Long-Molecule Sequencing: A New Approach for Identification of Clinically Significant DNA Variants in α-Thalassemia and β-Thalassemia Carriers.

Currently, carrier testing for thalassemia requires the application of different molecular tests to provide an accurate genotype. As an alternative method, long-molecule sequencing (LMS) was evaluated on the PacBio Sequel platform for genotyping carriers of α-thalassemia or β-thalassemia. Multiplex long PCR was used to generate representative amplicons for the α (HBA1/2) and β (HBB) gene loci. Following LMS, circular consensus sequencing reads were aligned to the hg19 reference genome and variants called using FreeBayes software. In a blinded study of 64 known carrier samples, all HBA1/2 and HBB variants detected by LMS were concordant with those independently assigned by targeted PCR assays. For HBA1/2 carrier samples, LMS accurately detected the common South East Asian, -α3.7, and -α4.2 deletions and four different rare single-nucleotide variants (SNVs). For HBB carrier samples, LMS accurately detected the most common Chinese insertion and deletion variant c.126_129delCTTT and 14 different SNVs/insertions and deletions and could discriminate compound heterozygous SNVs (trans configuration) and identify variants linked to benign SNPs (cis configuration). Overall, LMS displayed the hallmarks of a scalable, accurate, and cost-effective genotyping method. With further test coverage to additionally include detection of other clinically significant HBA1/2 copy number variations, such as the --THAI, --MED, and --FIL deletions, we propose that LMS will eventually serve as a comprehensive method for large-scale thalassemia carrier screening.