An evaluation of the polymerase chain reaction for detection of alpha-globin genes in the prenatal diagnosis of alpha zero-thalassaemia.

Homozygous alpha zero-thalassaemia results in the fatal disease Bart's hydrops foetalis and since 3-4% of Singaporeans carry the alpha-thalassaemia genes, prenatal diagnosis of thalassaemia is essential. The aim of this study was to establish the polymerase chain reaction (PCR), a method that enables selective amplification of the 136 base pair (bp) region within the alpha-globin gene cluster, as a routine test for the prenatal diagnosis of homozygous alpha zero-thalassaemia. Confirmation of PCR results was performed using DNA gene mapping and electrophoresis of cord blood. DNA was extracted from 24 chorionic villi samples and the presence of the alpha-globin genes was determined by PCR. The results showed that the optimal number of amplifications for accurate diagnosis was 50 cycles. Homozygous alpha zero-thalassaemia was detected in four foetuses and the pregnancies terminated. Confirmation of alpha zero-thalassaemia by DNA gene mapping and electrophoresis of cord blood showed absence of alpha-globin genes and only Hb Bart's respectively. The remaining 20 foetuses were correctly diagnosed as normal or possessing the alpha-thalassaemia trait. Using the PCR at less than 50 cycles of amplification (example 35 cycles), false positive results were obtained in 30% of cases. We conclude that DNA amplification using the PCR offers an accurate method of prenatal diagnosis of alpha zero-thalassaemia. Its advantages over the more establish gene mapping method include a more rapid analysis (three days compared with ten days by gene mapping) and the requirement of only minute amounts of DNA (1 microgram) for analysis. It is however, essential that the optimal number of amplification cycles be established so that false positive results may be avoided.