Hepatic and neuromuscular forms of glycogenosis type III: nine mutations in AGL

Glycogenosis type III (Cori disease) is an autosomal recessive disorder caused by the deficiency of the glycogen debranching enzyme, encoded by the AGL gene, and existing in six isoforms alternately spliced in a tissue-specific way. Generally, disease onset occurs early on starting from the first year of life, with hepatomegaly, hypoglycemia, hyperlipidemia, increased CK levels, and, in some cases, short stature and slight mental retardation. Frequently, hepatomegaly tends to resolve spontaneously and inexplicably during childhood, when myopathy, often associated with cardiomyopathy, arises. This disease is known to lack almost invariably clear links between the genotype and clinical phenotype. We describe nine new mutations in Italian patients: four nonsense (p.Arg285X, p.Lys422X, p.Arg910X, p.Arg977X), three frameshift (c.442delA, c.753_756delGACA, c.3963delG), and two missense (p.Ala1120Pro, p.Arg524His). Particularly, the nonsense p.Arg285X is linked to an exonic splicing enhancer and it was found to produce two species of transcripts at the same time. Moreover, we discuss a subgroup of subjects carrying c.2681+1G>A, which has proven to be the most frequent mutation among our patients. The previously described c.664+3A>G was also detected in two patients, both homozygous. The present work is yet another confirmation that the individual genetic background plays a pivotal role in influencing the phenotypes, as occurs in other metabolic diseases.

Glycogen storage disease type III (glycogen debranching enzyme deficiency): correlation of biochemical defects with myopathy and cardiomyopathy

OBJECTIVE

To determine whether a specific subtype of glycogen storage disease type III is associated with myopathy and cardiomyopathy.

DESIGN

Case series.

SETTING

Three referral medical centers.

PATIENTS

All patients with glycogen storage disease type III who were followed in 1990 and for whom both immunoblot analysis and clinical data were available.

MAIN OUTCOME MEASURES

Evaluation for myopathy and cardiomyopathy included determinations of serum creatine kinase activity; muscle strength testing; ischemic exercise testing; nerve conduction studies; and electromyographic, electrocardiographic, and echocardiographic studies.

RESULTS

Three patients with deficient debranching enzyme activity and deficient immunoreactive material in liver but normal debranching enzyme activity in muscle (glycogen storage disease IIIb) had no clinical evidence of myopathy or cardiomyopathy. Serum creatine kinase activity, muscle strength, ischemic exercise testing, electrocardiograms, and echocardiograms were normal in these patients. These studies and electromyograms were abnormal in seven patients with total debranching enzyme deficiency and an absence of immunoreactive material in both liver and muscle (glycogen storage disease IIIa) and in three patients who had debranching enzyme transferase deficiency but normal glucosidase activity in both liver and muscle (glycogen storage disease IIId). All 10 of these patients had progressive myopathy, and 6 had progressive cardiomyopathy.

CONCLUSION

Clinical features of glycogen storage disease type III correlate with the particular biochemical defect seen with the disorder. Assessments of debranching enzyme or debranching enzyme transferase activity in muscle can be used to predict whether patients with glycogen storage disease type III will develop myopathy and cardiomyopathy.

Immunoblot analyses of glycogen debranching enzyme in different subtypes of glycogen storage disease type III

To determine the tissue distribution of glycogen debranching enzyme, we used immunoblot analysis with a polyclonal antibody prepared against purified porcine muscle debranching enzyme. Debranching enzyme was identified in porcine brain, kidney, cardiac muscle, skeletal muscle, liver, and spleen; and in human liver, skeletal muscle, lymphocytes, lymphoblastoid cells, skin fibroblasts, cultured chorionic villi, and amniocytes. In each of these tissues the debranching enzyme band was 160 kd. To determine the molecular basis for glycogen storage disease type III at the protein level, tissues from 41 patients with glycogen storage disease type III were also subjected to immunoblot analysis. Three patients having isolated transferase deficiency with retention of glucosidase activity (type IIID disease) had nearly normal amounts of cross-reactive material. In the remaining patients (both transferase and glucosidase deficiency), debranching enzyme was either absent or greatly reduced. These latter patients included 31 with disease that appeared to involve both liver and muscle (type IIIA), four with disease that was present only in the liver (type IIIB), and three with unknown muscle status. In patients with both type IIIA and type IIIB disease, debranching enzyme protein was absent in skin fibroblasts, lymphoblastoid cells, and lymphocytes. The parents of two patients with type IIIA disease had an intermediate level of debranching enzyme protein, consistent with their presumed heterozygote state. An immunoblot analysis of cultured amniotic fluid cells from a woman whose fetus was at risk for type IIIA disease predicted an unaffected fetus; the prediction was confirmed postnatally. Thus Western blot analysis offers an alternate method of prenatal diagnosis for the most common form of glycogen storage disease type III.