A Case Report of Compound Heterozygosity for β0/β+-Thalassemia Resulting from under Diagnosed β-Thalassemia Found in a Hb A'2 Sample

Phenotypic Expression of Known and Novel Hemoglobin A2-Variants, Hemoglobin A2-Mae Phrik [Delta 52(D3) Asp > Gly, HBD:c.158A > G], Associated with Hemoglobin E [Beta 26(B8) Glu > Lys, HBB:c.79G > A] in Thailand

The interactions of δ-globin variants with α- and β-thalassemia or other hemoglobinopathies cause complex thalassemic syndromes and potential diagnostic problems. Understanding the molecular basis and phenotypic expression is crucial. Four unrelated Thai subjects with second hemoglobin (Hb) A2 fractions were studied. A standard automated cell counter was used to acquire initial hematological data. Hb analysis was carried out by capillary electrophoresis (CE) and high-performance liquid chromatography (HPLC) assays. Globin gene mutations and haplotype were identified by appropriate DNA analysis. An allele-specific polymerase chain reaction method was developed to provide a simple molecular diagnostic test. Hb analysis revealed a Hb A2 variant in all cases. DNA analysis of the δ-globin gene identified the Hb A2-Melbourne [δ43(CD2)Glu > Lys] variant in combination with Hb E in three cases. Analysis of the remaining case identified a novel δ-Hb variant, namely Hb A2-Mae Phrik [δ52(D3)GAT > GGT; Asp > Gly], found in association with Hb E and α+-thalassemia, indicative of the as yet undescribed combination of triple heterozygosity of globin gene defects. An allele-specific PCR-based assay was successfully developed to identify this variant. The β-haplotype of the Hb A2 Mae-Phrik allele was strongly associated with haplotype [+ − − − − ± +]. This study advanced our understanding of the phenotypic expression of known and novel δ-Hb variants coinherited with other globin gene defects, routinely causing problems with diagnosis. Therefore, knowledge and recognition of this Hb variant and molecular assessments are crucial to improving diagnosis.

δ-Hemoglobinopathies in Thailand: screening, molecular basis, genotype-phenotype interaction, and implication for prevention and control of thalassemia

The δ-globin gene defects are clinically silent but interaction with β-thalassemia can lead to a misdiagnosis of β-thalassemia carrier. We report an extensive molecular characterization of δ-hemoglobinopathies in Thailand. Study was done on 32,108 subjects, encountered at the thalassemia screening. Six different approaches based on the reduced Hb A₂ or appearance of Hb A₂-derivative were established for selective recruitment of subjects. Among 32,108 subjects, a total of 296 subjects were suspected of having δ-globin gene defects. Of these 296 subjects, Hb and DNA analyses identified δ-hemoglobinopathies with 10 different mutations in 34 (0.11%) of them. These included a novel mutation, [δ^{CD30(AGG>GGG)} (n = 1)], 5 previously undescribed in Thailand, [δ^-44(G>A) (n = 7), Hb A₂-Troodos (n = 5), δ^{IVSII-897(A>C)} (n = 4), δ^-68(C>T) (n = 2), and Hb A₂-Indonesia (n = 1)], and 4 mutations previously found in Thailand, [Hb A₂-Melbourne (n = 9), δ^-77(T>C) (n = 3), Hb A₂' (n = 1), and Hb A₂-Kiriwong (n = 1)]. Genetic heterogeneities seen included interactions of δ-globin gene defects with heterozygous Hb E, β-thalassemia, α-thalassemia, and in cis locations of the Hb A₂-Troodos and Hb E mutations found for the first time. Rapid identification methods of these δ-globin gene mutations were developed. The results should prove useful to a prevention and control program of hemoglobinopathies in the region.

Association of Hb A2 Variants with Several Forms of α- and β-Thalassemia in Thailand

In this study, Hb A₂ variants and their association with α- and β-thalassemia (α- and β-thal) were analyzed. We performed molecular analyses to identify α-thal [- -^SEA (Southeast Asian), - -^THAI (Thai), -α^3.7 (rightward) and -α^4.2 (leftward)] deletions, and Hb Constant Spring (Hb CS; HBA2: c.427T>C), Hb A₂-Melbourne (HBD: c.130G>A), Hb A₂' (HBD: c.49G>C), Hb A₂-Lampang (HBD: c.142G>A). β⁰-Thalassemia mutations included codon 17 (A>T) (HBB: c.52A>T), codons 41/42 (-TCTT) (HBB: c.126_129delCTTT), codons 71/72 (+A) (HBB: c.216_217insA) and IVS-I-1 (G>T) (HBB: c.92+1G>T) in 23 samples which had a Hb A₂ variant peak in zone 1 of the capillary electrophoresis (CE) electropherogram. Results showed that 20 patients (87.0%) carried Hb A₂-Melbourne with seven different genotypes for α- and β-thal, two (8.7%) carried Hb A₂' and one (4.3%) carried Hb A₂-Lampang. All three samples doubly heterozygous for Hb A₂-Melbourne/β⁰-thal had Hb A₂ levels lower than 4.0%, while summation of Hb A₂ and Hb A₂-Melbourne ranged from 4.9-5.3%, reaching the accepted range (4.0-10.0%) for β-thal trait. Hb A₂-Melbourne is the most common δ-globin variant in the Thai population. Hb A₂ variant and Hb A₂ levels must be combined in order to diagnose carriers of β-thal. β-Globin haplotype analysis showed an association with a single β-globin haplotype [+ - - - - + +] of Hb A₂-Melbourne, Hb A₂' and Hb A₂-Lampang, indicating that they were of the same origin. We developed a multiplex allele-specific polymerase chain reaction (ASPCR) for simultaneous detection of these three Hb A₂ variants.