[Diagnosis of glycogen storage disease type IIIA by detecting glycogen debranching enzyme activity, glycogen content and structure in muscle]

OBJECTIVE: Glycogen storage disease type III (GSD III) is an autosomal recessive disease caused by glycogen debranching enzyme (GDE) gene (AGL gene) mutation resulting in hepatomegaly, hypoglycemia, short stature and hyperlipidemia. GSD IIIA, involves both liver and muscle, and accounts for up to 80% of GSD III. The definitive diagnosis depends on either mutation analysis or liver and muscle glycogen debranching enzyme activity tests. This study aimed to establish enzymologic diagnostic method for GSD IIIA firstly in China by detecting muscular GDE activity, glycogen content and structure and to determine the normal range of muscular GDE activity, glycogen content and structure in Chinese children. METHOD: Muscle samples were collected from normal controls (male 15, female 20; 12-78 years old), molecularly confirmed GSD III A patients (male 8, female 4, 2-27 years old) and other myopathy patients (male 9, 2-19 years old). Glycogen in the muscle homogenate was degraded into glucose by amyloglucosidase and phosphorylase respectively. The glycogen content and structure were identified by glucose yield determination. The debranching enzyme activity was determined using limit dextrin as substrate. Independent samples Kruskal-Wallis H test, Nemenyi-Wilcoxson-Wilcox test, and Chi-square test were used for statistical analyses by SPSS 11.5. RESULT: (1) GSD III A patients' glycogen content were higher, but G1P/G ratio and GDE activity were lower than those of the other two groups (P < 0.01). In all of the three parameters, there were no significant difference between normal controls and other myopathy patients. (2) The range of normal values: glycogen content 0.31%-0.43%, G1P/G ratio 22.37%- 26.43%, GDE activity 0.234-0.284 micromol/(g. min). (3) Enzymologic diagnostic method had a power similar to that of gene analysis in diagnosis of GSD-IIIA patients. The sensitivity and specificity of enzymologic diagnostic method and mutation detection were 91.7% and 100% respectively. CONCLUSION: Enzymologic diagnostic method of GSD IIIA was firstly established in China. The range of normal values was determined. This method could be used in diagnosing suspected GSD IIIA patients in the clinic.