rs1801175 — G6PC1

G6PC1 R83C — The Most Common Cause of Glycogen Storage Disease Type Ia

After every meal, your liver stores glucose as glycogen. Between meals and during overnight fasting, it reverses this process — breaking glycogen back down and releasing glucose into the bloodstream to fuel the brain and other organs. The final, rate-limiting step of this release is controlled by glucose-6-phosphatase-alpha¹¹ glucose-6-phosphatase-alpha
G6Pase-α, encoded by G6PC1 on chromosome 17q21.31, a transmembrane enzyme anchored in the endoplasmic reticulum membrane that cleaves the phosphate group from glucose-6-phosphate to release free glucose into the blood. Without this enzyme, glucose-6-phosphate cannot exit the liver — it accumulates, is shunted into glycogen stores, lactate production, triglyceride synthesis, and uric acid generation, while blood glucose falls precipitously within minutes to hours of the last meal.

The c.247C>T variant (rs1801175) substitutes a cysteine for the arginine at position 83 of the G6Pase-α protein (p.Arg83Cys, commonly written R83C). This mutation is classified Pathogenic in ClinVar²² classified Pathogenic in ClinVar
VCV000011998, criteria provided by 27 of 31 submitters, no conflicts and is documented in OMIM as the defining G6PC1 allelic variant for glycogen storage disease type Ia³³ glycogen storage disease type Ia
OMIM 232200; also called von Gierke disease; an autosomal recessive disorder affecting approximately 1 in 100,000 births globally, with higher prevalence in Ashkenazi Jewish and certain European populations.

The Mechanism

Arg83 sits in a transmembrane helix of G6Pase-α that is critical for proper protein folding and membrane topology. The R83C substitution introduces a cysteine residue that disrupts both the hydrophobic packing of the helix bundle and the structural architecture of the enzyme's active site. Functional studies have demonstrated that the R83C mutation completely abolishes G6Pase-α activity⁴⁴ completely abolishes G6Pase-α activity
Shieh JJ et al. The molecular basis of glycogen storage disease type 1a: structure and function analysis of mutations in glucose-6-phosphatase. J Biol Chem, 2002 — there is no residual enzyme function at physiological temperature, in contrast to some other G6PC1 variants that retain partial activity.

The consequence of complete G6Pase-α loss is a metabolic dam. Glucose-6-phosphate cannot be dephosphorylated and released, so it accumulates and is diverted into four parallel pathways: (1) glycogen synthesis — causing progressive hepatomegaly and nephromegaly; (2) glycolysis to lactate — causing chronic lactic acidosis; (3) de novo lipogenesis and hypertriglyceridemia; and (4) purine catabolism to uric acid — causing hyperuricemia and eventually gout. A case report of an R83C homozygous patient illustrates this vividly: triglycerides of 3,860 mg/dL at diagnosis⁵⁵ triglycerides of 3,860 mg/dL at diagnosis
Sever S et al. Glycogen storage disease type Ia: linkage of glucose, glycogen, lactic acid, triglyceride, and uric acid metabolism. J Clin Lipidol, 2012, with simultaneous hypoglycemia, hepatic adenomas, and anemia — all downstream of the single enzymatic block.

The Evidence

R83C has been the most studied G6PC1 disease allele since the gene was cloned in 1993. In early molecular surveys, Lei et al.⁶⁶ Lei et al.
Lei KJ et al. Genetic basis of glycogen storage disease type 1a: prevalent mutations at the glucose-6-phosphatase locus. Am J Hum Genet, 1995 identified it as one of the two most common mutations in Caucasian patients, with concurrent prevalence in Hispanic GSD-Ia patients.

In the most comprehensive European mutation survey at the time, Rake et al.⁷⁷ Rake et al.
Rake JP et al. Glycogen storage disease type Ia: recent experience with mutation analysis, a summary of mutations reported in the literature. Eur J Pediatr, 2000 found R83C in 26.7% of all disease alleles among their GSD-Ia cohort — the single highest-frequency allele. This aligns with a subsequent population analysis (Matern et al.⁸⁸ Matern et al.
Matern D et al. Glycogen storage disease type I: diagnosis and phenotype/genotype correlation. Eur J Pediatr, 2002) that reviewed 130 patients and identified R83C consistently as the dominant allele across European ancestry groups.

The variant is dramatically enriched in the Ashkenazi Jewish population, where Ekstein et al.⁹⁹ Ekstein et al.
Ekstein J et al. Mutation frequencies for glycogen storage disease Ia in the Ashkenazi Jewish population. Am J Med Genet A, 2004 found it accounts for 93–100% of all pathogenic G6PC1 alleles — a founder effect that makes R83C carrier screening particularly informative in this community. The gnomAD allele frequency in Ashkenazi Jewish individuals is ~0.66%, approximately 20-fold higher than in Europeans and 100-fold higher than in East Asians.

Recent gene therapy work has used the R83C variant as the model mutation for developing GSD-Ia treatments, with both CRISPR/Cas9 and base-editing approaches demonstrating that correction of as little as 3–10% of liver cells is sufficient to prevent hypoglycemia and metabolic crisis (Arnaoutova et al. 2021¹⁰¹⁰ Arnaoutova et al. 2021
Arnaoutova I et al. Correction of metabolic abnormalities in a mouse model of glycogen storage disease type Ia by CRISPR/Cas9-based gene editing. Mol Ther, 2021, Arnaoutova et al. 2024¹¹¹¹ Arnaoutova et al. 2024
Arnaoutova I et al. Base-editing corrects metabolic abnormalities in a humanized mouse model for glycogen storage disease type-Ia. Nat Commun, 2024).

Practical Actions

For heterozygous carriers: a single functional G6PC1 copy is fully sufficient. G6Pase-α activity in carriers is approximately 50% of normal — well above any clinical threshold. Carriers have no metabolic symptoms and no dietary restrictions. The clinical relevance is entirely reproductive: if both parents carry a pathogenic G6PC1 allele, each pregnancy has a 25% chance of inheriting two defective alleles and developing GSD-Ia.

For individuals with GSD-Ia (homozygous or compound heterozygous): the cornerstone of management is preventing hypoglycemia through continuous glucose availability. Uncooked cornstarch (UCCS) — digested slowly by pancreatic amylase — is the standard approach, supplemented by fructose and galactose restriction (since neither can bypass the G6Pase block to generate free glucose). Liver transplantation corrects the hepatic defect but does not address the renal or other systemic involvement. Novel gene therapies targeting R83C specifically are in clinical development.

Interactions

GSD-Ia can also be caused by mutations in SLC37A4 (the glucose-6-phosphate transporter), which produces an overlapping phenotype with additional neutropenia (GSD-Ib). Individuals with clinical features of GSD-I who test negative for G6PC1 mutations should be evaluated for SLC37A4 variants. For heterozygous G6PC1 carriers, compound heterozygosity with a second G6PC1 pathogenic variant on the other chromosome produces GSD-Ia regardless of which two alleles are involved.