Molecular pathology of haemoglobin H disease in Sardinians

Alpha thalassemia allelic frequency in Lebanon

BACKGROUND

Hemoglobinopathies are the most common reported monogenic disorders worldwide. It is well established that Mediterranean and Arab countries are high risk areas for thalassemia in general, and for alpha thalassemia in particular. Reports of alpha thalassemia gene mutations from the Lebanese population are limited.

PROCEDURE

We investigated the spectrum of alpha thalassemia mutations in a sample of 70 unrelated Lebanese families. Six different mutations of alpha thalassemia gene were identified.

RESULTS

The most prevalent mutations were the single gene deletion -α(3.7) (43%) and the non-gene deletion α2 IVS1 [-5nt] (37%). The double deletional determinant -(MED) was detected only in 14% of thalassemic chromosomes.

CONCLUSION

We determined the mutational spectrum of alpha thalassemia which might be used in the future for molecular investigations of the disease in susceptible patients in our population.

Hemoglobin H (Hb H) disease in Canada: molecular diagnosis and review of 116 cases

Over the past decade, we have characterized at the DNA level a total of 116 hemoglobin H (Hb H) disease patients living in Canada. The majority of patients were of southeast Asian descent (Chinese, Filipino, Laotian, Vietnamese), with a small number being of Mediterranean, Middle Eastern or East Indian background. A total of 15 distinct genotypes were detected, all but one being compound heterozygotes for a two-gene cis deletion and a single-gene deletion (-alpha/-) or a non-deletion mutation of the alpha2-globin gene (alpha(T) alpha/-). Seven different two-gene cis deletions were encountered, along with nine single-gene deletions and point mutations. The wide range of mutations associated with Hb H disease in Canada is a reflection of the population heterogeneity. The diagnosis of Hb H disease at the molecular level is important with respect to genetic counseling and the identification of families at risk for having pregnancies affected with Hb Bart's hydrops fetalis syndrome and/or Hb H disease. Six of the Hb H disease patients in our cohort had spouses who carried single-gene deletions, making these couples at risk for having children with Hb H disease. More important, seven patients had partners who carried two-gene cis deletions. These couples are at reproductive risk for both Hb Bart's hydrops fetalis syndrome and Hb H disease.

Genetic and clinical features of hemoglobin H disease in Chinese patients

BACKGROUND

Normally, one pair of each of the two alpha-globin genes, alpha1 and alpha2, resides on each copy of chromosome 16. In hemoglobin H disease, three of these four alpha-globin genes are affected by a deletion, a mutation, or both. We studied the alpha1-globin gene abnormalities and the clinical and hematologic features of Chinese patients with hemoglobin H disease in Hong Kong.

METHODS

We assessed the clinical features, hematologic values, serum ferritin levels, and liver function of 114 patients with hemoglobin H disease. We also performed echocardiography and magnetic resonance imaging of the liver and examined the two pairs of alpha-globin genes.

RESULTS

Hemoglobin H disease in 87 of the 114 patients (76 percent) was due to the deletion of three of the four alpha-globin genes (--/-alpha), a combination termed the deletional type of hemoglobin H. The remaining 27 patients (24 percent) had the nondeletional type of hemoglobin H disease, in which two alpha-globin genes are deleted and a third is mutated (--/alphaalphaT). All 87 patients with the deletional type of hemoglobin H were double heterozygotes in whom there was a deletion of both alpha-globin genes from one chromosome, plus a deletion of the alpha1 or alpha2 gene from the other chromosome (--/alpha- or --/-alpha). A variety of mutated alpha-globin genes was found in the patients with nondeletional type of hemoglobin H disease. Patients with the nondeletional type of the H disease had more symptoms at a younger age, more severe hemolytic anemia, and larger spleens and were more likely to require transfusions than patients with deletional hemoglobin H disease. The severity of iron overload was not related to the genotype.

CONCLUSIONS

Chinese patients in Hong Kong with the nondeletional type of hemoglobin H disease have more severe disease than those with the deletional type of the disease. Iron overload is a major cause of disability in both forms of the disease.

