Molecular Characterization of β- and α-Globin Gene Mutations in Individuals with Borderline Hb A 2 Levels.

Elevated Hb A ₂ level (≥4.0%) is considered to be reliable parameter to identify β-thalassemia (β-thal) carriers. However, some β-thal carriers have been misdiagnosed as their Hb A ₂ levels are below 4.0%. In addition, coinheritance of α-thalassemia (α-thal) and β-thal might affect Hb A ₂ levels. Therefore, the aim of this study was to investigate the mutations of β- and α-globin genes in individuals with borderline Hb A ₂ levels in Thailand. Three hundred samples from individuals with Hb A ₂ levels of 3.5-3.9% were collected for molecular diagnosis of β-globin gene mutations. In addition, the α ⁰ -thal, α ⁺ -thal, Hb Constant Spring (Hb CS, HBA2 : c.427T>C), and Hb Paksé ( HBA2 : c.429A>T) diagnostics were also performed. Sixteen samples (5.33%) had β-globin gene mutations, and codon 41/42 (-TTCT) ( HBB : c.126_129delCTTT) was the most prevalent mutation. Ninety-eight samples (32.67%) had α-globin gene mutations including four Hb H (β4)-Hb CS disease, two Hb H disease, 13 heterozygous α ⁰ -thal, 11 homozygous α ⁺ -thal, two α ⁺ -thal/Hb CS, one α ⁺ -thal/Hb Paksé, 61 heterozygous α ⁺ -thal, and four Hb CS. Furthermore, seven cases of β-thal carriers coinheriting α-thal were observed, and five of them carried Hb H disease. High prevalence of both α- and β-thal in subjects with borderline Hb A ₂ levels suggested that molecular diagnosis of α- and β-thal should be performed, especially in a high prevalence area of thalasssemia carriers, for accurate diagnosis and genetic counseling to prevent and control new severe thalassemia cases. Moreover, β-thal carriers who coinherited Hb H disease might have reduced Hb A ₂ levels, leading to a misdiagnosis of β-thal in analysis programs.