Beta-thalassaemia in Campania: DNA polymorphism analysis in beta A and beta thal chromosomes and its usefulness in prenatal diagnosis

Hb Cardarelli [β86(F2)Ala→Pro]: A New Unstable and Hyperaffine Variant in Association with β+‐Thalassemia

Hb Cardarelli [beta86(F2)Ala-->Pro] is a new unstable and high oxygen affinity variant found in several members of a family from Naples, Southern Italy. A detailed structural and functional characterization of the variant was performed on two subjects, at both the protein and DNA level. The first patient exhibited 43% of the variant hemoglobin (Hb) without major hematological problems. The proband showed 82% of the abnormal Hb in association with beta(+)-thalassemia (thal) that caused relevant erythrocytosis requiring frequent phlebotomies. Structural investigation of the Hb variant by mass spectrometric methodologies identified the amino acid replacement as Ala-->Pro at beta86. The corresponding DNA mutation GCC-->CCC at codon 86 of the beta-globin gene was assessed by both DNA sequencing and amplification refractory mutation system (ARMS) techniques. Functional studies carried out on whole blood and diluted hemolysates from both patients demonstrated increased oxygen affinity, decreased Bohr effect, reduced heme-heme interaction and nearly halved 2,3-diphosphoglycerate (2,3-DPG) and chloride effects.

The significance of the hemoglobin A(2) value in screening for hemoglobinopathies

BACKGROUND AND OBJECTIVE

The inherited hemoglobinopathies are a large group of disorders that include thalassemias and hemoglobin variants. Accurate determination of the carrier phenotype is essential for detecting couples at risk for producing offspring with hemoglobinopathy. Heterozygous beta-thalassemia is usually silent at the clinical level. His phenotype is characterized by microcytosis and hypochromia with increased hemoglobin A(2) (HbA(2)) value. Therefore, HbA(2) determination plays a key role in screening programs for hemoglobinopathy. The aim of this review is to address and suggest an approach for reducing or abolishing hemoglobinopathy screening mistakes.

DESIGN AND METHODS

Quantitative methods for HbA(2) value determination, comment on the accuracy of the test and on the interpretation of data were discussed. The most probable diagnostic conclusion based on the HbA(2) level, hemoglobin pattern, hematological parameters and iron markers was suggested in this review.

RESULTS

Hemoglobinopathies are the only genetic disease where it is possible to detect carriers using hematological findings rather than DNA analysis. However, hematological diagnosis is sometimes presumptive, and in these cases, DNA analysis becomes necessary. Complete screening is based on the detection of red cell indices, HbA(2), HbF and hemoglobin variant values. In particular, HbA(2) determination plays a key role in screening programs for beta-thalassemia because a small increase in this fraction is one of the most important markers of beta-thalassemia heterozygous carriers.

CONCLUSION

Genetic factors both related and unrelated to the beta- and alpha-globin gene clusters, iron metabolism, endocrinological disorders, and some types of anemia, together with intra- and inter-laboratory variations in HbA(2) determination, may cause difficulties in evaluating this measurement in screening programs for hemoglobinopathies. Therefore, knowledge of all these issues is important for reducing or eliminating the risk of mistakes in screening programs for hemoglobinopathies.

HB City of Hope [β69(E13)GLY→SER] in Italy: Association of the Gene with Haplotype IX

Hb City of Hope [beta 69(E13)Gly----Ser] was detected by reversed phase high performance liquid chromatography in an asymptomatic carrier from Naples, Southern Italy. The amino acid substitution, identified by fast atom bombardment mass spectrometry, was due to a TGG----TGA substitution as assessed by DNA sequencing. Analysis of the chromosomal background indicates that the globin gene cluster containing the mutant gene has most probably been rearranged by a recombination event, since the mutation was associated with restriction fragment length polymorphism haplotype IX, instead of haplotype I, as previously reported.

Delayed decline of gamma-globin expression in infant age associated with the presence of Ggamma-158 (C-->T) polymorphism

Persistent production of fetal hemoglobin (HbF) in adult has ameliorative effects on hemoglobinopathies and great efforts are currently made to achieve an exhaustive understanding of the molecular mechanisms of the switching in globin gene expression. One of the factors reported to be associated with the expression of fetal globin genes is the Xmn I Ggamma-158 polymorphism, although it is still unclear if it is involved in this mechanism either by itself or in strong linkage disequilibrium with other loci. Here, we report a novel effect of the Xmn I Ggamma-158 site that was found associated with a significant delayed decline of HbF production in infant age. The prolonged decay trend was enhanced when the Ggamma-158 C-->T substitution was co-inherited with a beta-thalassemic trait. Our observations reinforce the hypothesis that this region plays an important role in the expression of the gamma-globin genes and give new insights on the intriguing and still poorly understood mechanisms of globin gene expression switching.

