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Detection and geographical mapping of beta-thalassaemia and delta-thalassaemia mutations

A detailed protocol has been prepared to define the known and unknown beta-thalassaemia mutations by a critical evaluation of the procedures now available in prenatal diagnostic laboratories throughout Europe. Mutations may be detected by a number of different procedures on deoxyribonucleic acid (DNA) enzymatically amplified by polymerase chain reaction (PCR). The methodologies in use may be divided into 2 groups for detection of known and unknown mutations. For known mutations, the simplest and most suitable methods for large scale application are primer specific amplification and reverse dot blot analysis. Both methods may be amenable to complete automation. For the former method, specific primers for each common beta-thalassaemic mutation were synthesized and for the latter, a series of oligonucleotide probes complementary to the common Mediterranean beta-thalassaemic mutations were prepared. These were made available to all participating members. The most widely used method to detect unknown mutations is denaturing gradient gel electrophoresis (DGGE), which was found to be a very powerful method for detection of mutations in the beta-globin gene. Dissemination of the technologies to directly detect the beta-thalassaemic mutations to all countries of the European Community has improved the capacity to make prenatal diagnoses by corionic villi analysis.

Detection of delta-thalassaemia mutations has been carried out by using a limited number of allele specific primers or probes. The clinical relevance of delta-thalassaemia is that double heterozygotes for delta- and beta-thalassaemia have thalassemia like red cell indices but normal haemoglobin A2 (HbA2) levels and they may thus be confused with heterozygous alpha-thalassaemia. A map of the frequency and distribution of the beta-thalassaemia and delta-thalassaemia mutations in Mediterranean at risk populations has been produced. Detailed analysis of the phenotype genotype correlation of those mutations occurring in Mediterranean populations indicates that the factors able to ameliorate the clinical picture of homozygous beta-thalassaemia are: homozygosity or double heterozygosity for a mild beta-thalassaemia mutation; coinheritance of alpha-thalassaemia; coinheritance of a genetic determinant able to increase the synthesis of gamma chains in adult life. Among these factors, however, only homozygosity for a mild mutation produces a consistent effect. The information has improved the capacity to predict the clinical phenotype in offspring and has aided genetic counselling.

Reported by

Università degli Studi di Cagliari
Via Jenner s/n
09100 Cagliari
Italy
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