p16INK4a gene promoter variation and differential binding of a repressor, the ras-responsive zinc-finger transcription factor, RREB

BALB/c mice are susceptible to the development of pristane-induced plasma cell tumors, and have a rare allelic variant in the coding region of the p16(INK4a) (p16) tumor suppressor gene that produces a protein with impaired activity. We have now found that the BALB/c p16 promoter has an allelic variant that may also compromise p16 activity. Following pristane treatment, BALB/c p16 mRNA levels in B cells were lower than that in DBA/2 or C.D2-Pctr1, a resistant BALB/c congenic strain that harbors DBA/2 chromatin surrounding the p16 locus. Four sequence variants were found between BALB/c and DBA/2 in the p16 promoter region. In reporter assays, the DBA promoter was at least four times more active in driving luciferase expression than the BALB/c promoter. Most of the difference in activity was localized to a single nucleotide deletion in BALB/c. This deletion created a consensus binding site for RREB, a ras-responsive transcriptional element with zinc-finger binding motifs. Transient transfections with RREB confirmed that the p16 promoter can be downregulated by RREB, in a Ras- or Mek-dependent manner, and that the BALB/c promoter is more sensitive than DBA/2 to regulation by RREB. BALB/c mice have both regulatory and coding region defects that may contribute to the impairment of p16 gene function.