A glycogen storage disease type 1a patient with type 2 diabetes

Background

Glycogen storage disease type 1a (GSD1a) is an inborn genetic disease caused by glucose-6-phosphatase-α (G6Pase-α) deficiency and is often observed to lead to endogenous glucose production disorders manifesting as hypoglycemia, hyperuricemia, hyperlipidemia, lactic acidemia, hepatomegaly, and nephromegaly. The development of GSD1a with diabetes is relatively rare, and the underlying pathogenesis remains unclear.

Case presentation

Here we describe a case of a 25-year-old Chinese female patient with GSD1a, who developed uncontrolled type 2 diabetes mellitus (T2DM) as a young adult. The patient was diagnosed with GSD1a disease at the age of 10 and was subsequently treated with an uncooked cornstarch diet. Recently, the patient was treated in our hospital for vomiting and electrolyte imbalance and was subsequently diagnosed with T2DM. Owing to the impaired secretory function of the patient’s pancreatic islets, liver dysfunction, hypothyroidism, severe hyperlipidemia, and huge hepatic adenoma, we adopted diet control, insulin therapy, and hepatic adenoma resection to alleviate this situation. The WES discovered compound heterozygous mutations at the exon 5 of G6PC gene at 17th chromosome in the patient, c.648G>T (p.L216 L, NM_000151.4, rs80356484) in her father and c.674T>C (p.L225 P, NM_000151.4, rs1555560128) in her mother. c.648G>T is a well-known splice-site mutation, which causes CTG changing to CTT at protein 216 and creates a new splicing site 91 bp downstream of the authentic splice site, though both codons encode leucine. c.674T>C is a known missense mutation that causes TGC to become CGC at protein 225, thereby changing from coding for leucine to coding for proline.

Conclusion

We report a rare case of GSD1a with T2DM. On the basis of the pathogenesis of GSD1a, we recommend attentiveness to possible development of fasting hypoglycemia caused by GSD and postprandial hyperglycemia from diabetes. As the disease is better identified and treated, and as patients with GSD live longer, this challenge may appear more frequently. Therefore, it is necessary to have a deeper and more comprehensive understanding of the pathophysiology of the disease and explore suitable treatment options.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-022-01344-3.

Classifying molecular phenotypes of G6PC variants for pathogenic properties and to guide therapeutic development

Due to advances in sequencing technologies, identification of genetic variants is rapid. However, the functional consequences of most genomic variants remain unknown. Consequently, variants of uncertain significance (VUS) that appear in clinical DNA diagnostic reports lack sufficient data for interpretation. Algorithms exist to aid prediction of a variant's likelihood of pathogenicity, but these predictions usually lack empiric evidence. To examine the feasibility of generating functional evidence in vitro for a given variant's role in disease, a panel of 29 coding sequence variants in the G6PC gene was assessed. G6PC encodes glucose-6 phosphatase enzyme, and reduction in its function causes the rare metabolic disease glycogen storage disease type 1a (GSD1a). Variants were heterologously expressed as fusion proteins in a hepatocyte-derived cell line and examined for effects on steady-state protein levels, biosynthetic processing, and intracellular distribution. The screen revealed variant effects on protein levels, N-linked glycosylation status, and cellular distribution. Of the eight VUS tested, seven behaved similar to wild-type protein while one VUS, p.Cys109Tyr, exhibited features consistent with pathogenicity for all molecular phenotypes assayed, including significantly reduced protein levels, alteration in protein glycosylation status, and abnormally diffuse protein localization pattern, and has recently been reported in a patient with GSD1a. Our results show that such a screen can add empiric evidence to existing databases to aid in diagnostics, and also provides further classification for molecular phenotypes that could be used in future therapeutic screening approaches for small molecule or gene editing strategies directed at specific variants.

G6PC mRNA Therapy Positively Regulates Fasting Blood Glucose and Decreases Liver Abnormalities in a Mouse Model of Glycogen Storage Disease 1a

Glycogen storage disease type Ia (GSD1a) is an inherited metabolic disorder caused by the deficiency of glucose-6-phosphatase (G6Pase). GSD1a is associated with life-threatening hypoglycemia and long-term liver and renal complications. We examined the efficacy of mRNA-encoding human G6Pase in a liver-specific G6Pase^−/− mouse model (L-G6PC^−/−) that exhibits the same hepatic biomarkers associated with GSD1a patients, such as fasting hypoglycemia, and elevated levels of hepatic glucose-6-phosphate (G6P), glycogen, and triglycerides. We show that a single systemic injection of wild-type or native human G6PC mRNA results in significant improvements in fasting blood glucose levels for up to 7 days post-dose. These changes were associated with significant reductions in liver mass, hepatic G6P, glycogen, and triglycerides. In addition, an engineered protein variant of human G6Pase, designed for increased duration of expression, showed superior efficacy to the wild-type sequence by maintaining improved fasting blood glucose levels and reductions in liver mass for up to 12 days post-dose. Our results demonstrate for the first time the effectiveness of mRNA therapy as a potential treatment in reversing the hepatic abnormalities associated with GSD1a.