Compound Heterozygosity of a W493C Substitution and R704/Premature Stop Codon within the γ-Glutamyl Carboxylase in Combined Vitamin K-Dependent Coagulation Factor Deficiency in a French Family.

Abstract 1302

Poster Board I-324

Combined vitamin K-dependent (VKD) coagulation factor deficiency is an autosomal recessive bleeding disorder associated with defects in either the γ-carboxylase (GGCX) which carboxylates VKD proteins to render them active or the vitamin K epoxide reductase (VKORC1) which supplies the reduced vitamin K cofactor required for carboxylation. Such deficiencies are rare, and due to mutations within either gene. Of note some mutations within the GGCX gene have recently been found associated with the pseudoxanthoma elasticum (PXE) syndrome, suggesting a role for GGCX in skin development. We report a new case of combined VKD coagulation factor deficiency resulting from two mutations in the GGCX gene, and the first identified in a French child, who exhibited impaired function in hemostatic VKD factors. The propositus exhibited bleeding at sites of venipuncture at birth, and at the age of 3 months was admitted in emergency for spontaneous multiple hematomas of the chest and thighs; recently, at the age of 1 year, he exhibited a hematoma of the wrist following a casual fall. Coagulation and VKD factors were low [PT (Control/patient): 12.8/>100 sec; II : 3 % ; VII :2% ; X :3%] while the non-VKD factor V was normal (105%). Vitamin K infusion corrected the bleeding tendency as well as coagulation paramenters [PT (Control/patient): 12.8/14.8 sec; II :72% ; VII+X : 62%; factor V remained normal at 102%]. Family analysis revealed that both parents and one brother were unaffected, both clinically and biologically, but that a brother had died of an unexplained abdominal hemorrhage, in the neonatal period. Taken together the clinical family history is consistent with a recessive trait. No sign of PXE was found in the family. DNA sequence analysis of the propositus did not identify any mutations in the VKORC1 gene but revealed two new heterozygous mutations in the carboxylase gene: a G10233T transversion in exon 11 (G1565U in mRNA) that caused an W493C substitution and a C12078T transition in the last exon (15) (C2196U in mRNA) that caused a premature R704stop, presumably deleting the last 55 C-terminal amino acids. W493 is a highly conserved amino acid and its homozygous mutation (for S493) has recently been reported in a case of VKD coagulation factor deficiency associated with PXE. R704 mutation has never been reported previously. Family analysis showed that W493C was transmitted by the father, R704stop by the mother, that the deceased brother carried both mutations, while the unaffected brother carried neither. Mutational analysis was carried out by site-directed mutagenesis of the GGCX cDNA subsequently subcloned into the BacPak8 baculovirus-based vector, and GGCX was expressed in the virally-infected SF21 insect cell line. Protein expression level of the enzyme was assessed by western blotting, and enzymatic activity was evaluated by measuring the incorporation of [14]C-CO2 within the Boc-Glu-Glu-Leu-OMe peptide. No difference in expression level was detected for either mutant protein compared to the wild type protein. However, while no difference in molecular weight was seen between the W493C mutant and wild type on wertern blotting, the R704stop mutation generated a shorter form of carboxylase than wild type, consistent with the predicted shorter reading frame. W493C mutant exhibited a much lower activity than wild type, suggesting a functional role for W493. The activity of the R704stop mutant is currently being assessed, as well as the activity of the coexpressed mutants. Since PXE has been correlated with a W493S mutation, a role for the W493C mutation in future development of PXE in this young patient will be assessed during follow-up.