Prolonged stalling of RNA Polymerase II at DNA damage explains phenotypical differences between Cockayne and UV-sensitive syndromes
Faithful transcription of eukaryotic genes by RNA polymerase II (Pol II) is essential for proper cell function. Nevertheless, the integrity of the DNA template of Pol II is continuously challenged by different sources of DNA damage, such as UV-light, that impede transcription. When unresolved, these transcription-blocking lesions (TBLs) can cause cellular dysfunction, senescence and apoptosis, eventually resulting in DNA damage-induced aging. Cells counteract these deleterious effects by Transcription-Coupled Nucleotide Excision Repair (TC-NER), which specifically removes TBLs, thereby safeguarding transcription. TC-NER initiation relies on the concerted actions of the CSB, CSA and UVSSA proteins, and loss of either of these factors results in a complete TC-NER deficiency. Although their TC-NER defect is similar, UVSSA loss results in UV-Sensitive Syndrome (UVSS), with only mild phenotypes like freckling and photosensitivity, while loss of CSA or CSB activity results in the severe Cockayne Syndrome (CS), characterized by premature aging, progressive neurodegeneration and mental retardation. Thus far the underlying mechanism for these striking differences in phenotypes remains unclear. Using live-cell imaging approaches, here we show that in TC-NER proficient cells lesion-stalled Pol II is swiftly resolved by repair of the TBL. However, in CSA and CSB knockout (KO) cells, elongating Pol II remains chromatin-bound. This lesion-stalled Pol II will obstruct other DNA transacting processes and will also shield the damage from repair by alternative pathways. In contrast, in UVSSA KO cells, Pol II is removed from the TBL by VCP-mediated proteasomal degradation, thereby, allowing alternative repair mechanisms to remove the TBL.