TAQI DIGESTION OF PCR PRODUCT INCREASES THE INFORMATIVITY OF ST14 VNTR FOR THE DIAGNOSIS OF HEMOPHILIA-A

Recently, a pair of PCR primers have been described that make it possi ble to amplify a highly polymorphic VNTR locus DX552 (St14). PCR produ cts range in size from approximately 650 to 3000 bp. Ninety X chromoso mes from unrelated Caucasian subjects were investigated. Digestion of the PCR products with TaqI underbar revealed the presence of a polymor phic TaqI underbar restriction site within the product 200 bp from the end. This restriction site is present on 60% and absent on 40% of all alleles, but the absence is confined solely to the alleles 1690 bp (3 9%) and 2100 bp (1%). Thus, there is a strong allelic association betw een the most frequent 1690 bp allele and the absence of the TaqI under bar restriction site. Determination of this polymorphisms within the S t14 VNTR region increases the expected heterozygosity at the DXS52 loc us from 72% to 80%. This increases the fraction of hemophilia A famili es where this marker is informative for indirect prenatal diagnosis an d carrier identification.