The EML4-ALK oncogene: targeting an essential growth driver in human cancer

Abstract

Targeting of essential growth drivers represents an ideal approach to cancer treatment. To identify such molecules in clinical specimens, we developed a highly sensitive functional screening system based on the preparation of retroviral cDNA expression libraries. By screening such a library of lung adenocarcinoma with a focus formation assay, we discovered the EML4-ALK fusion-type oncogene. A small chromosomal inversion thus leads to fusion of the amino-terminal portion of the microtubule-associated protein EML4 to the intracellular kinase domain of ALK, a receptor-type protein tyrosine kinase. Constitutive dimerization of EML4-ALK mediated by a dimerization motif of EML4 results in kinase activation. Specific inhibitors of the kinase activity of ALK have been developed as therapeutic drugs for EML4-ALK-positive lung cancer, three of which (crizotinib, ceritinib, and alectinib) have already been approved for clinical use. An overall clinical response rate of 93.5% for alectinib has shown that agents that target essential growth drivers can become magic bullets for cancer treatment.

Figure 1.

Two different approaches to molecularly targeted therapy for cancer. One class of drugs (upper) targets molecules that function in intracellular signaling pathways underlying regulation of cell proliferation. Although such agents are potentially applicable to a wide range of cancers, their efficacy may be modest. The other class of agents (lower) targets essential growth drivers. Such drugs are applicable only to cancers harboring the activated driver, but they may be of high efficacy.

Figure 2.

Functional screening for oncogenes with retroviral cDNA expression libraries. The cDNAs are prepared from cancer specimens and cloned into retroviral plasmids. Recombinant retroviruses generated from the plasmids are used to infect assay cells. The selection of fibroblast cell lines as recipient cells, for instance, allows the identification of transforming cDNAs on the basis of the formation of abnormal foci (lower left). The use of hematopoietic cell lines, on the other hand, allows the isolation of cDNAs whose protein products block terminal differentiation of the myeloid lineage (lower right).

Figure 3.

Protein structure of the EML4-ALK fusion kinase. The domain organization of each fusion partner is shown schematically. A chromosomal inversion, inv(2)(p21p23), leads to fusion of the amino-terminal portion of EML4 to the intracellular region of ALK. CC, coiled-coil domain; WD, tryptophan and aspartic acid repeats; TM, transmembrane domain.