The population genetics of the haemoglobinopathies

The haemoglobinopathies are the commonest single-gene disorders known, almost certainly because of the protection they provide against malaria, as attested by a number of observations. The geographical distributions of malaria and haemoglobinopathies largely overlap, and microepidemiological surveys confirm the close relationship between them. For two of the commonest disorders, haemoglobin S and alpha(+)-thalassaemia, there is also good clinical evidence for protection against malaria morbidity. However, not all the evidence appears to support this view. In some parts of the world malaria and haemoglobinopathies are not, and never have been, coexistent. It is also difficult to explain why the majority of haemoglobinopathies appear to be recent mutations and are regionally specific. Here we argue that these apparent inconsistencies in the malaria hypothesis are the result of processes such as genetic drift and migration and of demographic changes that have occurred during the past 10,000 years. When these factors are taken into account, selection by malaria remains the force responsible for the prevalence of the haemoglobinopathies.

A novel polyadenylation signal mutation in the alpha 2-globin gene causing alpha thalassaemia

In a family of Indian origin we have identified a deletion of two bases at the polyadenylation signal sequence of the alpha 2-globin gene (AATAAA-->AATA). Three individuals heterozygous for this mutation display an alpha o-thalassaemia-like phenotype. Single-stranded conformation analysis and automatic sequencing showed no additional mutations in either alpha 1- or alpha 2-globin genes. A previously described polyadenylation sequence mutation (AATAAA-->AATAAG), alpha TSaudi alpha, causes HbH disease in homozygotes. In this study the patients heterozygous for the AATA(-AA) mutation show a similar phenotype observed in the alpha TSaudi alpha heterozygotes. This confirms the observation that the inefficient transcriptional termination due to mutations of the polyadenylation sequence of the alpha 2-gene might interfere with the alpha 1-gene expression.

A base substitution (T-->C) in codon 29 of the alpha 2-globin gene causes alpha thalassaemia

We have identified three individuals of Greek or Greek Cypriot origin with an atypical form of HbH disease characterized by a severe hypochromic microcytic anaemia associated with relatively small amounts of HbH in the peripheral blood. Molecular analysis has shown that each is a compound heterozygote for a previously described mutation affecting the poly A addition signal (AATAAA-->AATAAG) and a previously undescribed mutation involving a T-->C transition in codon 29 of the alpha 2 gene causing a leucine-->proline substitution. Although this mutation would be expected to produce an unstable haemoglobin and hence a haemolytic anaemia, simple heterozygotes for the alpha 29Leu-->Pro mutation have the phenotype of alpha-thalassaemia trait.

An IVS-I-117 (G-->A) acceptor splice site mutation in the alpha 1-globin gene is a nondeletional alpha-thalassaemia-2 determinant in an Indian population

In 1991 we reported the identification of two deletional alpha-thalassaemia-2 determinants (-3.7 kb and -4.2 kb) and one nondeletional alpha-thalassaemia-2 determinant (Hb Koya Dora alpha 2 codon 142, TAA-->TCA) in a tribal population in Central India (Gupta et al, 1991). Evidence was obtained at that time for the possible presence of an additional nondeletional alpha-thalassaemia-2 because of low levels of Hb S (< 28%) in some Hb S heterozygotes with a simple alpha-thalassaemia-2 heterozygosity (-alpha/alpha alpha). This abnormality has now been identified as a G-->A mutation at IVS-I-117 of the alpha 1-globin gene (acceptor splice site) which makes this gene nonfunctional. Its frequency was established at approximately 6% which raises the total frequency of alpha-thalassaemia determinants in this population to approximately 60%. Subjects with a deletional alpha-thalassaemia-2 and the newly discovered alpha 1 acceptor splice junction mutation in trans appear to have an alpha chain deficiency similar to that of an alpha-thalassaemia-2 homozygote (-alpha/-alpha). An additional change (C-->G) at the Cap -4 site was observed in six alpha 1- and one alpha 2-globin genes; this polymorphism is not associated with a decrease in alpha chain synthesis and is not linked to the IVS-I-117 (G-->A) mutation.

