The Robust Tumoricidal Effects of Combined BET/HDAC Inhibition in Cutaneous T-Cell Lymphoma Can Be Reproduced by ANp73 Depletion

Combined BET inhibitor/histone deacetylase inhibitor treatment induces marked apoptosis of cutaneous T-cell lymphoma (CTCL) with minimal normal T-cell toxicity. At 96 hours when apoptosis was extensive, a majority of CTCL lines showed >2-fold suppression of T-cell survival factors (e.g., AKT1, BCL2 antiapoptotic factors, BIRC5, CD40, CD70, GADD45A, PRKCA, TNFRSF1B, ANp73) and >2-fold upregulation of proapoptotic factors and tumor suppressors (e.g., ATM, BAK, BIM, multiple caspases, FHIT, HIC1, MGMT, NOD1) (P < 0.05). The largest alter-ations were in TP73 isoform expression, resulting in increased TAp73/ANp73 ratios in CTCL lines and leukemic Se ' zary cells. Targeted ANp73 inhibition by small interfering RNA knockdown resulted in robust CTCL apoptosis comparable with that induced by BET inhibitor/histone deacetylase inhibitor with minimal normal T-cell toxicity. Chromatin immunoprecipitation analysis showed that BET inhibitor/histone deacetylase inhibitor treatment reduced RNA polymerase II binding to DNp73, MYC, and AKT1 while increasing its binding to TAp73. CTCL skin lesions expressed both TAp73 and ANp73 isoforms in situ. In aggregate, these findings implicate TAp73/ANp73 balance as a major factor governing CTCL survival, show that the expression of p73 isoforms can be altered by molecular biological and pharmaceutical means, show that p73 isoforms are expressed across the entire CTCL clinical spectrum, and identify the p73 pathway as a potential target for therapeutics.