Nuclear targeting and cell cycle regulatory function of human BARD1

Area of Science:

• Molecular Biology
• Cancer Genetics

Background:

• Mutations in the BARD1 gene are linked to breast and ovarian cancers, suggesting its role as a tumor suppressor.
• BARD1 has a BRCA1-dependent ubiquitin E3 ligase activity and a BRCA1-independent proapoptotic function.
• Previous work established BARD1's nuclear-cytoplasmic shuttling, a process crucial for its functions.

Purpose of the Study:

• To identify the specific sequences responsible for BARD1 nuclear import.
• To investigate the role of these nuclear localization signals (NLSs) in BARD1's regulation of the cell cycle.

Main Methods:

• Deletion mapping and site-directed mutagenesis were used to define BARD1's NLSs.
• Flow cytometry and 5-bromo-2-deoxyuridine incorporation assays assessed cell cycle effects.

Main Results:

• Two active NLSs in human BARD1 were identified, which are absent in rodent BARD1.
• Mutating the primary bipartite NLS blocked nuclear import and led to cytoplasmic accumulation of BARD1.
• BARD1 induces a p53-independent G1 phase cell cycle arrest, dependent on its NLS but not its nuclear export signal.

Conclusions:

• BARD1's cell cycle regulation is a nuclear event mediated by its NLSs.
• These findings link BARD1's nuclear function to its expression during mitosis and potential roles in DNA damage checkpoints.