Purpose: Responses to EGFR -targeted therapy are generally temporary, due to inevitable drug resistance. The prevalence and characteristics of receptor tyrosine–kinase (RTK) fusion as acquired resistance to EGFR tyrosine–kinase inhibitors (TKIs) are rarely investigated. Methods: We retrospectively reviewed genomic profiling data of 3873 EGFR (exons 18–21)-mutant lung cancer patients with more than once next-generation sequencing detection. A total of 16 patients who acquired RTK fusions during EGFR-TKI treatment with paired pre- and post-EGFR-TKI samples were identified. Their treatment history was collected. Results: Newly acquired RTK fusions during EGFR-TKI treatment included RET (n=6, 37.5%), ALK (n=5, 31.3%), NTRK1 (n=4, 25.0%), ROS1 (n=1, 6.3%), and FGFR3 (n=1, 6.3%). All RET and EML4 – ALK fusions were uncommon variants of KIF5B-RET and E2:A20 (V5), respectively. Interestingly, RET fusion occurred only after osimertinib treatment, and contributed to drug resistance in 50% (6 of 12) of patients treated with osimertinib, indicating that fusions had different prevalence when functioning as resistance mechanisms to EGFR TKIs. Moreover, we found that in all patients developing drug resistance to EGFR TKIs due to fusion emergence (n=16), those that had a treatment history of third-generation EGFR TKIs accounted for 75% (n=12). Conclusion: We have extended the current knowledge of resistance mechanisms to EGFR TKIs in non-small-cell lung cancer. Detection of RTK fusions should be included in genomic profiling panels to uncover potential resistance mechanisms of EGFR TKIs, which might inform therapeutic strategies, such as combination-therapy approaches, to circumvent tumorigenesis.

Background: Genomic instability is a universal hallmark of all cancers. Many of the most commonly used chemotherapeutic agents target this genomic instability by directly damaging the DNA, which results in tumour cell death. Our previous work has revealed that loss of SASH1 is associated with impaired apoptosis and increased cellular proliferation. A new generation of drugs have been developed that target the DNA repair enzyme PARP to induce DNA damage and cell death. SASH1 (SAM and SH3 domain containing protein 1) has been described as a tumour suppressor and in support of this SASH1 mRNA levels are decreased in lung, breast, thyroid and colorectal cancers. Our data demonstrates that SASH1 functions in the repair of DNA damage and loss of SASH1 protein expression could be used as a companion diagnostic for PARP inhibitors. Method: SASH1 IHC staining of lung cancer was correlated with patient survival. DNA damage repair was assessed following the depleted of SASH1 (siRNA). SASH1 protein levels in cell lines were correlated to PARP inhibitor sensitivity. Result: A lung cancer tissue microarray (TMA) of 225 patients was assessed for SASH1 protein level. Low SASH1 levels were associated with an improved patient prognosis in adenocarcinoma on univariate analysis (p ¼ 0.03). Analysis of DNA repair pathways demonstrated that SASH1 plays a role in homologous recombination (HR). Based on this observation, the impact of SASH1 expression on sensitivity to PARP inhibitors was explored. An inverse correlation between SASH1 levels and sensitivity to Olaparib was identified in lung cancer cell lines Figure 1 (R 2 ¼ 0.882). We subsequently analysed Olaparib sensitivity in a panel of SASH1 depleted lung cancer cells that demonstrated increased Olaparib sensitivity. Conclusion: Our results indicate that SASH1 protein expression is a prognostic factor in lung cancer, high levels being associated with a worse prognosis in adenocarcinoma. Low SASH1 expression