CRISPR/Cas9 genome editing of SLC37A4 gene elucidates the role of molecular markers of endoplasmic reticulum stress and apoptosis in renal involvement in glycogen storage disease type Ib

Abstract

Glycogen storage disease type Ib (GSD Ib) is an autosomal recessive disorder, caused by a deficiency of ubiquitously expressed SLC37A4 protein. Deficiency of SLC37A4 leads to abnormal storage of glycogen in the liver and kidneys, resulting in long-term complications of renal disease and hepatocellular adenomas, whose mechanisms are poorly understood. Molecular markers of the adaptive responses to the metabolic stress caused by a deficiency of SLC37A4, such as markers related to the endoplasmic reticulum (ER) stress and unfolded protein response (UPR), have not been extensively studied. The aim of this study was to investigate the expression of molecular markers of the UPR response and apoptosis related to a deficiency of SLC37A4 in kidney cells. For that purpose, we intended to establish a human kidney cell model system for GSD Ib. The novel variant c.248G>A, found in GSD Ib patients, was introduced into the Flp-In™T-REx™-293 cell line using CRISPR/Cas9-mediated precise gene editing method, resulting in significant decrease of SLC37A4 gene expression. In this model system we used RT-qPCR analysis to investigate the expression of molecular markers of the UPR response (ATF4, DDIT3, HSPA5, and XBP1s) and apoptosis (BCL2, BAX). We demonstrated that under chronic metabolic stress conditions caused by SLC37A4 deficiency, the ER stress-induced UPR was triggered, resulting in suppression of the UPR molecular markers and cell survival promotion (decreased expression levels of ATF4, DDIT3, HSPA5, with the exception of XBP1s). However, persistent metabolic stress overrides an adaptation and induces apoptosis through increased expression of pro-apoptotic markers (decreased ratio of BCL2/BAX genes). We established a cellular model system characterized by a deficiency of SLC37A4, which presents pathological manifestations of GSD Ib in the kidney. Expression analysis in a novel model system supports the hypothesis that renal dysfunction in the GSD Ib is partly due to the ER stress and increased apoptosis.