Analysis of the Genotypes in a Chinese Population with Increased Hb A2and Low Hematological Indices

Reliability of hemoglobin A2 value as measured by the Premier Resolution system for screening of β-thalassemia carriers

OBJECTIVES

Accurate quantification of hemoglobin (Hb) A2 is vital for diagnosing β-thalassemia carriers. This study aimed to assess the precision and diagnostic utility of HbA2 measurements using the new high-performance liquid chromatography (HPLC) method, Premier Resolution, in comparison to capillary electrophoresis (CE).

METHODS

We analyzed 418 samples, previously identified as A2A by CE, using Premier Resolution-HPLC. We compared the results, established correlations, and determined an optimal HbA2 cutoff value for β-thalassemia screening. Additionally, we prospectively evaluated the chosen cutoff value in 632 samples. Mutations in the β- and α-globin genes were identified using polymerase chain reaction (PCR) techniques and DNA sequencing.

RESULTS

HbA2 levels were consistently higher with Premier Resolution, yet there was a significant correlation with CE in all samples (bias, -0.33; r, 0.991), β-thalassemia (bias, -0.27; r, 0.927), and non-β-thalassemia carriers (bias, -0.36; r, 0.928). An HbA2 cutoff value of ≥4.0 % for β-thalassemia screening achieved 100 % sensitivity and 99.6 % specificity. Further validation yielded sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of 97.3 , 99.8, 97.3, 99.8, and 99.7 %, respectively. We also identified a rare β-Hb variant, Hb La Desirade [HBB:c.389C>T], associated with β-thalassemia and co-inherited with a single α-globin gene.

CONCLUSIONS

The Premier Resolution HPLC is a reliable and accurate method for routine β-thalassemia carrier screening, aligning with existing CE methods.

Clinical application of targeted long read sequencing in prenatal beta-thalassemia testing and genetic counseling

BACKGROUND

Beta thalassemia, related to HBB mutation and associated with elevated hemoglobin A2 (HbA2), is an important genetic hemoglobinopathy with high incidences of disease and carrier rates in Singapore. Carrier screening is essential to facilitate prenatal counseling and testing. However, when individuals with elevated HbA2 do not have an identifiable HBB disease-associated variant, there is ambiguity on risk to their offspring.

METHODS

We describe a case report of a proband with elevated HbA2, no identifiable HBB disease-associated variant, whose partner was a beta thalassemia carrier. Through clinical HBB gene sequencing, multiplex ligation-dependent probe amplification (MLPA) analysis, as well as targeted Nanopore long read sequencing of selected genes, we performed a complete analysis of HBB including the promoter region, 5'UTR and coding gene sequence, as well as evaluation for potential modifier variants and other rare structural variants.

RESULTS

This process identified that the proband was heterozygous for KLF1:c.544T>C (p.Phe182Leu), a potential functional polymorphism previously known to be associated with benign elevated HbA2 levels. The presence of disease variants in the HBB locus was excluded.

CONCLUSION

This finding provided clarity and enabled family planning for the proband and her family.

Application of real-time PCR-based multicolor melting curve with automatic analysis system in pregestational and prenatal thalassemia diagnoses

BACKGROUND

To evaluate the value of the real-time PCR-based multicolor melting curve analysis (MMCA) with an automatic analysis system used in a mass thalassemia screening and prenatal diagnosis program.

METHODS

A total of 18,912 peripheral blood samples from 9456 couples and 1150 prenatal samples were detected by MMCA assay. All prenatal samples were also tested by a conventional method. Samples with unknown melting peaks, unusual peak height ratios between a wild allele and a mutant allele, or a discordant phenotype-genotype match were further studied by using multiplex ligation-dependent probe amplification (MLPA) or Sanger sequencing. All MMCA results were automatically analyzed and manually checked. The consistency between MMCA assay and conventional methods among prenatal samples was investigated.

RESULTS

Except for initiation codon (T > G) (HBB:c.2T > G), all genotypes of thalassemia inside the scope of conventional methods were detected by MMCA assay. Additionally, 27 carriers with 10 rare HBB variants, 13 with α fusion gene, 1 with a rare deletion in α globin gene, and 1 with rare HBA variant were detected by using MMCA assay.

CONCLUSION

MMCA can be an alternative approach used in routine thalassemia carrier screening and prenatal diagnosis for its high throughput, sufficient stability, low cost, and easy operation.

