The role of the RNA exosome in atypical HUS

Abstract

Haemolytic uraemic syndrome (HUS) is a rare kidney disease characterised by microangiopathic haemolytic anaemia, thrombocytopenia, and acute kidney injury, histologically this is as a result of thrombotic microangiopathy (TMA). Work over the last 25 years has identified inherited and acquired dysregulation of the complement system as the underlying cause in complement associated HUS (C-aHUS); leading to the introduction of the complement inhibitor, eculizumab. It is now clear that there is a subgroup of patients who do not respond to eculizumab. The National Renal Complement Therapeutic Centre (NRCTC) maintains one of the world’s largest repositories of HUS cases, with approximately 2000 individuals. This project aimed to review eculizumab non-responsive patients for an alternative genetic diagnosis. Using a combination of Whole Exome and Sanger sequencing eleven patients with variants in five RNA surveillance genes (EXOSC3, POLR3B, POLR3H, RNU4ATAC and TSEN2) were identified. These patients uncover a previously undescribed pathway for TMA pathogenesis. To further understand the effects of RNA surveillance in a conditional knockout mouse of one of these genes (ExoscS™) was utilised. A whole-body knockout model (Exosc3^p/3fp.Rosa26CreERT2/ER1'2) did not develop HUS but did show evidence of ribosomal dysfunction in the form of bone marrow and intestinal failure secondary to cell cycle arrest and apoptosis. P53 inhibition resulted in significant improved survival. To determine whether TMA would develop in an endothelial specific knockout model, Exosc3fl/fl were crossed with an endothelial specific ere model (Exosc39fp/9fp.Tie2CreERT2/ERT2), unfortunately these mice did not show evidence of recombination. Overall, this project has identified five genes in the RNA surveillance pathway that result in TMA via complement non-responsive pathways. Whilst the mouse models did not develop TMA, the mice demonstrated evidence of ribosomal dysfunction that was partially rescued with a p53 antagonist; suggesting ribsomopathy is central to the pathogenesis of EXOSC3-mediated TMA and offering new insights in to other forms of TMA.