Altered expression of FANCL confers mitomycin C sensitivity in Calu-6 lung cancer cells

Fanconi Anemia (FA) results from mutations in a group of genes whose products, including BRCA2 and BACH1/BRIP1, are known to function in one common pathway (the FA-BRCA pathway) to guard genome integrity, especially when challenged by DNA crosslinking agents, such as Cisplatin and mitomycin C (MMC). The extremely high incidence of cancer in FA patients reveals the essentialness of this pathway in tumor suppression. However, this pathway's involvement in nonFA cancers is not well understood. To evaluate the contribution of the FA-BRCA pathway to cancer, we investigated the integrity of the FA-BRCA pathway in ten human cancer cell lines. We found that the Calu-6 lung cancer cell line carries a defective FA-BRCA pathway. In this cell line, the examination of six FA proteins, essential for the activation of the FA-BRCA pathway, detected substantially reduced expression of FANCL, a catalytic subunit of the ubiquitin ligase/E3-complex. Reconstitution of FANCL in these cells restored the activation of the FA-BRCA pathway, but MMC sensitivity of the cells with a complemented FA-BRCA pathway was decreased as compared to the cells with an impaired FA-BRCA pathway. Collectively, the abnormal FANCL expression is the cause leading to a defective FA-BRCA pathway, which confers the sensitivity of Calu-6 cells to MMC. This suggests that the correlation of an intact FA-BRCA pathway with MMC resistance may emerge as a common mechanism underlying resistance to DNA crosslinking agents in cancer patients.