Factor H-Related 1 and Heparan Sulfate Architecture Contribute to Complement Dysregulation in C3 Glomerulopathy

Amanda K Slagle^1,2*Nicolo Ghiringhelli Borsa¹Kai Wang³Amanda Taylor¹Nic Meyer¹Michael Jones¹William D Walls¹Angela FM Nelson¹Sarah Roberts¹Mingyao Sun⁴Elena Goicoechea De Jorge⁵Santiago Rodriguez De Cordoba⁵Diana Jalal^4,6Carla Marie Nester¹Yuzhou Zhang¹Richard JH Smith^1,2*

• ¹Molecular Otolaryngology and Renal Research Laboratories, The University of Iowa, Iowa City, Iowa, United States
• ²Interdisciplinary Program in Immunology, Graduate College, The University of Iowa, Iowa City, Iowa, United States
• ³Department of Biostatistics, College of Public Health, The University of Iowa, Iowa City, Iowa, United States
• ⁴Department of Internal Medicine, Carver College of Medicine, The University of Iowa, Iowa, Iowa, United States
• ⁵Margarita Salas Center for Biological Research, Spanish National Research Council (CSIC), Madrid, Catalonia, Spain
• ⁶Iowa City VA Health Care System, Veterans Health Administration, United States Department of Veterans Affairs, Iowa City, Iowa, United States

Dysregulation of the alternative pathway of complement underlies the pathogenesis of C3 glomerulopathy (C3G). Because Factor H (FH) prevents excessive alternative pathway activity while Factor H-related protein 1 (FHR-1) is believed to enhance this response, we investigated the balance between FH and FHR-1 in C3G. To assess the role of FHR-1 in C3G pathogenicity, we used a multiplex ligation-dependent probe amplification to detect copy number variants in CFHR3-CFHR1 and enzyme linked immunosorbent assays to measure circulating protein levels in C3G patients compared to controls. Additionally, an in vitro C3b deposition assay was used to characterize the functional impact of FHR-1 on local complement activity. In this study, we confirm that CFHR3-CFHR1 copy number impacts C3G risk. In C3G patients with two copies of CFHR3-CFHR1, the FHR-1:FH protein ratios are increased compared to controls; however, this increase is not disease specific. Rather, it is reflective of deteriorating renal function and was also observed in a second cohort of patients with chronic kidney disease from a variety of other causes. Functional studies showed that FHR-1 competes with FH to increase C3b deposition on mouse mesangial cell surfaces, an effect enhanced by heparan sulfate cleavage. Altogether, we show that as renal function declines, a change in the FHR-1:FH ratio combined with changes in heparan sulfate architecture increase complement activity. These findings suggest that complement activity may contribute to the chronic inflammation and progression of renal damage associated with chronic kidney disease.