Molecular characterization of genomic breakpoints of ALK rearrangements in non-small cell lung cancer

ALK rearrangement is called the 'diamond mutation' in non-small cell lung cancer (NSCLC). Accurately identifying patients who are candidates for ALK inhibitors is a key step in making clinical treatment decisions. In this study, a total of 783 ALK rearrangement-positive NSCLC cases were identified by DNA-based next-generation sequencing (NGS), including 731 patients with EML4-ALK and 52 patients with other ALK rearrangements. Diverse genomic breakpoints of ALK rearrangements were identified. Approximately 94.4% (739/783) of the cases carried ALK rearrangements with genomic breakpoints in the introns of ALK and its partner genes, and 2.8% (21/739) of these cases resulted in frameshift transcripts of ALK. Meanwhile, 5.6% (44/783) of the ALK rearrangement-positive cases had breakpoints in the exons that would be expected to result in abnormal transcripts. RNA-based NGS was performed to analyse the aberrant fusions at the transcript level. Some of these rearranged DNAs were not transcribed, and the others were fixed by some mechanisms so that the fusion kinase proteins could be expressed. Altogether, these findings emphasize that, when using DNA-based NGS, functional RNA fusions should be confirmed in cases with uncommon/frameshift rearrangement by RNA-based assays.