Complement factor B mutations in atypical hemolytic uremic syndrome-disease-relevant or benign?

Area of Science:

• Genetics
• Complement System Biology
• Renal Disease Pathogenesis

Background:

• Atypical hemolytic uremic syndrome (aHUS) is a rare kidney disease driven by complement system overactivation.
• Genetic mutations in Factor B (FB) can lead to a hyperactive C3 convertase, contributing to aHUS pathogenesis.
• Previous studies identified four disease-associated FB mutations, but the relevance of other identified variants remained unclear.

Purpose of the Study:

• To functionally characterize ten newly identified Factor B (FB) genetic variations in patients with aHUS.
• To determine the disease relevance of these FB mutations by assessing their impact on complement convertase activity and regulation.
• To evaluate the accuracy of in silico prediction algorithms for FB mutation pathogenicity.

Main Methods:

• Assessed alternative C3 and C5 convertase formation and regulation using various functional assays.
• Analyzed ligand binding and enzymatic activity of FB mutants.
• Compared experimental findings with predictions from in silico algorithms.

Main Results:

• Identified two gain-of-function FB mutations (M433I and K298Q) associated with aHUS pathogenesis due to overactive or FH-resistant convertase formation.
• One mutation (R178Q) resulted in a non-functional protein.
• Seven FB variants showed near-normal or slightly reduced function, similar to the common R7 polymorphism.
• In silico prediction algorithms did not fully align with experimental data.
• Combined data suggest 9 out of 15 identified FB mutations are likely unrelated to aHUS.

Conclusions:

• Functional assessment is mandatory to confirm the disease association of identified FB nucleotide changes in aHUS.
• Experimental validation is critical, as in silico predictions for FB mutation relevance in aHUS may be unreliable.
• Most identified FB genetic changes in aHUS patients may not be causative, highlighting the importance of precise genetic diagnostics.