Synthetic lethality: general principles, utility and detection using genetic screens in human cells

Functional drug-gene interactions in lung cancer

Despite the dawn of the genomic information era, the challenges of cancer treatment remain formidable. Particularly for the most prevalent cancer types, including lung cancer, successful treatment of metastatic disease is rare and escalating costs for modern targeted drugs place an increasing strain on healthcare systems. Although powerful diagnostic tools to characterize individual tumor samples in great molecular detail are becoming rapidly available, the transformation of this information into therapy provides a major challenge. A fundamental difficulty is the molecular complexity of cancer cells that often causes drug resistance, but can also render tumors exquisitely sensitive to targeted agents. By using lung cancer as an example, we outline the principles that govern drug sensitivity and resistance from a genetic perspective and discuss how in vitro chemical-genetic screens can impact on patient stratification in the clinic.

Metformin is synthetically lethal with glucose withdrawal in cancer cells

Comment on: Menendez JA, et al. Cell Cycle 2012; 11: 2782-92.

Inhibiting homologous recombination for cancer therapy

We review the rationale for seeking inhibitors of homologous recombination (HR) repair for use in cancer therapy. Cells use HR as one way to repair DNA double-strand breaks that arise directly from treatments such as radiotherapy, or indirectly during replication when forks encounter other damage. HR occurs during the S and G 2 phases of the cell cycle and is therefore more significant in dividing cancer cells than in non-dividing cells of healthy tissue, giving a potential therapeutic advantage to inhibiting the process. Also, some tumors consist of cells that are defective in other DNA repair pathways, and such cells may be sensitive to HR repair inhibitors because of synthetic lethality, in which blocking two alternative pathways that a cell can use to reach a needed end-point has a much bigger impact than blocking either pathway alone. We review strategies for identifying HR inhibitors and discuss current progress.

Olaparib maintenance therapy in platinum-sensitive relapsed ovarian cancer

BACKGROUND

Olaparib (AZD2281) is an oral poly(adenosine diphosphate [ADP]-ribose) polymerase inhibitor that has shown antitumor activity in patients with high-grade serous ovarian cancer with or without BRCA1 or BRCA2 germline mutations.

METHODS

We conducted a randomized, double-blind, placebo-controlled, phase 2 study to evaluate maintenance treatment with olaparib in patients with platinum-sensitive, relapsed, high-grade serous ovarian cancer who had received two or more platinum-based regimens and had had a partial or complete response to their most recent platinum-based regimen. Patients were randomly assigned to receive olaparib, at a dose of 400 mg twice daily, or placebo. The primary end point was progression-free survival according to the Response Evaluation Criteria in Solid Tumors guidelines.

RESULTS

Of 265 patients who underwent randomization, 136 were assigned to the olaparib group and 129 to the placebo group. Progression-free survival was significantly longer with olaparib than with placebo (median, 8.4 months vs. 4.8 months from randomization on completion of chemotherapy; hazard ratio for progression or death, 0.35; 95% confidence interval [CI], 0.25 to 0.49; P<0.001). Subgroup analyses of progression-free survival showed that, regardless of subgroup, patients in the olaparib group had a lower risk of progression. Adverse events more commonly reported in the olaparib group than in the placebo group (by more than 10% of patients) were nausea (68% vs. 35%), fatigue (49% vs. 38%), vomiting (32% vs. 14%), and anemia (17% vs. 5%); the majority of adverse events were grade 1 or 2. An interim analysis of overall survival (38% maturity, meaning that 38% of the patients had died) showed no significant difference between groups (hazard ratio with olaparib, 0.94; 95% CI, 0.63 to 1.39; P=0.75).

CONCLUSIONS

Olaparib as maintenance treatment significantly improved progression-free survival among patients with platinum-sensitive, relapsed, high-grade serous ovarian cancer. Interim analysis showed no overall survival benefit. The toxicity profile of olaparib in this population was consistent with that in previous studies. (Funded by AstraZeneca; ClinicalTrials.gov number, NCT00753545.).

Back to the future: mechanism-based, mutation-specific combination chemoprevention with a synthetic lethality approach

There is an increasing recognition that the mutations accompanying carcinogenesis may provide a window of therapeutic advantage designated synthetic lethality, an example of which is reported in this issue of the journal by Huang and colleagues (beginning on page 666). First discovered and studied in yeast, synthetic lethality has basic principles that have encouraged its development for treatment and now prevention in animal models of human cancer, especially malignancies refractory to standard approaches. The pros and cons of this approach and challenges in implementing it clinically are discussed.

A chemical-genetic screen reveals a mechanism of resistance to PI3K inhibitors in cancer

Linking the molecular aberrations of cancer to drug responses could guide treatment choice and identify new therapeutic applications. However, there has been no systematic approach for analyzing gene-drug interactions in human cells. We establish a multiplexed assay to study the cellular fitness of a panel of engineered isogenic cancer cells in response to a collection of drugs, enabling the systematic analysis of thousands of gene-drug interactions. Applying this approach to breast cancer revealed various synthetic-lethal interactions and drug resistance mechanisms, some of which were known, thereby validating the method. NOTCH pathway activation, which occurs frequently in breast cancer, unexpectedly conferred resistance to PI3K inhibitors, which are currently undergoing clinical trials in breast cancer patients. NOTCH1 and downstream induction of c-MYC overrode the dependency of cells on the PI3K/mTOR pathway for proliferation. These data reveal a novel mechanism of resistance to PI3K inhibitors with direct clinical implications.

Oncogene addiction as a foundational rationale for targeted anti-cancer therapy: promises and perils

A decade has elapsed since the concept of oncogene addiction was first proposed. It postulates that – despite the diverse array of genetic lesions typical of cancer – some tumours rely on one single dominant oncogene for growth and survival, so that inhibition of this specific oncogene is sufficient to halt the neoplastic phenotype. A large amount of evidence has proven the pervasive power of this notion, both in basic research and in therapeutic applications. However, in the face of such a considerable body of knowledge, the intimate molecular mechanisms mediating this phenomenon remain elusive. At the clinical level, successful translation of the oncogene addiction model into the rational and effective design of targeted therapeutics against individual oncoproteins still faces major obstacles, mainly due to the emergence of escape mechanisms and drug resistance. Here, we offer an overview of the relevant literature, encompassing both biological aspects and recent clinical insights. We discuss the key advantages and pitfalls of this concept and reconsider it as an illustrative principle to guide post-genomic cancer research and drug development.

Investigating a macromolecular complex: the toolkit of methods

Structural biologists studying macromolecular complexes spend considerable effort doing strictly "non-structural" work: investigating the physiological relevance and biochemical properties of a complex, preparing homogeneous samples for structural analysis, and experimentally validating structure-based hypotheses regarding function or mechanism. Familiarity with the diverse perspectives and techniques available for studying complexes helps in the critical assessment of non-structural data, expedites the pre-structural characterization of a complex and facilitates the investigation of function. Here we survey the approaches and techniques used to study macromolecular complexes from various viewpoints, including genetics, cell and molecular biology, biochemistry/biophysics, structural biology, and systems biology/bioinformatics. The aim of this overview is to heighten awareness of the diversity of perspectives and experimental tools available for investigating complexes and of their usefulness for the structural biologist.