The skin phenotype is caused by the biallelic deficiency to γ-carboxylate GRP
Six VKCFD1 patients were reported with a severe PXE-like phenotype with skin hyper-laxity and skin folds (Goldsmith, Pence, Ratnoff, Adelstein, & Furie, 1982; Li, Grange et al., 2009; Li, Schurgers et al., 2009; Vanakker et al., 2007). Out of these, four patients were found to be compound heterozygous, where one allele has a loss-of-function or a nonsense variant. This results into monoallelic disability to γ-carboxylate VKD proteins. The other alleles harbor variants GGCX:p.(R83W), GGCX:p.(V255M), GGCX:p.(G537A) or GGCX:p.(G558R) that exhibited levels between 18 – 32% of γ-carboxylated GRP at 10 µM K1 when compared to wt in our assay (Figure 2a, Table 1). Patients harboring variants with greater ability to γ-carboxylate GRP > 32% did not show severe skin folds. Levels of γ-carboxylated MGP varied between 2.9 – 102 % for all these variants (Figure 2a, Table 1). BGLAP, PRGP1, TMG4, and GAS6 are not likely to be causative proteins since all types of dose-responses were detected (Table S1).
In addition, a mild skin phenotype with broadened and agglutinated bundles of fragmented elastic fibres at the age of 47 years was diagnosed in a patient harboring variants GGCX:p.(H404P);p.(R485P) (Watzka et al., 2014). Levels of γ-carboxylated GRP for these variants were reduced to 35% and 58%, respectively indicating that patients with greater ability to γ-carboxylate GRP might exhibit a mild skin phenotype (Figure 2b, Table 1). Levels of γ-carboxylated MGP are 9 % and 87 %, respectively (Figure 2b, Table 1).
Due to low levels of γ-carboxylated GRP, we are expecting for two more patients with the genotypes GGCX:p.(R83P);p.(R83P) and GGCX:p.(L394R);p.(L394R) the development of a skin phenotype (Figure 2c, Table 1). These patients are currently infants (Brenner et al., 1998; Watzka et al., 2014) and the age of onset was reported to be around 18 years (De Vilder, Eva Y G et al., 2017 Jan 25).
There are two patients, who are heterozygous for GGCX:p.(R476C) or GGCX:p.(R476H), which atypically developed a PXE-like phenotype at the age of 3 or 18 years, respectively (Vanakker et al., 2007). Values of γ-carboxylated GRP are 42 % or 96 %, respectively. Moreover, the other allele in these patients’ is wt harboring 100 % activity (Figure S2, Table 1).