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SMA is an autosomal recessive neuromuscular disease caused by mutations in the multifunctional protein SMN. Within the nucleus, SMN localizes to Cajal bodies (CBs), which have been shown to be associated with nucleoli, nuclear organelles dedicated to the first steps of ribosome biogenesis. The highly organized structure of the nucleolus can be dynamically altered by genotoxic agents. After a genotoxic stress, RNAP1, Fibrillarin (FBL) and nucleolar DNA are exported to the periphery of the nucleolus and once DNA repair is fully completed the organization of the nucleolus is restored. We found that SMN is required for the restoration of nucleolar structure after genotoxic stress. Unexpectedly, during DNA repair, SMN shuttles from the CBs to the nucleolus. This shuttling is important for the nucleolar homeostasis and relies on the presence of Coilin, FBL and the activity of PRMT1.

Nucleotide Excision Repair is one of the five DNA repair systems. More than 30 proteins are involved in this process, including the seven XP proteins. When mutated, the genes coding for these proteins are provoking the rare disease Xeroderma Pigmentosum, which causes cutaneous defects and a high prevalence of skin cancers in patients. The CSA and CSB proteins are also involved in Nucleotide Excision Repair, and their mutation leads to Cockayne Syndrome, another rare disease, causing dwarfism, neurodegeneration, and ultimately early death, but without high skin cancer incidence. Some mutations of ERCC5, the gene coding for XPG, may give rise to a combined Xeroderma Pigmentosum and Cockayne Syndrome. A defect in Nucleotide Excision Repair alone cannot explain all these phenotypes. XPG has been located in the nucleolus, where ribosome biogenesis happens. This energy-consuming process starts with the transcription of the ribosomal DNA in a long ribosomal RNA, the pre-rRNA 47S, by RNA Polymerase 1. 47S pre-rRNA undergoes several cleavages and modifications to form three mature products: the ribosomal RNAs 18S, 5.8S and 28S. In the cytoplasm, these three products will enter the ribosomes’ composition, the producers of protein in our cells. Our work aimed to observe ribosome biogenesis in presence of an unstable XPG protein. By working on Xeroderma Pigmentosum/Cockayne Syndrome cell lines, meaning in the absence of XPG, we uncovered that the binding of UBF, as well as the number of unresolved R-loops, is increased along the ribosomal DNA gene body and flanking regions. Furthermore, ribosomal RNA maturation is impaired, with increased use of alternative pathways of maturation as well as an increase of immature precursors. These defective processes may explain the neurodegeneration observed when the XPG protein is heavily truncated, as ribosomal homeostasis and R-loops resolution are critical for proper neuronal development.

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