Abstract 4250: 5′ deletion on ALK break-apart FISH and risk of false positive results

Abstract

Abstract Purpose: Break-apart fluorescence in situ hybridization (FISH) is the FDA-approved assay for detecting anaplastic lymphoma kinase (ALK) rearrangements in non-small cell lung cancer (NSCLC), identifying patients (pts) who may benefit from ALK tyrosine kinase inhibitors (TKIs). A case is considered positive for rearrangement either based on split 3′ and 5′ signals or loss of the 5′ signal (5′ deletion). We hypothesized that 5′ deletion on FISH may not be completely specific for presence of rearrangement, making the assay vulnerable to false positive results. Methods: An institutional IRB-approved database of NSCLC pts was queried for those with ALK-rearrangements. Clinical characteristics and response to ALK TKIs were reviewed. Immunohistochemistry (IHC) and next-generation sequencing (NGS) were obtained to further characterize ALK status. Results: Of 1614 NSCLC pts with ALK testing, 97 (6.0%) patients had abnormal ALK FISH: 30 with split signals, 24 with 5′ deletion, 3 with 3′ deletion, 1 with atypical rearrangement, and 39 with details unavailable. Pts with 5′ deletion were older (p = 0.01), had a more extensive smoking history (p = 0.03), and were more likely to harbor mutations in EGFR or KRAS (2 cases vs 0). Pts with 5′ deletion treated with ALK TKIs had a trend toward less tumor shrinkage (p = 0.07). ALK IHC and FISH were discordant in 3 of 21 pts with 5′ deletion and 0 of 26 pts with split signals (p = 0.03). NGS was concordant with ALK IHC, but not FISH, in 2 of 7 samples with 5′ deletions. All three testing modalities were concordant in 3 samples with split signals. Conclusions: NSCLC pts with 5′ deletion on ALK FISH have clinical and molecular features less typical of ALK-rearranged tumors, suggesting an unappreciated risk of false positives with this assay. While the majority of patients with 5′ deletion on ALK FISH harbor a true rearrangement and respond to ALK TKI, confirming these cases with IHC or NGS could reduce the false positive rate. Table 1.Clinicopathologic Features of NSCLC patients with positive ALK FISHSplit Signals (N = 30)5″ Deletion (N = 24)p-valueAge at Diagnosis - yearsp = 0.01MedianRange22-8228-76Age Distribution - No. (%)<40517%14%40-491033%28%50-59827%1042%60-69413%833%>70310%313%Race - No. (%)p = 0.03Caucasian2480%2188%Asian27%28%Other413%14%Smoking History - No. (%)p = 0.03Never Smoker1963%1042%Light Smoker (<10py)620%729%Heavy Smoker517%625%Presence of Co-MutationsEGFR00%14%KRAS00%14%None30100%2292%ALK Immunohistochemistryp = 0.03Positive26100%1886%Negative00%314%Next-Generation SequencingPositive3100%571%Negative00%229% Citation Format: Xin Gao, Lynette M. Sholl, Mizuki Nishino, Jennifer Heng, Pasi A. Janne, Geoffrey R. Oxnard. 5′ deletion on ALK break-apart FISH and risk of false positive results. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4250. doi:10.1158/1538-7445.AM2015-4250