Abstract
Knockout of any of several noncanonical open reading frames (ORF) impaired cancer cell survival.
Major Finding: Knockout of any of several noncanonical open reading frames (ORF) impaired cancer cell survival.
Concept: Expression of one such ORF, renamed GREP1, was specifically required in breast cancer cells.
Impact: This study provides evidence for functional roles of noncanonical ORFs such as GREP1 in cancer.
In addition to known protein-coding open reading frames (ORF), it has been suggested that translation producing functional proteins may occur from mRNAs transcribed from noncanonical ORFs, which can be located in genomic regions conventionally recognized as pseudogenes, noncoding RNA–encoding sequences, or the 5′ or 3′ untranslated regions of protein-coding genes. To investigate this, Prensner and colleagues performed CRISPR–Cas9-based loss-of-function screens in eight cancer cell lines, specifically targeting 553 noncanonical ORFs selected based on prior evidence or predictions of ORF translation. These screens revealed that for more than 10% of the selected noncanonical ORFs, loss of function impaired cell viability. Deeper investigation using CRISPR tiling and start-codon mutagenesis provided evidence that the observed cell viability deficits were not attributable to disruption of genomic regulatory regions or noncoding RNAs but rather to the putative proteins produced from the noncanonical ORFs. One of the noncanonical ORFs identified as being functional, G029442 (renamed GREP1, for glycine-rich extracellular protein-1), was shown to encode a secreted protein highly and specifically expressed in breast cancer cells grown in vitro and in patient breast cancer samples. Functional studies revealed that GREP1 overexpression induced increased production of the protumorigenic cytokine GDF15. Furthermore, GDF15 supplementation restored viability to GREP1-knockout cells, but this effect was not complete in some cell lines, suggesting that additional factors are involved. In summary, this work provides support for the notion that many noncanonical ORFs may give rise to functional proteins and reveals GREP1 as a previously unknown and potentially targetable dependency of breast cancer cells.
Note: Research Watch is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details. For more Research Watch, visit Cancer Discovery online at http://cancerdiscovery.aacrjournals.org/CDNews.
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