Abstract
Purpose :
Factor I (FI) is a serine protease inhibitor of the alternative pathway of the complement system. Genetic variants in complement FI (CFI) are associated with advanced age-related macular degeneration (AAMD). The clinical impact of these variants however is largely unknown since a majority of the variants have not been characterized. This study assessed the functional significance of CFI variants using a serum-based assay.
Methods :
In this cross-sectional study, individuals with and without rare CFI genetic variants were evaluated. Carriers of rare variants included individuals with (n=77) and without (n=28) AAMD. Our comparison population included non-carriers with (n=49) and without (n=44) AAMD. Serum specimens were obtained from our biorepository. The function of a variant FI was assessed in serum by an assay that measures the proteolytic cleavage of complement protein C3b to iC3b using ELISA and Western blotting. Serum was diluted to 0.04% and supplemented with 1.9 µM C3b and 290 nM factor H. Assay specificity was determined by comparing the results in the presence or absence of exogenous C3b.
Results :
Our study determined that the CFI variants in AAMD could be categorized into three types. Type 1 variants (n=17; 31 patients) had reduced antigenic levels of serum FI and a corresponding (~50%) decrease in function (haploinsufficiency). These variants cause a quantitative deficiency of FI as the variant protein is not secreted. Type 2 variants (n=5; 8 patients) had normal FI antigenic levels but decreased degradation of C3b to iC3b. This group is in keeping with a CFI variant that is secreted but has markedly decreased function - again leading to haploinsufficiency. Type 3 variants (n=16; 65 individuals) had normal antigenic levels and apparently normal degradation of C3b to iC3b. However, iC3b generation was reduced if measured per antigenic unit of FI. This group includes variants that demonstrate a more modest deficiency in function, i.e, they are less dysfunctional than Types 1 and 2 but are not equivalent to wild-type. Therefore, an individual carrying this type of a variant is at risk of developing AAMD (37/65 have AAMD in our cohort).
Conclusions :
We present a high throughput functional assay that detects impaired FI activity in serum. This assay may aid in stratifying patients with a rare variant in CFI and could impact early therapeutic decisions.
This is a 2020 ARVO Annual Meeting abstract.