Hypofibrinogenemia Associated With a Heterozygous Missense Mutation γ153Cys to Arg (Matsumoto IV): In Vitro Expression Demonstrates Defective Secretion of the Variant Fibrinogen

Abstract

We genetically analyzed a case of hypofibrinogenemia that showed no bleeding or thrombotic tendency. Direct sequencing of a polymerase chain reaction-amplified γ-chain gene segment showed a novel nucleotide substitution. This heterozygous mutation encodes both Cys (TGT) and Arg (CGT) at residue 153. To examine the basis for the fibrinogen deficiency, we prepared expression vectors containing mutant γ-chain DNAs encoding γ153R and γ153A for in vitro expression in Chinese hamster ovary (CHO) cells. Enzyme-linked immunosorbent assay and immunoblot analysis of the culture media and cell lysates showed that CHO cells transfected with γ153R or γ153A synthesized the variant γ-chain, but did not secrete variant fibrinogen into the culture medium. Metabolic pulse-chase experiments showed that fibrinogen assembly was impaired when either variant γ-chain was expressed. In cells expressing normal fibrinogen, assem- bly intermediates and intact fibrinogen were seen in cell lysates prepared after short (3 minutes) or long (1 hour) incubation with 35S-methionine. Neither intermediates nor intact fibrinogen was seen with the variant γ-chains. These data suggest that γ-chains have an important early role in fibrinogen assembly. Thus, our results support the model for fibrinogen assembly proposed by Huang et al (J Biol Chem 268:8919, 1993), in which the first step in assembly is the formation of γ or βγ dimers, or both. This model implies that γCys153 has a critical role in the formation of these early assembly intermediates. We concluded that the γ153Cys→Arg substitution does not allow fibrinogen assembly and secretion, and this is manifest in vivo as a fibrinogen deficiency. We designated this variant as fibrinogen Matsumoto IV.