The T-box repressors TBX2 and TBX3 specifically regulate the tumor suppressor gene p14ARF via a variant T-site in the initiator

The murine tumor suppressor p19(ARF) (p14(ARF) in humans) is thought to fulfill an important protective role in preventing primary cells from oncogenic transformation via its action in the p53 pathway. Several disease-implicated regulators of p19(ARF) are known to date, among which are the T-box genes TBX2, which resides on an amplicon in primary breast tumors, and TBX3, which is mutated in the human developmental disorder Ulnar-Mammary syndrome. Here we identify a variant T-site, matching 13 of 20 nucleotides of a consensus T-site, as the essential TBX2/TBX3-binding element in the human p14(ARF) promoter. Mutant analysis indicates that both the consensus T-box and a C-terminal conserved repression domain are essential for p14(ARF) repression. Whereas the core nucleotides required for interaction of the archetypal T-box protein Brachyury with a consensus T-site are conserved in the variant site, additional flanking nucleotides contribute to the specificity of TBX2 binding. This is illustrated by the inability of TBX1A or Xbra to activate via the variant p14(ARF) T-site. Importantly, this suggests a hitherto unsuspected level of specificity associated with T-box factors and corresponding recognition sites in regulating their target genes in vivo.