Evaluating the Clinical Utility of a Long-Read Sequencing-Based Approach in Prenatal Diagnosis of Thalassemia

Case report: Identification of a novel triplication of alpha-globin gene by the third-generation sequencing: pedigree analysis and genetic diagnosis

BACKGROUND

Thalassemia, a common autosomal hereditary blood disorder worldwide, mainly contains α- and β-thalassemia. The α-globin gene triplicates allele is harmless for carriers, but aggravates the phenotype of β-thalassemia. Therefore, it is particularly crucial to accurately detect the structural variants of α-globin gene clusters.

CASE REPORT

We reported a 28-year-old man, the proband, with microcytic hypochromic anemia. From pedigree analysis, his mother and sister had hypochromic microcytosis, and his father was normal. Genetic testing of thalassemia identified a novel α-globin gene triplicate named αααanti4.2del726bp (NC_000016.10:g.170769_174300dupinsAAAAAA) by third-generation sequencing (TGS) in the proband and his father, which was further validated by multiplex ligation-dependent probe amplification (MLPA) and Sanger sequencing. The genotypes of the proband's mother and sister were both -α3.7/αα compounded with heterozygous HBB:c.126_129delCTTT. They were categorized as silent α-thalassemia with co-inheritance of β-thalassemia trait. The proband's genotype additionally had the α-globin gene triplicates compared with his mother and sister, which increased the imbalance between α/β-globin, so the proband had more severe hematological parameters. The proband's wife was diagnosed as HBA2:c.427T > C heterozygosis, and his daughter had the novel α-globin gene triplicates compounded with HBA2:c.427T > C, therefore the girl might be asymptomatic.

CONCLUSION

The identification of the novel α-globin gene triplicates provides more insight for the research of thalassemia variants and indicates that TGS has significant advantages on genetic testing of thalassemia for the reliability, accuracy and comprehensiveness.

The comprehensive analysis of thalassemia alleles (CATSA) based on single-molecule real-time technology (SMRT) is a more powerful strategy in the diagnosis of thalassemia caused by rare variants

Thalassemia is one of the most widely distributed monogenic disorders in the world and affects the largest number of people. It can manifest a wide spectrum of phenotypes from asymptomatic to fatal, which is associated with the degree of imbalance between α- and β-globin chains. Therefore, individuals with different genotypes could present with a similar phenotype. Genetic analysis is always needed to make a correct diagnosis. However, routine genetic analysis of thalassemia used in the Chinese population identifies only 23 common variants, resulting in many cases undiagnosed or being misdiagnosed. In this study, we applied a long-read sequencing-based approach termed comprehensive analysis of thalassemia alleles (CATSA) to 30 subjects whose hematologic screening results could not be explained by the routine genetic test results. The identification of additional variants and the correction of genotypes allowed the interpretation of the clinical phenotype in 24 subjects, which have been confirmed to be correct by independent experiments. Moreover, we identified a novel 8.4-kb deletion containing the entire HBB and HBD genes as well as part of the HBBP1 gene, expanding the genotype spectrum of β-thalassemia. CATSA showed a great advantage over other genetic tests in the diagnosis of thalassemia caused by rare variants.

Molecular characterization of a novel 83.9-kb deletion of the α-globin upstream regulatory elements by long-read sequencing

Inherited deletions of upstream regulatory elements of α-globin genes give rise to α-thalassemia, which is an autosomal recessive monogenic disease. However, conventional thalassemia target diagnosis often fails to identify these rare deletions. Here we reported a family with two previous pregnancies of Hb Bart's hydrops fetalis and was seeking for prenatal diagnosis during the third pregnancy. Both parents had low level of Hemoglobin A2 indicating α-thalassemia. Conventional Gap-PCR and PCR-reverse dot blot showed the father carried -SEA deletion but did not identify any variants in the mother. Multiplex ligation-dependent probe amplification identified a deletion containing two HS-40 probes but could not determine the exact region. Finally, a long-read sequencing (LRS)-based approach directly identified that the exact deletion region was chr16: 48,642-132,584, which was located in the α-globin upstream regulatory elements and named (αα)JM after the Jiangmen city. Gap-PCR and Sanger sequencing confirmed the breakpoint. Both the mother and fetus from the third pregnancy carried heterozygous (αα)JM, and the fetus was normally delivered at gestational age of 39 weeks. This study demonstrated that LRS technology had great advantages over conventional target diagnosis methods for identifying rare thalassemia variants and assisted better carrier screening and prenatal diagnosis of thalassemia.

