Molecular defect in factor IX Tokyo: substitution of valine-182 by alanine at position P2' in the second cleavage site by factor XIa resulting in impaired activation

F9 missense mutations impairing factor IX activation are associated with pleiotropic plasma phenotypes

BACKGROUND

Circulating dysfunctional factor IX (FIX) might modulate distribution of infused FIX in hemophilia B (HB) patients. Recurrent substitutions at FIX activation sites (R191-R226, >300 patients) are associated with variable FIX activity and antigen (FIXag) levels.

OBJECTIVES

To investigate the (1) expression of a complete panel of missense mutations at FIX activation sites and (2) contribution of F9 genotypes on the FIX pharmacokinetics (PK).

METHODS

We checked FIX activity and antigen and activity assays in plasma and after recombinant expression of FIX variants and performed an analysis of infused FIX PK parameters in patients (n = 30), mostly enrolled in the F9 Genotype and PK HB Italian Study (GePKHIS; EudraCT ID2017-003902-42).

RESULTS

The variable FIXag amounts and good relation between biosynthesis and activity of multiple R191 variants results in graded moderate-to-mild severity of the R191C>L>P>H substitutions. Recombinant expression may predict the absence in the HB mutation database of the benign R191Q/W/K and R226K substitutions. Equivalent changes at R191/R226 produced higher FIXag levels for R226Q/W/P substitutions, as also observed in p.R226W female carrier plasma. Pharmacokinetics analysis in patients suggested that infused FIX Alpha distribution and Beta elimination phases positively correlated with endogenous FIXag levels. Mean residence time was particularly prolonged (79.4 h, 95% confidence interval 44.3-114.5) in patients (n = 7) with the R191/R226 substitutions, which in regression analysis were independent predictors (β coefficient 0.699, P = .004) of Beta half-life, potentially prolonged by the increasing over time ratio between endogenous and infused FIX.

CONCLUSIONS

FIX activity and antigen levels and specific features of the dysfunctional R191/R226 variants may exert pleiotropic effects both on HB patients' phenotypes and substitutive treatment.

Arg260-Cys mutation in severe factor XIII deficiency: conformational change of the A subunit is predicted by molecular modelling and mechanics

To explore the implications of the structure/ function relationships in factor XIII. a patient with severe A subunit deficiency was examined at the DNA and RNA levels. Nucleotide sequence analysis of the patient's DNA amplified by PCR revealed that the patient had a replacement of C by T in the codon for Arg260. RT-PCR analysis demonstrated that only one kind of mRNA coding for the Arg260-Cys mutation was expressed in the patient at a normal level. Another possible defective allele of the A subunit gene with a G-A polymorphism was not expressed (null allele). The substitution of Arg260 by Cys located on the interface of two A subunits would preclude the reciprocal ionic interaction (salt bridge) between Arg260 and Asp404. Molecular modelling and, for the first time, molecular mechanics calculated that Cys260 changed the local conformation of the A subunit and reduced the electrostatic interaction between two monomers, suggesting destabilization of the molecule's dimer.

Heteroduplex screening for molecular defects in factor IX genes from haemophilia B families

Heteroduplex screening of amplified fragments containing sequences of all known small haemophilic mutations in the factor IX gene localized mutations in 18 new families: 12 were at common recurrent sites; three were novel. Carriers and/or patients from each of 41 families with mutations in 7 exons and 5' and 3' non-coding regions were positive.