Identification of double heterozygous -α^4.2Ⅰ/-α^4.2Ⅱ using third-generation sequencing

The 4.2 kb deletion (-α^4.2/) is a common a⁺-thalassemia with a carrier rate, followed by the South-East Asian deletion (-^SEA) and the 3.7 kb deletion (-α^3.7/). There are few reports about 4.2 kb deletion sub-types. Herein, we present a patient with double heterozygous -α^4.2Ⅰ/-α^4.2Ⅱwho was identified using third-generation sequencing (TGS).

Hematology and hemoglobin fraction analysis were carried out by complete blood count (CBC) and capillary electrophoresis (CE). Gap-PCR was used to detect the common deletional α-thalassemia, and multiple ligation-dependent probe amplification (MLPA) was performed to screen the large deletion. Sanger sequencing identified the variant. The different deletions were confirmed by TGS.

CBC showed the patient with microcytic hypochromic anemia, and CE indicated the presence of a Hb variant. Gap-PCR and MLPA detected 4.2 kb deletion homozygotes (-α^4.2/-α^4.2). The Hb variant was confirmed as Hb Q-Thailand by Sanger sequencing. The patient was identified as compound heterozygous of 4.2 kb deletion and Hb Q-Thailand (-α^4.2/-α^{4.2-Q-Thailand}, -α^4.2Ⅰ/-α^4.2Ⅱ) using TGS.

Hb Q-Thailand (-α^{4.2-Q-Thailand}/) complex 4.2 kb deletion heterozygote (-α^4.2/) is easily misdiagnosed as 4.2 kb homozygous using Gap-PCR and MLPA. The TGS enables the identification of the two different 4.2 kb deletion sub-types.