GGCX-Associated Phenotypes: An Overview in Search of Genotype-Phenotype Correlations

Abstract

Gamma-carboxylation, performed by gamma-glutamyl carboxylase (GGCX), is an enzymatic process essential for activating vitamin K-dependent proteins (VKDP) with important functions in various biological processes. Mutations in the encoding GGCX gene are associated with multiple phenotypes, amongst which vitamin K-dependent coagulation factor deficiency (VKCFD1) is best known. Other patients have skin, eye, heart or bone manifestations. As genotype-phenotype correlations were never described, literature was systematically reviewed in search of patients with at least one GGCX mutation with a phenotypic description, resulting in a case series of 47 patients. Though this number was too low for statistically valid correlations-a frequent problem in orphan diseases-we demonstrate the crucial role of the horizontally transferred transmembrane domain in developing cardiac and bone manifestations. Moreover, natural history suggests ageing as the principal determinant to develop skin and eye symptoms. VKCFD1 symptoms seemed more severe in patients with both mutations in the same protein domain, though this could not be linked to a more perturbed coagulation factor function. Finally, distinct GGCX functional domains might be dedicated to carboxylation of very specific VKDP. In conclusion, this systematic review suggests that there indeed may be genotype-phenotype correlations for GGCX-related phenotypes, which can guide patient counseling and management.

Keywords: GGCX; VKCFD1; cutis laxa; elastic fibers; gamma-carboxylation; pseudoxanthoma elasticum.

Figure 1

The Vitamin K cycle. Glu-residues are gamma-carboxylated by GGCX to Gla-residues, an enzymatic process using VKH₂, O₂ and CO₂ as cofactors. During this process, VKH₂ is oxidized to VK epoxide, which is then reduced to VK and in a second reduction step to VKH₂ by VKORC1. Then, VKH₂ can be reused, the reason for which this process is called the VK cycle. Gamma-carboxylation is only performed in VKDP and is essential for their activation and downstream functioning in multiple biological processes, such as blood clotting, bone formation, inflammation and cell proliferation. Warfarin inhibits the VK cycle by preventing VK reduction. C: carbon; GGCX: gamma-glutamyl carboxylase; Gla: gamma-carboxyglutamate; Glu: glutamate; H: hydrogen; O: oxygen; R: attached hydrogen or a hydrocarbon side chain of any length; VK: vitamin K (quinone); VKDP: VK-dependent proteins; VKH₂: vitamin K hydroquinone; VKORC1: vitamin K epoxide reductase complex, subunit 1.

Figure 2

Predicted gamma-glutamyl carboxylase (GGCX) topology. Adapted from [9]. This figure gives an overview of the predicted localization of known or predicted GGCX domains on a GGCX topology model. Green circles depict amino acid residues which undergo glycosylation. Connect: hydrophobic domains important for interaction with vitamin K. ER: endoplasmic reticulum; HTTM: horizontally transferred transmembrane domain; Gla: gamma-carboxyglutamate; Glu-BS: glutamate binding site; PP-BS: propeptide binding site; RmlC: deoxythymidine-6-deoxy-d-xylo-4-hexulose 3,5 epimerase (EC5.1.3.13).

Figure 3

Skin and eye symptoms of the known gamma-glutamyl carboxylase (GGCX)-related disease entities. In each panel, two photos depict different aspects of the respective skin phenotypes (left and middle), the right image shows a fundus typical for the disease entity: (A) PXE; (B) PXE-like disorder with combined coagulation factor deficiency; (C) patient with PXE/PXE-like overlap; and (D) PXE-like syndrome with pigmentary retinopathy. *: yellowish skin papules, °: skin plaques; arrowheads: angioid streaks; spherical diagram: peau d’orange; +: skin loosening and excessive skin folds; arrow: reticular rash; PXE: pseudoxanthoma elasticum; PXE-like: PXE-like disorder with combined coagulation factor deficiency.

Figure 4

Overview of the pipeline used for the systematic search of the literature. GGCX: gamma-glutamyl carboxylase, KO: knockout, n: number, P1: first patient criterium, P2: second patient criterium.