Screening and Diagnosis of Rare Thalassemia Variants: Is Third-Generation Sequencing Enough?

CONTEXT.—Rare thalassemia subtypes are often undiagnosed because conventional testing methods can only identify 23 common types of α- and β-thalassemia.OBJECTIVE.—To assess a comprehensive approach for the screening and diagnosis of rare thalassemia.DESIGN.—The study cohort included 72 individuals with suspected rare thalassemia variants. Screening was conducted by next-generation sequencing (NGS) combined with third-generation sequencing (TGS) and chromosomal microarray analysis (CMA)/copy number variation sequencing.RESULTS.—Of the 72 individuals with suspected rare thalassemia, 49 had rare α- or β-gene variants. NGS combined with gap polymerase chain reaction detected a total of 42 cases, resulting in a positive detection rate of 58.3%. Additionally, 4 α-globin genetic deletions were identified by TGS, which increased the variant detection rate by 5.6%. Two samples with a microdeletion of chromosome 16 or 11 were detected by CMA, which increased the detection rate by 2.8%. For one sample, reanalysis of the NGS and TGS data confirmed the presence of the β41-42/βN and βN/βN mosaic. The HBB:c.315 + 2delT mutation was initially reported in Guangdong Province, China. Two HBB gene mutations (HBB:c.315 + 5G>C and HBB:c.295G>A) and 4 rare HBA gene deletions (-11.1, -α27.6, -α2.4, and -α21.9) were initially identified in the Zhonshan region. The hematologic phenotypes of all rare cases in this study were clarified.CONCLUSIONS.—Rare thalassemia variants are more common than previously thought. Despite advancements in TGS, there is still no foolproof method for detection of all types of thalassemia. Thus, a comprehensive approach is necessary for accurate screening and diagnosis of rare thalassemia variants.