Thalassaemia intermedia in Cyprus: the interaction of α and β thalassaemia

Epistasis and the sensitivity of phenotypic screens for beta thalassaemia

Genetic disorders of haemoglobin, particularly the sickle cell diseases and the alpha and beta thalassaemias, are the commonest inherited disorders worldwide. The majority of affected births occur in low-income and lower-middle income countries. Screening programmes are a vital tool to counter these haemoglobinopathies by: (i) identifying individual carriers and allowing them to make informed reproductive choices, and (ii) generating population level gene-frequency estimates, to help ensure the optimal allocation of public health resources. For both of these functions it is vital that the screen performed is suitably sensitive. One popular first-stage screening option to detect carriers of beta thalassaemia in low-income countries is the One Tube Osmotic Fragility Test (OTOFT). Here we introduce a population genetic framework within which to quantify the likely sensitivity and specificity of the OTOFT in different epidemiological contexts. We demonstrate that interactions between the carrier states for beta thalassaemia and alpha thalassaemia, glucose-6-phosphate dehydrogenase deficiency and Southeast Asian Ovalocytosis have the potential to reduce the sensitivity of OTOFTs for beta thalassaemia heterozygosity to below 70%. Our results therefore caution against the widespread application of OTOFTs in regions where these erythrocyte variants co-occur.

beta-thalassemia intermedia caused by compound heterozygosity for Hb Malay (beta codon 19 AAC-->AGC; asn-->Ser) and codons 41/42 (-CTTT) beta(0)-thalassemia mutation

We report a case of beta-thalassemia intermedia caused by compound heterozygosity for hemoglobin (Hb) Malay and codon 41/42 (-CTTT) beta(0)-thalassemia mutation in a 38-year-old Chinese woman. This patient has long-standing anemia with a baseline Hb level of around 70 g/L. She worked as a full-time cashier and had not required regular blood transfusions. Nevertheless, she had splenomegaly necessitating splenectomy, cholelithiasis, and iron overload. This case illustrates the varied phenotypic expression associated with compound heterozygosity for Hb Malay and other beta-thalassemia mutations. Since Hb Malay migrates as Hb A on electrophoresis and chromatography, this variant Hb mutation ought to be included in the differential diagnosis for beta-thalassemia major or intermedia patients of Southeast Asian descent who are reported to have Hb A on the basis of Hb analysis. The possible presence of this mutation should also be considered in appropriate cases for genetic counseling in couples at risk of conceiving fetuses with beta-thalassemia major or intermedia.

Genotype-phenotype correlations in beta-thalassemias

In this paper we review the molecular basis of the marked heterogeneity of the thalassemia syndromes as well as the relative implications for carrier screening and prenatal diagnosis. The classical phenotype of heterozygous beta-thalassemia may be modified by a number of environmental and genetic interacting factors--among which the most relevant are: (1) coinheritance of alpha-thalassemia, which may normalize the red blood cell indices; (2) the presence of a mild beta-thalassemia mutation; (3) cotransmission of delta-thalassemia which may reduce the increase of HbA2 typical of heterozygous beta-thalassemia to normal values and (4) the presence of a silent mutation which can be defined only by imbalanced beta-globin chain synthesis. A number of molecular mechanisms are able to produce the non transfusion dependent attenuated forms of thalassemia syndromes referred to as thalassemia intermedia. The most common are homozygosity for mild beta-thalassemia mutations, coinheritance with homozygous beta-thalassemia of alpha-thalassemia or genetic determinants able to sustain a continuous production of HbF in adult life or the presence of heterozygosity for hyperunstable globin variants.