The Role of RNA Splicing Factors in Cancer: Regulation of Viral and Human Gene Expression in Human Papillomavirus-Related Cervical Cancer

Abstract

The spliceosomal complex components, together with the heterogeneous nuclear ribonucleoproteins (hnRNPs) and serine/arginine-rich (SR) proteins, regulate the process of constitutive and alternative splicing, the latter leading to the production of mRNA isoforms coding multiple proteins from a single pre-mRNA molecule. The expression of splicing factors is frequently deregulated in different cancer types causing the generation of oncogenic proteins involved in cancer hallmarks. Cervical cancer is caused by persistent infection with oncogenic human papillomaviruses (HPVs) and constitutive expression of viral oncogenes. The aberrant activity of hnRNPs and SR proteins in cervical neoplasia has been shown to trigger the production of oncoproteins through the processing of pre-mRNA transcripts either derived from human genes or HPV genomes. Indeed, hnRNP and SR splicing factors have been shown to regulate the production of viral oncoprotein isoforms necessary for the completion of viral life cycle and for cell transformation. Target-therapy strategies against hnRNPs and SR proteins, causing simultaneous reduction of oncogenic factors and inhibition of HPV replication, are under development. In this review, we describe the current knowledge of the functional link between RNA splicing factors and deregulated cellular as well as viral RNA maturation in cervical cancer and the opportunity of new therapeutic strategies.

Keywords: RNA; cervical cancer; heterogeneous nuclear ribonucleoproteins (hnRNPs); human papillomavirus (HPV); serine/arginine-rich proteins (SR); splicing factors.

FIGURE 1

Steps of the splicing process. The snRNP U1, SF1 and U2AF bind to the intron 5′-end SD site, to an intronic branch point site (A) and to the intron 3′-end SA site, respectively. The U2 displaces SF1 and the U4/U5/U6 snRNPs trimer interacts with snRNPs U1 and U2 causing U1 and U4 release. The activated spliceosome catalyzes the cleavage of the intron 5′-end, the formation of a lariat, the intron the cleavage of the intron 3′-end, the release of the lariat/U2/U5/U6 complex release and joining of exons (Shi, 2017). Exons are defined as “E”; the intron is represented as a black solid line.

FIGURE 2

Mechanisms of alternative splicing. The spliceosome generates distinct mRNA isoforms by alternative usage of splice donor (SD) and acceptor (SA) sites, located at the 5′ and 3′ end of introns, respectively. The splicing products include: (I) exon skipping (the partial or total removal of exons), (II) inclusion of mutually exclusive exons, (III) intron retention and the use of alternative (IV) SD or (V) SA sites (Baralle and Giudice, 2017). In the figure, black solid lines represent introns; black and red dashed lines represent alternative splicing mechanisms. Exons are indicated as “E” and introns are indicated as “I.”

FIGURE 3

Regulation of alternative splicing. Alternative splicing is finely regulated by trans-acting hnRNPs and SRSFs protein families. The hnRNPs generally bind exonic (ESSs) and intronic splicing silencers (ISSs), antagonize SRs activity and inhibit spliceosome assembly on the splicing sites (red lines). The SRSFs bind to exonic (ESEs) and intronic (ISEs) splicing enhancers and increase the splicing efficiency by favoring the spliceosome recruitment and assembly on the splicing sites (green lines) (Chen and Manley, 2009; Wang et al., 2015).

FIGURE 4

A schematic representation of the HPV16 genome and transcripts. The scheme is based on the HPV episteme (van et al., 2017). (A) The P97 derived early mRNAs are polyadenylated at the early polyadenylation site (pAE). The P670 and PE8 derived late mRNAs are polyadenylated at the late polyadenylation site (pAL). Both the early (B) and late (C) transcripts are polycistronic and subjected to alternative splicing (Zheng and Baker, 2006; Graham, 2010; Graham and Faizo, 2017). The donor (DS) and the acceptor (AS) splice sites in the HPV16 genome are indicated as black and white triangles, respectively. The coding potential of each transcript is also indicated.

FIGURE 5

Splicing regulation of HPV16 mRNAs. Activities of (A) hnRNPs and (B) SRSFs and their binding motifs on HPV16 transcripts are shown. In particular: red lines indicate silencing activities; green arrows indicate enhancing activities; red sequences are silencing elements; while green sequences are enhancer elements. The donor (SD) and acceptor (AS) splice sites are indicated as black and white triangles, respectively.