The av/41 polymorphism of erythroid a-spectrin has been characterized initially by an increased susceptibility to proteolysis of the aIV-aV domain junction (Alloisio N

Hereditary combined vitamin K-dependent (VKD) coagulation factor deficiency is an autosomal recessive bleeding disorder associated with defects in either the ␥-carboxylase, 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 we report the fourth case resulting from mutations in the carboxylase gene, identified in a Tunisian girl who exhibited impaired function in hemostatic VKD factors that was not restored by vitamin K administration. Sequence analysis of the proposita did not identify any mutations in the VKORC1 gene but, remarkably, revealed 3 heterozygous mutations in the carboxylase gene that caused the substitutions Asp31Asn, Trp157Arg, and Thr591Lys. None of these mutations have previously been reported. Family analysis showed that Asp31Asn and Thr591Lys were coallelic and maternally transmitted while Trp157Arg was transmitted by the father, and a genomic screen of 100 healthy individuals ruled out frequent polymorphisms. Mutational analysis indicated wild-type activity for the Asp31Asn carboxylase. In contrast, the respective Trp157Arg and Thr591Lys activities were 8% and 0% that of wildtype carboxylase, and their compound heterozygosity can therefore account for functional VKD factor deficiency. The implications for carboxylase mechanism are discussed. IntroductionHereditary combined vitamin K-dependent (VKD) factor deficiency is a bleeding disorder characterized by the reduced activities of the procoagulant factors II, VII, IX, and X and anticoagulant proteins C, S, and Z. [1][2][3][4][5][6][7][8] The inheritance of the disease is autosomal recessive and is due to mutations in the genes for either the ␥-carboxylase 9-12 or the vitamin K epoxide reductase (VKORC1). 13 The carboxylase converts clusters of Glus to ␥-carboxylated Glus (Glas) in the Gla domains of VKD proteins, which renders them active by generating a calcium-binding module that binds either to anionic phospholipids that become exposed on cell surfaces or to hydroxyapatite in the extracellular matrix. 14,15 The carboxylase uses reduced vitamin K (KH 2 ) as a cofactor to drive Glu carboxylation, and the KH 2 becomes oxygenated to a vitamin K epoxide (KO) product that must be recycled for continuous carboxylation. Recycling is accomplished by VKORC1, which is the target of anticoagulant therapy with coumarin derivatives like warfarin that block KH 2 regeneration and consequently inhibit VKD protein carboxylation. Both VKORC1 and the carboxylase are integral membrane enzymes that reside in the endoplasmic reticulum (ER), where the VKD hemostatic factors are modified during their secretion from the cell. The concerted action of these 2 enzymes can therefore explain why congenital defects in either the carboxylase or VKORC1 lead to combined functional deficiency of the VKD factors.The interactions between VKD proteins and the carboxylase are complex and not well understood. 16,17 All VKD prot...

Spectrin a-chain mutants associated with hereditary elliptocytosis are highly variable in their level of expression. It has been assumed that the degree of elliptocytosis can be increased when the spectrin a chain, encoded by the a gene in trans to the variant, is expressed at a low level. We now provide strong evidence for the existence of low-level expression of spectrin a chains. This condition is referred to as the aV/41 polymorphism.