Expression of the C-terminal domain of novel human SR-A1 protein: Interaction with the CTD domain of RNA polymerase II

Phenotypic screens identify SCAF1 as critical activator of RNAPII elongation and global transcription

Transcripts produced by RNA polymerase II (RNAPII) are fundamental for cellular responses to environmental changes. It is therefore no surprise that there exist multiple avenues for the regulation of this process. To explore the regulation mediated by RNAPII-interacting proteins, we used a small interfering RNA (siRNA)-based screen to systematically evaluate their influence on RNA synthesis. We identified several proteins that strongly affected RNAPII activity. We evaluated one of the top hits, SCAF1 (SR-related C-terminal domain-associated factor 1), using an auxin-inducible degradation system and sequencing approaches. In agreement with our screen results, acute depletion of SCAF1 decreased RNA synthesis, and showed an increase of Serine-2 phosphorylated-RNAPII (pS2-RNAPII). We found that the accumulation of pS2-RNAPII within the gene body occurred at GC-rich regions and was indicative of stalled RNAPII complexes. The accumulation of stalled RNAPII complexes was accompanied by reduced recruitment of initiating RNAPII, explaining the observed global decrease in transcriptional output. Furthermore, upon SCAF1 depletion, RNAPII complexes showed increased association with components of the proteasomal-degradation machinery. We concluded that in cells lacking SCAF1, RNAPII undergoes a rather interrupted passage, resulting in intervention by the proteasomal-degradation machinery to clear stalled RNAPII. While cells survive the compromised transcription caused by absence of SCAF1, further inhibition of proteasomal-degradation machinery is synthetically lethal.

Chemo-Phosphoproteomic Profiling with ATR Inhibitors Berzosertib and Gartisertib Uncovers New Biomarkers and DNA Damage Response Regulators

The ATR kinase protects cells against DNA damage and replication stress and represents a promising anti-cancer drug target. The ATR inhibitors (ATRi) berzosertib and gartisertib are both in clinical trials for the treatment of advanced solid tumors as monotherapy or in combination with genotoxic agents. We carried out quantitative phospho-proteomic screening for ATR biomarkers that are highly sensitive to berzosertib and gartisertib, using an optimized mass spectrometry pipeline. Screening identified a range of novel ATR-dependent phosphorylation events, which were grouped into three broad classes: (i) targets whose phosphorylation is highly sensitive to ATRi and which could be the next generation of ATR biomarkers; (ii) proteins with known genome maintenance roles not previously known to be regulated by ATR; (iii) novel targets whose cellular roles are unclear. Class iii targets represent candidate DNA damage response proteins and, with this in mind, proteins in this class were subjected to secondary screening for recruitment to DNA damage sites. We show that one of the proteins recruited, SCAF1, interacts with RNAPII in a phospho-dependent manner and recruitment requires PARP activity and interaction with RNAPII. We also show that SCAF1 deficiency partly rescues RAD51 loading in cells lacking the BRCA1 tumor suppressor. Taken together these data reveal potential new ATR biomarkers and new genome maintenance factors.

Expanding the SRI domain family: a common scaffold for binding the phosphorylated C-terminal domain of RNA polymerase II

The SRI domain is a small three-helix domain originally discovered near the C-terminus of both histone methyltransferase SETD2 and helicase RECQL5. The SRI domain binds to the C-terminal repeat domain of the largest subunit of RNA polymerase II, allowing SETD2 and RECQL5 to regulate various mechanisms associated with RNA transcription. Using original tools to detect common patterns in distantly related sequences, we have identified SRI domains in several additional proteins, most of which are involved in RNA metabolism. Combining sequence analysis with structural prediction, we show that this domain family is more diverse than previously thought and we predict critical structural and functional features.

Alternative splicing in cancer: Noise, functional, or systematic?

Pre-messenger RNA splicing is a fine-tuned process that generates multiple functional variants from individual genes. Various cell types and developmental stages regulate alternative splicing patterns differently in their generation of specific gene functions. In cancers, splicing is significantly altered, and understanding the underlying mechanisms and patterns in cancer will shed new light onto cancer biology. Cancer-specific transcript variants are promising biomarkers and targets for diagnostic, prognostic, and treatment purposes. In this review, we explore how alternative splicing cannot simply be considered as noise or an innocent bystander, but is actively regulated or deregulated in cancers. A special focus will be on aspects of cell biology and biochemistry of alternative splicing in cancer cells, addressing differences in splicing mechanisms between normal and malignant cells. The systems biology of splicing is only now applied to the field of cancer research. We explore functional annotations for some of the most intensely spliced gene classes, and provide a literature mining and clustering that reflects the most intensely investigated genes. A few well-established cancer-specific splice events, such as the CD44 antigen, are used to illustrate the potential behind the exploration of the mechanisms of their regulation. Accordingly, we describe the functional connection between the regulatory machinery (i.e., the spliceosome and its accessory proteins) and their global impact on qualitative transcript variation that are only now emerging from the use of genomic technologies such as microarrays. These studies are expected to open an entirely new level of genetic information that is currently still poorly understood.

