Specific USP7 inhibitors stabilized p53 and exhibited toxicity in tumor cells in vitro and in vivo.

  • Major finding: Specific USP7 inhibitors stabilized p53 and exhibited toxicity in tumor cells in vitro and in vivo.

  • Approach: Co-crystal structures reveal the mechanism of inhibition of USP7 inhibitors in complex with USP7.

  • Impact: Specific pharmacologic inhibition of USP7 may be feasible and promote stabilization of p53 in cancer.

Stabilization of reactivation of the tumor suppressor p53 is a potential strategy for cancer treatment. One potential approach to stabilize p53 is to target the ubiquitin E3 ligase MDM2, which ubiquitinates and destabilizes p53. The deubiquitinase USP7 protects MDM2 from degradation, suggesting the potential for targeting USP7 in cancer. However, specific USP7 inhibitors have been challenging to develop. Kategaya, Di Lello, Rougé, and colleagues used nuclear magnetic resonance–based screening and structure-based design to develop two selective USP7 inhibitors, GNE-6640 and GNE-6776, which attenuated ubiquitin binding to USP7, thereby suppressing its deubiquitinase activity. Both compounds increased MDM2 ubiquitination and degradation, and resulted in cytotoxicity in a number of cancer cell lines, including acute myeloid leukemia cells. Further, GNE-6640 and GNE-6776 enhanced sensitivity to chemotherapeutic agents and targeted inhibitors such as PIM kinase inhibitors. Structural analyses showed that the USP7 inhibitors bind the USP7 ubiquitin-binding site, and found that USP7 preferentially interacts with free Lys48 side chains, resulting in sequential depolymerization of Lys48-linked ubiquitin chains. In a related study, Turnbull, Ioannidis, Krajewski, and colleagues used a ubiquitin-rhodamine assay to screen 500,000 compounds to identify potential USP7 inhibitors, and further compound optimization yielded the selective inhibitors FT671 and FT827. Co-crystal structures of FT671 and FT827 with USP7 revealed plasticity in the USP domains, and showed that both compounds bound competitively at the ubiquitin binding site. In cancer cell lines FT671 destabilized MDM2, increasing levels of p53 and enhancing transcription of p53 target genes. Further, in a multiple myeloma xenograft model, FT671 was well tolerated and suppressed tumor growth. Taken together, these studies identified specific USP7 inhibitors that exhibited cytotoxicity in cancer cells, suggesting the feasibility of targeting USP7 as a strategy to stabilize p53 in cancer.

Kategaya L, Di Lello P, Rougé L, Pastor R, Clark KR, Drummond J, et al. USP7 small-molecule inhibitors interfere with ubiquitin binding. Nature 2017;550:534–8.

Turnbull AP, Ioannidis S, Krajewski WW, Pinto-Fernandez A, Heride C, Martin AC, et al. Molecular basis of USP7 inhibition by selective small-molecule inhibitors. Nature 2017;550:481–6.

Note: Research Watch is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details. For more Research Watch, visit Cancer Discovery online at http://cancerdiscovery.aacrjournals.org/content/early/by/section.

©2017 American Association for Cancer Research.
2017
American Association for Cancer Research.