Editors' ChoiceCancer

Precision medicine by synthetic lethality

Sci. Signal.  24 Nov 2015:
Vol. 8, Issue 404, pp. ec345
DOI: 10.1126/scisignal.aad8630

Synthetic lethality is defined as perturbations (genetic mutation, chemical or drug exposure, or environmental conditions) that individually have little effect on cell viability but that when combined result in cell death. The principle of synthetic lethality can be leveraged to screen for drugs that selectively target and kill cancer cells that have specific alterations. Using a synthetic-lethality screen, Pfister et al. determined that cancer cells deficient in SETD2, a histone methyltransferase that trimethylates histone H3 at Lys26 (H3K26me3), died in response to chemical inhibition of WEE1, a “checkpoint” kinase that inhibits premature entry into mitosis. Compared with cancer cells that did not have SETD2 deficiency, renal carcinoma cell lines with deficient in SETD2 died at a lower dose of AZD1775, a WEE1 inhibitor that is in clinical trials. AZD1775 arrested both SETD2-deficient renal carcinoma cells and SETD2-knockout U2OS cells in S phase. Progression of DNA replication forks was reduced in U2OS cells with reduced amounts of SETD2 or in U2OS cells with normal amounts SETD2 that were exposed to AZD1775, but only in the SETD2-depleted cells did AZD1775 completely abolish replication fork progression and increase fork-stalling, which suggested that inhibition of WEE1 in the context of SETD2 depletion generated replication stress. Lack of deoxyribonucleoside triphosphates (dNTPs) could result in stalled DNA replication. Indeed, the abundance of RRM2, a subunit of ribonucleotide reductase that catalyzes the formation of dNTPs, was reduced in SETD2-knockdown U2OS cancer cells, and exposure of these cells to AZD1775 further reduced the abundance of RRM2. SETD2-deficient cancer cells had less RRM2 mRNA, consistent with an effect of SETD2 on RRM2 transcription. In addition, RRM2 depletion by siRNA in U2OS cells or exposure of U2OS cells to hydroxyurea or gemcitabine, chemicals that inhibit RRM2, sensitized the cells towards the cytotoxic effects of AZD1775. STED2-deficient cells exposed to AZD1775 exhibited increased activity of the cyclin-dependent kinases, CDK1 and CDK2, which in turn increased the phosphorylation of RRM2 at Thr33, a site that when phosphorylated promotes the ubiquitylation and degradation of RRM2. Western blot analysis of the abundance of wild-type RRM2 or a T33A mutant showed that abundance of wild-type RRM2 was decreased upon WEE1 inhibition with AZD1775, whereas the abundance of T33A mutant did not change. In addition, inhibition of WEE1 resulted in abnormal firing of replication origins that could further enhance replicative stress. Last, AZD1775 treatment regressed SETD2-deficient xenografted tumors in nude mice but did not have a significant effect on xenografted tumor growth from SETD2-proficient U2OS cells. Thus, this study demonstrates how synthetic lethality screens can identify treatments that trigger death of cancers with specific attributes (see Shoaib and Sørensen).

S. X. Pfister, E. Markkanen, Y. Jiang, S. Sarkar, M. Woodcock, G. Orlando, I. Mavrommati, C.-C. Pai, L.-P. Zalmas, N. Drobnitzky, G. L. Dianov, C. Verrill, V. M. Macaulay, S. Ying, N. B. La Thangue, V. D’Angiolella, A. J. Ryan, T. C. Humphrey, Inhibiting WEE1 selectively kills histone H3K36me3-deficient cancers by dNTP starvation. Cancer Cell 28, 557–568 (2015). [Online Journal]

M. Shoaib and C. S. Sørensen, Epigenetic deficiencies and replicative stress: Driving cancer cells to an early grave. Cancer Cell 28, 545–547 (2015). [PubMed]