Phenotype–genotype correlations in hemophilia A carriers are consistent with the binary role of the phase between F8 and X‐chromosome inactivation

Abstract

The recessive X-linked disorder hemophiliaA (HA) is rarely expressed in female carriers, most of whom express about half of normal factorVIII activity ( C). To propose an integrative assessment model for the binary role of the phase between the mutated F8 and the active X-chromosome (Xa) in C in HA carriers. We studied 67 females at risk of severe HA, comprising five symptomatic females ( C<1.5IUdL(-1) ) and 14 controls. A correlation study between C (observed vs. expected) and X-chromosome inactivation (XCI) patterns (XIPs; androgen receptor gene [AR] system) in blood leukocyte DNA was performed in carriers, by comparison of a model correlating C and XIP with arbitrary models devoid of biological significance, and with C levels in non-carriers (mean model) as a proxy from background data dispersion not influenced by XIP. We provide proof-of-concept example from a family presenting with extremely skewed XIPs in which the severe HA phenotype appeared in a heterozygous carrier of a crossover between AR and F8 loci that phased the mutated F8 with the maternally inherited Xa. Furthermore, four cases of severe HA affected women who had a combination of a heterozygous F8 mutation and extremely skewed XIPs in leukocytes or oral mucosa are presented. Correlation analyses between C levels and XIPs in carriers (n=38) but not in non-carriers (n=20) showed highly significant differences between the proposed correlation model and models without biological significance. The data support a binary influence of XCI, either increasing or decreasing the C, subject to the underlying phase set between the F8 mutation and XCI. Our evidence suggests that the phase between XCI and mutated F8 acts as a molecular switch conditioning C levels and HA expression in carriers.