Recurrent Fusion Genes in Gastric Cancer: CLDN18-ARHGAP26 Induces Loss of Epithelial Integrity

Genome rearrangements, a hallmark of cancer, can result in gene fusions with oncogenic properties. Using DNA paired-end-tag (DNA-PET) whole-genome sequencing, we analyzed 15 gastric cancers (GCs) from Southeast Asians. Rearrangements were enriched in open chromatin and shaped by chromatin structure. We identified seven rearrangement hot spots and 136 gene fusions. In three out of 100 GC cases, we found recurrent fusions between CLDN18, a tight junction gene, and ARHGAP26, a gene encoding a RHOA inhibitor. Epithelial cell lines expressing CLDN18-ARHGAP26 displayed a dramatic loss of epithelial phenotype and long protrusions indicative of epithelial-mesenchymal transition (EMT). Fusion-positive cell lines showed impaired barrier properties, reduced cell-cell and cell-extracellular matrix adhesion, retarded wound healing, and inhibition of RHOA. Gain of invasion was seen in cancer cell lines expressing the fusion. Thus, CLDN18-ARHGAP26 mediates epithelial disintegration, possibly leading to stomach H(+) leakage, and the fusion might contribute to invasiveness once a cell is transformed.

ZO-1 regulates Erk, Smad1/5/8, Smad2, and RhoA activities to modulate self-renewal and differentiation of mouse embryonic stem cells

ZO-1/Tjp1 is a cytosolic adaptor that links tight junction (TJ) transmembrane proteins to the actin cytoskeleton and has also been implicated in regulating cell proliferation and differentiation by interacting with transcriptional regulators and signaling proteins. To explore possible roles for ZO-1 in mouse embryonic stem cells (mESCs), we inactivated the ZO-1 locus by homologous recombination. The lack of ZO-1 was found to affect mESC self-renewal and differentiation in the presence of leukemia-inhibiting factor (LIF) and Bmp4 or following removal of the growth factors. Our data suggest that ZO-1 suppresses Stat3 and Smad1/5/8 activities and sustains extracellular-signal-regulated kinase (Erk) activity to promote mESC differentiation. Interestingly, Smad2, critical for human but not mESC self-renewal, was hyperactivated in ZO-1(-/-) mESCs and RhoA protein levels were concomitantly enhanced, suggesting attenuation of the noncanonical transforming growth factor β (Tgfβ)/Activin/Nodal pathway that mediates ubiquitination and degradation of RhoA via the TJ proteins Occludin, Par6, and Smurf1 and activation of the canonical Smad2-dependent pathway. Furthermore, Bmp4-induced differentiation of mESCs in the absence of LIF was suppressed in ZO-1(-/-) mESCs, but differentiation down the neural or cardiac lineages was not disturbed. These findings reveal novel roles for ZO-1 in mESC self-renewal, pluripotency, and differentiation by influencing several signaling networks that regulate these processes. Possible implications for the differing relevance of Smad2 in mESC and human ESC self-renewal and how ZO-1 may connect to the different pathways are discussed.