Variation in hemoglobin A2

A New Mutation, Hb A2-Canakkale [δ10(A7)Ala→Val; HBD: c.32C>T], and Other Well-Known δ Variants Identified in a Selected Cohort with Low Hb A2 Levels

Hemoglobinopathies are the most common single-gene disorders, and β-thalassemia (β-thal) imposes a tremendous health burden on Turkey. Thus, premarital carrier screening is obligatory in Turkey, as it is in some other countries. As a result of this mandatory procedure, at routine clinical checkups, many individuals who had undergone premarital screening but did not have any clinical symptoms and/or hematological findings, have compulsorily been required to undergo further evaluation due to abnormal levels of hemoglobin (Hb) fractions (Hb A, Hb A₂ and Hb F). Many consequences, such as mutations in unrelated gene(s) or someone's nutritional status, have been reported to affect the Hb fractions levels. In the present study, we aimed to determine whether HBD has a molecular causative role in patients with low Hb A₂ levels (below 1.8%). The study was conducted with 20 individuals with low Hb A₂ levels who had applied to our outpatient clinic. All DNA samples were analyzed for the HBD gene. Nineteen of the 20 subjects were diagnosed to carry a mutation with one of four different δ-globin variants. Three of them had been described previously [Hb A₂-Yialousa (HBD: c.82G>T), Hb A₂-Bornova (HBD: c.350G>C) and Hb A₂-Yokoshima (HBD: c.77G>A)]. The novel [δ10(A7)Ala→Val, HBD: c.32C>T] mutation was defined as a new δ variant and reported to the HbVar database as Hb A₂-Canakkale. In conclusion, the molecular characterization of Hb A₂ low levels has been suggested to be significant for a definite diagnosis and counseling.

Twelve Cases of Hb Manitoba [α102(G9)Ser→Arg]: the Fluctuation in the Variant Expression

Hb Manitoba [α102(G9)Ser→Arg] is a rare α chain variant with diverse ethnic origins. It is mildly unstable with an expression of around 10.0-14.2% in the heterozygous state in most literature. In this study, 12 cases of Hb Manitoba [11 cases carried Hb Manitoba II (HBA1: c.309C>A) and one case carried Hb Manitoba IV (HBA1: c.307A>C)] were detected during a wide-spectrum study of α chain variants in the UK. Fluctuation in variant expression from 6.9 to 15.2% of total Hb on high performance liquid chromatography (HPLC) would pose a diagnostic dilemma in routine laboratories. Focusing on the variant expression, the median of Hb Manitoba was around 11.5% of total Hb in three cases, apparently with normal hemoglobin (Hb), and normal red blood cell (RBC) indices. Two cases showed a higher expression (13.9 and 15.2%) and five cases showed a lower expression (6.9-9.9%). The common α-thalassemia (α-thal) -α^3.7 (rightward) deletion coexisted with one case of increased Hb Manitoba expression. Iron (or other nutrient) deficiency was likely the cause of decreased Hb Manitoba percentage in this study. The α73(EF2)Val→Val (α2) (HBA2: c.222G>T) polymorphism is published for the first time and coexisted with two cases. The Cap +14 (C>G) (HBA2: c.-24C>G) polymorphism coexisted with another case in a heterozygous state. In conclusion, the fluctuation in variant expression can cause a diagnostic dilemma, especially in routine laboratories. Screening for the common -α^3.7 deletion and iron deficiency is recommended when an α chain variant is suspected.

Haemoglobin A2' and its significance in beta thalassaemia diagnosis

We have studied, haemoglobin A(2)' (A(2) prime), a delta chain variant haemoglobin occurring in a small percentage of individuals of African ancestry. In heterozygotes, the percentage of haemoglobin A(2)' was found to be slightly lower then the percentage of haemoglobin A(2), suggesting that the variant delta chain is synthesized at a reduced rate. When quantifying haemoglobin A(2) for the diagnosis of beta thalassaemia heterozygosity, it is essential to add together the A(2) and A(2)' to give 'total haemoglobin A(2)'. However, it not necessary to use a different reference range for total haemoglobin A(2) in A(2)' heterozygotes. When using microcolumn chromatography, A(2)' was found to be measured with A(2). On the high-performance liquid chromatography instrument studied, haemoglobin A(2)' fell in the haemoglobin S window but its retention time differed from that of haemoglobin S.

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.

Nonrandom patterns of codon usage and of nucleotide substitutions in human alpha- and beta-globin genes: an evolutionary strategy reducing the rate of mutations with drastic effects?

Nucleotide substitutions within a structural gene can cause two principal "drastic" phenotypic effects at the protein level: translatable leads to untranslatable and nonpolar hydrophobic in equilibrium hydrophilic amino acid substitutions. The sequence of nucleotides in the structural human alpha- and beta-globin genes and their variants were examined to determine whether codon usage, patterns of nucleotide substitutions, or both, reduced the relative and absolute rates of these unfavorable mutations. Based on translation of abnormal hemoglobins, it is likely that all 61 nontermination codons are potentially translatable, though only 47 are normally used. Moreover, codons that can mutate to a termination codon are never used whenever the corresponding amino acid is specified also by triplets that cannot mutate to termination by a single-step mutation. Thus, the number of opportunities to mutate to an untranslatable codon is reduced to the minimum compatible with the amino acid composition of these chains. The relative rates of U in equilibrium non-U substitutions were much lower than those of other substitutions. Because U residues must be involved in most termination mutations and in all nonpolar hydrophobic in equilibrium hydrophilic amino acid substitutions, there is a considerable reduction of mutational events, causing drastic phenotypic effects. These findings are likely to be the end result of evolutionary selection by yet unknown mechanisms.

Haemoglobin Lepore trait: haematological and structural studies on the Italian population

Haematological data on 59 heterozygotes for haemoglobin (Hb) Lepore and 10 double heterozygotes for Hb Lepore and beta thalassaemia from 36 Italian families are reported. The red cell indices are defined and compared with those of groups of non-thalassaemic and beta thalassaemic subjects of comparable number, age and sex distribution. The relative level of each haemoglobin fraction and the absolute production of single polypeptide chains are calculated in order to compare the expression of the non-alpha chain genes in Hb Lepore trait and beta thalassaemia. Structural studies demonstrate that the haemoglobin Lepore is of the Boston type (delta 87 beta 116) in all subjects, confirming that this type of fusion variant is probably the only one which occurs in Mediterranean populations. The distribution and incidence of the Lepore haemoglobinopathy are discussed.