Is the poly A (T>C) mutation a causative factor for misdiagnosis in second trimester prenatal diagnosis of β-thalassemia by fetal blood analysis on high performance liquid chromatography?

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.

Prenatal Diagnosis of HbE-β-Thalassemia: Experience of a Center in Western India

The clinical presentation of HbE-β-thalassemia is extremely variable, however, many cases are severe and transfusion dependent. We offered prenatal diagnosis to 108 couples, 20 of whom came prospectively. CVS was done in 93 cases (9.5-13 weeks of gestation) while amniocentesis/cordocentesis was done for 15 cases in the second trimester. Diagnosis was done by reverse dot blot hybridization, ARMS, DNA sequencing and in a few cases by HPLC analysis of fetal blood. The genetic combinations in the couples at-risk were the following: HbE trait/β-thal trait-95, HbE-thal/HbE trait-5, HbE homozygous/β-thal trait-3, HbE-thal/β-thal trait-3, HbE Lepore/β-thal trait-1, HbE trait/HbD^Punjab trait-1. IVS1-5(G>C) was the commonest β-thalassemia mutation followed by codon15(G>A), codon30(G>C), codons41/42(-CTTT), the 619 bp deletion and codon8/9(+G) in the β-thalassemic parent. However, several rare mutations seen in India like -90(C>T), -88(C>T),codon15(-T), IVS1-129(A>C), IVS1-130(G>C), IVSII-1(G>A), IVSII-837(C>T) and IVSII 848(C>A) were also encountered. Twenty-one fetuses were affected (HbE-β-thal-20, β-thal major-1) and all the couples opted for termination of the pregnancies. Couples with affected children wish to undergo prenatal testing for HbE-β-thal in subsequent pregnancies. More regional centers are needed for these services, particularly in West Bengal and the North-East where HbE is very common.

Variable haematological and clinical presentation of β-thalassaemia carriers and homozygotes with the Poly A (T→C) mutation in the Indian population

OBJECTIVES

To study the varied clinical and haematological profile of β-thalassaemia homozygotes, compound heterozygotes and heterozygotes with the Poly A (T→C) mutation and its implication in prenatal diagnosis.

MATERIALS AND METHODS

Forty individuals were included in the study. Peripheral smear examination, complete blood count and haemoglobin analysis were carried out. β-thalassaemia mutation analysis was carried out by reverse-dot-blot hybridization, amplification refractory mutation system and DNA sequencing of the β-globin gene.

RESULTS

Five of the six β-thalassaemia homozygotes with the Poly A (T→C) mutation and five individuals who were compound heterozygous for the Poly A (T→C) mutation along with another common Indian β-thalassaemia mutation showed a severe β-thalassaemia major phenotype, while one individual presented as a thalassaemia intermedia. Majority of the 28 heterozygous individuals with this mutation showed borderline HbA₂ (mean HbA₂ = 3.7 ± 0.4%) levels as compared to individuals with common β-thalassaemia mutations (mean HbA₂ = 5.2 ± 1.4%). The Mean Corpuscular Volume (MCV) levels in individuals heterozygous for the Poly A (T→C) mutation (mean MCV 70.0 ± 5.2 fl) were significantly higher than in individuals with other common β-thalassaemia mutations (mean MCV 60.7 ± 7.7 fl) (P < 0.001).

CONCLUSION

It is important to identify these often silent carriers of β-thalassaemia for prenatal diagnosis as homozygotes have a severe disease.