Diagnosis of congenital disorders of glycosylation type‐I using protein chip technology

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Abstract
A method for the diagnosis of the congenital disorders of glycosylation type I (CDG‐I) by SELDI‐TOF‐MS of serum transferrin immunocaptured on protein chip arrays is described. The underglycosylation of glycoproteins in CDG‐I produces glycoforms of transferrin with masses lower than that of the normal fully glycosylated transferrin. Immobilisation of antitransferrin antibodies on reactive‐surface protein chip arrays (RS100) selectively enriched transferrin by at least 100‐fold and allowed the detection of patterns of transferrin glycoforms by SELDI‐TOF‐MS using approximately 0.3 µL of serum/plasma. Abnormal patterns of immunocaptured transferrin were detected in patients with known defects in glycosylation (CDG‐Ia, CDG‐Ib, CDG‐Ic, CDG‐If and CDG‐Ih) and in patients in whom the basic defect has not yet been identified (CDG‐Ix). The correction of the N‐glycosylation defect in a patient with CDG‐Ib after mannose therapy was readily detected. A patient who had an abnormal transferrin profile by IEF but a normal profile by SELDI‐TOF‐MS analysis was shown to have an amino acid polymorphism by sequencing transferrin by quadrupole‐TOF MS. Complete agreement was obtained between analysis of immunocaptured transferrin by SELDI‐TOF‐MS and the IEF profile of transferrin, the clinical severity of the disease and the levels of aspartylglucosaminidase activity (a surrogate marker for the diagnosis of CDG‐I). SELDI‐TOF‐MS of transferrin immunocaptured on protein chip arrays is a highly sensitive diagnostic method for CDG‐I, which could be fully automated using microtitre plates and robotics.