DC have significantly shorter telomeres than age-matched normal controls suggesting that both forms of the disease are associated with rapid telomere shortening in hemopoietic stem cells. The further characterization of these genes will not only lead to a better understanding of the biology of DC but may also provide further insights into the maintenance of telomeres and the biology of aplastic anemia, ageing, and cancer.

Skewed X-inactivation in carriers of X-linked dyskeratosis congenita.

A gene causing Dyskeratosis Congenita (DC), a rare genetic disorder associated with bone marrow failure, has been mapped to chromosome Xq28, but autosomal inheritance of the disease has also been reported. We have investigated the pattern of X-inactivation in the peripheral blood of carriers of DC using the methylation-sensitive Hpa II site in the androgen receptor gene (HUMARA). In 5 different families in which the inheritance of DC appears to be X-linked, all 16 carriers showed skewed X-inactivation patterns. These cases indicate that, in the hematopoiesis of heterozygous females, cells expressing the normal DC allele have a growth advantage over cells that express the mutant allele. In 7 other families with sporadic cases of DC or with an uncertain pattern of inheritance, both skewed and normal patterns of X-inactivation were observed. In these families or where crucial family members are unavailable, the study of X-inactivation patterns will add to linkage analysis in providing information about carrier status.

Organization of the human protein 4.1 genomic locus: new insights into the tissue-specific alternative splicing of the pre-mRNA.

Protein 4.1 is a globular 80-kDa component of the erythrocyte membrane skeleton that enhances spectrin-actin interaction via its internal 10-kDa domain. Previous studies have shown that protein 4.1 mRNA is expressed as multiple alternatively spliced isoforms, resulting from the inclusion or exclusion of small cassette sequences called motifs. By tissue screening for protein 4.1 isoforms, we have observed new features of an already complex pattern of alternative splicing within the spectrin/actin binding domain. In particular, we found a new 51-nt exon that is present almost exclusively in muscle tissue. In addition, we have isolated multiple genomic clones spanning over 200 kb, containing the entire erythroid and nonerythroid coding sequence of the human locus. The exon/intron structure has now been characterized; with the exception of a 17-nt motif, all of the alternatively spliced motifs correspond to individual exons. The 3'-untranslated region (UTR) has also been completely sequenced using various PCR and genomic-sequencing methods. The 3' UTR, over 3 kb, accounts for one-half of the mature mRNA.

Linkage disequilibrium of polymorphic sites in the G6PD gene in African populations and the origin of G6PD A-.

So far, four polymorphic sites are known to exist in the glucose 6-phosphate dehydrogenase (G6PD) gene of people of African origin: the mutation in G6PD A (which creates a FokI restriction enzyme site); the additional mutation in G6PD A- (which creates an NlaIII site); and two restriction fragment length polymorphisms (PvuII and PstI). We have investigated the status of these four sites in 78 men of African descent and found them to exist in linkage disequilibrium--only five of the ten haplotypes expected at least twice under a random assortment regimen were observed. The mutation of G6PD A- is found only in the presence of the PvuII and PstI sites and we therefore suggest that it has arisen only once. We propose a likely sequence for the evolution of these different mutations in the G6PD gene.

G6PD Mediterranean accounts for the high prevalence of G6PD deficiency in Kurdish Jews.

The Jews of Kurdistan are a small inbred population with a high incidence of beta-thalassaemia and glucose-6-phosphate dehydrogenase (G6PD) deficiency. Recently, it was reported that the beta-thalassaemia in this population shows an unusual mutational diversity; 13 different mutations were identified, of which 4 had not previously been observed in any other population. In contrast, we now report that the G6PD deficiency, which has the highest known incidence in the world, and which affects about 70% of males, is almost entirely attributable to a single widespread mutation, G6PD Mediterranean.

A PvuII restriction fragment length polymorphism of the glucose-6-phosphate dehydrogenase gene is an African-specific marker.

The site of a PvuII restriction fragment length polymorphism (RFLP) of the human glucose-6-phosphate dehydrogenase (G6PD) gene has been located in intron V, 60 bp upstream of G6PD exon VI. A population survey shows this RFLP to be specific for African populations, with frequencies of the rarer allele (PvuII type 2 site present) of 0.32-0.40 in Kenyans, Nigerians, Zambians, and West Indians. This allele has not been found in the European, Asian and Middle Eastern populations studied. Such population-specific markers may be useful in the study of population affinities and may provide insight into prehistoric migrations of peoples.

