BLM Potentiates c-Jun Degradation and Alters Its Function as an Oncogenic Transcription Factor

Mutations in BLM helicase predispose Bloom syndrome (BS) patients to a wide spectrum of cancers. We demonstrate that MIB1-ubiquitylated BLM in G1 phase functions as an adaptor protein by enhancing the binding of transcription factor c-Jun and its E3 ligase, Fbw7 alpha. BLM enhances the K48/K63-linked ubiquitylation on c-Jun, thereby enhancing the rate of its subsequent degradation. Functionally defective Fbw7 alpha mutants prevalent in multiple human cancers are reactivated by BLM. However, BS patient-derived BLM mutants cannot potentiate Fbw7 alpha-dependent c-Jun degradation. The decrease in the levels of c-Jun in cells expressing BLM prevents effective c-Jun binding to 2,584 gene promoters. This causes decreases in the transcript and protein levels of c-Jun targets in BLM-expressing cells, resulting in attenuated c-Jun-dependent effects during neoplastic transformation. Thus, BLM carries out its function as a tumor suppressor by enhancing c-Jun turnover and thereby preventing its activity as a proto-oncogene.