Defective transcription elongation in human cancers imposes targetable proteotoxic vulnerability

Successful cancer therapy is contingent on identifying cancer-specific aberrant phenotypes and their associated vulnerabilities. We recently reported that a subset of almost every cancer type contains a genome-wide defect in RNA Polymerase II-mediated transcription elongation (TE^def), which impairs the expression of long genes and confers resistance to anti-tumor immune attack. Using a combination of computational analysis and laboratory experiments, we report that tumor cells with TE^def have widespread overexpression of the components of the protein homeostasis machinery (mostly composed of short genes), including protein folding and clearance. Accordingly, TE^def cells were characterized by abnormally high levels of insoluble protein aggregates in the cytoplasm and autophagy influx. We present evidence that TE^def cells exhibit impaired clearance of misfolded protein aggregates through the ubiquitin-proteasome system, and thus rely on autophagy for their degradation. As such, while these cells were highly resistant to proteasome inhibitors, they were acutely sensitive to inhibitors of autophagy in vitro and in vivo. This study reveals a major aberrant phenotype that is observed in ∼15-25% of all cancers and characterizes a unique cellular vulnerability that can be readily exploited in the clinic to improve treatment efficacy.