The production of normal and variant human glucose-6-phosphate dehydrogenase in cos cells.

Full-length cDNA coding for human glucose-6-phosphate dehydrogenase (G6PD) was inserted into a eukaryotic expression vector containing the immediate early promoter of cytomegalovirus. When this plasmid was introduced into cos cells by transfection it led to the production of high levels of human G6PD. cDNAs containing mutations found in G6PD-deficient individuals were constructed by in vitro mutagenesis and expressed in the same system. Characterization of the G6PD proteins obtained in this way confirmed the primary structure inferred for the variant enzymes. An enzyme in which lysine-205 had been mutated to threonine was produced and found to have no G6PD activity, proving that this lysine residue is essential for enzyme activity in human G6PD.

Identification of novel DKC1 mutations in patients with dyskeratosis congenita: implications for pathophysiology and diagnosis.

Dyskeratosis congenita (DC) is characterised by the failure of those tissues that are rapidly dividing in the adult, particularly the skin and haemopoietic system. The X-linked form of the disease is caused by mutations in the DKC1 gene. To date the only DKC1 mutations detected result in alterations in the amino acid sequence of dyskerin. Dyskerin is the catalytic subunit of the H+ACA box small nucleolar RNA particles responsible for the site-specific pseudouridination of rRNA and in humans is also a component of the telomerase complex. In order to further characterise the disease at the molecular level, male DC patients from 25 families were screened for mutations in the DKC1 gene. Sequence variations were detected in 10 of these families. In five families, previously identified mutations were detected. Of the five novel sequence changes, three were coding changes: R158 W, S280R and P384L. A fourth sequence change was detected in the 5'-flanking region that disrupts a putative Spl transcription factor binding site. An intronic change was also detected that resulted in the partial incorporation of a portion of intron 1 into the mRNA. The identification of this mutation highlights the importance of screening for mutations that cause the partial aberrant splicing of mRNA. This is the first report of DKC1 mutations that are predicted to affect the level of expression of dyskerin. This suggests that a decrease in the amount of the normal protein may cause the disease.

Polymorphic sites in the African population detected by sequence analysis of the glucose-6-phosphate dehydrogenase gene outline the evolution of the variants A and A-.

The human X chromosome-linked gene encoding glucose-6-phosphate dehydrogenase (G6PD; EC 1.1.1.49) is known to be highly polymorphic from the biochemical characterization of enzyme variants. The variant A (with enzyme activity in the normal range) and the variant A- (associated with enzyme deficiency) each have a frequency of about 0.2 in several African populations. Two restriction fragment length polymorphisms have also been found in people of African descent, but not in other populations, whereas a silent mutation has been shown to be polymorphic in Mediterranean, Middle Eastern, African, and Indian populations. We report now on two additional polymorphisms that we have detected by sequence analysis, one in intron 7 and one in intron 8. The analysis of 54 African male subjects for the seven polymorphic sites, clustered within 3 kilobases of the G6PD gene, has revealed only 7 of the 128 possible haplotypes, indicating marked linkage disequilibrium. These data have enabled us to suggest an evolutionary pathway for the different mutations, with only a single ambiguity. The mutation underlying the A variant is the most ancient and the mutation underlying the A- variant is the most recent. Since it seems reasonable that the A- allele is subject to positive selection by malaria, whereas the other alleles are neutral, G6PD may lend itself to the analysis of the role of random genetic drift and selection in determining allele frequencies within a single genetic locus in human populations.

G6PD haplotypes spanning Xq28 from F8C to red/green color vision.

