ALK Knock-In Reporter Reveals APE1 as a Negative Regulator of EML4-ALK Formation

Chromosomal rearrangements that lead to the formation of oncogenic gene fusions, such as EML4-ALK, are thought to arise from incorrect repair of double-strand breaks in DNA. However, the mechanisms and factors driving rearrangement formation remain poorly understood, and analysis of these processes is limited by detection methods that are labor-intensive, low-throughput, and not readily quantitative at single-cell resolution. Here, we developed a genetically encoded ALK reporter based on A549 lung adenocarcinoma cells, created by inserting an ALK-P2A-mCherry cassette into the endogenous ALK locus, so that induced EML4-ALK fusion activated mCherry fluorescence. Reporter activation yielded a readily quantifiable mCherry-positive subpopulation that could be measured and enriched by flow cytometry and correlated with EML4-ALK levels. Using this platform, we combined CRISPR-mediated rearrangement induction with knockdown of DNA repair factors using RNA interference. Of the factors involved in base excision repair, homologous recombination-related pathways and canonical non-homologous end joining, knockdown of the APEX1 gene encoding apurinic endonuclease 1 (APE1) selectively increased EML4-ALK levels both in the reporter cell line and in parental A549 cells. Together, this work provides a sensitive, single-cell A549-based ALK reporter platform and a framework for future studies aimed at identifying cellular and environmental factors that modulate oncogenic EML4-ALK rearrangement formation.