Clinical studies show that cetuximab alone is effective for only about 10% of mCRC patiets(13). Many efforts have been made to find biomarkers or drivers of drug resistance mechanism to allow as many patients as possible to benefit from cetuximab treatment. However, application of cetuximab on patients with KRAS G13D mutation remains controversial. By using KRAS G13D CRC PDX model, we explored the therapeutic efficacy of cetuximab. The tumor growth in mice model was initially suppressed, but developed resistance not long after. As our result shows, cetuximab may be an available selection for KRAS G13D mutated patients. However, we used a mice model and did not combined cetuximab with traditional chemotherapy, which is inconsistent with clinical practice. Nonetheless, our results provide clues for further studies of cetuximab in such patients.
Cetuximab targets EGFR on the cell membrane, which is a member of the RTK family. Previous studies on the acquired resistance to cetuximab focus on the mutations or amplifications of several RTK family genes, including KRAS, NRAS, HER2 and MET(14–16). By using WES and RNA sequencing technology, we firstly explored resistance mechanism in KRAS G13D mutated tumors. In our analysis, 145 genes showed significant changes in the development course of drug resistance. Indeed, the results of our study are inconsistent with the results previously reported of KRAS wild-type patients. Our results did not detect previously reported common mutations or amplifications in NRAS, HER2 or MET, Among the 145 genes, RTK family related genes include JAK2, PRKAA1, FGFR2 and RALBP1. Most of the other genes have not been studied and reported specifically. As cetuximab has been reported to have some immune influence in CRC patients by increase the number of CD3 + T, CD8 + T and natural killer (NK) cells and reduced T-regulatory cells(17), we mapped 145 interest genes to 1040 immune genes, and 10 immune genes were filtered out for subsequent studies about their association with treatment efficacy or drug resistance.
According to mRNA, the evolution of mRNA of SWAP70 was consistent with the gene evolution and was consistent with observed drug resistance process, which suggests SWAP70 may be a very important gene for cetuximab resistance. SWAP70 is a protein that has been suggested to be involved in regulation of actin rearrangement. A study reported that mutation of SWAP-70 can transform mice embryo fibroblasts and promote the growth of tumor cells. Thus, SWAP-70 is supposed to be a new type of oncogene(18). Another study found SWAP-70 may colocalize with the G proteins in a membrane signaling cluster and regular sphingosine 1-phosphate to influence immune system by affecting dendritic cell motility and endocytosis(19). All the above information suggested that SWAP-70 is closely related to the development of tumor, and SWAP-70 presumed to be an acquired resistance gene in KRAS G13D mutation colorectal cancer. The functions and mechanisms of miRNA in acquired resistance are largely unknown. Our study did not find miRNA changes in 5 passages, which suggests that changes in the genes themselves may be the main cause of resistance.
Taken together, we depicted dynamic genome and transcriptome alterations in tumors by a cetuximab treated KRAS G13D mutated CRC PDX model. To our knowledge, this is the first genome and transcriptome profiling for resistance mechanism in this type patients. The conclusion is preliminary due to animal study and cetuximab monotherapy. Nonetheless, our results still provide clues for subsequent studies on cetuximab application in KRAS G13D mutated CRC patients.