A group of patients with non-small cell lung cancer (NSCLC) have tumors that contain an inversion in chromosome 2 that juxtaposes the 5' end of the echinoderm microtubule-associated protein-like 4 (EML4) gene with the 3' end of the anaplastic lymphoma kinase (ALK) gene, resulting in the novel fusion oncogene EML4-ALK. This fusion oncogene rearrangement is transforming both in vitro and in vivo and defines a distinct clinicopathologic subset of NSCLC.
The echinoderm microtubule-associated protein-like 4-anaplastic lymphoma kinase (EML4-ALK) fusion oncogene arises from an inversion on the short arm of chromosome 2 (Inv(2)(p21p23)) that joins exons 1-13 of EML4 to exons 20-29 of ALK. The resulting chimeric protein, EML4-ALK, contains an N-terminus derived from EML4 and a C-terminus containing the entire intracellular tyrosine kinase domain of ALK.
Since the discovery of this fusion oncogene in 2007, multiple variants of EML4-ALK have been reported, all of which encode the same cytoplasmic portion of ALK but contain different truncations of EML4. In addition, fusions of ALK with other partners including TRK-fused gene TFG and KIF5B have also been described in lung cancer patients, but appear to be much less common than EML4-ALK.
For EML4-ALK, the EML4 fusion partner mediates ligand-independent dimerization and/or oligomerization of ALK, resulting in constitutive kinase activity. In cell culture systems, EML4-ALK possesses potent oncogenic activity. In transgenic mouse models, lung-specific expression of EML4-ALK leads to the development of numerous lung adenocarcinomas.
The oncogenic role of the ALK fusion oncogene provides a potential avenue for therapeutic intervention. Cancer cell lines harboring the EML4-ALK translocation are effectively inhibited by small molecule inhibitors that target the ALK tyrosine kinase (TK). In vivo, treatment of EML4-ALK transgenic mice with ALK inhibitors results in tumor regression, supporting the notion that ALK-driven lung cancers are "addicted" to the fusion oncogene
Anaplastic lymphoma kinase (ALK) gene rearrangements or the resulting fusion proteins may be detected in tumor specimens using fluorescence in situ hybridization (FISH), immunohistochemistry (IHC), and reverse transcription polymerase chain reaction of cDNA (RT-PCR).
- FISH – The gold standard assay for diagnosing ALK-positive NSCLC is FISH. The commercial break-apart probes include two differently colored (red and green) probes that flank the highly conserved translocation breakpoint within ALK. In non-rearranged cells, the overlying red and green probes result in a yellow (fused) signal; in the setting of an ALK rearrangement, these probes are separated, and splitting of the red and green signals is observed. Atypical patterns of rearrangement have also been identified, and these are also responsive to ALK inhibition. ALK gene amplification alone is not predictive of responsiveness to these agents and does not carry the same significance as rearrangement.
- IHC – Multiple monoclonal antibodies have been developed to use for the IHC detection of the ALK fusion oncogene, and IHC using these antibodies is highly sensitive and specific. Thus, IHC can potentially be used to screen for and identify the presence of ALK positivity. In the United States, FISH is the only approved test to diagnose ALK-positive NSCLC, and thus FISH should be used to confirm the IHC results. In Europe, immunohistochemistry is widely used to detect ALK rearrangement.
- RT-PCR – RT-PCR of cDNA is another commonly used screening strategy for detecting ALK gene rearrangements in NSCLC. A number of multiplex assays have been developed to simultaneously capture all possible in-frame fusions between EML4 and ALK in which the kinase domain of ALK would be preserved. While different breakpoints in EML4 or ALK may be detected, novel ALK fusion partners will not. Furthermore, this method is frequently limited by the quality of the RNA that can be isolated from archival tissue.
Advanced non-small cell lung cancer (NSCLC) associated with the anaplastic lymphoma kinase (ALK) fusion oncogene is highly sensitive to ALK tyrosine kinase (TK) inhibitors. Treatment with ALK TK inhibitors should be limited to patients whose tumors contain this abnormality as demonstrated by fluorescence in situ hybridization (FISH). An ALK inhibitor is preferred as the initial therapy for patients whose tumor contains this genetic abnormality in countries where it is approved for this indication.
Cytotoxic chemotherapy appears to have a similar level of activity in anaplastic lymphoma kinase (ALK)-positive patients with NSCLC compared with those with ALK-negative disease. The choice of a specific chemotherapy agent or regimen is based upon the same criteria applied in other cases of advanced NSCLC.
EGFR tyrosine kinase inhibitors — Patients with the ALK fusion oncogene do not appear to respond to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs). In a retrospective cohort of 19 patients with the ALK fusion oncogene, there were no clinical responses to EGFR TKIs, and the median time to progression was only five months.