De novo initiation codon mutation (ATG-->ACG) of the beta-globin gene causing beta-thalassemia in a Swiss family

Investigation of microcytic anemia with normal ferrous status in two members (father and daughter) of a Swiss family originating from Bern revealed high levels of HbA2 (4%, 7.3%) and HbF (3.2%, 3.1%). Direct sequence analysis of asymmetrically amplified DNA showed the ATG-->ACG mutation in the intiation codon of the beta-globin gene. Heterozygous beta-thalassemia was not found in either of the propositus's parents or in any of his brothers and sisters. Extended restriction fragment length polymorphism haplotyping of the beta chromosomes led us to the conclusion of a recent spontaneous mutation in the paternal germ cell. The results of routine HLA and blood group testing supported the stated paternity. We also found that the intragenic sequence polymorphisms (frameworks) are not always in linkage disequilibrium with the Bam HI polymorphism downstream from the beta-globin gene as previously observed. This is the second family found to carry this initiation codon mutation in the beta-globin gene. Unlike the first reported family, of Yugoslavian origin, our patients have high HbF levels and this in the absence of a C-->T substitution at -158 site 5' to G gamma.

The population genetics of the haemoglobinopathies

The haemoglobinopathies are the commonest single gene disorders known, and are so common in some regions of the world that the majority of the population carries at least one genetic abnormality affecting the structure or synthesis of the haemoglobin molecule. The prevalence of the common haemoglobinopathies (the alpha- and beta-thalassaemias, HbS, HbC and HbE) is almost certainly a result of the protection they provide against malaria, as the epidemiological evidence reviewed in this chapter shows. World-wide, the distributions of malaria and the common haemoglobinopathies largely overlap, and micro-epidemiological surveys have confirmed the close relationship between the disorders. However, there are complications to this picture which appear to undermine the malaria hypothesis. First, in some areas, malaria and haemoglobinopathies are not coincident. Second, the malaria hypothesis does not easily explain why no two regions of the world have the same haemoglobinopathy or combination of haemoglobinopathies. The majority of mutations have arisen only once and are regionally specific. By using molecular characterization of mutations and the analysis of haplotypes on haemoglobinopathy-bearing chromosomes it is possible to show how a combination of selection by malaria, genetic drift and population movements can explain the first complication. In order to explain the second, we have argued that malaria selection has operated relatively recently on human populations (within the last 5000 years). The present distribution is then seen as the result of selection elevating sporadic mutations in local populations. In the absence of sufficient gene flow to spread all mutations to all populations, the consequence is a patchwork distribution of haemoglobinopathies. Given time, we would expect the mutations that protect and do not compromise the health of their carriers to become widely disseminated, but it is likely that human intervention will alter this process of natural selection.

Prenatal diagnosis of Hb H disease due to compound heterozygosity for South-east Asian deletion and Hb constant spring by polymerase chain reaction

A pregnant woman has two children affected by moderately severe Hb H disease due to compound heterozygosity of South-east Asian deletion and Constant Spring mutation. In her third pregnancy, transabdominal chorionic villus sampling was performed at the tenth gestational week to obtain fetal DNA. The polymerase chain reaction was used for detection of both the South-east Asian deletion and the Constant Spring mutation. Hb H disease was diagnosed in the fetus. After genetic counselling, the couple elected to have the pregnancy terminated.

Hb H disease in a Turkish family resulting from the interaction of a deletional alpha-thalassaemia-1 and a newly discovered poly A mutation

We have analysed the alpha-globin gene defects present in several members of a large family from Southern Turkey. One deletional alpha-thalassaemia-1 (type MED-II) was found in 10 subjects: this deletion is in excess of 26.5 kb and includes all zeta- and alpha-globin genes. Besides the common types of deletional alpha-thalassaemia-2 (-3.7 kb and -4.2 kb) we observed a nondeletional alpha-thalassaemia-2 that results from an A----G mutation (AATAAA----AATGAA) in the polyadenylation signal of the alpha 2-globin gene: the same A----G replacement is present in the psi alpha l gene. The mutation must cause a considerable alpha-chain deficiency as is evidenced by the haematological data for five members with Hb H disease due to a compound heterozygosity for alpha-thalassaemia-1 (MED-II) and the newly discovered poly A mutation. Several members had additional beta-chain abnormalities (Hb S, Hb D-Los Angeles, beta-thalassaemia); the 11 persons with a Hb S heterozygosity and various alpha-globin gene defects (-alpha/alpha alpha; alpha T alpha/alpha alpha, - -/alpha alpha, -alpha/-alpha and - -/alpha T alpha) showed a decrease in the level of Hb S that was directly related to the severity of the alpha-chain deficiency.

