Mucin Type Galactosyltransferase: Enzymology and Deficiency in the Tn-Syndrome.:Enzymology and Deficiency in the Tn-Syndrome

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Tn-syndrome (also referred to as Permanent mixed-field polyagglutinability) is due to an acquired defect of N-acetylgalactosaminide ß1, 3galactosyltransferase activity (ß1, 3Gal-T, E. C. 2.4. 1.122) in all blood cells. The deficiency encompasses a constant number of cells in an affected individual; the proportion of deficient cells, however, shows considerable variation among individuals. The Tn-syndrome is characterized by the co-expression of the Tn- and sialosyl-Tn-antigens whose molecular nature are GalNAcα(1-R) or Neu5Acα(2-6)GalNAcα(1-R), respectively, linked O-glyco-sidically to thr/ser on plasma membrane glycoproteins. The molecular cause is a repressed activity of ß1, 3Gal-T in affected cells as shown by its reversibility after treating them with 5-azacytidine. Clinically, the idiopathic form of the Tn-syndrome is correlated with hemolytic anemia and thrombopenia of variable clinical severity. Expression of Tn- and sialosyl-Tn is also found as an epiphenomenon in a variety of neoplastic disorders for which multiple causes for altered glycosylation may be responsible. These conditions are not reviewed here.The Tn-syndrome recalls in many respects paroxysmal nocturnal hemoglobinuria (PNH). However, 5-azacytidine treatment did not reverse absence of CD48 expression in a case with idiopathic PNH.
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