Glucose-6-phosphatase (G6PC1) promoter polymorphism associated with glycogen storage disease type 1a among the Indian population

Abstract

Glycogen storage disease type 1a (GSD-1a) is an autosomal recessive inherited metabolic disorder caused by mutations in the G6PC1 gene leading to a deficiency of the glucose-6-phosphatase enzyme. To date more than 110 mutations have been identified in the coding regions as well as splice junctions, however only two polymorphisms have been so far identified in the promoter of the G6PC1 gene. Thirteen unrelated Indian patients (9 boys and 4 girls) with GSD-1a were screened for mutations through PCR amplification of genomic DNA for the promoter region of G6PC1 gene, followed by direct sequencing. Mutation analysis had identified a heterozygous polymorphism (g.-225C>G; rs559748047) with two unrelated GSD-1a patients, which had yielded a target site for the restriction enzyme HpyCH4III, hence RFLP was done for further confirmation. One hundred healthy Indian subjects were genotyped for this variant rs559748047 with an allele frequency of 13.5%. This polymorphism was found located in the AE-II region that consists of binding sites for several transcription factors including CRE. As a result, the transcriptional activity of the promoter construct containing mutant G allele exhibited an 8.43 fold decrease in activity than wild-type C allele as determined functionally by the transfection of luciferase reporter constructs containing the promoter region in HepG2 cells. Further, hormonally inductive transcriptional activity was significantly reduced for dexamethasone, dbcAMP and insulin with mutant G allele in the G6PC1 promoter. Thus, the promoter polymorphism (g.-225C>G) of G6PC1 gene was characterized to be associated with the decrease in both basal and hormonal transcriptional activity of G6Pase. These findings suggest that g.-225C>G, rs559748047 polymorphism of the G6PC1 promoter may be crucial in the disease development and progression of GSD-1a.