The Complement System and Genetics of Disease
The complement system plays a major role in immunological protection by providing for opsonization and lysis of microorganisms, removal of foreign particles and dead cells, recruitment and activation of inflammatory cells, regulation of antibody production, and elimination of immune complexes. Each of the complement cascade’s three activation pathways (the alternative, classic and lectin pathways) converges upon a common terminal pathway that culminates in the formation of the cytolytic membrane attack complex (MAC).
In recent years, a variety of complement pathway proteins have been identified as molecular constituents of drusen, the hallmark extracellular deposits associated with early-stage (‘dry’) age-related macular degeneration (AMD). Discoveries related to the compositional profile of drusen formed the basis for a new paradigm of AMD disease pathology in which complement activation and immune responsiveness are important facets of AMD pathogenesis and progression. Definitive support for this model of AMD pathogenesis was provided by recent genetic studies that revealed highly significant associations between AMD and four complement pathway-associated genes; Complement Factor H (CFH), Complement Factor B (CFB), Complement Component 3 (C3), and Complement Factor H-related 1 and 3 (CFHR1/3).
Discoveries published in 2005 showed that approximately 50% of patients with age-related macular degeneration (AMD) possess specific sequence variants in the Complement Factor H (CFH) gene, which encodes the CFH protein, the primary inhibitor of the alternative complement pathway. Data now support the concept that this 'risk' variant of CFH exhibits significantly diminished functionality which results in inappropriate control of complement activation and damage to healthy cells and tissues. In AMD, this contributes to the destruction of the macula, that part of the retina responsible for central, fine acuity vision. Ultimately, blindness can result.
Importantly, the scientific discoveries of 2005 also identified a 'protective', highly functional, naturally-occurring form of CFH. That is, a form of the CFH protein that is associated with protection against developing AMD, perhaps because it is particularly effective at controlling over-activation of the complement system.
Since these original CFH discoveries, specific variants on other genes have also been associated with AMD including Complement Factor B (CFB)/Complement Component 2 (C2), genes on chromosome 10 (LOC387715 and HTRA1), CFH-related 1 and 3 (CFHR1/3), and C3. In contrast to CFH, CFB protein binds to C3b, serving to amplify the complement cascade. The precise role of CFB/C2, C3, CFHR1/3, and the chromosome 10-associated variants in AMD has yet to be fully elucidated. While approximately 50% of patients with AMD suffer from variations in the CFH gene, approximately 75-80% of the disease can be explained by variations in the CFH, CFB/C2 and the chromosome 10 genes combined. Individuals who carry two copies (homozygotes) of each of these three gene risk variants have an odds ratio, or lifetime probability of developing the disease, that is greater than 100-fold above those who are not homozygous for all three regions.
Separately, two 'orphan' renal diseases that are often associated with kidney failure have been linked with variations in the CFH gene and corresponding protein. Some 80% of patients with dense deposit disease (DDD, also known as membranoproliferative glomerulonephritis, MPGNII) have the same CFH 'risk' genetic disposition that is associated with AMD. Many DDD patients also have macular drusen, abnormal retinal deposits that are indistinguishable from those observed in patients with AMD. aHUS has long been associated with various CFH point mutations which result in CFH insufficiency or compromised protein functionality.



