Molecular and clinical characterization of −α 4.2 subtypes in Shenzhen, Southern China

Objectives: The 4.2-kb deletion (−α4.2) is a common cause of α-thalassemia in southern China. However, the molecular characteristics and clinical phenotypes of its subtypes remain incompletely characterized. This study aims to investigate these knowledge gaps for −α4.2 subtypes, explore their formation mechanisms, and determine their association with HBA1:c.223G>C (p.Asp75His; Hb Q-Thailand).Methods: We retrospectively analyzed α-thalassemia testing data from 18,050 individuals, identifying 112 −α4.2 carriers previously characterized by Multiplex Ligation-dependent Probe Amplification (MLPA). Those MLPA data were used for subtyping. Sanger sequencing detected HBA1:c.223G>C and deletion breakpoints. Bioinformatics analyzed subtype distinctions. Associated hematological parameters were evaluated for clinical significance.Results: MLPA subtyped 98.2% (110/112) carriers: 88.2% (97/110) as −α4.2 (C) and 11.8% (13/110) as −α4.2 (B). HBA1:c.223G>C displayed strong linkage with −α4.2 (B) [11/13 cases; termed −α4.2-Q (B)], but not with −α4.2 (C). Notably, we recognized two novel −α4.2 (B) cases lacking this mutation [−α4.2-without Q association (B)], representing the first report of such variants. Breakpoint analyses showed distinct structural features: −α4.2 (C) has an A-rich sequence with single-stranded characteristics, while −α4.2 (B) predictedly forms complex stable secondary structures. Hematological analysis showed that −α4.2-Q (B) exhibited the characteristic Hb Q-Thailand pattern, while −α4.2-without Q association (B) had parameters similar to −α4.2 (C).Conclusion: Based on this systematic analysis, we proposed a dual-mechanism hypothesis (‘functional trigger + linkage maintenance’) to explain the Hb Q-Thailand/−α4.2 (B) association. Hematological findings were consistent. These results deepen our understanding of the molecular and clinical characteristics of −α4.2 subtypes, helping clinicians provide more precise counseling.