Simple SummaryThe cell architecture is created and maintained via adhesion molecules, called cadherins, that keep similar cells bound to each other. E-cadherin is a cadherin encoded by theCDH1gene, essential in normal epithelia. P-cadherin, encoded by theCDH3gene, exerts a similar function but is mainly present in tumours. E- to P-cadherin switch is a common event in several epithelial tumours. We unveiled a mechanism explaining the E- to P-cadherin switch (CDH1toCDH3switch), which we found to occur in many gastric cancers. We induced this switch by depletingCDH1in gastric cancer cells, and, as they started to produceCDH3, this rescued cell-adhesion. Despite this rescue, cells bearing the switch showed migratory and proliferative features, commonly observed in aggressive tumours. We observed thatCDH1depletion leads to novel genomic interactions between theCDH1andCDH3gene sequences. We identified the sequences needed for these interactions to occur, which seem also responsible for controlling the amount of E- or P-cadherin molecules in the cell. Our data provide evidence that loss of E-cadherin is able to compromise the normal interactions between theCDH1toCDH3gene sequences, allowing the expression of P-cadherin in gastric cancers.AbstractCadherins are cell–cell adhesion molecules, fundamental for cell architecture and polarity. E-cadherin to P-cadherin switch can rescue adherens junctions in epithelial tumours. Herein, we disclose a mechanism for E-cadherin to P-cadherin switch in gastric cancers.CDH1andCDH3mRNA expression was obtained from 42 gastric tumours’ RNA-seq data. CRISPR-Cas9 was used to knock outCDH1and a putative regulatory element.CDH1-depleted and parental cells were submitted to proteomics and enrichment GO terms analysis; ATAC-seq/4C-seq with aCDH1promoter viewpoint to assess chromatin accessibility and conformation; and RT-PCR/flow cytometry to assessCDH1/E-cadherin andCDH3/P-cadherin expression. In 42% of gastric tumours analysed,CDH1toCDH3switch was observed.CDH1knockout triggeredCDH1/E-cadherin complete loss andCDH3/P-cadherin expression increase at plasma membrane. This switch, likely rescuing adherens junctions, increased cell migration/proliferation, commonly observed in aggressive tumours. E- to P-cadherin switch accompanied increasedCDH1promoter interactions withCDH3–eQTL, absent in normal stomach and parental cells.CDH3–eQTL deletion promotesCDH3/CDH1reduced expression. These data provide evidence that loss ofCDH1/E-cadherin expression alters theCDH3locus chromatin conformation, allowing aCDH1promoter interaction with aCDH3-eQTL, and promotingCDH3/P-cadherin expression. These data highlight a novel mechanism triggering E- to P-cadherin switch in gastric cancer.