Background:ALK-rearrangements are a powerful and highly prevalent driver in cancer biology. The advent of potent tyrosine kinase inhibitors has led to the development of numerous laboratory tests and FDA approved therapies for the detection and treatment of ALK-positive cancers. The vast majority of reported ALK rearrangements involve exon 20 with fusions involving exons 17-19 occasionally reported. While less common, ALK-rearrangements occurring earlier in the ALK gene have not been well characterized. This is largely because most clinically available biomarker testing is only optimized for detection of rearrangements involving the common regions. To this end, we aimed to investigate the prevalence of early exon ALK rearrangements (eALKr).
Methods:Two pathology databases, including the Tempus deidentified database, were retrospectively queried to characterize eALKr, which were defined as any rearrangement that involved ALK exons 1-16. Demographic information and tumor type were recorded. Identification of eALKr was performed via targeted amplicon-based RNA sequencing, targeted hybrid capture DNA sequencing and whole transcriptome sequencing. Select specimens with early ALK fusions were also evaluated for ALK detection using available FDA-approved methods (immunohistochemistry [IHC] and fluorescent in-situ hybridization [FISH]).
Results:Out of the specimens that underwent NGS testing in the two databases, the prevalence of eALKr was 0.05% (73/143,959) across the databases and 10.3% (73/709) for all tumors with ALK rearrangements. Across the 73 rearrangements, there were 58 unique 5’ partner genes of which 46 were only identified once. eALK rearrangements were identified in all exons from exons 1 to 16 except for exon 13, with exon 4 being the most common. Prostatic, breast and ovarian serous carcinoma were the most common tumor types (Table 1). In three cases IHC and FISH were unreliable due to the early rearrangement location (0/3 IHC positive, 1/3 FISH positive). FISH was negative for samples with rearrangements in exon 2 and 4 and showed an atypical (deletion) pattern with a rearrangement involving exon 12.
Conclusions:A non-trivial number of eALKr were detected within two database using next generation sequencing (0.05% of total cancer cases; 10.3% of ALK fusion cases). These novel eALK rearrangements were predominantly in tumor types not normally associated with ALK fusions. FDA approved testing modalities are only optimized to detect common ALK rearrangements, which in part may be a reason why eALK-rearrangements are poorly characterized, with limited prevalence and treatment data available.