Familial autoimmune thyrogastric syndrome: clinical, genetic and in vitro studies

Introduction Autoimmune thyrogastric syndrome (ATGS) is an autoimmune polyendocrinopathy type IIIB associating autoimmune thyroiditis and chronic gastritis. We previously described eight families with ATGS (Valdes-Socin et al. Acta Clin Belg 2015). Genetics and physiopathology of ATGS is currently unknown.
Patients and methods We collected clinical and genetic data (lymphocyte DNA) from four Belgian families (10 females, mean age: 42 ± 11 years). All patients harbored autoimmune gastritis and autoimmune thyroiditis (Hashimoto or Graves disease). Five autoimmune thyrogastric syndrome patients (involving autoimmune gastritis or gNETs, and Graves’ or Hashimoto’s disease) were included in the first gene discovery series and initially studied by Whole Exome Sequencing (WES). WES uncovered different pathogenic variants in SLC4A2, SLC26A7 and SLC26A9, which cotransport together with ATP4A. Preliminary candidate genes found in this study were included in a custom 12 genes achlorhydria panel for targeted Next Generation sequencing (tNGS) studies. Belgian patients and 66 other familial and sporadic Spanish patients (not presented here) were then studied with the custom panel. In addition, 40 healthy individuals were recruited as controls. In vitro studies were designed to test the pathogenicity of SLC26A7, SLC26A9 SLC4A2 genes. Therefore, wild type (WT) HEK293 and knock-out cell lines were cultured in an enriched and a restrictive. Moreover, a colony-forming assay and flow cytometry studies was designed to test ROS damage-mediated apoptosis.
Results The WES study revealed new variants in the ATP4A (c.719C>A, p.Pro240His), SLC26A7 (c.643A>G, p.Ile215Val ) and also in SLC26A9 genes (c.514G>A, p.Val172Met and c.2546G>A, p.Arg849Gln) in Belgian families. According to in silico analysis, these are possible/probably missense damaging mutations. ATP4A encodes for the gastric hydrogen potassium ATPase, responsible for the acidification of the stomach. SLC26A7 is an epithelial Solute Carrier that regulates chloride and bicarbonate transport both in thyroid and gastric cells. SLC26A9 is another Solute Carrier that regulates iodine, chloride and bicarbonate transport in thyroid cells. The inactivation of SLC26A7, SLC26A9 SLC4A2 genes in HEK293T cells determined oxidative stress apoptosis (H2O2), suggesting a primary role for these genes in ROS control, cellular acid-base balance and the pathogenesis of ATGS. Previous studies demonstrated that mutations in the gastric ATP4A proton pump triggered gastric achlorhydria, activating autoimmunity (Calvete et al. Hum Mol Genet. 2015)
Discussion: The current clinical classification of autoimmune polyendocrinopathy does not assist the patient’s needs. We describe in a large international and collaborative study (1), a constellation of genes in the setting of familial and sporadic autoimmune polyendocrinopathy. These genes are involved in a novel pathogenic mechanism, based in the alteration of cellular the acid-base balance and gastric aclhorhydria, Indeed, SLC26A7, SLC26A9 and ATP4A genes are strongly associated with thyroid and gastric disease, suggesting a monogenic model for ATGS. Our data pave the way for the diagnosis and clinical management of patients with ATGS. Additional studies are needed to precise possible renal and respiratory phenotypes in our patients, and to investigate the pathogenic consequences of gene variants herein described.

References
1Calvete, Reyes, Valdes-Socin & al. Cell 2021 Dec; 10(12): 3500