The nondeletional types of Hb H disease in Guangxi

Three primers were designed, one specific for alpha 1-globin DNA, a second for alpha 2-globin DNA, and a third that is common for both alpha 1- and alpha 2-globin DNA. These three primers can be applied for selective amplification of the two globin DNA fragments, which is useful for identification of nondeletional types of Hb H disease. Fifty-nine DNA samples of Hb H patients from Guangxi were studied by selective amplification, and 27 cases (45.8%) were confirmed as nondeletional types. Of these, 22 (81.5%) had the Hb Constant Spring (CS) mutation and one had the Hb Quong Sze (QS) mutation; both were identified by hybridization with synthesized oligonucleotide probes. Nondeletional Hb H disease in Guangxi seems to be more severe than the deletional types. The average hemoglobin level of the nondeletional Hb H/CS (--/alpha alpha) is 6.8 g/dl, which is lower than that of the deletional types (7.9 g/dl), while the levels of Hb H and Hb Bart's were much higher in the patients with Hb H/CS than in those with the deletional types.

Locus assignment of human alpha globin mutations by selective amplification and direct sequencing

We describe a simple approach for molecular characterization and locus assignment of structural mutants by direct sequencing of enzymatically amplified DNA selective to alpha 1 and alpha 2 globin gene regions. Nucleotide substitution of two structural variants (Stanleyville II alpha 2(78Lys) and J Mexico alpha 2(54Glu) were determined and their encoding loci were specified. The amplified segment encompasses sequences upstream of the CAAT box to downstream of the Poly(A) addition signal. Hence all of the alpha globin structural variants and most of the nondeletion alpha thalassaemic mutants should be characterizable by this approach.

The C-T substitution in the distal CACCC box of the beta-globin gene promoter is a common cause of silent beta thalassaemia in the Italian population

This paper describes four families of Italian descent in each of which the propositus had the clinical phenotype of thalassaemia intermedia, resulting from the compound heterozygous state for high HbA2 beta thalassaemia and type I silent beta thalassaemia. Direct sequencing on amplified DNA and/or oligonucleotide analysis detected, in all families but one, the compound heterozygous state for codon 39 nonsense mutation and the C-T substitution at position -101 in the distal CACCC box of the beta-globin gene promoter (beta th-101). Members of these families who are heterozygous for high HbA2 beta thalassaemia showed the codon 39 nonsense mutation, while those with the clinical phenotype of silent beta thalassaemia had the beta th-101 mutation. In the remaining family, the propositus and one of his siblings had the compound heterozygous state for a molecularly undefined high HbA2 beta thalassaemia and the beta th-101 mutation in combination with the triple alpha globin gene arrangement. These patients showed a more severe thalassaemia intermedia like clinical phenotype as compared to those with the same beta-globin genotype and a normal alpha-globin gene arrangement. In the families investigated the beta th-101 was always associated with haplotype I. A group of patients with thalassaemia intermedia from Southern Italy, either homozygous or heterozygous for haplotype I and in whom previous studies had failed to define the mutation in one of the beta thalassaemia globin genes, were screened by oligonucleotide analysis for the presence of the beta th-101. Three out of nine were positive. These results indicate that the beta th-101 mutation is a common cause of the type I silent beta thalassaemia phenotype in the Southern Italian population.

