Characterization based on genotype–biochemical phenotype association in fructose-1,6-bisphosphatase deficiency

Abstract

Purpose Fructose-1,6-bisphosphatase (FBPase) deficiency, caused by an FBP1 mutation, is an autosomal recessive disorder characterized by hypoglycemic lactic acidosis. The mechanism by which the mutations cause enzyme activity loss is uncertain. Methods We performed whole-exome sequencing in an adult patient with severe hypoglycemic lactic acidosis and identified that the patient carried compound heterozygous missense mutations of FBP1 with c.491G > A (p.G164D) and c.581T > C (p.F194S). Results Biochemical analysis revealed that FBP1 mutant (G164D or F194S) decreased protein expression and enzyme activity loss. The interactome analysis for binding partners demonstrated that G164D and F194S mutants interact with the proteins involved in unfolded protein response. Additionally, G164D and F194S mutants aggregated in the endoplasmic reticulum, suggesting the involvement of protein misfolding in its pathogenesis. All FBP1 missense mutations previously reported were classified into three functional categories: Type 1 mutations, located at pivotal residues in enzyme activity motifs with no effects on protein expression; Type 2 mutations, which mediate changes in amino acid hydrophobicity and structurally cluster around the substrate-binding pocket, are associated with aggregation in the endoplasmic reticulum, and decreased protein expression; and Type 3 mutations, which are likely non-pathogenic mutations. Conclusion Protein misfolding contributes to FBPase deficiency pathogenesis, particularly in Type 2 mutations.