Genotype Analysis of Patients with Hereditary Factor X Deficiency Enrolled in 2 Phase 3 Studies of Pdfx, a New High-Purity Factor X Concentrate

Introduction: Hereditary factor X (FX) deficiency is a rare, autosomal recessive bleeding disorder of variable severity, with an estimated prevalence of 1:500,000 to 1:1,000,000. Like hemophilia A and B, patients with severe FX deficiency commonly present with bleeding into joints, muscles, or mucous membranes. However, unlike the X-linked disorders of hemophilia A and B, hereditary FX deficiency occurs equally in both sexes due to the location of the FX gene (F10) on chromosome 13q34. A novel, high-purity, high-potency, plasma-derived FX concentrate (pdFX) has been developed for replacement therapy in patients with hereditary FX deficiency. This analysis examines the genotypic and phenotypic characteristics of subjects with hereditary FX deficiency enrolled in 2 prospective, open-label, multicenter phase 3 studies of pdFX.

Methods: In study 1, subjects aged ≥12 years with moderate or severe FX deficiency (basal plasma FX activity [FX:C] of ≥1 and <5 IU/dL or <1 IU/dL, respectively) who required treatment with replacement therapy for ≥1 spontaneous or menorrhagic bleed in the past 12 months received pdFX as on-demand treatment or short-term preventative therapy for 6 months to 2 years. In study 2, subjects aged ≥12 years with mild to severe FX deficiency (basal plasma FX:C of <20 IU/dL) with a history of unusual bleeding received pdFX during and after surgery, until no longer at risk of postoperative bleeding. F10 genotyping was performed for each subject and, for study 1, F10 mutations were compared with bleed frequency to assess potential genotype-phenotype associations.

Results: Study 1 enrolled 16 subjects (aged 12–58 years [mean 27.1 years], 62.5% female) from the United Kingdom (n=3), Spain (n=4), the United States (n=2), Turkey (n=6), and Germany (n=1); two patients had moderate and 14 had severe FX deficiency. Among the 16 subjects, 13 separate mutations were identified, of which 6 were novel. Nine mutations were missense mutations, 2 were deletions, 1 was a nonsense mutation, and 1 was a splice-site mutation. Consistent with the low FX:C of <5 IU/dL, all subjects either were homozygous for a single mutation (n=11) or had compound heterozygous mutations (n=5).

Study 2 enrolled 2 male subjects (ages 55 years [United States] and 59 years [United Kingdom], respectively), both with mild FX deficiency (basal FX:C of 6 and 8 IU/dL, respectively) and compound heterozygous mutations. Of the 4 mutations identified in these 2 patients, 3 were novel.

Of the subjects with moderate or severe FX deficiency (study 1), all 6 Turkish subjects had homozygous mutations resulting in an identical amino acid substitution (p.Gly262Asp). Two subjects in Spain and 1 each in the United States and Germany had mutations resulting in an identical amino acid substitution (p.Gly21Arg); one of these Spanish subjects was a compound heterozygote, with an additional missense mutation of p.Cys246Arg, while the other 3 subjects’ mutations were homozygous. Two UK subjects were each homozygous for 2 different missense mutations (p.Phe71Ser and p.Ile451Phe, respectively), and the remaining 4 subjects each had unique compound heterozygous mutations (p.Val298Met and p.Tyr384Leufs*57 [United Kingdom], p.Glu350Lys and p.Gly450Arg [Spain], p.Cys57Phe and c.70+4A>G splice site mutation [Spain], and p.Gln411* and exon 2 deletion [United States], respectively). The basal level of expressed FX protein (FX:Ag) in 1 patient (87 U/dL) was within the normal range (73–127 U/dL), whereas FX:Ag levels in all other patients (range, <1–17 U/dL) were below normal. Due to the small numbers of each type of mutation, no conclusions could be drawn regarding the F10 genetic variants and bleed frequency.

Each subject with mild FX deficiency (study 2) had unique compound heterozygous mutations (p.Tyr319His and c.71-1G>C splice site mutation [United Kingdom] and p.Cys90Arg and p.Gln416Leu [United States]). Basal FX:Ag levels in these patients (55 and 48 U/dL, respectively) were close to the normal range.

Conclusions: In this analysis, 17 separate F10 mutations were identified in 18 subjects with mild to severe hereditary FX deficiency. Of the mutations identified, 9 are novel and have not previously been characterized.

Support: Bio Products Laboratory Ltd.