β-globin cluster haplotypes in normal individuals and β039-thalassemia carriers from Sardinia, Italy

Seven polymorphic sites in the beta-globin cluster in association with specific thalassemia mutations were analyzed in a sample from Sardinia, Italy. In order to verify previous works carried out on normal samples (beta(A)/beta(A)) and family studies on beta-thalassemia homozygotes individuals, the haplotype frequencies in both normal individuals (beta(A)/beta(A)) and beta(0)39-thalassemia carriers (beta(A)/beta0) were studied. In our work chromosomes carrying beta(0)39 mutation are characterized by a prevalence of haplotype II (- + + - + + +) (52%) relative to haplotype I (+- - - - + +) (29%), in contrast, among chromosomes with beta(A) the frequency of haplotype I is much greater than that of haplotype II. These data confirm what was found by other authors. Nevertheless, our results disagree with those of previous studies of Sardinians, both in frequencies values and in the numbers of haplotypes identified. Population analysis performed with samples carrying the beta-thalassemic mutation highlighted the peculiarity of Sardinians with respect to other Mediterranean populations. The Corsican population is most similar to the Sardinian population, confirming previous analyses performed with both classical markers and mitochondrial and genomic DNA.

β -thalassaemia-87 C→G: relationship of the Hb F modulation and polymorphisms in compound heterozygous patients

A clinical, haematological, biochemical and molecular study was carried out in 17 patients affected with thalassaemia intermedia, who were compound heterozygotes for the beta-thalassaemia mutation beta-87 C-->G to determine the genetic basis of their clinical heterogeneity. The beta-87 was found associated with haplotype VIII (beta-87/VIII) or V (beta-87/V). The 10 patients with the beta-87/VIII showed milder clinical conditions, with significantly higher levels of haemoglobin (Hb) (9.8 +/- 1.1 g/dl vs. 8.5 +/- 1.3 g/dl) and fetal haemoglobin (Hb F) (6.2 +/- 1.5 g/dl vs. 2.6 +/- 1.5 g/dl; P = 0.0034) and higher synthesis of (G)gamma ((G)gamma/(Total)gamma 69.4 +/- 2.6% vs. 42.8 +/- 16.2%; P = 0.0042) than the seven patients with the beta-87/V. The beta-87/VIII showed a configuration of rare polymorphisms in the 5' sub-haplotype, which have been reported to exert an increasing effect on Hb F. They were "T"-158 (G)gamma-globin gene, T-A-G in pre-(G)gamma framework, (TG)(11)(CG)(3) in the (G)gamma-IVS2, (AT)(9)N(12)(AT)(10) in LCR-HS2; in contrast, the haplotype V had, respectively, "C", T-G-A (TG)(19)(CG)(2)CACG in the (G)gamma-IVS2, and (AT)(10)N(12)(AT)(11). In all patients the beta-87 was associated with the (AT)(9)T(5) motif 5' beta-globin gene with increased affinity for the BP-1 protein, and with the (TG)(13) in the (A)gamma-IVS2. The high increase of the Hb F, mostly of the (G)gamma-type, strongly suggests the hypothesis that the 'T'-158 (G)gamma plays a principal role and that the other polymorphisms could exert a cooperative role in the modulation of Hb F in patients with erythropoietic stress.

Beta+45 G --> C: a novel silent beta-thalassaemia mutation, the first in the Kozak sequence

A family from the Southeast of Italy was found to have a novel beta-globin mutant, beta+45 G-->C, with the features of a silent beta-thalassaemia mutation. It was asymptomatic in two heterozygotes, but its interaction with the severe thalassaemia mutation beta-IVS-II-654 C-->T worsened the haematological and biosynthetic phenotype in two compound heterozygotes; moreover, another compound heterozygote, who was also heterozygote for the alphaalphaalpha(anti3.7), suffered from thalassaemia intermedia. The mutation was found associated in cis with the IVS-II-754 T-->C substitution, which did not lead to abnormally spliced mRNA. Furthermore, the amount of beta+45 mRNA was the same as the betaA mRNA in the reticulocytes of the carriers. In vitro transcription/translation experiments demonstrated that the beta+45 G-->C decreased the efficiency of translation of the beta-globin chain by about 30%: this slight impairment was consistent with the observed clinical phenotype. The beta+45 G-->C is the first mutation found in the Kozak sequence (GACACCATGG) of the beta-globin gene and the first one at the position -6 upstream the ATG. The Kozak consensus sequence plays a major role in the initiation of translation process. The present finding supports the hypothesis that the G in position -6 is important in this process.

