Evolutionary insights into coagulation factor IX Padua and other high-specific-activity variants

Area of Science:

• Biochemistry
• Genetics
• Hematology

Background:

• High-specific-activity factor IX (FIX) Padua variant (R338L) shows promise for hemophilia B (HB) gene therapy.
• Amino acid substitutions at conserved position R338 are underrepresented in HB databases.

Purpose of the Study:

• To conduct a comprehensive 20-amino acid scan at position 338 of human and canine FIX.
• To determine the specific activity of these FIX variants.
• To investigate evolutionary pressures on FIX activity.

Main Methods:

• Systematic amino acid substitution scan at position 338 in human and canine factor IX.
• Measurement of specific activity for each generated FIX variant.
• In vivo gene therapy study in hemophilia B dogs using a novel variant (cFIX-R338I).
• Screening of pediatric thromboembolic disease patients for F9 R338 variants.

Main Results:

• Human FIX-R338L exhibited the highest activity; canine FIX-R338L showed sevenfold higher activity than wild-type (WT) canine FIX.
• WT human and canine FIX were among the least active variants.
• In vivo studies confirmed increased specific activity of cFIX-R338I compared to WT cFIX.
• No F9 R338 variants were identified in 232 pediatric patients with thromboembolic disease.

Conclusions:

• Evolutionary pressure may favor limiting FIX activity rather than maximizing it, contrary to expectations for gene therapy.
• The FIX Padua variant (R338L) remains a highly promising candidate for hemophilia B gene therapy due to its superior activity.
• Wild-type FIX activity levels appear to be under selective pressure, suggesting a balance between clotting and potential thrombotic risks.