Identification of mutations that causes glucose-6-phosphate transporter defect in Tunisian patients with glycogenosis type 1b

Abstract

Abstract Background: Glycogen storage disease type 1b (GSD1b) is an autosomal recessive lysosomal storage disease caused by defective glucose-6-phosphate transporter encoded by SLC37A4 leading to the accumulation of glycogen in various tissues. The high rate of consanguineous marriages in Tunisian population provides an ideal environment to facilitate the identification of homozygous pathogenic mutations. We aimed to determine the clinical and genetic profiles of patients with GSD1b to evaluate SLC37A4 mutations spectrum in Tunisian patients. Methods: All the 9 exons and flanking intron regions of SLC37A4 gene were screened by direct sequencing to identify mutations and polymorphisms in three unrelated families with GSD1b. Bioinformatics tools were then used to predict the impacts of identified mutations on the structure and function of protein in order to propose a function-structure relationship of the G6PT1 protein. Results: Three patients (M.T, M.B and S.I) in Families I, II and III who had the severe phenotype were homoallelic for the two identified missense mutation and frameshift mutations: p.R300H (famillies I, II) and p.Trp393X (Family III), respectively. These two mutations were identified and previously reported. One of the alterations was a missense mutation p.R300H of exon 6 in SLC37A4 gene. The analysis of the protein structure flexibility upon p.R300H mutation using DynaMut tool and CABS-flex 2.0 server showed that the reported mutation increase the molecule flexibility of in the cytosol region and would probably lead to significant conformational changes. The other deleterious SLC37A4 gene alteration was a nonsense mutation p.Trp393X in exon 8 previously found homozygous. Conclusion: This is the first Tunisian report of SLC37A4 mutations identified in Tunisia causing the glycogenosis type Ib disease. Bioinformatics analysis allowed us to establish an approximate structure-function relationship for the G6PT1 protein, thereby providing better genotype/phenotype correlation knowledge.