Cellular sensitivity to UV-irradiation is mediated by RNA polymerase I transcription

Ribosomal Dysfunction Is a Common Pathomechanism in Different Forms of Trichothiodystrophy

Mutations in a broad variety of genes can provoke the severe childhood disorder trichothiodystrophy (TTD) that is classified as a DNA repair disease or a transcription syndrome of RNA polymerase II. In an attempt to identify the common underlying pathomechanism of TTD we performed a knockout/knockdown of the two unrelated TTD factors TTDN1 and RNF113A and investigated the consequences on ribosomal biogenesis and performance. Interestingly, interference with these TTD factors created a nearly uniform impact on RNA polymerase I transcription with downregulation of UBF, disturbed rRNA processing and reduction of the backbone of the small ribosomal subunit rRNA 18S. This was accompanied by a reduced quality of decoding in protein translation and the accumulation of misfolded and carbonylated proteins, indicating a loss of protein homeostasis (proteostasis). As the loss of proteostasis by the ribosome has been identified in the other forms of TTD, here we postulate that ribosomal dysfunction is a common underlying pathomechanism of TTD.

Temporal Bone Histopathology in Cockayne Syndrome

: Cockayne syndrome (CS) is a rare autosomal recessive syndrome resulting in defective DNA repair. Its features include cachectic dwarfism, hearing loss, skin hypersensitivity to sunlight, premature aging, and dementia. Presented is a right temporal bone of a patient who died at the age of 29 years. The clinical course was compatible with type 1 CS, the classical form. Homozygous missense variant in the ERCC6 gene (Excision Repair Cross-Complementation group 6) was found, compatible with CS complementation group B. Five years before his death he complained of tinnitus. An audiogram 3 and a 1/2 years before his death demonstrated a moderate symmetrical sensorineural hearing loss at 2 to 8 kHz. The speech reception threshold was 20 dB, and the word recognition score was 100% on the right.Histopathology revealed a near normal population of inner hair cells except in the basal 5 mm of the cochlea, and mild loss of outer hair cells particularly at the base of the cochlea. Severe atrophy of the spiral ligament and atrophy of stria vascularis and spiral prominence was present. There was loss of Claudius cells, outer sulcus cells, and mesenchymal cells on the scala tympani side of the basilar membrane and loss of cellularity of the limbus. There was a moderate loss of Scarpa's and spiral ganglion neurons, with the most severe loss in the basal segment. The vestibular neuro-epithelium was nearly intact, with the exception of mild loss in the saccule. The vestibular perilymphatic, and to a lesser extent endolymphatic spaces, were filled with filamentous material and osteoid. The patient had better hearing and a larger complement of neurons compared with the few published case reports.Neurodegenerative symptoms are likely attributed to the effect of intramitochondrial reactive oxygen species. The pathogenesis of hearing loss in CS may shed light on other causes of hearing loss, such as that induced by noise.

Structural basis of RNA polymerase I stalling at UV light-induced DNA damage

RNA polymerase I (Pol I) transcribes ribosomal DNA (rDNA) to produce the ribosomal RNA (rRNA) precursor, which accounts for up to 60% of the total transcriptional activity in growing cells. Pol I monitors rDNA integrity and influences cell survival, but little is known about how this enzyme processes UV-induced lesions. We report the electron cryomicroscopy structure of Pol I in an elongation complex containing a cyclobutane pyrimidine dimer (CPD) at a resolution of 3.6 Å. The structure shows that the lesion induces an early translocation intermediate exhibiting unique features. The bridge helix residue Arg1015 plays a major role in CPD-induced Pol I stalling, as confirmed by mutational analysis. These results, together with biochemical data presented here, reveal the molecular mechanism of Pol I stalling by CPD lesions, which is distinct from Pol II arrest by CPD lesions. Our findings open the avenue to unravel the molecular mechanisms underlying cell endurance to lesions on rDNA.