The C-terminal domain of RNA Pol II helps ensure that editing precedes splicing of the GluR-B transcript

The C-terminal domain (CTD) of the large subunit of RNA polymerase II (Pol II) influences many steps in the synthesis of an mRNA and helps control the final destiny of the mature transcript. ADAR2 edits RNA by converting adenosine to inosine within double-stranded or structured RNA. Site-selective A-to-I editing often occurs at sites near exon/intron borders, where it depends on intronic sequences for substrate recognition. It is therefore essential that editing precedes splicing. We have investigated whether there is coordination between ADAR2 editing and splicing of the GluR-B pre-mRNA. We show that the CTD is required for efficient editing at the R/G site one base upstream of a 5'-splice site. The results suggest that the CTD enhances editing at the R/G site by preventing premature splicing that would remove the intronic recognition sites for ADAR2. Editing at the GluR-B Q/R site, 24 bases upstream of the intron 11 5'-splice site, stimulates splicing at this intron. Furthermore, unlike previously studied introns, the CTD actually inhibits excision of intron 11, which includes the complementary recognition sequences for the Q/R editing site. In summary, these results show that the CTD and ADAR2 function together to enforce the order of events that allows editing to precede splicing, and they furthermore suggest a new role for the CTD as a coordinator of two interdependent pre-mRNA processing events.

Effect of bleomycin and cisplatin on the expression profile of SRA1, a novel member of pre-mRNA splicing factors, in HL-60 human promyelocytic leukemia cells

Recently, a new member of the human SR (Ser/Arg-rich) superfamily of pre-mRNA splicing factors, SRA1 (SR-A1), has been discovered and cloned by members of our group, the gene for which was found to be overexpessed in a series of human tumors. In the present study, we investigated the significance of alterations at the mRNA expression levels of the SRA1 gene after treatment of HL-60 human promyelocytic leukemia cells with the anticancer drugs cisplatin and bleomycin. The kinetics of apoptosis and cell toxicity were investigated by DNA laddering and the MTT and trypan blue assays, respectively. Total RNA was extracted and cDNA was prepared by reverse transcription. The splicing-related genes SRA1 and SC35, as well as the apoptosis-related gene BCL2 (Bcl-2), were amplified by PCR using gene-specific primers. The results showed that mRNA levels of SRA1 were up-regulated upon treatment with the antibiotic bleomycin, whereas they were down-regulated by treatment of HL-60 human promyelocytic leukemia cells with cisplatin. Our results support the hypothesis that mRNA expression analysis of SRA1 may serve as a new prospective molecular marker, playing an important role in chemotherapy outcome in human leukemia.

Expression analysis and prognostic significance of the SRA1 gene, in ovarian cancer

The SR-related-CTD-associated-factors (SCAFs) have the ability to interact with the C-terminal domain of the RNA polymerase II, linking this way transcription to splicing. SRA1 (SR-A1) gene, encoding for a human high-molecular weight SCAF protein, is located on chromosome 19, between the IRF3 and the R-RAS oncogene and it has been demonstrated from members of our group that SRA1 is constitutively expressed in most of the human tissues, while it is overexpressed in a subset of ovarian tumors. In this study, we examine the expression of SRA1 gene in 111 ovarian malignant tissues and in the human ovarian carcinoma cell lines OVCAR-3, TOV21-G, and ES-2, using a semi-quantitative RT-PCR method. SRA1 gene was overexpressed in 61/111 (55%) of ovarian carcinomas. This higher expression was positively associated to the size of the tumor (p<0.001), the grade and the stage of the disease (p=0.003 and p=0.006, respectively), and the debulking success (p<0.001). Kaplan-Meier survival analysis revealed that lower SRA1 expression increases the probability of both the longer overall and the progression free survival of the patients. Multivariate Cox regression analysis revealed that SRA1 may be used as an independent prognostic biomarker in ovarian cancer. Our results suggest that SRA1 is associated with cancer progression and may possibly be characterized as a new marker of unfavorable prognosis for ovarian cancer.

Prognostic significance of the expression of SR-A1, encoding a novel SR-related CTD-associated factor, in breast cancer

SR-A1 is a human high-molecular-weight SR-related CTD-associated factor that links the machineries of transcription and mRNA splicing. In this study we examined the prognostic value of SR-A1 gene expression using a semi-quantitative RT-PCR method. High SR-A1 expression was observed in 31/81 (38.3%) breast cancer tissues and was found to be more frequent in patients with tumors of large size (p=0.027), as well as in lymph node-positive patients (p=0.035). Follow-up analysis revealed that low SR-A1 expression increases the probability of both overall and disease-free survival of patients. Our results suggest that SR-A1 may possibly be characterized as a new marker of unfavorable prognosis for breast cancer.