Program: Oral and Poster Abstracts
Session: 322. Disorders of Coagulation or Fibrinolysis: Poster II
Methodology: FVIII levels were assessed by one stage (FVIII:C) and chromogenic (FVIII:CR) assays. At least two plasma samples spanning >3 years from each female were tested in duplicate with each FVIII assay. To address XCI skewing, we performed methylation-based assays at the Androgen Receptor (AR) and Fragile X Mental Retardation 1 (FMR1) loci. At least three independent PBMC DNA samples from each female were evaluated. Additionally, we screened VWF regions known to influence FVIII:C (exons 18-20, 24-27).
Results: For each female, results between XCI assays were indistinguishable (r2 = 0.99). Two of four females had pronounced skewing (≥80:20); a third had measurable skewing (67:33). Importantly, all three predominantly expressed the mutant paternal allele. Familial XCI skewing argues for a genetic cause. However, XIST, the major regulator of XCI, lacked promoter alterations. Importantly, there was linear correlation between XCI skewing and FVIII:C measured by FVIII:C or FVIII:CR assays (r2 = 0.77 and 0.83, respectively). One female with random XCI, had FVIII:C considered hemostatic (median 51%, range 43-67), whereas the other females with skewed XCI had levels <40% (16%, range 14-20, 28%, range 26-32, and 30%, range 23-35). Notably, two females had similar FVIII:C (30% and 28%) but a greater XCI skewing discrepancy (80:20 vs. 67:33). While these two females were heterozygotes for VWF p.Thr789Ala, reported to be associated with 7% higher FVIII:C, neither ABO blood type nor any additional VWF variants known to affect FVIII binding differentiated these two individuals. Therefore, it is likely that XCI skewing primarily explains their bleeding tendency.
Conclusions: Our results indicate that HA carrier bleeding phenotypes are multifaceted, and the major determinant of FVIII:C is XCI. These results also suggest that even moderate XCI skewing could influence clinical bleeding in HA carriers. However, random XCI in one female explains FVIII:C but does not fully negate bleeding tendency, emphasizing the complexity of carrier phenotype. These findings provide justification for an expanded study of carriers in unrelated pedigrees using a comprehensive approach to include XCI assays.
Disclosures: No relevant conflicts of interest to declare.
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