F8 p.Thr74=

The c.222G>A (p.Thr74=) variant is a synonymous change that may result in abnormal gene splicing. The Grpmax filtering allele frequency in gnomad V4.1.0 is 0.0001184 (BS1). This variant has been reported in at least nine males with mild hemophilia A (PMIDs: 35770352, 35743412, 30690819; Fernández-Artazcoz (2024), and Internal Laboratory Data), including at least one patient who underwent comprehensive testing with F8 sequencing and CNV analysis, and at least one patient with type 2N von Willebrand disease excluded by normal 2N binding. Patients have also been reported with reduced factor VIII activity levels >40% (PMIDs: 25652415, Fernández-Artazcoz (2024)), indicating that the variant is associated with a spectrum of factor VIII reduction extending into the low-normal range. Patient mRNA analyses show variable splicing patterns, with some samples containing both full length and exon 2 skipped transcripts, while others predominantly express the wildtype transcript. (PMIDs: 25652415, 34962362, 23088352). This variability is consistent with incomplete or leaky splicing, likely reflecting differences in individual sensitivity to the splice defect. No PS4 or PP4 points have been applied due to the variant meeting BS1. A minigene assay showed normal exon 2 splicing for c.222G>A in HeLa and in Huh7 hepatocyte derived cells (PMID: 30690819). In summary, the clinical significance of this variant is uncertain. ACMG/AMP criteria applied, as specified by the Coagulation Factor Deficiency Variant Curation Expert Panel for F8/F9: BS1.