Nucleotide excision repair (NER) guarantees genome integrity against UV light-induced DNA damage. After UV irradiation, cells have to cope with a general transcriptional block. To ensure UV lesions repair specifically on transcribed genes, NER is coupled with transcription in an extremely organized pathway known as transcription-coupled repair. In highly metabolic cells, more than 60% of total cellular transcription results from RNA polymerase I activity. Repair of the mammalian transcribed ribosomal DNA has been scarcely studied. UV lesions severely block RNA polymerase I activity and the full transcription-coupled repair machinery corrects damage on actively transcribed ribosomal DNAs. After UV irradiation, RNA polymerase I is more bound to the ribosomal DNA and both are displaced to the nucleolar periphery. Importantly, the reentry of RNA polymerase I and the ribosomal DNA is dependent on the presence of UV lesions on DNA and independent of transcription restart.

Prader-Willi Syndrome (PWS) is a neurodevelopmental disorder caused by mutations affecting paternal chromosome 15q11-q13, and characterized by hypotonia, hyperphagia, impaired cognition, and behavioural problems. Psychotic illness is a challenging problem for individuals with PWS and has different rates of prevalence in distinct PWS genotypes. Previously, we demonstrated behavioural and cognitive endophenotypes of relevance to psychiatric illness in a mouse model for one of the associated PWS genotypes, namely PWS-IC, in which deletion of the imprinting centre leads to loss of paternally imprinted gene expression and over-expression of Ube3a. Here we examine the broader gene expression changes that are specific to the psychiatric endophenotypes seen in this model. To do this we compared the brain transcriptomic profile of the PWS-IC mouse to the PWS-cr model that carries a deletion of the PWS minimal critical interval spanning the snoRNA Snord116 and Ipw. Firstly, we examined the same behavioural and cognitive endophenotypes of relevance to psychiatric illness in the PWS-cr mice. Unlike the PWS-IC mice, PWS-cr exhibit no differences in locomotor activity, sensory-motor gating, and attention. RNA-seq analysis of neonatal whole brain tissue revealed a greater number of transcriptional changes between PWS-IC and wild-type littermates than between PWS-cr and wild-type littermates. Moreover, the differentially expressed genes in the PWS-IC brain were enriched for GWAS variants of episodes of psychotic illness but, interestingly, not schizophrenia. These data illustrate the molecular pathways that may underpin psychotic illness in PWS and have implications for potential therapeutic interventions.