c.376A>G, (p.Ser126Gly) Alpha-Galactosidase A mutation induces ER stress, unfolded protein response and reduced enzyme trafficking to lysosome: Possible relevance in the pathogenesis of late-onset forms of Fabry Disease

Fabry Disease (FD) (OMIM 301500) is a metabolic X-linked inherited lysosomal storage disorder that results from the deficient activity of Alpha-Galactosidase A (Alpha-Gal), a lysosomal hydrolase that cleaves neutral glycosphingolipids with terminal N-linked galactosyl moieties, mainly globotriaosylceramides (Gb3). The enzyme, encoded by a 12-kb gene mapping on the long arm (Xq22.1 region) of the X chromosome, is constituted by a glycosylated subunit of approximately 55 kD, synthesized as an inactive precursor that undergoes maturation in endoplasmic reticulum (ER) and Golgi apparatus before being delivered to the lysosome to form a functional dimer. The gene is comprised of seven exons and, so far, >1000 different mutations have been described as associated to FD (www.dbfgp.org/dbFgp/fabry/FabryGP.htm). Clinical phenotypes are divided in two main classes, classic or non-classic, based on clinical and biochemical findings. Non-classic FD, usually recognized as late-onset forms with oligosymptomatic phenotype, presents with symptoms restricted solely to cardiocytes, kidneys or brain associated to missense misfolding mutations. In the group of the non-classic FD, special attention should be given to patients carrying the c.376A > G (p.Ser126Gly) mutation. The lack of clear experimental evidences on its pathogenetic role, despite the clinical pictures of the patients with severe ischaemic lesions, renal involvement and acroparesthesias, led many authors to classify this mutation as inconsistent, non-pathogenetic, and consequently not eligible to the current pharmacological treatments for FD. To shed light on the cellular processes affected by this mutation and to assess if the biochemical pathways involved with, could really have a significant pathogenetic impact, we studied the mutation in silico and in COS-7 and HEK 293 cell models. We found p.Ser126Gly, even retaining both high degree of synthesis and residual activity, is mostly stacked into the ER inducing unfolded protein response (UPR) with reduced trafficking to the lysosome. These data strongly suggest that p.Ser126Gly could trigger a pathogenetic mechanism different from the classic and well assessed increased turnover with loss of biological activity described for other missense mutations. This mechanism seems mainly related to a negative gain of function, with ER retention and UPR activation and could lead, via inflammation and/or apoptosis, to irreversible cell damage.