Homozygous beta-thalassaemia resulting in the beta-thalassaemia carrier state phenotype

Clinical, laboratory, and molecular characteristics of a cohort of children with hemoglobinopathy S/beta-thalassemia

INTRODUCTION

Hemoglobinopathy Sβ-thalassemia (HbSβ-thal) has a wide range of clinical and laboratory severity. There is limited information on the natural history of HbSβ-thal and its modulating factors. We described the molecular, hematological, and clinical characteristics of a cohort of children with HbSβ-thal and estimated its incidence in Minas Gerais, Brazil.

METHODS

Laboratory and clinical data were retrieved from medical records. Molecular analysis was performed by HBB gene sequencing, PCR-RFLP, gap-PCR, and MLPA.

RESULTS

Eighty-nine children were included in the study. Fourteen alleles of β-thal mutations were identified. The incidence of HbSβ-thal in the state was 1 per 22,250 newborns. The most common βS-haplotypes were CAR and Benin. The most frequent βthal-haplotypes were V, II, and I. Coexistence of 3.7 kb HBA1/HBA2 deletion was present in 21.3 % of children. β-thalassemia mutations were associated with several clinical and laboratory features. In general, the incidence of clinical events per 100 patient-years was similar for children with HbSβ0-thal, IVS-I-5 G>A, and IVS-I-110 G>A. Children with HbSβ+-intermediate phenotypes had a more severe laboratory and clinical profile when compared with those with HbSβ+-mild ones. βS-haplotypes and α-thalassemia did not meaningfully influence the phenotype of children with HbSβ-thal.

CONCLUSION

The early identification of β-thalassemia alleles may help the clinical management of these children.

Hb S/-Thalassemia in the REDS-III Brazil Sickle Cell Disease Cohort: Clinical, Laboratory and Molecular Characteristics

We described the clinical, laboratory and molecular characteristics of individuals with Hb S (HBB: c.20A>T)/β-thalassemia (Hb S/β-thal) participating in the Recipient Epidemiology and Donor Evaluation Study (REDS-III) Brazil Sickle Cell Disease cohort. HBB gene sequencing was performed to genotype each β-thal mutation. Patients were classified as Hb S/β⁰-thal, Hb S/β⁺-thal-severe or Hb S/β⁺-thal based on prior literature and databases of hemoglobin (Hb) variants. Characteristics of patients with each β-thal mutation were described and the clinical profile of patients grouped into Hb S/β⁰-thal, Hb S/β⁺-thal and Hb S/β⁺-thal-severe were compared. Of the 2793 patients enrolled, 84 (3.0%) had Hb S/β⁰-thal and 83 (3.0%) had Hb S/β⁺-thal; 40/83 (48.2%) patients with Hb S/β⁺-thal had mutations defined as severe. We identified 19 different β-thal mutations, eight Hb S/β⁰-thal, three Hb S/β⁺-thal-severe and eight Hb S/β⁺-thal. The most frequent β⁰ and β⁺ mutations were codon 39 (HBB: c.118C>T) and IVS-I-6 (T>C) (HBB: c.92+6T>C), respectively. Individuals with Hb S/β⁰-thal had a similar clinical and laboratory phenotype when compared to those with Hb S/β⁺-thal-severe. Individuals with Hb S/β⁺-thal-severe had significantly lower total Hb and Hb A levels and higher Hb S, white blood cell (WBC) count, platelets and hemolysis markers when compared to those with Hb S/β⁺-thal. Likewise, individuals with Hb S/β⁺-thal-severe showed a significantly higher occurrence of hospitalizations, vaso-occlusive events (VOE), acute chest syndrome (ACS), splenic sequestration, blood utilization, and hydroxyurea (HU) therapy.

Mild β(+)-thalassemia associated with two linked sequence variants: IVS-II-839 (T>C) and IVS-II-844 (C>A)

We report four unrelated families with a mild β(+)-thalassemia (β(+)-thal) allele consisting of two sequence variants at the 3' end of IVS-II: IVS-II-839 (T>C) (HBB: c.316-12T>C) and IVS-II-844 (C>A) (HBB: c.316-7C>A). These sequence variants alter the conserved polypyrimidine tract of the consensus splice acceptor sequence (Y11NYAG/G), which could reduce splicing efficiency. This may represent a common, yet under-diagnosed β(+)-thal allele in African populations.

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?

We report the problems in diagnosis faced by two families referred for prenatal diagnosis of thalassemia where cordocentesis and fetal blood analysis by high performance liquid chromatography (HPLC) had to be done. The Hb A levels of the fetal blood measured by HPLC on the VARIANT™ Hemoglobin Testing System were 1.2 and 6.7%, respectively, suggestive of a heterozygous β-thalassemia (β-thal) fetus in the first case and a normal fetus in the second case. In one family, one of the parents had a borderline Hb A(2) level and in the other, one parent had normal RBC indices. However, DNA sequencing, done later, showed that in the first case the fetus was a compound heterozygote for the IVS-I-5 (G>C) and the polyadenylation signal site [poly A (T>C)] mutation, while in the second case, the fetus was homozygous for the poly A mutation. This emphasizes that characterization of β-thal mutations must be done whenever one of the parents has a borderline Hb A(2) level or normal RBC indices, and one should not rely on fetal blood analysis by HPLC for prenatal diagnosis of β-thal so as to avoid misdiagnosis.

