Person:

Ellisen, Leif

The p53 family transcription factors p53, p63 and p73 make diverse contributions in development and cancer. Mutation or deletion of p53 is observed in the majority of human cancers. In contrast, p63 and p73 are not lost in cancer but mediate distinct genetic roles in normal and tumor-specific contexts: p73 promotes genome stability and mediates chemosensitivity, while p63 largely lacks these p53-like functions and instead promotes proliferation and cell survival. We recently uncovered a mechanism which maintains p63/p73 homeostasis within the epithelium through direct transcriptional regulation of microRNAs (miRs). We discovered that several of the top p63-regulated miRs target p73 for inhibition, including miR-193a-5p, a direct p63/p73 transcriptional target which is repressed by p63 and activated by p73 both in vitro and in vivo. The resulting feed-forward circuit involving p63, miR-193a-5p and p73 controls p73 levels, cell viability and DNA damage susceptibility in certain cancers including squamous cell carcinoma. Here, we discuss the evolutionary implications of this regulatory circuit, which may point to a general mechanism of miR-mediated cross-talk within transcription factor gene families. Additionally, we suggest that inducible chemoresistance mediated by this miR-dependent mechanism might be an attractive target for therapeutic intervention.

Targeted cancer therapy requires the rapid and accurate identification of genetic abnormalities predictive of therapeutic response. We sought to develop a high-throughput genotyping platform that would allow prospective patient selection to the best available therapies, and that could readily and inexpensively be adopted by most clinical laboratories. We developed a highly sensitive multiplexed clinical assay that performs very well with nucleic acid derived from formalin fixation and paraffin embedding (FFPE) tissue, and tests for 120 previously described mutations in 13 cancer genes. Genetic profiling of 250 primary tumours was consistent with the documented oncogene mutational spectrum and identified rare events in some cancer types. The assay is currently being used for clinical testing of tumour samples and contributing to cancer patient management. This work therefore establishes a platform for real-time targeted genotyping that can be widely adopted. We expect that efforts like this one will play an increasingly important role in cancer management.

Macroautophagy (autophagy) is a critical cellular stress response; however, the signal transduction pathways controlling autophagy induction in response to stress are poorly understood. Here we reveal a new mechanism of autophagy control whose deregulation disrupts mitochondrial integrity and energy homeostasis in vivo. Stress conditions including hypoxia and exercise induce reactive oxygen species (ROS) through upregulation of a protein complex involving REDD1, an mTORC1 inhibitor and the pro-oxidant protein TXNIP. Decreased ROS in cells and tissues lacking either REDD1 or TXNIP increases catalytic activity of the redox-sensitive ATG4B cysteine endopeptidase, leading to enhanced LC3B delipidation and failed autophagy. Conversely, REDD1/TXNIP complex expression is sufficient to induce ROS, suppress ATG4B activity and activate autophagy. In Redd1−/− mice, deregulated ATG4B activity and disabled autophagic flux cause accumulation of defective mitochondria, leading to impaired oxidative phosphorylation, muscle ATP depletion and poor exercise capacity. Thus, ROS regulation through REDD1/TXNIP is physiological rheostat controlling stress-induced autophagy.

Considering the study by William Kim and colleagues, this commentary reflects on advances in genome‐directed cancer therapy, the organization of molecular tumor boards, and progress toward informatics‐based matching of tumor molecular profiles with effective treatments.

In this issue of JEM, Sundaram et al. (https://doi.org/10.1084/jem.20170354) report a mechanism by which the normal epithelial wound healing response is “hijacked” to promote invasion and metastasis in head and neck squamous carcinomas (HNSCCs), a finding that unveils new markers of poor outcomes and potential targets for therapeutic intervention.