Going beneath the tip of the iceberg. Identifying and understanding EML4-ALK variants and TP53 mutations to optimize treatment of ALK fusion positive (ALK+) NSCLC

Since the discovery of echinoderm microtubule-associated protein-like 4 (EML4) and anaplastic lymphoma kinase (ALK) gene fusion in non-small cell lung carcinoma (NSCLC) in 2007, more than 10 EML4-ALK variants based on the exon breakpoints in EML4 have been identified. Unlike other receptor tyrosine kinase fusion positive NSCLC such as ROS1 or RET fusion, EML4-ALK is the dominant fusion variant in ALK+ NSCLC accounting for approximately 85 % of all fusion variants in ALK+ NSCLC. Currently, eight EML4-ALK variants are generally recognized with a number (1, 2, 3a/b, 4', 5a/b, 5', 7, 8) with EML4-ALK variants 1 and 3 being the two most common variants accounting for 75-80 % of the total EML4-ALK variants. Preclinical, retrospective analyses of institutional databases, and global randomized phase 3 trials have demonstrated differential clinical response (overall response rate, progression-free survival) to ALK tyrosine kinase inhibitors (TKIs) between the "short" (v3 and v5) and "long" (v1, v2, v5', v7, and v8) EML4-ALK variants. We discuss in more details how EML4-ALK variant structure influences protein stability and response to ALK TKIs. Additionally, the most recalcitrant single solvent-front mutation ALK G1202R is more prone to develop among EML4-ALK v3 following sequential use of next-generation ALK TKIs. Furthermore, TP53 mutations being the most common genomic co-alterations in ALK+ NSCLC also contribute to the heterogeneous response to ALK TKIs. Recognizing ALK+ NSCLC is not one homogeneous disease entity but comprised of different ALK fusion variants with different underlying genomic alterations in particular TP53 mutations that modulate treatment response will provide insight into the further optimization of treatment of ALK+ NSCLC patients potentially leading to improvement in survival.