Glycogen storage disease type 1B (GSD-1B) is a rare genetic disorder that belongs to a group of conditions known as glycogen storage diseases. It is characterized by the inability of the liver to produce adequate amounts of glucose due to a deficiency in the enzyme glucose-6-phosphate translocase. The incidence of GSD-1B is approximately 1 case per 100.000 individuals.
Key features of glycogen storage disease type 1B include:
- Impaired Glucose Production: The deficiency of glucose-6-phosphate translocase results in impaired transport of glucose-6-phosphate from the cell cytoplasm into the endoplasmic reticulum, where it would usually be converted to glucose. This leads to an inability to release glucose into the bloodstream during periods of fasting.
- Hypoglycemia: Individuals with GSD-1B experience hypoglycemia (low blood sugar), especially during fasting or between meals. This can lead to weakness, fatigue, irritability, and, in severe cases, seizures.
- Neutropenia: GSD-1B is associated with neutropenia, a condition characterized by a reduced number of neutrophils, a type of white blood cell. Neutropenia increases the risk of infections, particularly bacterial infections.
- Impaired Immune Function: Neutropenia can impair immune function, making individuals with GSD-1B more susceptible to recurrent infections.
- Inflammatory Bowel Disease (IBD): Some individuals with GSD1B may develop inflammatory bowel disease, which can include diarrhea, abdominal pain, and poor growth.
- Delayed Growth and Development: Growth may be delayed in individuals with GSD1B, and puberty may be delayed.
Managing glycogen storage disease type 1B involves a carefully controlled diet that includes frequent meals and nocturnal feeding to prevent prolonged fasting. Uncooked cornstarch may be used as a slow-release glucose source during fasting periods. Neutropenia and immune-related issues may require specific medical interventions, including granulocyte colony-stimulating factor (G-CSF) treatment.
Regular monitoring, including blood glucose levels, blood counts, and metabolic parameters, is essential for individuals with GSD-1B. Genetic counseling is necessary for affected individuals and their families to understand the inheritance pattern and assess the risk of having affected children.
While GSD-1B is a lifelong condition, appropriate management can help individuals lead relatively everyday lives. Early diagnosis and intervention are crucial for optimizing outcomes and preventing complications associated with hypoglycemia and immune-related issues.
More Information
Glucose-6-phosphatase is located in the membrane of the endoplasmic reticulum. Together with the glucose-6-phosphate transporter, known as SLC37A4, it forms a molecular complex responsible for glucose production by catalyzing the final stage of glycogenolysis and gluconeogenesis.
Pathogenic variants in SLC37A4 are responsible for GSD-1B. More than 100 pathogenic variants have been identified in SLC37A4 that can cause GSD-1B in homozygosis and compound heterozygosis. The two most frequent variants are c.1042_1043del and c.1015G>T, with the c.1042_1043del deletion accounting for 31% of alleles in European patients and c.1015G>T for 13%. The presence of c.1015G>T in Germany is higher than in the rest of Europe, accounting for 29% of mutant alleles.
The most frequent variant in the Japanese population is c.352T>C, which is implicated in every second case of GSD-1B.
Glycogen storage disease type 1B genetic test analyzes the 5 most frequent pathogenic mutations of the SLC37A4 gene.
With the technique used for genetic testing, only the gene's specific mutations, which are the most important and frequent in the literature, are analyzed. However, it should be noted that there are likely other gene or chromosomal mutations in the gene to be tested, which cannot be identified with this method. Different analysis techniques can be used for these cases, such as e.g. next-generation sequencing (NGS).