The levels of zeta, gamma, and delta chains in patients with Hb H disease

Details are given of a study of blood samples from 24 patients with Hb H disease from different Mediterranean countries and from the Far East. Four different types of alpha-thal-1 (--) were observed, namely -(alpha) (approximately 20.5-kb deletion); --MED-I (approximately 17.5-kb deletion); --MED-II (greater than 26.5-kb deletion); and --SEA (approximately 18-kb deletion, in Orientals only). The alpha-thal-2 was mainly of the deletion type (16 with the 3.7-kb deletion; 1 with the 4.2-kb deletion), while 4 of the 7 patients with a nondeletional type had the five-nucleotide deletion at the donor splice site of the first intron of the alpha 2 gene. All patients had a mild-to-moderate hemolytic anemia; no significant differences in hematology were observed between the groups. Hb A2 was decreased to about one-third of the normal level. The Hb H formation varied considerably and its quantitation was not always satisfactory. Patients with Hb H disease due to any alpha-thal-1 combined with a nondeletional alpha-thal-2 had the highest Hb H levels and a more marked anemia. The zeta chain production was small and absent in patients with the MED-II type of alpha-thal-1 because this deletion included the zeta and psi zeta genes. The highest zeta chain levels were present in the four patients with the SEA type of alpha-thal-1. The gamma chain production was increased, particularly in patients with a mutation of C----T at position -158 to the G gamma globin gene. This gamma chain was primarily present as Hb Bart's (or gamma 4) and only about 15% was recovered as Hb F or alpha 2 gamma 2. The evaluation of the rate of gamma chains produced in these patients was greatly facilitated by data from one patient who had Hb H disease and a heterozygosity for the A gamma-beta+-HPFH. The low levels of Hb A2 and of Hb F (relative to Hb Bart's) can be explained by a decreased affinity of alpha chains for delta and gamma chains as compared with beta chains in conditions of severe alpha chain deficiency.

Interaction of heterozygous beta zero-thalassemia with single functional alpha-globin gene

In this study, we analyzed the phenotypic manifestations resulting from the interaction of heterozygous beta zero-thalassemia(beta zero-39 nonsense mutation) with the functional loss of three alpha-globin structural genes in six subjects, of whom four had the [-alpha/--]alpha-globin genotype and two the [--/alpha Th alpha] alpha-globin genotype. The beta-thalassemia defect was in all cases the nonsense mutation at codon 39. The nondeletion alpha-thalassemia alpha th was the initiation codon mutation (AUG----GUG) of the alpha-2 gene. In all these subjects hypochromia and microcytosis were more marked than in beta zero-thalassemia heterozygotes with a full complement of four alpha-globin genes. All but one had moderate anemia. The alpha:beta globin chain synthesis ratios were consistently decreased. No cases had Hb H on electrophoresis. Subjects with [--/alpha Th alpha] alpha-globin genotype had more severe thalassemia-like manifestations than those with [--/-alpha] alpha-globin genotype.

Correlation of clinical phenotype to genotype in haemoglobin H disease

Clinical assessment, haematological studies, and globin gene mapping were performed in 21 Greek subjects with haemoglobin H disease. Clinical phenotypes ranged from mild, and virtually asymptomatic, to severe cases requiring transfusion. The severe clinical phenotype was exclusively associated with non-deletion genotypes, whereas the mild and intermediate phenotypes occurred with deletion genotypes. Patients with non-deletion genotypes had higher levels of Hb H. For deletion genotypes of haemoglobin H disease, the value of antenatal diagnosis is questionable. In non-deletion genotypes, antenatal diagnosis should be considered, because of the more severe clinical course observed in these patients.

Non-deletion haemoglobin H disease in Papua New Guinea

Analysis of DNA from members of a Melanesian family from Papua New Guinea with haemoglobin (Hb) H disease revealed that all four alpha globin genes are intact in affected subjects. Study of restriction enzyme site and length polymorphisms and the use of oligonucleotide probes indicated that the molecular basis of this Papuan form of non-deletion Hb H disease differs from the previously described Middle Eastern and Mediterranean types.

An initiation codon mutation (AUG----GUG) of the human alpha 1-globin gene. Structural characterization and evidence for a mild thalassemic phenotype

alpha-globin is encoded by two adjacent genes, alpha 1 and alpha 2. Recent evidence suggests that these genes are not equally expressed and that the alpha 2-globin gene encodes the majority of alpha-globin. This finding would predict that a thalassemic mutation of the alpha 2-globin gene would result in a more severe loss of alpha-chain synthesis than a similar mutation in the alpha 1-globin gene. In a previous study we described a nondeletion alpha-thalassemia defect in the alpha 2-globin gene resulting from an AUG----ACG initiation codon mutation. In the present study we describe a different initiation codon mutation, AUG----GUG, present in the alpha 1-globin gene. The alpha 1- and alpha 2-globin gene initiation codon mutations result in similarly lowered levels of encoded mRNA. Despite the similarity of these two mutations, the alpha 2 mutant results in a more severe loss of alpha-globin synthesis and a more severe clinical alpha-thalassemia phenotype than the corresponding alpha 1-globin gene mutation. This difference reflects the dominant role of alpha 2-globin gene in overall alpha-globin synthesis.