Clinical and genetic variability of glycogen storage disease type IIIa: seven novel AGL gene mutations in the Mediterranean area.

Deficiency of amylo-1,6-glucosidase, 4-alpha-glucanotransferase enzyme (AGL or glycogen debrancher enzyme) is responsible for glycogen storage disease type III, a rare autosomal recessive disorder of glycogen metabolism. The AGL gene is located on chromosome 1p21, and contains 35 exons translated in a monomeric protein product. The disease has recognized clinical and biochemical heterogeneity, reflecting the genotype-phenotype heterogeneity among different subjects. The clinical manifestations of GSD III are represented by hepatomegaly, hypoglycemia, hyperlipidemia, short stature and, in a number of subjects, cardiomyopathy and myopathy. In this article, we discuss the genotypic-phenotypic heterogeneity of GSD III by the molecular characterization of mutations responsible for the disease on a collection of 18 independent alleles from the Mediterranean area. We identified by heteroduplex band shift, DNA direct sequencing, and restriction analysis, seven novel mutations (four nonsense point-mutations: R34X, S530X, R1218X, W1398X; two microinsertions: 1072insT and 4724insAA; and one bp deletion: 676DeltaG), together with two new cases carrying a IVS21 + 1 G --> A splicing site mutation previously described in Italian patients. Altogether, 15 alleles were characterized. The correlation between type of mutation and clinical severity was studied in six patients in whom both mutated alleles were detected. Our data confirm the extreme genetic heterogeneity of this disease, thus precluding a strategy of mutation finding based on screening of recurrent common mutations.