Genetic mechanism of Cis-AB inheritance. II. Cases associated with structural mutation of blood group glycosyltransferase.

The genetic mechanism of the rare occurrence of Cis-AB expression, that is, AB and/or O offspring from AB X O parents, has not been fully understood. The synthesis of blood group A and B substances are controlled by N-acetylgalactosaminyltransferase (A-enzyme) and galactosyltransferase (B-enzyme). Therefore, the genetic mechanism of Cis-AB expression may be elucidated by examining the characteristics of A- and B-enzymes in Cis-AB plasma. In a previous study, we presented evidence that Cis-AB expression in one case examined was due to unequal chromosomal crossing over producing a single chromosome with the genes for A. and B-enzymes, rather than to a structural mutation producing a single abnormal enzyme with bifunctional activity. In contrast to the previous case, the present two Cis-AB plasma contained a single abnormal enzyme that can transfer both N-acetylgalactosamine (GalNAc) and galactose (Gal). Moreover, the subjects' plasma also contained an enzymatically inactive, but immunologically cross-reacting material. Therefore, Cis-AB expression in the present two cases is due to a structural mutation in either the A or B gene producing a single abnormal enzyme with bifunctional activity.