Leukocyte Phosphomannomutase Activity in Diagnosis of Congenital Disorder of Glycosylation Ia

Abstract

Phosphomannomutase (PMM; EC 5.4.2.8) is a cytosolic enzyme that catalyzes the reversible conversion of mannose 6-phosphate to mannose 1-phosphate, a substrate for the synthesis of GDP-mannose, which is a nucleotide sugar required in glycosylation. Previously known in yeast (1)(2), Van Schaftingen and Jaeken (3) described the enzyme in 1995 in humans and its deficiency in patients with congenital disorders of glycosylation (CDG) Ia, an autosomal recessive disorder characterized by psychomotor retardation and multisystemic involvement (4). PMM deficiency is related to the presence of mutations in the corresponding PMM2 gene (5). Most patients bear the very common R141H mutation, which is never found in the homozygous state because it is probably lethal (6). Diagnosis of CDG Ia, screening for which involves evidence of abnormally glycosylated serum N-glycoproteins (7), is completed by measurement of cellular PMM activity. Authors who have published PMM activity results (3)(8)(9)(10)(11)(12)(13) have worked primarily on cultured fibroblasts and lymphoblasts or peripheral blood mononucleated cells (PBMCs). However, in most studies, the analytical conditions are described rather succinctly, no indication is given of the preanalytical conditions, and well-defined reference values are lacking. The aim of this study was to determine preanalytical conditions for the PMM assay measured in PBMCs from healthy individuals and the corresponding reference values, and to compare them with the results obtained for CDG Ia patients and their relatives. Deindentified blood samples were obtained (from January 1999 to January 2001) from 414 individuals without CDG I (control group; age range, 1 month to 87 years), 25 CDG Ia patients for whom diagnosis was confirmed by the identification of PMM2 mutations, and 35 of their parents or relatives bearing one mutation on the PMM2 gene. PBMCs were first isolated on a Ficoll gradient …