Targeting MYC Transcription with Small Peptide Derived from KSHV Transactivator (RNA-Seq analysis of PDX tumors)

Herpesviruses rely on host cell transcription and translation machineries for replication. Viral proteins thus function to redirect multiple cellular proteins for viral replication. In herpesvirus replicating cells, host cell gene transcription is frequently down-regulated because important transcriptional apparatuses are appropriated by viral transcription factors. Here we show that an evolutionally-shaped viral protein sequence is a great starting material for unique drug development to modulate cellular transcription. Cellular c-Myc protein (MYC) is overexpressed in over 70% of all types of cancer cells and therefore a very attractive target to control cancer cell growth. We identified a small functional peptide derived from the Kaposi's sarcoma-associated herpesvirus transactivator (K-Rta), which strongly attenuates MYC expression, reduces cell proliferation, and selectively kills cancer cells in both tissue culture and a xenograft tumor mouse model. Mechanistically, the peptide blocks promoter-enhancer interactions by preventing coactivator complex consisting of Nuclear receptor coactivator 2, p300, and SWI/SNF proteins from engaging the MYC promoter in leukemia cells. Target gene profiling with SLAM-seq suggests that the viral peptide attenuates MYC expression through a mechanism likely different from that of BET bromodomain inhibitors. Furthermore, fusing the 13 amino acids peptide with humanized anti-CD22 single chain armed the antibody drug with cell killing ability, and inhibited cell growth in soft agar. Our studies thus demonstrate the utility of the peptide sequence as a therapeutics module, which may be used to modulate MYC activity in a cell type-specific manner. Overall design: The goal of these studies was to utilize comprehensive transcriptomic profiling in order to determine the gene expression and pathway alterations mediated by treatment of human xenograft tumors with MBK50, a novel small functional peptide therapeutic derived from the Kaposi's sarcoma-associated herpesvirus (KSHV)-transactivator (K-Rta), and which strongly attenuates MYC expression. This was performed in the context of xenograft tumors generated by intraperitoneal injection of KSHV-infected human primary effusion lymphomas (PEL) BCBL-1 cells, into NRG mice. For this study, NRG mice were randomized into three groups and then treated with either PBS vehicle control, the MBK50 wild type peptide (10 mg/kg), or an inactive mutant peptide (10 mg/kg). The mice were treated once daily for 10 days. Following treatment, tumors were harvested for isolation of total RNA and followed by library preparation and next-generation sequencing (NGS).