Triagem de hemoglobinopatias em doadores de sangue de Caxias do Sul, Rio Grande do Sul, Brasil: prevalência em área de colonização italiana

A alta prevalência de beta-talassemia em italianos e a participação dos mesmos na formação étnica da cidade de Caxias do Sul e arredores, Rio Grande do Sul, Brasil, conduziram-nos à investigação de hemoglobinopatias em uma amostra de 608 doadores de sangue do Hemocentro Regional de Caxias do Sul. Apesar da influência étnica, encontramos 1,81% de hemoglobinas anormais (0,16% Hb AC, 0,99%, Hb AS e 0,66% Hb AH), um padrão similar com o estudo do interior do Estado do Rio Grande do Sul para alterações qualitativas. Para as talassemias, as técnicas mais comuns, cruzadas com seqüenciamento de DNA, em nossas mãos, não foram capazes de esclarecer anormalidades quantitativas da hemoglobina. Esse resultado pode ser atribuído a alterações genéticas ainda não conhecidas, a limitações técnicas ou, mais simplesmente, à miscigenação.

Genetic modifying factors in beta-thalassemia

The beta-thalassemia is probably the most extensively studied genetic disease. Essentially any molecular defect that has been first described in association with the globin genes has been later implicated as a molecular determinant of newly discovered genes. Accordingly, the thalassemias have always represented a model genetic disease, especially in relation to the development of programs for population screening, genetic counseling and prenatal diagnosis. Here we will review the present knowledge on the genetics of thalassemia and of the relevant modifying factors. Major categories of the carrier state, the genotypes, the clinical phenotypes and the correlation between genotype and phenotype will be discussed.

Molecular, haematological and clinical studies of the -101 C --> T substitution of the beta-globin gene promoter in 25 beta-thalassaemia intermedia patients and 45 heterozygotes

We report the clinical, haematological, biosynthetic and molecular data of 25 double heterozygote beta-thalassaemia intermedia patients and 45 beta-thalassaemia heterozygotes with the C --> T substitution at nucleotide position -101 from the Cap site, in the distal CACCC box of the beta-globin gene promoter. This mutation is considered the most common amongst the silent beta-thalassaemia mutations in Mediterranean populations. Of the 25 compound heterozygotes for the beta -101 C --> T and common severe beta-thalassaemia mutations, all but one had mild thalassaemia intermedia preserving haemoglobin levels around 9.5 g/dl and haemoglobin F levels < 25%. The only transfused patient was characterized to have an additional alpha-globin gene. Strict assessment of haematological and biosynthetic findings in the heterozygotes for the beta -101 C --> T mutation (excluding six cases with an alpha-globin gene defect) demonstrated that less than half of them had completely normal (silent) haematology; the remainder had either high haemoglobin A2 values (in the range of 3.7-5.1%) and/or low red cells indices and/or raised haemoglobin F values. The alpha/non-alpha-globin chain synthesis ratios were generally raised, with mean 1.44 (1.07-2.10). Amongst the parents of the compound heterozygotes, who were not selected for molecular analysis following haematological screening, half of the cases were completely silent. Interaction with severe beta-thalassaemia mutations always resulted in the clinical phenotype of mild non-transfusion-dependent thalassaemia intermedia.

Molecular, haematological and clinical studies of a silent beta-gene C-->G mutation at 6 bp 3' to the termination codon (+1480 C-->G) in twelve Greek families

We report the clinical, haematological, biosynthetic and molecular data of 12 beta-thalassaemia intermedia patients and their heterozygous parents, all of whom carried a rare C-->G mutation at nucleotide position 6 3' to the termination codon (term. cd +6 C-->G) in the 3' untranslated region (3' UTR) of the beta-globin gene (+1480 C-->G). This mutation has been reported previously in a single beta-thalassaemia intermedia patient of Greek origin. The 12 patients of the present study had the clinical phenotype of mild non-transfusion-dependent thalassaemia intermedia, preserving haemoglobin levels around 9 g/dl and haemoglobin F levels <25%. All were compound heterozygotes for the +1480 C-->G mutation and common severe beta-thalassaemia mutations. The haematological parameters of heterozygotes with this mutation were within the normal range with the exception of a slightly raised alpha/non-alpha-globin chain synthesis (1.2-1.9). mRNA analysis demonstrated a 20-34% reduction in mRNA levels associated with the +1480 C-->G mutation compared to normal beta-globin alleles. These findings confirm that the C-->G mutation at position 6 3' to the termination codon is a mild beta-thalassaemia mutation causing slight reduction in beta-globin mRNA levels and beta-globin chain synthesis. It becomes clinically relevant when co-inherited with a severe beta-thalassaemia mutation in trans.