June 2020
Volume 61, Issue 7
Free
ARVO Annual Meeting Abstract  |   June 2020
Functional Analysis of Genetic Variants in Complement Factor I in Advanced Age-related Macular Degeneration using a Serum-based Assay
Author Affiliations & Notes
  • Anuja Java
    Internal Medicine , Washington University School of Medicine, St. Louis, Missouri, United States
  • Peter Baciu
    Biology, Allergan Inc, California, United States
  • David Kavanagh
    National Renal Complement Therapeutics Center, Newcastle, United Kingdom
  • John Atkinson
    Internal Medicine , Washington University School of Medicine, St. Louis, Missouri, United States
  • Johanna M Seddon
    Ophthalmology and Visual Sciences, University of Massachusetts Medical School, Massachusetts, United States
  • Footnotes
    Commercial Relationships   Anuja Java, None; Peter Baciu, None; David Kavanagh, None; John Atkinson, Achillion Pharmaceuticals (C), AdMiRx, Inc (I), Annexon Biosciences (C), Biomarin Pharmaceutical (C), Celldex Therapeutics (C), Clinical Pharmacy services, CDMI (C), Compliment Corporation (I), Gemini therapeutics, INc (I), Kypha, Inc (I), Kypha, Inc (S); Johanna Seddon, Gemini Therapeutics (E), Laboratoires Théa (R)
  • Footnotes
    Support  NIH R01-EY011309; R01-EY028602; American Macular Degeneration Foundation, Northampton, MA. USA; The Macular Degeneration Center of Excellence, University of Massachusetts Medical School, Department of Ophthalmology and Visual Sciences, Worcester, MA, USA
Investigative Ophthalmology & Visual Science June 2020, Vol.61, 3651. doi:
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    • Get Citation

      Anuja Java, Peter Baciu, David Kavanagh, John Atkinson, Johanna M Seddon; Functional Analysis of Genetic Variants in Complement Factor I in Advanced Age-related Macular Degeneration using a Serum-based Assay. Invest. Ophthalmol. Vis. Sci. 2020;61(7):3651.

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      © ARVO (1962-2015); The Authors (2016-present)

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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.

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