Tissue consistent correlation of factor VIII activity levels with the pattern of X-chromosome inactivation in female carriers of a severe pathogenic variant

BETIANA ZIEGLER; LILIANA ROSSETTI; MARTÍN ABELLEYRO; VANINA MARCHIONE; DANIELA NEME; ENRIQUE MEDINA ACOSTA; CARLOS DE BRASI; C. PAMELA RADIC

Montreal

Congreso; WFH2022 World congress; 2022

Introduction: In female carriers (FC), the skewing of X-chromosome inactivation (XCI) against the normal F8 allele is the most typical cause of the haemophilia phenotype. The FVIII:C level inversely correlates with the XCI status in symptomatic FC. In a prior study, we devised a Vshaped correlation model that incorporates the variance in FVIII:C levels in non-carriers. Here, we implemented a biphasic V-model to assess the strength of the relationship between the FVIII:C levels and the patterns of XCI in a cohort of FC for whom the haplotype phase of the causative variant is unknown. Methods: The XCI status was investigated in DNA from peripheral blood (PB) from 91 FC, 36 non-carriers, and oral mucosa (OM) from 17 FC by genotyping the XCI surrogate markers at the AR (Xq) and RP2 (Xq) genes. Results: The patterns of XCI reported by the AR and RP2 surrogate markers were highly concordant in either PB (n = 84, Spearman r = .7843 [CI95:.6817?.8567], p < .0001) or OM (n = 22, Spearman r = .8238 [CI95:.6080?.9263], p < .0001). The observed combined heterozygosity for the two markers achieved 100% (n = 155). In PB, we observed no difference (Mann-Whitney-test p > .05) in the FVII:C/XCI correlation using the V-model in FC (Observed-Expected differences [O-E]: 11.48, n = 91) versus the mean-model in non-carriers (O-E:10.25, n = 36).We expect this because in non-carriers the skewing of XCI does not affect F8 expression. However, significant differences were observed from the V-model in FC as weighted against the meanmodel in FC (O-E:22.17, p < .004) or the A-model in FC (without biological basis) (O-E:18.06, p < .0007). In OM, we also see no difference from the V-model in FC (O-E:8.58, n = 17) versus the mean-model in non-carriers. The difference was significant when weighed against the mean-model in FC (O-E:15.94, p < .03) or the A-model in FC (O-E:24.24, p < .0095). Conclusions: Because XCI skewing may act for-or-against the normal F8-allele in heterozygous carriers, we planned a V-shaped FVIII:C/XCI correlation model centred on (FVIII:C- carrier- mean; XIP:50%) in which each line of V (descending/ascending) represents an X-chromosome haplotype phase. The V-model accurately reports the correlation between the whole range of FVIII:C levels and XCI?s extent in FC, as tested in two tissue types.