LAUSR

Pathogenic variants in the FVIII gene ( F8 ) cause haemophilia A. The most frequent variant is the intron 22 gene inversion, which causes nearly 45% of severe cases. 1 Factor VIII (FVIII) replacement ther-apy is used to treat haemophilia A and is effective unless patients developaneutralizingalloantibody(inhibitor)againstexogenousFVIII. Inhibitor development is the most severe treatment complication and is associated with significant morbidity and a decreased quality of life. It is estimated that around 20%–30% of severe haemophilia A patients develop an inhibitor during their lifetime. 2 It is recognized thatthedevelopmentofaninhibitorresultsfromacomplexinteraction between a patient’s immune system and genetic and environmental risk factors. The causative F8 variant is considered the most impor-tant factor conditioning inhibitor development. 3 Other risk factors are ancestry, family history, ABO blood group, HLA alleles and common DNA variants in genes for immune-regulatory cytokines (e.g. IL-10, TNF-alpha and CTLA4). 4 DNA methylation has never been studied in haemophilia A patients as a potential risk factor for inhibitor development while it is known that this process is sensitive to environmental changes.Theonlymethylationstudyperformedsofarhasdetecteddif-ferentially methylated CpGs in the F8 locus for patients with genomic inversions. 5 We here tested for differentially methylated CpGs in association with inhibitor development in 35 patients with severe haemophilia A duetointron22geneinversion.ThisstudywasapprovedbytheUZLeu-ven Ethical Committee (S59352). Leukocyte DNA from nine patients with a standard inhibitor (high titre inhibitor of > 5