A novel protein encoded by the circular form of the SHPRH gene suppresses glioma tumorigenesis

Abstract

Circular RNAs (circRNAs) are recognized as functional non-coding transcripts in eukaryotic cells. Recent evidence has indicated that even though circRNAs are generally expressed at low levels, they may be involved in many physiological or pathological processes, such as gene regulation, tissue development and carcinogenesis. Although the 'microRNA sponge' function is well characterized, most circRNAs do not contain perfect trapping sites for microRNAs, which suggests the possibility that circRNAs have functions that have not yet been defined. In this study, we show that a circRNA containing an open reading frame (ORF) driven by the internal ribosome entry site (IRES) can translate a functional protein. The circular form of the SNF2 histone linker PHD RING helicase (SHPRH) gene encodes a novel protein that we termed SHPRH-146aa. Circular SHPRH (circ-SHPRH) uses overlapping genetic codes to generate a 'UGA' stop codon, which results in the translation of the 17 kDa SHPRH-146aa. Both circ-SHPRH and SHPRH-146aa are abundantly expressed in normal human brains and are down-regulated in glioblastoma. The overexpression of SHPRH-146aa in U251 and U373 glioblastoma cells reduces their malignant behavior and tumorigenicity in vitro and in vivo. Mechanistically, SHPRH-146aa protects full-length SHPRH from degradation by the ubiquitin proteasome. Stabilized SHPRH sequentially ubiquitinates proliferating cell nuclear antigen (PCNA) as an E3 ligase, leading to inhibited cell proliferation and tumorigenicity. Our findings provide a novel perspective regarding circRNA function in physiological and pathological processes. Specifically, SHPRH-146aa generated from overlapping genetic codes of circ-SHPRH is a tumor suppressor in human glioblastoma.

Translation of yes-associated protein (YAP) was antagonized by its circular RNA via suppressing the assembly of the translation initiation machinery

“…In addition, circRNA could also bind with proteins in the related signal pathways [19][20][21][22]. While circRNAs can be translated to protein peptides [23,24], it is not known whether circRNA could regulate protein translation, especially the protein from their parental RNA. Our present study was designed to discover a new regulation mechanism for Yap during tumorigenesis by investigating whether Yap circular RNA (circYap) is able to directly and selectively regulate Yap translation.…”

Non-coding AUG circRNAs constitute an abundant and conserved subclass of circles

“…Moreover, our results do not necessarily exclude inefficient translation of circRNAs or restricted translation solely under specific conditions. Consequently, it is possible that published circRNAs, such as circMbl in Drosophila (Pamudurti et al 2017), and circZNF609, circSHPRH, circFBXW7, and circLINC-PINT in human cells (Legnini et al 2017; Zhang et al 2018a; Yang et al 2018; Zhang et al 2018b), indeed engage in protein production, however, our data strongly point towards translation of circRNAs as being a rare and uncommon process.…”

“…Additionally, synthetic circRNAs have been engineered to express protein by the use of internal ribosome entry sites (IRESs) allowing cap-independent translation (Wang and Wang 2015). Recently, it was shown that open reading frames (ORFs) within endogenously expressed circRNAs give rise to circRNA-specific peptides (Legnini et al 2017; Pamudurti et al 2017; Yang et al 2017; Zhang et al 2018a; Yang et al 2018; Zhang et al 2018b) suggesting that circRNAs are not necessarily exclusively non-coding.…”

Depolarization-Associated CircRNA Regulate Neural Gene Expression and in Some Cases May Function as Templates for Translation

“…They revealed that circ-Mbl encodes a protein that was detected by mass spectrometry and was found to be enriched in synaptosomes of fly heads. A novel protein derived from circ-SHPRH was also found to be abundantly expressed in human brains and dysregulated in glioblastoma [21]. The circ-SHPRH protected the host protein from degradation, thus increasing the tumor suppressive activity of the gene.…”

“…It was recently suggested that circRNAs are not exclusively noncoding RNA; they can serve as templates for the translation of bio-active polypeptides [17][18][19]. The presence of open reading frames (ORFs) as well as internal ribosome entry sites (IRESs) within endogenous circRNAs enables these molecules to produce functional peptides through a cap-independent mechanism [20,21]. However, the scope of circRNA translation is not yet clear.…”