Molecular basis of USP7 inhibition by selective small-molecule inhibitors.

Ubiquitination controls the stability of most cellular proteins, and its deregulation contributes to human diseases including cancer. Deubiquitinases remove ubiquitin from proteins, and their inhibition can induce the degradation of selected proteins, potentially including otherwise 'undruggable' targets. For example, the inhibition of ubiquitin-specific protease 7 (USP7) results in the degradation of the oncogenic E3 ligase MDM2, and leads to re-activation of the tumour suppressor p53 in various cancers. Here we report that two compounds, FT671 and FT827, inhibit USP7 with high affinity and specificity in vitro and within human cells. Co-crystal structures reveal that both compounds target a dynamic pocket near the catalytic centre of the auto-inhibited apo form of USP7, which differs from other USP deubiquitinases. Consistent with USP7 target engagement in cells, FT671 destabilizes USP7 substrates including MDM2, increases levels of p53, and results in the transcription of p53 target genes, induction of the tumour suppressor p21, and inhibition of tumour growth in mice.

Tightly regulated long-term erythropoietin expression in vivo using tet-inducible recombinant adeno-associated viral vectors.

Recombinant adeno-associated viral (rAAV) vectors containing an improved tetracycline (tet) system of transcriptional regulation are an efficient strategy for the control of long-term therapeutic gene expression. In vivo studies with the original tet-off and tet-on vectors, while promising, have failed to demonstrate complete repression in the uninduced state. To address this issue, we incorporated the tTS(kid) fusion of the tet repressor and a KRAB-derived transcriptional silencer into the tet-on system in the context of rAAV vectors. The tTS(kid) repressor and rtTA activator were expressed constituitively from a regulator vector, and the repressor and an erythropoietin (Epo) transgene were expressed inducibly via a second vector. Following intramuscular co-injection of these vectors, we observed repeated induction of serum Epo protein following drug administration and undetectable levels after its withdrawal. Four cycles of regulation were achieved over a 32-week period. Thus, the tet-on system plus the tTS(kid) repressor delivered via nonpathogenic rAAV vectors is a powerful tool for controlling the in vivo expression of therapeutic transgenes. In a clinical setting, the repressor could provide a mechanism for abolishing transgene expression if it were no longer needed or if the safety of a patient became compromised.