A stepwise haematological screening and whole-exome sequencing reveal multiple mutations from SUPT5H causing an elevation of Hb A2 from a cohort of 47336 individuals

INTRODUCTION

Though an increase in Hb A₂ is one of the most key markers of β-thal carriers, a few independent cases are reported to show elevated Hb A₂ levels caused by mutations in other genes beyond β-globin gene.

METHODS

We reviewed the haematological indices of 47336 individuals to analyse the phenotype-genotype correlation and identified 1439 individuals (3.04%) positive in the elevation of Hb A₂ . Globin and KLF1 genes analysis was performed, and further whole-exome sequencing was carried to dissect the genetic causes of those positive samples without β-thalassemic or KLF1 mutations.

RESULTS

Of these 1439 individuals with elevated Hb A₂ , 1381 had a molecular defect in globin genes, and most were β-thalassemic mutation; 10 had a molecular defect in KLF1 gene. Finally, among the 38 individuals without β-thalassemic or KLF1 mutations, 7 were identified to carried a loss-of-function mutation in SUPT5H.

CONCLUSION

This study has provided a mutation spectrum of SUPT5H in a cohort screening leading to the elevation of Hb A₂ . According to the previous observations that individuals with a combination of β-thal mutation and a SUPT5H variant might present moderate β-thaelassemia, these findings emphasized the importance of comprehensive molecular diagnosis to prevent birth defects of β-thaelassemia caused by rare mutations from modifier genes.

Borderline HbA2 levels: Dilemma in diagnosis of beta-thalassemia carriers

There is inconsistency in the exact definition of diagnostic levels of HbA₂ for β thalassemia trait. While many laboratories consider HbA₂ ≥4.0 % diagnostic, still others consider HbA₂ ≥3.3 % or HbA₂ ≥3.5 % as the cut-off for establishing β thalassemia carrier diagnosis. This is because, over the years, studies have described β thalassemia carriers showing HbA₂ levels that lie above the normal range of HbA₂ but below the typical carrier range of β thalassemia. These, "borderline HbA₂ levels", though not detrimental to health, are significant in β thalassemia carrier diagnosis because they can lead to misinterpretation of results. In this review, we have evaluated the prevalence of borderline HbA₂ levels and discussed the causes of borderline HbA₂ values. We have also compiled an extensive catalogue of β globin gene defects associated with borderline HbA₂ levels and have discussed strategies to avoid misdiagnosing borderline HbA₂ β thalassemia carriers. Our analysis of studies that have delineated the cause of borderline HbA₂ levels in different populations shows that 35.4 % [626/1766] of all individuals with borderline HbA₂ levels carry a molecular defect. Among the positive samples, 17 % [299/1766] show β globin gene defects, 7.7 % [137/1766] show α thalassemia defects, 2.7 % [49/1766] show KLF1 gene mutations, 2.3 % [41/1766] show the co-inheritance of β and α thalassemia, 2.0 % [37/1766] show the co-inheritance of β and δ thalassemia and 1.8 % [32/1766] show α globin gene triplication. It appears that a comprehensive molecular work up of the β globin gene is the only definite method to detect borderline HbA₂ β thalassemia carriers, especially in populations with a high prevalence of the disease. The presence of associated genetic or acquired determinants may subsequently be assessed to identify the cause of borderline HbA₂.

Elevated Hb A2 is Not Always Indicative of β-Thalassemia

Hb A₂ levels are usually high in carriers of β-thalassemia (β-thal). These levels also provide a sensitive marker for the identification of hemoglobin (Hb) variants. In this study, we aimed to examine two patients from two Chinese families who showed elevated Hb A₂ levels but did not show any signs of β-thal. The HBB variants were analyzed using direct sequencing of HBB and in silico prediction analysis. Moreover, the family's genetic history was investigated. We examined two probands from different Chinese families with elevated Hb A₂ levels who were not afflicted with β-thal, although several nucleotide changes were found at codon 81 (CTC>CTA) (HBB: c.246C>A) in Family 1 and a compound heterozygosity for codon 40 (AGG>AAG) (HBB: c.122G>A) and IVS-II-478 (C>A) (HBB: c.316-373C>A) in Family 2. After investigating the genetic history of both families including the β-thal aspect, we found that these mutations were not responsible for the elevated Hb A₂ levels. It is rarely reported that high Hb A₂ level is not indicative of β-thal. In contrast, low or normal Hb A₂ level is always found with β-thal due to other molecular defects that mask their β-thal genotype. Our results highlight the importance of considering the genetic factors related and unrelated to β-thal to improve the accuracy of future genetic counseling.