Clinical characteristics and genetic backgrounds of Japanese patients with atypical hemolytic uremic syndrome

Background: Atypical hemolytic uremic syndrome (aHUS) is caused by complement overactivation, and its presentation and prognosis differ according to the underlying molecular defects. The aim of this study was to characterize the genetic backgrounds of aHUS patients in Japan and to elucidate the associations between their genetic backgrounds, clinical findings, and outcomes. Methods: We conducted a nationwide epidemiological survey of clinically diagnosed aHUS patients and examined 118 patients enrolled from 1998 to 2016 in Japan. We screened variants of seven genes related to complement and coagulation, as well as positivity for anti-CFH antibodies, and assessed clinical manifestations, laboratory findings, and clinical course. Results: The most frequent genetic abnormalities were in C3 (31%) and the frequency of CFH variants was relatively low (10%) compared to Western countries. The predominant variant in this cohort was C3 p.I1157T (23%), which was related to favorable outcomes despite frequent relapses. A total of 72% of patients received plasma therapy, while 42% were treated with eculizumab. The prognosis of Japanese aHUS patients was relatively favorable, with a total mortality rate of 5.4% and a renal mortality rate of 15%. Conclusions: The common occurrence of genotype C3, especially the p.I1157T variant was the characteristic of the genetic backgrounds of Japanese aHUS patients that differed from those of Caucasian patients. In addition, the favorable prognosis of patients with the unique C3 p.I1157T variant indicates that understanding the clinical characteristics of individual gene alterations is important for predicting prognosis and determining therapeutic strategies in aHUS.