Genetic dissection of clinical heterogeneity in hemoglobin H patients by targeted long-read sequencing

Hemoglobin H (HbH) disease is a subtype of α-thalassemia typically caused by genetic defects in three out of four α-globin genes. To date, the genetic factors contributing to the highly heterogeneous clinical severity of HbH disease remain largely unknown. In this study, we perform targeted long-read sequencing (T-LRS) on a cohort of 591 HbH patients, aiming to profile the genomic variants and their haplotypes within the α/β-globin gene clusters and key erythroid genes. Phenotypic analysis confirms that non-deletional HbH patients generally exhibit more severe clinical manifestations compared to deletional ones. Moreover, we identify the co-inheritance of β-thalassemia mutations to be a mitigating factor for HbH patients, as reflected by higher hemoglobin levels and lower serum ferritin, suggesting the less imbalanced synthesis of α/β-globin among these patients. Furthermore, through haplotype phasing using long-sequencing reads, we find a haplotype of HS40 associated with milder clinical symptoms of HbH patients, validated by reporter gene assay, and that functional mutations in erythroid transcription factors BCL11A and MYB-HBS1L exert significant effects on β-thalassemia but not on HbH patients. This study presents the largest T-LRS study for α-thalassemia patients, which may provide insight into precise clinical diagnosis and phenotyping of HbH diseases.