Third-generation sequencing for genetic disease

Third-generation sequencing (TGS) has led to a brave new revolution in detecting genetic diseases over the last few years. TGS has been rapidly developed for genetic disease applications owing to its significant advantages such as long read length, rapid detection, and precise detection of complex and rare structural variants. This approach greatly improves the efficiency of disease diagnosis and complements the shortcomings of short-read sequencing. In this paper, we first briefly introduce the working mechanism of one of the most important representatives of TGS, single-molecule real-time (SMRT) sequencing by Pacific Bioscience (PacBio), followed by a review and comparison of the advantages and disadvantages of different sequencing technologies. Finally, we focused on the progress of SMRT sequencing applications in genetic disease detection. Future perspectives on the applications of TGS in other fields were also presented. With the continuous innovation of the SMRT technologies and the expansion of their fields of application, SMRT sequencing has broad clinical application prospects in genetic diseases detection, and is expected to become an important tool for the molecular diagnosis of other diseases.

Case report: A novel 10.8-kb deletion identified in the β-globin gene through the long-read sequencing technology in a Chinese family with abnormal hemoglobin testing results

Background

Thalassemia is a common inherited hemoglobin disorder caused by a deficiency of one or more globin subunits. Substitution variants and deletions in the HBB gene are the major causes of β-thalassemia, of which large fragment deletions are rare and difficult to be detected by conventional polymerase chain reaction (PCR)-based methods.

Case report

In this study, we reported a 26-year-old Han Chinese man, whose routine blood parameters were found to be abnormal. Hemoglobin testing was performed on the proband and his family members, of whom only the proband's mother had normal parameters. The comprehensive analysis of thalassemia alleles (CATSA, a long-read sequencing-based approach) was performed to identify the causative variants. We finally found a novel 10.8-kb deletion including the β-globin (HBB) gene (Chr11:5216601-5227407, GRch38/hg38) of the proband and his father and brother, which were consistent with their hemoglobin testing results. The copy number and exact breakpoints of the deletion were confirmed by multiplex ligation-dependent probe amplification (MLPA) and gap-polymerase chain reaction (Gap-PCR) as well as Sanger sequencing, respectively.

Conclusion

With this novel large deletion found in the HBB gene in China, we expand the genotype spectrum of β-thalassemia and show the advantages of long-read sequencing (LRS) for comprehensive and precise detection of thalassemia variants.

Third generation sequencing transforms the way of the screening and diagnosis of thalassemia: a mini-review

Thalassemia is an inherited blood disorder imposing a significant social and economic burden. Comprehensive screening strategies are essential for the prevention and management of this disease. Third-generation sequencing (TGS), a breakthrough technology, has shown great potential for screening and diagnostic applications in various diseases, while its application in thalassemia detection is still in its infancy. This review aims to understand the latest and most widespread uses, advantages of TGS technologies, as well as the challenges and solutions associated with their incorporation into routine screening and diagnosis of thalassemia. Overall, TGS has exhibited higher rates of positive detection and diagnostic accuracy compared to conventional methods and next-generation sequencing technologies, indicating that TGS will be a feasible option for clinical laboratories conducting in-house thalassemia testing. The implementation of TGS technology in thalassemia diagnosis will facilitate the development of effective prevention and management strategies, thereby reducing the burden of this disease on individuals and society.

Applications of advanced technologies for detecting genomic structural variation

Chromosomal structural variation (SV) encompasses a heterogenous class of genetic variants that exerts strong influences on human health and disease. Despite their importance, many structural variants (SVs) have remained poorly characterized at even a basic level, a discrepancy predicated upon the technical limitations of prior genomic assays. However, recent advances in genomic technology can identify and localize SVs accurately, opening new questions regarding SV risk factors and their impacts in humans. Here, we first define and classify human SVs and their generative mechanisms, highlighting characteristics leveraged by various SV assays. We next examine the first-ever gapless assembly of the human genome and the technical process of assembling it, which required third-generation sequencing technologies to resolve structurally complex loci. The new portions of that "telomere-to-telomere" and subsequent pangenome assemblies highlight aspects of SV biology likely to develop in the near-term. We consider the strengths and limitations of the most promising new SV technologies and when they or longstanding approaches are best suited to meeting salient goals in the study of human SV in population-scale genomics research, clinical, and public health contexts. It is a watershed time in our understanding of human SV when new approaches are expected to fundamentally change genomic applications.