The most telomeric region of the human X chromosome within band Xq28 consists of a gene-rich region of about 3 Mb which contains the genes for coagulation factor VIIIc, glucose-6-phosphate dehydrogenase (G6PD), and red/green color vision. We have studied five polymorphic sites from this region, in a sample of normal people from the Cosenza province of Southern Italy. These sites, which span a distance of some 350 kb, are in strong linkage disequilibrium. Of the 32 possible haplotypes only 10 were found, and 4 of these account for 80% of all X chromosomes analyzed. In addition, we found that all G6PD-deficient people with the G6PD Mediterranean mutation belong to only two haplotypes. One of these (Med 1) is found only within a small subregion of the area investigated, west of the Appennine mountain range. Most remarkably, all Med 1 G6PD-deficient individuals also had red/green color blindness. The more frequent haplotype (Med 2) is the same in Calabria and in Sardinia, where it accounts for about 90% of the G6PD Mediterranean mutations, despite the fact that gene flow between the populations of Sardinia and Southern Italy must have been limited. These data do not enable us to determine whether the two types of G6PD Mediterranean have arisen through two separate identical mutational events or through a single mutational event followed by recombination. However, the data indicate relatively little recombination over an extended region of the X chromosome and they suggest that the G6PD Mediterranean mutation is recent by comparison to the other polymorphisms investigated.

Human hexose-6-phosphate dehydrogenase (glucose 1-dehydrogenase) encoded at 1p36: coding sequence and expression.

Using the published protein sequence from a rabbit microsomal glucose-6-phosphate dehydrogenase G6PD we have isolated and sequenced a cDNA clone coding for its human equivalent, which is also known as hexose-6-phosphate dehydrogenase (H6PD) and glucose dehydrogenase. The corresponding genomic sequence is in the databases enabling its localization to chromosome 1p36. The gene spans 37 kb and consists of 5 exons, the fifth of which codes for more than half of the 89 kDa protein. The first intron is a 10 kb insertion in the 5' untranslated sequence. The predicted mRNA has an exceptionally long (6.5 kb) 3' untranslated sequence. The predicted protein shows extensive homology with X-linked G6PD, suggesting the two genes share a common ancestor but no intron positions are conserved between the two genes suggesting the gene duplication was an ancient event. The C-terminal portion of the protein is not homologous with G6PD but shows limited homology with proteins of unknown function found throughout evolution and encoded next to G6PD in various micro-organisms. Intriguingly this C-terminal portion has some homology with the N-terminal sequence of Plasmodium falciparum G6PD.

Dyskeratosis congenita caused by a 3' deletion: germline and somatic mosaicism in a female carrier.

X-linked dyskeratosis congenita (DC) is a bone marrow failure syndrome caused by mutations in the DKC1 gene located at Xq28. By 20 years of age, most affected boys develop bone marrow failure, whereas female carriers show a skewed pattern of X-chromosome inactivation. The gene product, dyskerin, is homologous to a yeast protein involved in ribosomal RNA biogenesis, providing a unique insight into a cause of aplastic anemia. Whereas most causative mutations are single amino acid substitutions, and nonsense or frameshift mutations have not been observed, we present here a case of DC caused by a 2-kb deletion that removes the last exon of the gene. Normal levels of mRNA are produced from the deleted gene, with the transcripts using a cryptic polyadenylation site in the antisense strand of the adjacent MPP1 gene, normally located 1 kb downstream of DKC1 in a tail to tail orientation. The predicted truncated protein lacks a lysine-rich peptide that is less conserved than the rest of the dyskerin molecule and is dispensable in yeast, supporting the contention that it may retain some activity and that null mutations at this locus may be lethal. The affected boy had an unaffected brother with the same haplotype around the DKC1 gene and a sister who was heterozygous for the deletion. We conclude therefore that the mother must be a germline mosaic with respect to this deletion. Investigation of her blood cells and other somatic tissues showed that a small proportion of these cells also carried the deletion, making her a somatic mosaic and indicating that the deletion took place early in development.

Glucose 6-phosphate dehydrogenase mutations causing enzyme deficiency in a model of the tertiary structure of the human enzyme.

Human glucose 6-phosphate dehydrogenase (G6PD) has a particularly large number of variants resulting from point mutations; some 60 mutations have been sequenced to date. Many variants, some polymorphic, are associated with enzyme deficiency. Certain variants have severe clinical manifestations; for such variants, the mutant enzyme almost always displays a reduced thermal stability. A homology model of human G6PD has been built, based on the three-dimensional structure of the enzyme from Leuconostoc mesenteroides. The model has suggested structural reasons for the diminished enzyme stability and hence for deficiency. It has shown that a cluster of mutations in exon 10, resulting in severe clinical symptoms, occurs at or near the dimer interface of the enzyme, that the eight-residue deletion in the variant Nara is at a surface loop, and that the two mutations in the A- variant are close together in the three-dimensional structure.