Abstract 802 Poster Session IV, Tuesday, 5/4 (poster 206)

Carbohydrate-deficient glycoprotein syndromes (CDGs) are progressive multisystemic disorders characterized by variable clinical and biochemical features allowing a classification in four different types, among which the type 1A is the most frequent.

The CDGs are characterized by a heterogenous deficiency of the carbohydrate moietites in various structural and circulating glycoproteins. We investigated the carbohydrate structure of serum thyroid-related glycoproteins in two patients with CDGs type 1 (one with normal and the other with altered PMM2 activity), by means of Concanavalin A (Con-A) and Ricin lectin affinity chromatography. Con-A binds exposed mannose residues and allows the separation of glycoproteins in 3 different fractions (unbound=UB, weakly bound=WB and firmly bound=FB) depending on the degree of glycosylated chain maturation (UB>WB>FB); Ricin binds terminal galactose residues, and the difference in glycoprotein binding before and after neuraminidase (NAase) treatment gives an estimation of the sialylation degree of the molecule. Thyroid function tests revealed normal serum TSH, FT4, FT3, thyroglobulin (Tg) and free alfa-SU levels. In both CDG patients this study showed a significant prevalence (P<0.05) of less mature hormonal isoforms vs normal controls in the Con-A analysis of carbohydrate branching of some thyroid-related glycoproteins, such as TSH and Tg(case 1:FB-TSH=48.7+3.1%, case 2:53.1+4%; controls:38.3+4.6%; case 1:FB-Tg=74.4+2.6%, case 2:72.3+8%, controls:26+18.6%). In contrast alfa-SU binding pattern showed a prevalence of UB fractions in CDGs vs controls (case 1:60.3+3.7%, case 2:60.4+6%, controls:45+4.8%):this different binding pattern is perhaps related to the presence of a supplementary O-linked chain at Thr39 in free alfa-SU vs TSHbeta-bound alfa-SU. Ricin analysis of immuno-purified TSH showed in both cases a nearly complete lack of lectin binding both before and after NAase treatment, indicating a lack of sialic acid and galactose terminal residues. Nevertheless, TSH biological properties are not severely altered as similar and normal values of thyroid function tests were found in both cases.

In conclusion, we characterized the abnormal processing of the carbohydrate branching of thyroid-related glycoproteins in two CDGs patients; the immunopurified-TSH binding pattern to Ricin appears to be specific of CDGs type 1.