(A) Histone acetyltransferases (HATs) acetylate chromatin, leading to less compacted chromatin and transcriptional activation. Nuclear YAP/TAZ interact with TEAD transcription factors to strongly increase nascent RNA synthesis by RNA Pol I and Pol II, which promotes replication stress. (D) Knockout of the Hippo pathway LATS1/2 kinases leads to hypophosphorylation and nuclear translocation of the YAP/TAZ transcriptional coactivators. Ewing sarcoma cell lines, or cell lines expressing EWS–FLI1, display increased RNA synthesis, increased R-loops, and replication stress. (C) Expression of the EWS–FLI1 fusion oncoprotein, which drives Ewing sarcoma, increases RNA Pol II C-terminal domain (CTD) phosphorylation via an unknown mechanism. This is accompanied by increased R-loop formation, replication stress, and DNA breakage. (B) Estrogen activates the estrogen receptor (ER) transcription factor, causing a rapid burst in transcription at a large number of responsive genes, including those transcribed by RNA Pol I and Pol III. TBP activates all three RNA polymerases (RNA Pols) thus, RAS-induced replication stress could result from increased RNA synthesis by RNA Pol I and Pol III as well as Pol II. Elevated RNA synthesis is accompanied by an increase in R-loops and by replication stress. (A) H-RAS activation increases global RNA synthesis, at least in part through increased expression of the general transcription factor (GTF) TBP. Abbreviations: RNAP, RNA polymerase TF, transcription factor. Oncogenic signaling activates BRF1 and inhibits the RNA Pol III transcriptional repressor MAF1. TFIIIC recruits TFIIIB, comprising BRF1 or BRF2, BDP1, and TBP, which in turn recruits RNA Pol III. (C) RNA Pol III transcription initiation requires TFIIIC binding at tRNA genes or TFIIIA and TFIIIC binding at 5s rRNA genes. Oncogenic activation of RNA Pol II increases the expression of GTF and RNA Pol subunits involved in RNA Pol I-, II-, and III-dependent transcription. Oncogenic signaling stabilizes MYC through direct phosphorylation or preventing GSK3-directed degradation, allowing MYC to recruit coactivators to target genes. Bromodomain-containing protein 4 (BRD4) recruits P-TEFB from its inhibitory 7SK complex, allowing phosphorylation of RNA Pol II CTD serine 2 and transcriptional elongation. RNA Pol II phosphorylation on serine 5 of its C-terminal domain (CTD) by TFIIH facilitates the transition from initiation to elongation. (B) Recruitment and activation of RNA Pol II is aided by a preinitiation complex (PIC) formed by the general transcription factors (GTFs) TFIIDA, B, F, E, and H. Both UBF1 and SL1 help to mediate elongation in combination with other RNA Pol I-specific elongation factors. Phosphorylation of RRN3 allows its release from RNA Pol and relieves promoter-proximal pausing.

UBF1 phosphorylation increases UBF1-Pol I binding. UBF1 recruits the SL1 complex and RRN3, mediating RNA Pol I recruitment and promoter binding. (A) Schematic of growth factor signaling pathways downstream of RAS and PI3K and the regulation of RNA polymerase I (Pol I).

Here we discuss our mechanistic understanding of hypertranscription-induced replication stress and the resulting cellular responses, in the context of oncogenes and targeted cancer therapies. A growing number of recent studies are reporting that oncogenes, such as RAS, and targeted cancer treatments, such as bromodomain and extraterminal motif (BET) bromodomain inhibitors, increase global transcription, leading to R-loop accumulation, transcription–replication conflicts, and the activation of replication stress responses. Despite the widely accepted importance of oncogene-induced hypertranscription, its study remains neglected compared with other causes of replication stress and genomic instability in cancer. Oncogenic signaling can promote global increases in transcription activity, also termed hypertranscription. Replication stress results from obstacles to replication fork progression, including ongoing transcription, which can cause transcription–replication conflicts.

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