EML4-ALK is a new oncogene in non-small cell lung carcinoma showing wild-type EGFR and K-RAS from non-smokers

Abstract

Lung cancers that develop in chronic smokers result from tobacco carcinogenesis but the etiology of those in non-smokers remain unclear. Although environmental tobacco smoke from passive smoking is recognized to be a causative factor, genetic evidences suggest distinct carcinogenic pathways in the two cancer populations. Mutations involving the epidermal growth factor receptor (EGFR) prevail in non-smokers, particularly those of East Asian descent, while various genes, e.g. K-RAS, are involved in lung cancers of smokers. Genetic translocation with gene fusion is mainly found in lympho-haematological and connective tissue malignancies, but in recent years, its role in epithelial cancers such as prostate carcinoma is beginning to be recognized. The echinoderm microtubule-associated protein-like 4 (2p21) - anaplastic lymphoma kinase (2p23) (EML4-ALK) fusion gene that results from chromosome 2 inversion has been identified recently in non-small cell lung cancers (NSCLC) from Japan. Its transforming power has been demonstrated in cultured mouse fibroblasts and its oncogenic potential in nude mice. We investigated the frequency, genetics and clinicopathological profile of EML4-ALK in 240 resected primary NSCLC (192 adenocarcinomas [AD], 30 squamous cell carcinoma [SCC], 18 other types) from 117 men, 123 women. Using reverse-transcription polymerase chain reaction, 13 (5.42%) cases were found involving 4 variants of fusion transcripts in which a variable length of EML4 is fused to exon 20 of ALK. This included 2 cases of variant 1 (V1) (EML4 exon 13), 2 of V2 (exon 20), 8 of V3 (exon 6) and 1 of V4 (exon 18). The genomic breakpoints of all the variants were identified by direct DNA sequencing and were consistent with the results of the fusion transcripts. The cases showing EML4-ALK included 11 AD and 2 SCC. There were statistically significant association with never (10/115) and passive (2/17) smokers together, compared to current smokers (1/74) (Chi square test, P = 0.013). Patients with EML4-ALK were apparently younger (mean 56.8±10.9 years) compared to those without the fusion gene (mean 62.8±10.2 years) (t test, P = 0.039). No significant association was found between the fusion gene and patient gender, tumour size, grade, lymph node or pathological stages. All patients harboring the fusion gene showed wild-type EGFR (13/123) and K-RAS (13/216). The association between EML4-ALK and wild-type EGFR was statistically significant (Chi square test, P <0.001). To the best of our knowledge, this series encompasses the largest number of NSCLC that harbor EML4-ALK reported in the literature. EML4-ALK fusion gene is a new oncogene in NSCLC with wild-type EGFR and K-RAS from patients who are not chronic smokers.