Development of molecular diagnostic platform for α0 -thalassemia 44.6 kb (Chiang Rai, --CR ) deletion in individuals with microcytic red blood cells across Thailand

INTRODUCTION

The α⁰ -thalassemia 44.6 kb or Chiang Rai (--^CR ) deletion has been reported in northern Thailand and is capable of causing hemoglobin (Hb) H disease and a lethal α-thalassemia genotype, Hb Bart's hydrops fetalis, in this region. However, there are no current data regarding the frequency of --^CR nationwide due to a lack of effective diagnostic assay. Therefore, this study aimed to develop a reliable platform for simultaneous genotyping of --^CR and two common α⁰ -thalassemias in Thailand (--^SEA and --^THAI ) and investigate the frequency of --^CR across Thailand.

METHODS

Multiplex gap-PCR assay and five renewable plasmid DNA controls for --^CR , --^SEA , --^THAI , α2-globin (HBA2), and β-actin (ACTB) were newly developed and validated with reference methods. The developed assay was further tested on 1046 unrelated individuals with a reduced mean corpuscular volume (MCV) of less than 75 fl for investigating genotypic and allelic spectrum of --^CR .

RESULTS

Our developed assay showed 100% concordance with reference methods. The results were valid and reproducible throughout hundreds of reactions. Comparison of the genotypic and allelic spectra revealed that heterozygous --^SEA (--^SEA /αα) and --^SEA alleles were dominant with the frequency of 22.85% (239/1046) and 13.34% (279/2092), respectively. Of these, --^THAI and --^CR were relatively rare in this population and comparable to each other with the allelic frequency of 0.14% (3/2092).

CONCLUSION

This study successfully established a reliable molecular diagnostic platform for genotyping of --^CR , --^SEA , and --^THAI in a single reaction. Additionally, we demonstrated the frequency of --^CR in Thailand for the first time and provided knowledge basis for the planning of severe α-thalassemia prevention and control programs in Thailand, where thalassemia is endemic.

Diagnosis of α0-thalassemia Chiang Rai (--CR) deletion by melt curve analysis in Northern Thailand

A large novel 44.6 kb deletion named α⁰-thalassemia Chiang Rai (--^CR) was first described in the individuals with uncommon Hb Bart's hydrops fetalis and HbH disease. This study aimed to develop a real-time gap PCR and melt curve analysis for the detection of --^CR and investigate its frequency in northern Thailand. Among 4,952 blood samples, the assay was performed in 525 samples with a mean corpuscular volume (MCV) < 80 fL, HbA₂ < 3.5%, HbA₂+E < 25%, and negative for common deletional α⁰-thalassemia --^SEA and --^THAI. The developed method showed T_m values of 85.8 ± 0.0 °C and 91.5 ± 0.1 °C, which were specific for --^CR and wild-type alleles, respectively. Nine (0.18% of 4,952 or 1.71% of 525) were positive for --^CR, in which two were HbH disease and the rest were heterozygous for --^CR. This study demonstrated the success of real-time gap PCR with melt curve analysis for --^CR diagnosis. Additionally, the prevalence of --^CR in the northern Thai population was comparable to --^THAI. Thus, this study implies the importance of --^CR in northern Thailand. Moreover, the developed real-time gap PCR with melt curve analysis is simple and highly accurate, and may be considered as an additional tool for routine α⁰-thalassemia --^CR diagnosis in this region.