Activity values of the enzyme phosphomanomutase 2 for diagnosing the CDG Ia glycosylation defect

Congenital disorders of glycosylation (CDG) are biochemically characterized by abnormal carbohydrate assembly. These disorders can result from mutations in the genes encoding different enzymes, leading to a partial or complete reduction in catalytic activity. They can be inherited in an autosomal recessive, dominant, or X-linked manner. Approximately 130 congenital disorders are currently known, within a multisystemic clinical spectrum with high heterogeneity between family members and between different types of CDG. Deficiencies of the cytosolic enzyme phosphomannomutase 2 (PMM2, E.C.5.4.2.8) cause a type-Ia glycosylation defect (PMM2-CDG or CDG Ia), which results in neurological damage, coagulation disorders, and gastrointestinal, hepatic, cardiac, and ocular problems, among others. This enzyme catalyzes the isomerization of mannose-6-phosphate to mannose-1-phosphate.

Our aim was to determine PMM2 activity levels using a microspectrophotometric method.

We collected 50 whole blood samples from healthy volunteer donors, including 24 females and 26 males aged 1.94-26. Leukocytes were extracted using the dextran-heparin method and then lysed. Moreover, protein was quantified via the Folin-Lowry method. We used the Van Schaftingen and Jaeken method, which we modified and standardized.

The study established a reference value of 6.546-48.023 nmol/h*mg protein for PMM2.

This study allowed detecting a 24-day-old girl with CDG Ia who showed no residual phosphomannomutase 2 activity.