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One of the most common and unwanted side effects during oral anticoagulant therapy (OAT) is bleeding complications. In rare cases, vitamin K antagonist (VKA)-related bleeding events are associated with mutations affecting the F9 propeptide at amino acid position 37 due to a substitution of alanine to either valine or threonine. Based on our actual cohort of 18 patients, we update the knowledge on this rare phenotype and its origin. A founder mutation for both variants was reconfirmed by haplotype analysis of intronic and extragenic short tandem repeat (STR) polymorphisms with a higher prevalence in Switzerland than in other regions of Europe. Screening of healthy individuals for the presence of these F9 gene mutations did not identify any of these variants, thus proving the rare occurrence of this genotype. Furthermore, both variants were expressed in vitro and warfarin dose responses were studied. Our warfarin dose response analysis confirmed higher sensitivity of both variants to warfarin with the effect being more apparent for Ala37Thr. Thus, although F9 propeptide mutation-associated hypersensitivity to VKA is a rare phenomenon, awareness towards this bleeding phenotype is important to identify patients at risk.

γ‐Glutamyl carboxylase (GGCX) catalyzes the γ‐carboxylation of 15 different vitamin K dependent (VKD) proteins. Pathogenic variants in GGCX cause a rare hereditary bleeding disorder called Vitamin K dependent coagulation factor deficiency type 1 (VKCFD1). In addition to bleedings, some VKCFD1 patients develop skin laxity and skeletal dysmorphologies. However, the pathophysiological mechanisms underlying these non‐hemorrhagic phenotypes remain elusive. Therefore, we have analyzed 20 pathogenic GGCX variants on their ability to γ‐carboxylate six non‐hemostatic VKD proteins in an in vitro assay, where GGCX variants were expressed in GGCX‐/‐ cells and levels of γ‐carboxylated co‐expressed VKD proteins were detected by a functional ELISA. We observed that GGCX variants causing markedly reduced γ‐carboxylation of Gla rich protein (GRP) in vitro were reported in patients with skin laxity. Reduced levels of γ‐carboxylated Matrix gla protein (MGP) are not exclusive for causing skeletal dysmorphologies in VKCFD1 patients. In silico docking of vitamin K hydroquinone on a GGCX model revealed a binding site, which was validated by in vitro assays. GGCX variants affecting this site result in disability to γ‐carboxylate VKD proteins and hence are involved in the most severe phenotypes. This genotype‐phenotype analysis will help to understand the development of non‐hemorrhagic phenotypes and hence improve treatment in VKCFD1 patients.

γ-Glutamyl carboxylase (GGCX) catalyses γ-carboxylation of 15 different vitamin K dependent (VKD) proteins. Pathogenic variants in GGCX cause a rare hereditary bleeding disorder called Vitamin K dependent coagulation factor deficiency type 1 (VKCFD1). In addition to bleedings, some VKCFD1 patients develop skin laxity and skeletal dysmorphologies. However, the pathophysiological mechanisms underlying these non-haemorrhagic phenotypes remain elusive. Therefore, we analyzed the effect of 22 GGCX pathogenic variants on γ-carboxylation of six non-haemostatic VKD proteins (UCMA/GRP, MGP, BGLAP, GAS6, PRGP1, TMG4) in a GGCX-/- HEK293T cell line by a functional ELISA. We observed that biallelic deficiency to γ-carboxylate Gla-rich protein lead to the development of skin laxity. Markedly reduced level of γ-carboxylated MGP is crucial but not exclusive for causing facial dysmorphologies. Moreover, we identified the vitamin K hydroquinone binding site in GGCX in an in silico model by docking studies, which was further validated by functional assays. Variants affecting this site result into loss-of-function or severely diminished ability to γ-carboxylate VKD proteins and hence are involved in the most severe phenotypes. This genotype-phenotype analysis will help to develop new treatment options for VKCFD1 patients, where individualized therapy with γ-carboxylated VKD proteins may represent a promising strategy.

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