LE DIAGNOSTIC PRENATAL DANS LES HEMOGLOBINOPATHIES HUMAINES

Abstract

About 1000 pregnancies risking sickle cell disease or thalassemia have now been diagnosed in 13 centers throughout the world. Up to now,the classical method for diagnosing these antenatal hemoglobinopathies has used fetal red blood cells obtained from the fetal blood in utero by fetoscopy or by aminocentesis when the former technique was impossible. The fetal blood sample, rich in reticulocytes, is incubated with a radioactive amino acid incorporated into the hemoglobin synthesized in the reticulocytes. Using biochemical techniques, the globin chains are separated by chromatography and the proportion of adult globin chains determined. The homozygous state in β thalassemia is characterized by a complete depression of the β (A) chain synthesis or a β (A/γ) ratio of globin chain synthesis below 0.02. Fetuses heterozygous for β thalassemia show a β/γ ratio above 0.03, while normal fetuses have a ratio higher than 6%. Sickle cell disease in the homozygous state is characterized by complete depression of β (A) chain synthesis with only β(S) chain synthesis present as the adult globin chain. When the fetal blood is contaminated by maternal blood, the adult red cells have to be discarded. We have developed a new techniqe isoelectrically focusing the hemoglobins on acrylamide slab gels. This method is faster and easier than the classical one and eliminates the use of radioactive meterial. It can be used in diagnosing prenatal sickle cell disease and thalassemias, but any maternal cells present must be completely eliminated. Genetic engineering is a fascinating new approach which can already be applied in many cases of sickle cell disease diagnosis. The DNA of amniotic fluid cells is digested at specific sites by restriction enzymes, and the fragments are analysed according to size. The globin genes can be selectively characterized by specific and radioactive probes complementary to the β globin genes. This method is not yet applicable to β thalassemias, but has a great potential in the future.