Amniotic fluid secretor typing: validation for use in prenatal prediction of myotonic dystrophy

Use of the well-established linkage of the secretor locus (Se) to the myotonic dystrophy locus (Dm) as an indirect means for prenatal prediction of myotonic dystrophy requires that the phenotypic expression of the fetal secretor locus be accurately and reliably assessed in amniotic fluid. Secretor status determinations on 89 amniotic fluids obtained by second trimester amniocentesis were compared with results of postnatal secretor typing of saliva samples collected from the resulting 89 offspring. The secretor types of the paired amniotic fluid-saliva samples were in agreement in all cases. The only unusual typing result was on an amniotic fluid from a blood group O secretor fetus in which the level of soluble H antigen was estimated to be about one-third that observed for other H secretors. Though it is possible that rare secretor fetuses with very low titers of soluble antigen at the time of amniocentesis could be mistyped as non-secretors, our results indicate that such an erroneous typing result would only occur in about 1-2% of all amniotic fluids tested. Constructed mixtures of 10% heparinized blood in non-secretor amniotic fluid or of 10% serum in amniotic fluid derived from secretor fetuses did not introduce sufficient levels of soluble antigen or of antibody to interfere with accurate secretor typing, thus providing reassurance that maternal and/or fetal blood contamination of amniotic fluid does not compromise accuracy of fetal secretor typing. This study documents the accuracy and reliability of amniotic fluid secretor typing for prenatal prediction of fetal risk for later development of myotonic muscular dystrophy.