Diagnostic Challenges in a Low a₂ β-Thalassemia Case

Inheritance of two β-thal alleles results in transfusion dependent anemia, growth retardation, and skeletal abnormalities (β-thal major), whereas heterozygosity (β-thal minor or β-thal trait) is clinically asymptomatic. β-thal trait leads to hypochromia, microcytosis, and in some cases, mild anemia. The characteristic finding in β-thal trait, however, is an elevated level of Hb A₂ (>3.5%). Many factors are known to influence Hb A₂ levels: α-thalassemia and Fe deficiency are two well-established factors. However, co-inheritance of α-thal with β-thal trait or concurrent Fe deficiency does not decrease Hb A₂ to normal levels (1.8-3.5%). The differential diagnosis of “normal Hb A₂ β-thal trait” includes δβ thalassemias resulting in the deletion of both δ and β globin genes, rare, mild, “silent” β-thal alleles (such as the -101 C→T mutation in the β-globin promoter), and rarely coinheritance of δ-thal alleles.

We present a novel case of low Hb A₂ β-thalassemia trait, wherein the value of Hb A₂ is affected by the presence of δ gene mutation. The proband was never diagnosed to have a β-thalassemia trait until recently when her child was observed to have Hb FXA (variant hemoglobin + β-thalassemia compound heterozygosity).

A 6-week-old Caucasian baby was referred to the pediatric hematology/oncology section of North Carolina Baptist Hospital for routine evaluation of Hb FXA pattern observed on newborn screening by electrophoresis. The parents were counseled by the physician to learn that the father is a known case of Hb D-Los Angeles trait but the mother was never diagnosed to have β-thalassemia. Initial CBC of the child revealed: Hb 11.7g/dL, Hct 34.9%, MCV 88.3fl, MCH 29.5pg, and MCHC 33.4g/dl. An EDTA blood sample on the child, mother, and father was sent to the T.H.J. Huisman Hemoglobinopathy Laboratory at Georgia Regents University in Augusta, Georgia for definitive identification of the Hb variant.

Hemoglobin electrophoresis was performed and different hemoglobin fractions were calculated using cation exchange high performance liquid chromatography (HPLC) on a Synchropak CM-300 column Eprogen, Darien, IL, USA]. Genetic testing was performed to characterize the specific β-globin gene mutation. Amplification of the β-globin gene by polymerase chain reaction (PCR) and direct sequencing were used as a fast and reliable method for identification of the β-globin gene mutations. The HEP and DNA results are shown below.

The Hemoglobin phenotype and genotype of the child is consistent with Hb D-Los Angeles - β⁺thalassemia. A heterozygous β⁺ thalassemia mutation was found [IVS I-5, G→T] along with a heterozygous Hb D-Los Angeles mutation [β121(GH4)Glu→Gln, GAA→CAA] on the infant’s sample. The father’s phenotype and genotype is consistent with Hb A – D Los Angeles trait; however, the mother’s genotype analysis is not consistent with the phenotype analysis. Her HEP shows a normal hemoglobin pattern but a heterozygous β⁺ thalassemia mutation [IVS I-5, G→T] was observed in her DNA. IVS I-5, G→T mutation is known to produce a high Hb A₂ phenotype, but in our case, it was found to be within normal range. We further investigated to look for the presence of α-thalassemia that turned out to be negative (αα/αα). We further amplified and sequenced the δ globin gene and found a heterozygous Hb A₂-Yialousa mutation [δ27(B9)Ala → Ser; HBD: c.82G→T] leading to δ⁺ thalassemia. This mutation activates a cryptic splice site at the boundary of Exon-1 and intron 1, thus leading to a decrease in correctly spliced δ-globin mRNA and hence, δ-thal phenotype.

Hb A₂-Yialousa explains the presence of a normal level of Hb A₂ in the mother’s sample. The phenotype of increased Hb A₂ typical of β-thalassemia carriers can be reduced to normal or borderline values because of the coinheritance of a δ-globin gene mutation, which may lead to misinterpretation of diagnostic results. Thus, Hb A₂ values alone cannot be relied upon for the diagnosis of β-thalassemia cases.