Novel homozygous CD46 variant with C‐isoform expression affects C3b inactivation in atypical hemolytic uremic syndrome. Issue 10 (31st August 2022) - Schack, Vivien R.
Abstract: Atypical hemolytic uremic syndrome (aHUS) is a thrombotic microangiopathy that may lead to organ failure. Dysregulation of the complement system can cause aHUS, and various disease‐related variants in the complement regulatory protein CD46 are described. We here report a pediatric patient with aHUS carrying a hitherto unreported homozygous variant in CD46 (NM_172359.3:c.602C>T p.(Ser201Leu)). In our functional analyses, this variant caused complement dysregulation through three separate mechanisms. First, CD46 surface expression on the patient's blood cells was significantly reduced. Second, stably expressing CD46(Ser201Leu) cells bound markedly less to patterns of C3b than CD46 WT cells. Third, the patient predominantly expressed the rare isoforms of CD46 (C dominated) instead of the more common isoforms (BC dominated). Using BC1 and C1 expressing cell lines, we found that the C1 isoform bound markedly less C3b than the BC1 isoform. These results highlight the coexistence of multiple mechanisms that may act synergistically to disrupt CD46 function during aHUS development. Abstract : Atypical hemolytic uremic syndrome (aHUS) may in part be caused by impairment of C3b‐binding to CD46. Three separate mechanisms are shown to contribute: (1) a CD46 variant; (2) reduced CD46 expression; and (3) predominant expression of CD46 C isoforms, with reduced C3b‐binding capabilities when compared with the more frequent BC isoforms.