Acquired JHDM1D–BRAF Fusion Confers Resistance to FGFR Inhibition in <i>FGFR2</i> -Amplified Gastric Cancer

Abstract

<i>FGFR2</i> gene is frequently amplified in gastric cancer. Recently, targeting FGFR2 has drawn attention as a form of gastric cancer therapy, and FGFR-selective inhibitors have shown promising efficacy in clinical studies. Because overcoming acquired resistance is a common problem with molecular targeting drugs, we investigated a resistant mechanism of FGFR inhibitors using the gastric cancer cell line SNU-16, which harbors <i>FGFR2</i> amplification. We established single-cell clones of FGFR inhibitor-resistant SNU-16 (AZD-R) by continuous exposure to AZD4547, a selective FGFR inhibitor. To screen the genetic alterations acquired in AZD-R, we ran a comparative genomic hybridization assay and found an amplification of Chr7q34 region. The chromosomal breakpoints were located between the 12th and the 13th exon of jumonji C domain containing histone demethylase 1 homolog D (<i>JHDM1D</i>) and between the 3rd and the 4th exon of <i>BRAF</i> We sequenced cDNA of the AZD-R clones and found fusion kinase <i>JHDM1D-BRAF</i>, which has previously been identified in primary ovarian cancer. Because JHDM1D-BRAF fusion lacks a RAS-binding domain, the dimerization of JHDM1D-BRAF was enhanced. A cell growth inhibition assay using MEK inhibitors and RAF-dimer inhibitors indicated the dependence of AZD-R clones for growth on the MAPK pathway. Our data provide a clinical rationale for using a MEK or RAF dimer inhibitor to treat <i>FGFR2</i>-amplified gastric cancer patients who have acquired resistance through the JHDN1D-BRAF fusion. <i>Mol Cancer Ther; 17(10); 2217-25. ©2018 AACR</i>.