Measurement of factor H variants in plasma using variant-specific monoclonal antibodies: application to assessing risk of age-related macular degeneration.

PURPOSE The Y402H polymorphism in the complement regulator factor H (fH) is strongly associated with age-related macular degeneration (AMD) across diverse populations. Persons homozygous for histidine at this position have up to 12-fold greater risk for AMD than those homozygous for tyrosine. Knowledge of fH-Y402H status is, therefore, valuable in predicting risk and focusing preventive measures in the elderly. This knowledge requires genetic analysis, which is unavailable in most laboratories and which provides no information about the levels of fH protein, a putative linked determinant of disease risk. METHODS The authors describe novel monoclonal antibodies that distinguish the two fH allelic variants in plasma. ELISA with these antibodies not only reliably identifies the fH-Y402H status, confirmed by genotyping, but also quantifies the concentration of total fH and the fH-Y402 and fH-H402 variants. RESULTS In young adult control subjects, mean fH concentration was 233 mg/L. In elderly control subjects, mean fH concentration was 269 mg/L, whereas in a matching AMD cohort, mean fH concentration was 288 mg/L. Total fH concentration was similar in each subgroup of young and elderly control subjects; however, in the AMD group, fH concentration was significantly higher in the heterozygous subgroup. Measurement of the two variants in this subgroup showed that both were elevated to a similar degree. CONCLUSIONS The novel monoclonal antibody MBI-7 was used to develop a robust assay for measurement of fH and the variants in plasma. The simplicity of the assay means that it may be used by any clinical laboratory to identify polymorphic status and to quantify plasma levels in persons at risk for AMD.