Molecular Genetics of a Complement Factor H homolog

Project Summary Age-related macular degeneration (AMD) is the leading cause of vision loss in the developed world, affecting 11% of adults over the age of 85. In the United States alone, the disease currently afflicts 10 million individuals, with health care costs in the billions of dollars. The 2005 discovery of a Y402H variant in complement factor H (CFH) as a risk factor was a major advance in AMD genetics, however the functional significance of CFH and Y402H and the pathogenic mechanisms that initiate and promote the disease remain poorly understood. The canonical model of CFH function is that it binds to heparan sulfate (HS) on cell surfaces where it inhibits alternative complement pathway activity by two distinct mechanisms. Thus, the prevailing hypothesis for AMD pathogenesis is that missense mutations in HS binding sites in CFH, that include Y402H, result in defective binding to cell surfaces and ectopic activation of the alternate complement pathway in the retina. As a consequence of this activation, increased inflammation, cytolytic activity, and the accumulation of lipid-rich deposits (i.e. drusen) promote AMD pathogenesis. In stark contrast to this canonical model, preliminary studies by the PI and co-workers indicate that CFH-1, a CFH homolog in C. elegans, localizes on the ciliary membrane of C. elegans mechanosensory neurons in a heparan sulfate dependent manner where it maintains inversin/NPHP-2 within its eponymous cilia compartment. Additional data indicate that the inversin/NPHP-2 compartment is compromised in human Y402H photoreceptors, suggesting that this novel function for CFH-1 is conserved in human CFH. On the basis of this data, the PI proposes the radical hypothesis that CFH is an essential structural component of sensory neuron cilia in the human retina and that structural defects in photoreceptor cilia promote AMD pathogenesis in patients with CFH loss-of-function mutations. The aims of this proposal will extend these preliminary studies and use classic molecular and genetic techniques to dissect the mechanism of CFH-1 localization and function in restricting inversin/NPHP-2 distribution in C. elegans mechansensory neuron cilia. This novel model of CFH-1 function in C. elegans will be tested in human photoreceptors using a retina organoid model derived from induced pluripotent stem cells and genotyped post-mortem retina. Together, the proposed work will provide substantial insight into previously unappreciated CFH functions that are likely to contribute to a unique new model of AMD pathogenesis.