Molecular Pathology of Thalassemia and Triplicated Locus in North Indian Population; Interaction with Heterozygous Thalassemia

Hematological and genetic profiles of persons with co-inherited heterozygous β-thalassemia and supernumerary α-globin genes

INTRODUCTION

Thalassemias are common monogenic autosomal recessive hemoglobin disorders. The usually asymptomatic heterozygotes (β-thalassemia traits, βTT) may rarely develop non-transfusion-dependent-thalassemia (NTDT) due to co-inheritance of supernumerary α-globin genes. Literature on phenotypic/genotypic features of these rare combinations is limited.

MATERIALS AND METHODS

We studied the demographic, clinical, and laboratory data from 47 persons with co-inherited βTT + supernumerary α-globin genes. HBB mutations were tested for by ARMS-PCR and/or Sanger sequencing, ααα^(anti3.7) /ααα^(anti4.2) and deletional α-thalassemia testing by multiplex gap-PCRs, and Xmn1^G γ genotyping by PCR-RFLP.

RESULTS

The 47 cases comprised 0.08% of 61 010 hemoglobinopathy screenings during the study period. Mean age was 31.9 ± 14.7 years (range 5.5-83 years), with 57.4% males. Thirty (63.8%) had NTDT-phenotype, 16 (34%) were asymptomatic/minimally symptomatic, and 1 became transfusion-dependent at the age of 20 years. Anemia/pallor and jaundice were the commonest complaints (76% each); 40% had required blood transfusions. Twenty-one had splenomegaly, 14 had hepatomegaly. Mean hemoglobin was 9.0 ± 1.9 g/dl (range 4.0-13.0). HbA2 was 5.1 ± 0.7% (3.4%-6.3%) and HbF% 4.2 ± 3.2% (0.5%-18.4%). Forty-four (93.6%) had ααα^anti3.7 , while 3 (6.4%) had ααα^anti4.2 triplications. HBB:c.92+5G>C (47%), HBB:c.27_28insG (14.9%), and HBB:c.47G>A (8.5%) were the commonest β-globin mutations. One case showed HBB:c.-138C>T (β⁺⁺ ), while the rest had β⁰ or severe-β⁺ mutations. Symptomatic cases had significantly lower hemoglobins and higher HbF% than asymptomatic ones.

CONCLUSION

This largest Indian and globally second-largest study reports the βTT + ααα^4.2 state for the first time in such genotypically-complex Indian cases. Supernumerary α-genes should be suspected in all βTT with disproportionate clinical symptoms, mild-to-moderately elevated HbF, and unexplained anisopoikilocytosis.

Alpha-globin gene triplication and its effect in beta-thalassemia carrier, sickle cell trait, and healthy individual

The genotype and phenotype correlation between coinheritance of heterozygous beta-thalassemia with the alpha-globin triplication is unclear. In this study we have investigated and reviewed alpha triplication frequency in beta-thalassemia carriers, sickle cell trait, and healthy individuals and its effect on hematological and phenotypical changes. In this study, 4005 beta-thalassemia carriers, 455 sickle cell trait, and 2000 healthy individuals were included. Molecular characterization of beta and alpha-thalassemia was performed. The frequencies of alpha-globin triplication in beta-thalassemia carriers, sickle cell trait, and healthy individuals were 67 (1.67%), 4 (0.88%), and 18 (0.9%), respectively. In total, the frequency of alpha-triplications is approximately 89 (1.39%) in Khuzestan province, South of Iran population. We have compared the average hematological parameters of beta-thalassemia carriers, sickle cell trait, and healthy individuals with and without alpha gene triplication. This mutation did not show any significant effect on the change of blood indices, neither in healthy individuals nor in sickle cell trait and beta-thalassemia carriers. Therefore, there is no need to take more notice of anti 3.7 mutation in beta-thalassemia carriers is opposed with some studies reported that the presence of excess alpha-globin genes in beta-thalassemia carriers can lead to the phenotype of beta-thalassemia intermedia. Therefore, not every individual with triplicated alpha globin coinherited with beta-thalassemia trait will have a significantly lower Hb than normal, and it is highly likely that none of them will need transfusion.

A decision support scheme for beta thalassemia and HbE carrier screening

The most effective way to combat β-thalassemias is to prevent the birth of children with thalassemia major. Therefore, a cost-effective screening method is essential to identify β-thalassemia traits (BTT) and differentiate normal individuals from carriers. We considered five hematological parameters to formulate two separate scoring mechanisms, one for BTT detection, and another for joint determination of hemoglobin E (HbE) trait and BTT by employing decision trees, Naïve Bayes classifier, and Artificial neural network frameworks on data collected from the Postgraduate Institute of Medical Education and Research, Chandigarh, India. We validated both the scores on two different data sets and found 100% sensitivity of both the scores with their respective threshold values. The results revealed the specificity of the screening scores to be 79.25% and 91.74% for BTT and 58.62% and 78.03% for the joint score of HbE and BTT, respectively. A lower Youden's index was measured for the two scores compared to some existing indices. Therefore, the proposed scores can obviate a large portion of the population from expensive high-performance liquid chromatography (HPLC) analysis during the screening of BTT, and joint determination of BTT and HbE, respectively, thereby saving significant resources and cost currently being utilized for screening purpose.

Jaundice and alpha gene triplication in beta-thalassemia: Association or causation?

There are few studies investigating alpha globin gene triplications in beta-thalassemia in Asian Indians and its effect on phenotype, which was the primary aim of this study. Gap-PCR was performed in order to detect common alpha thalassemia determinants (-alpha(3.7), -alpha(4.2) and alpha alpha alpha(anti 3.7) triplication). Alpha-triplication was detected in 15.4% (10/65) of patients with thalassemia intermedia, 8.8% (4/45) of those with thalassemia minor and in 2.7% (2/74) of healthy controls. The severity of jaundice was higher in thalassemia intermedia cases with alpha-triplication and two of the alpha-triplication cases had a marked increase in serum bilirubin following intercurrent illness. Thus, alpha globin gene triplication is important genetic determinant underlying thalassemia intermedia in North Indians. Patients with alpha-triplication may develop prominent jaundice with marked increase in serum bilirubin following antecedent aggravating factors.

Nucleotide -88 (C-T) promoter mutation is a common beta-thalassemia mutation in the Jat Sikhs of Punjab, India

A study of beta-gene mutations in Jat Sikhs, a subcaste of Punjabis, revealed a very high prevalence (46%: 41/88) of the mild beta++ promoter region mutation -88 (C-T). Sixteen individuals presenting in homozygous form were clinically mild. Un-transfused patients had characteristic hematological findings: high Hb F (38.1-68.6%, mean 47.4%), high Hb A2 (5.7-9.8%, mean 6.88%), and the rest had adult hemoglobin. The 19 subjects with compound heterozygosity for -88 (C-T) and another beta-gene mutation presented both as thalassemia intermedia (four cases) and as thalassemia major (15 cases). One of the four patients with the milder phenotype had a second mild mutation, CAP+1 (beta++). In the other three cases with the milder phenotype, the second mutation was associated with the presence of the XmnI Ggamma polymorphism. Notably, the XmnI Ggamma was negative in all the -88 (C-T) alleles. None of the patients had associated alpha-thalassemia even in the thalassemia intermedia group. Haplotype analysis of the -88 (C-T) homozygous cases showed a single haplotype (+ - - - - + -) in all but two individuals. This haplotype is distinct from those described in the Africans with homozygous -88 (C-T), suggesting that the mutation in our population occurred independently.