A Novel Gastrokine, Gkn3, Marks Gastric Atrophy and Shows Evidence of Adaptive Gene Loss in Humans

Abstract

Gastrokines are stomach mucus cell-secreted proteins; 2 gastrokines are known, GKN1 and GKN2. Gastrokine expression is lost in gastric cancer, indicating a possible function in tumor suppression. We have identified a third gastrokine gene in mammals. Gkn3 was characterized by studies of molecular structure, evolutionary conservation, and tissue expression as well as transcriptional/translational outcome in mouse genetic models of gastric pathology. The functional consequences of Gkn3 overexpression were evaluated in transfected cell lines. Gkn3 encodes a secreted (approximately 19 kilodalton) protein that is co-expressed with trefoil factor (Tff)2 in the distal stomach and discriminates a Griffinia simplicifolia lectin (GS)-II-positive mucus neck cell (MNC) subpopulation in the proximal stomach. In humans, widespread homozygosity for a premature stop codon polymorphism, W59X, has likely rendered GKN3 non-functional. Population genetic analysis revealed an ancestral GKN3 read-through allele that predominates in Africans and indicates the rapid expansion of W59X among non-Africans during recent evolution. Mouse Gkn3 expression is strongly up-regulated in (Tff2-deficient) gastric atrophy, a pre-cancerous state that is typically associated with Helicobacter pylori and marks a non-proliferative, GS-II positive lineage with features of spasmolytic polypeptide-expressing metaplasia (SPEM). Gkn3 overexpression inhibits proliferation in gastric epithelial cell lines, independently of incubation with recombinant human TFF2 or apoptosis. Gkn3 encodes a novel, functionally distinct gastrokine that is overexpressed and might restrain epithelial cell proliferation in gastric atrophy. Spread of the human GKN3 stop allele W59X might have been selected for among non-Africans because of its effects on pre-neoplastic outcomes in the stomach.