Multiple transport protein defects in a patient with glycogen storage disease type 1: GSD 1b/1c beta.

A male child presented at 5 months of age with vomiting, diarrhoea, hypoglycaemia and hepatomegaly. Histology on a frozen liver biopsy suggested glycogen storage disease (GSD), while biochemical analyses confirmed an elevated glycogen content and normal activities of the GSD enzymes with the proviso that a variant of GSD 1 should be considered. The patient presented at 9 months of age with severe lactic acidosis and hypoglycaemia. A glucagon tolerance test and galactose load test on the patient produced no glycaemic response. A second biopsy was obtained and appropriately handled for the investigation of variants of the glucose-6-phosphatase enzyme (G6Pase) complex. Results showed that the patient had a deficiency of two transport proteins of the G6Pase complex, namely glucose-6-phosphate translocase and pyrophosphate translocase, i.e. GSD 1b/1c beta. These results were confirmed by additional kinetic analyses which provided confirmation of the double translocase deficiency. Evidence for inhibitors to these translocases was not found. The patient's treatment has resulted in the hypoglycaemia now being well controlled; however, at 3 years of age, height and weight are markedly lagging and he is moderately developmentally delayed. Neutropenia has not been found and neutrophil function is normal. Double enzyme deficiencies are very rare and possible explanations which might lead to this phenotype are considered. This, to the authors' knowledge, is the first report of a double translocase deficiency causing GSD type 1.