GGCX variants leading to biallelic deficiency to γ‐carboxylate GRP cause skin laxity in VKCFD1 patients

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. (a) Under physiological conditions, GGCX γ‐carboxylates specific Glu residues of VKD proteins to Gla residues rendering them as functional proteins. (b) The pathological condition due to GGCX variants leads to inactive uncarboxylated or partially γ‐carboxylated VKD proteins. Patients harboring these GGCX variants develop a rare bleeding disorder called VKCFD1 (Vitamin K dependent coagulation factor deficiency type 1). Some of the VKCFD1 patients are diagnosed with additional non‐hemorrhagic phenotypes as for example skin laxity (PXE‐like phenotype) or skeletal dysmorphologies. We identified that the biallelic deficiency by GGCX variants to γ‐carboxylate GRP is associated with the reported patients’ genotype who developed a PXE‐like phenotype. The biallelic reduction of γ‐carboxylated MGP by GGCX variants is not the only risk factor for the development of skeletal defects in VKCFD1 patients