Identification of candidates for driver oncogenes in scirrhous-type gastric cancer cell lines.

Scirrhous-type gastric cancer (SGC) is one of the most intractable cancer subtypes in humans, and its therapeutic targets have been rarely identified to date. Exploration of somatic mutations in the SGC genome with the next-generation sequencers has been hampered by markedly increased fibrous tissues. Thus, SGC cell lines may be useful resources for searching for novel oncogenes. Here we have conducted whole exome sequencing and RNA sequencing on 2 SGC cell lines, OCUM-8 and OCUM-9. Interestingly, most of the mutations thus identified have not been reported. In OCUM-8 cells, a novel CD44-IGF1R fusion gene is discovered, the protein product of which ligates the amino-terminus of CD44 to the transmembrane and tyrosine-kinase domains of IGF1R. Furthermore, both CD44 and IGF1R are markedly amplified in the OCUM-8 genome and abundantly expressed. CD44-IGF1R has a transforming ability, and the suppression of its kinase activity leads to rapid cell death of OCUM-8. To the best of our knowledge, this is the first report describing the transforming activity of IGF1R fusion genes. However, OCUM-9 seems to possess multiple oncogenic events in its genome. In particular, a novel BORCS5-ETV6 fusion gene is identified in the OCUM-9 genome. BORCS5-ETV6 possesses oncogenic activity, and suppression of its message partially inhibits cell growth. Prevalence of these novel fusion genes among SGC awaits further investigation, but we validate the significance of cell lines as appropriate reagents for detailed genomic analyses of SGC.