Epidemiology of the delta globin alleles in southern Italy shows complex molecular, genetic, and phenotypic features

We characterized mutations and haplotypes of the delta-globin gene (HBD, MIM# 142000) in two regions of southern Italy. Mutations were discovered by screening for individuals with Hb A2<2%. In Basilicata, about 10,000 students were screened and 53 carriers in 43 unrelated families were diagnosed; in Campania, cases were referred through a routine thalassemia counseling service. Twelve alleles were detected. Four were novel variants [Hb A2-Metaponto (g.238C>A), Hb A2-Campania (g.302C>A), Hb A2-Lucania (g.393C>G), and Hb A2-Capri (g.443G>T)]. Hb A2-Lucania was not inherited but had arisen in the propositus. Two were novel mutations in the noncoding regions: the substitutions IVS2+6T>A, presumably affecting the splicing, and g.-126A>T in the GATA motif presumably affecting transcription. All novel alleles were found associated with haplotypes common in the Mediterranean area. The remaining six were alleles already described. The Hb A2-Yialousa (g.82G>T) was the most prevalent (42/63 families). Recurrent homologous crossing-over events have, most likely, linked this allele to Haplotypes IX (24 families), IV (10 families), or III (seven families). The ratio of Haplotypes IX:IV:III was about the same in the two regions. The rare allele Hb A2-NYU (g.39T>A) was found in 11 families from Basilicata associated with Haplotype I. All the 11 families lived in a restricted area extending from the Ionian Coast for 15 km along the Angri and Sinni Rivers. A founder effect most probably gave origin to this isolated group. The remaining four alleles were rare: the 7.2-kb deletion Corfù type (HBD g.-5946_1262del), Hb A2-Mitsero (g.14C>T), Hb A2-Etolia (g.385T>C), Hb A2-Coburg (g.1376G>A). Correlation between genotype and phenotype was established in 103 carriers.

Hb Vila Real [beta36(C2)Pro-->His] in Italy: characterization of the amino acid substitution and the DNA mutation

A rare high oxygen affinity hemoglobin variant was identified in a 22-year-old male patient from Napoli (Naples, Italy) affected by erythrocytosis. A detailed structural characterization of the variant hemoglobin was carried out, both at the protein and DNA levels essentially by mass spectrometric procedures and allele-specific amplification techniques. The amino acid substitution was determined by liquid chromatography tandem mass spectrometric analysis of the tryptic digest as beta36(C2)Pro-->His; the corresponding DNA mutation was identified as C-->A at the second position of codon 36 of the beta chain (CCT-->CAT). These variations identified the presence of Hb Vila Real, described only once before in a Portuguese woman. Haplotype analysis of DNA polymorphisms showed that the beta-globin gene of Hb Vila Real was associated with haplotype I.

Hb Rainier [beta145(HC2)Tyr-->Cys] in Italy. Characterization of the amino acid substitution and the DNA mutation

A high oxygen affinity hemoglobin variant was identified in a 53-year-old male patient from Napoli (Italy), suffering from pulmonary thromboembolism and polycythemia. A detailed structural characterization of the variant hemoglobin was carried out, both at the protein and DNA levels, by a combination of DNA sequencing and allele-specific amplification techniques with mass spectrometric procedures. The amino acid substitution was found to be Tyr-->Cys, and the corresponding DNA mutation was identified as A-->G at the second position of codon 145 of the beta chain. These variations indicated the presence of Hb Rainier. Haplotype analysis of DNA polymorphisms showed that the beta-globin gene from Hb Rainier was associated with haplotype II. Moreover, structural analyses provided direct identification of an intramolecular disulphide bridge joining the newly inserted beta145Cys with beta93Cys. This is the first report of the occurrence of Hb Rainier in Italy.

