The Robust Tumoricidal Effects of Combined BET/HDAC Inhibition in CTCL Can Be Reproduced by ΔNp73 Depletion

ABSTRACT

Combined BET-inhibitor/HDAC-inhibitor (BETi/HDACi) 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 anti-apoptotic factors, BIRC5, CD40, CD70, GADD45A, PRKCA, TNFRSF1B, ΔNp73) and ≥ 2-fold upregulation of pro-apoptotic factors and tumor suppressors (e.g. ATM, BAK, BIM, multiple caspases, FHIT, HIC1, MGMT, NOD1) (p<0.05). The largest alterations were in TP73 isoform expression resulting in increased TAp73/ΔNp73 ratios in CTCL lines and leukemic Sézary cells. Targeted ΔNp73 inhibition by si-RNA knockdown resulted in robust CTCL apoptosis comparable to that induced by BETi/HDACi with minimal normal T-cell toxicity. ChIP analysis showed that BETi/HDACi treatment reduced RNA polymerase II binding to ΔNp73, MYC and AKT1 while increasing its binding to TAp73. CTCL skin lesions expressed both TAp73 and ΔNp73 isoforms in situ. In aggregate, these findings implicate TAp73/ΔNp73 balance as a major factor governing CTCL survival, show that expression of p73 isoforms can be altered by molecular biological and pharmaceutical means, demonstrate that p73 isoforms are expressed across the entire CTCL clinical spectrum, and identify the p73 pathway as a potential target for therapeutics.