Genetic determinants of immunogenicity to factor IX during the treatment of haemophilia B.

Inhibitors are an impediment to the effective management of haemophilia B (HB), but there is limited understanding of the underlying genetic risk factors. In this study we aim to understand the role of F9 gene mutations on inhibitor development in patients with HB. Mutations in the F9 gene were identified and HLA typing performed for five boys with severe HB. Data from the CDC Haemophilia B Mutation Project (CHBMP) database were used to assess association between F9 gene mutation type and inhibitor development. Analysis of the CHBMP database showed that larger disruptions in the F9 gene are associated with a higher life-time prevalence of inhibitors. We detected the following mutations in the five subjects, including four novel mutations: Nonsense in three patients (c.223 C>T; p.Arg75* in two siblings, c.553 C>T; p.Glu185*); Splice site in two patients (c.723 + 1 G>A, c.278-27 A>G); Missense in one patient (c.580 A>G, p.Thr194Ala; c.723 G>T; p.Gln241His). Of the two siblings only one responded to immune tolerance induction (ITI). These siblings have identical F9 gene mutations but differ with respect to the HLA alleles. Interestingly, an analysis of peptide-MHC binding affinities shows a significantly higher (one-sided unpaired t-test, P = 0.0018) median affinity for FIX-derived peptides in the sibling that responded to ITI. We conclude that the nature of the F9 gene mutation may be an important risk factor for the development of inhibitors. In addition, the HLA alleles of the individual patients, in conjunction with the mutation type, could be a predictor for the development of inhibitors as well as the response to ITI.

Factor IX oligomerization underlies reduced activity upon disruption of physiological conditions.

Coagulation factor IX (FIX) is a serine protease that plays a pivotal role in the blood coagulation cascade. FIX deficiency leads to a blood clotting disorder known as haemophilia B. FIX, synthesized as a prepro-peptide of 461 amino acids, is processed and secreted into plasma. The protein undergoes numerous modifications, including, but not limited to glycosylation, γ-carboxylation and disulphide bond formation. Upon processing and limited proteolysis, the protein is converted into an active protease. Under physiological conditions, the FIX zymogen is a monomer. The purpose of this work was to analyse the conditions that may affect FIX monomeric state and promote and/or reduce oligomerization. Using native gel electrophoresis and size exclusion chromatography, we found that under decreased pH and ionic strength conditions, the FIX zymogen can oligomerize, resulting in the formation of higher molecular weight species, with a concomitant reduction in specific activity. Similarly, FIX oligomers formed readily with low bovine serum albumin (BSA) concentrations; however, increased BSA concentrations impeded FIX oligomerization. We hypothesize that normal blood physiological conditions are critical for maintaining active FIX monomers. Under conditions of stress associated with acidosis, electrolyte imbalance and low albumin levels, FIX oligomerization is expected to take place thus leading to compromised activity. Furthermore, albumin, which is commonly used as a drug stabilizer, may enhance the efficacy of FIX biological drugs by reducing oligomerization.

Analysis of F9 point mutations and their correlation to severity of haemophilia B disease.

Haemophilia B is an X-linked recessive disorder caused by deficiency of functional coagulation factor IX, which results almost exclusively from mutations in the F9 gene. We sought to determine features, which could distinguish between mutations that cause severe disease symptoms from those that cause non-severe disease symptoms. Towards this objective, we have performed a statistical analysis of reported point mutations in F9. These include: potential local changes in mRNA free energy, codon usage, charge and type of mutated amino acid, location of the mutation with regard to protein secondary structure and functional domain and amino acids' evolutionary conservation scores. Wilcoxon signed-rank tests showed highly significant differences between severe and non-severe disease causing mutations in their effect on free energy of small mRNA fragments and evolutionarily conserved amino acids. Our results suggest that information at the mRNA level as well as conservation of the amino acid correlate well with disease severity. This study demonstrates that computational tools may be used to characterize the severity of haemophilia B associated with point mutations and suggests their utility in predicting the outcome of sequence changes in recombinant proteins.

TNF-alpha-converting enzyme (TACE/ADAM17)-dependent loss of CD30 induced by proteasome inhibition through reactive oxygen species.

Combinations with proteasome inhibitors are currently being investigated to improve the therapy of hematological malignancies. We previously found that proteasome inhibition by bortezomib failed to sensitize anti-CD30 antibody (Ab)-based lymphoma cell killing. In this study, we demonstrate in L540 Hodgkin's lymphoma cells that proteasome inhibition not only communicates apoptosis but also more rapidly causes a loss of CD30 antigen from cell membrane and a simultaneous release of soluble CD30, a targeting competitor. This shedding was catalyzed by the tumor necrosis factor (TNF)-alpha-converting enzyme (TACE, ADAM17) and blocked by the ADAM17-selective inhibitor, Ro32-7315. In parallel with CD30 shedding, bortezomib caused the generation of reactive oxygen species (ROS). As apoptosis and shedding were inhibited by the radical scavenger, N-acetyl-L-cysteine, ROS might have a pivotal function in both effects. In contrast, the pan-caspase inhibitor, zVAD-fmk, blocked bortezomib-induced apoptosis but not CD30 shedding, and Ro32-7315 blocked shedding but allowed apoptosis. This suggests independent terminal signaling pathways that are conflicting in Ab-based immunotherapy. Consequently, shedding inhibition substantially improved the synergistic antitumor efficacy of the human anti-CD30 Ab, MDX-060, and bortezomib. As proteasome inhibition also stimulated loss of TNF receptors, interleukin-6 receptor and syndecan-1 in different leukemia and lymphoma cell lines, we concluded that proteasome inhibition might impede targeted therapy against antigens susceptible to shedding.