Hb O-Arab [beta 121(GH4)Glu-->Lys]: association with DNA polymorphisms of African ancestry in two Mediterranean families

Hb O-Arab [beta 121(GH4)Glu-->Lys] was detected in two Mediterranean families, one from Southern Italy and the other from Albania. The GAA-->AAA mutation at codon 121 was characterized by DNA sequencing. The mutant genes were associated with the same beta-globin gene framework variant and with the rare Hpa I/3' beta polymorphic restriction site generating a 7.0 kb fragment. However, at 5' the gene of the Italian family was associated with the restriction fragment length polymorphism subhaplotype [+ - - - +] and the Taq I/3'G gamma polymorphic site, while that of the Albanian family was associated with subhaplotype [- - - - +] but not with the Taq I/3'G gamma site. The particular features of these polymorphisms support the hypothesis of an African origin for the Hb O-Arab gene and a subsequent recombination event leading to the haplotype found in the Italian family.

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.

Why are some genetic diseases common? Distinguishing selection from other processes by molecular analysis of globin gene variants

Various processes (selection, mutation, migration and genetic drift) are known to determine the frequency of genetic disease in human populations, but so far it has proved almost impossible to decide to what extent each is responsible for the presence of a particular genetic disease. The techniques of gene and haplotype analysis offer new hope in addressing this issue, and we review relevant studies of three haemoglobinopathies: sickle cell anaemia, and alpha and beta thalassaemia. We show how for each disease it is possible to recognize a pattern of regionally specific mutations, found in association with one or a few haplotypes, that is best explained as the result of selection; other patterns are due to population migration and genetic drift. However, we caution that such conclusions can be drawn in special circumstances only. In the case of the haemoglobinopathies it is possible because a selective agent (malaria) was already suspected, and the investigations could be carried out in relatively genetically homogenous populations whose migratory histories are known. Moreover, some data reviewed here suggest that gene conversion and the haplotype composition of a population may affect the frequency of a mutation, making interpretation of gene frequencies difficult on the basis of standard population genetics theory. Hence attempts to use the same approaches with other genetic diseases are likely to be frustrated by a lack of suitably untrammelled populations and by difficulties accounting for poorly understood genetic processes. We conclude that although this combination of molecular and population genetics is successful when applied to the study of haemoglobinopathies, it may not be so easy to apply it to the study of other genetic diseases.

DNA polymorphisms associated with Hb D-Los Angeles [beta 121(GH4)Glu-->Gln] in southern Italy

We detected Hb D-Los Angeles [beta 121(GH4)Glu-->Gln], the most common hemoglobin variant after Hb S and Hb Lepore-Boston, in six unrelated families in Southern Italy. Ten patients were studied; eight patients were heterozygotes and two were compound heterozygotes for the hemoglobin variant and the beta-thalassemia codon 39 (C-->T) nonsense mutation. The beta-globin gene sequence was characterized by polymerase chain reaction direct sequencing; restriction fragment length polymorphisms were defined by Southern blot analysis. The gene variant, due to the GAA-->CAA substitution at codon 121, was found in association with the 5' subhaplotype [+ - - - -] and the beta-globin gene framework 1; in addition, it was found to be associated with the absence of Ava II/phi beta and Xmn I/5'G gamma, and with the presence of Hpa I/3' beta. This restriction fragment length polymorphism haplotype is common in the Mediterranean area as well as in other populations. The findings are equally compatible with an independent origin in the Mediterranean area or with origin in Asia and subsequent spread to Italy.

ααααanti-3.7 type II: a new α-globin gene rearrangement suggesting that the α-globin gene duplication could be caused by intrachromosomal recombination

We report here a new human alpha-globin gene rearrangement carrying the two normal, alpha 2 and alpha 1, and two hybrid, alpha 1/alpha 2, globin genes in the order 5'-alpha 2-alpha 1/alpha 2-alpha 1/alpha 2-alpha 1-3'. Both the hybrid genes, subtyped with ApaI and RsaI restriction enzymes, were found to be of the uncommon anti 3.7 type II. The hybrid genes were expressed at the biosynthetic level and their interaction with the beta-thalassaemia IVS 1 nt 1 G----A mutation caused thalassaemia intermedia. We also report a case of an alpha alpha alpha-globin gene rearrangement in the twin of one of the alpha alpha alpha alpha-globin gene carriers; the duplicated gene was of the anti 4.2 type and was associated with the absence of RsaI polymorphism. The singular finding of an alpha alpha alpha alpha-anti 3.7 cluster with two identical rare hybrid genes suggests that the reciprocal unequal recombination causing the alpha-globin gene rearrangements could be of the intrachromosomal rather than the interchromosomal type.