SummaryWe investigated the presence of the gene mutation of factor V, FV R506Q or factor V Leiden, responsible for activated protein C resistance, in DNA samples of 127 probands and 188 relatives from 128 families with antithrombin deficiency. The factor V mutation was identified in 18 families. Nine families were available to assess the mode of inheritance and the clinical relevance of combined defects.The factor V and antithrombin genes both map to chromosome 1. Segregation of the defects on opposite chromosomes was observed in three families. Co-segregation with both defects on the same chromosome was demonstrated in four families. In one family a de novo mutation of the antithrombin gene and in another a crossing-over event were the most likely explanations for the observed inheritance patterns.In six families with type I or II antithrombin deficiency (reactive site or pleiotropic effect), 11 of the 12 individuals with both antithrombin deficiency and the factor V mutation developed thrombosis. The median age of their first thrombotic episode was 16 years (range 0-19); this is low compared with a median age of onset of 26 years (range 20-49) in 15 of 30 carriers with only a defect in the antithrombin gene. One of five subjects with only factor V mutation experienced thrombosis at 40 years of age. In three families with type II heparin binding site deficiencies, two of six subjects with combined defects experienced thrombosis; one was homozygous for the heparin binding defect.Our results show that, when thrombosis occurs at a young age in antithrombin deficiency, the factor V mutation is a likely additional risk factor. Co-segregation of mutations in the antithrombin and factor V genes provides a molecular explanation for severe thrombosis in several generations. The findings support that combinations of genetic risk factors underly differences in thrombotic risk in families with thrombophilia.

Antithrombin is the principle regulator of thrombin and other blood coagulation proteinases. It is a member of the serpin family of proteinase inhibitors. The genomic sequence of the antithrombin locus has been completed, revealing a gene spanning 13,477 base pairs from the transcription start site to the poly(A) addition signal. Nine complete and one partial Alu repeat elements were identified within the introns of the gene, with all but one orientated in the reverse direction. Inherited deficiency of antithrombin is associated with a venous thrombotic tendency. Restriction fragment mapping of the antithrombin genes in an individual with type I antithrombin deficiency identified an intragenic deletion in one allele. Localization of the deletion breakpoints involved restriction analysis and direct sequencing of amplified DNA spanning the deletion site. The deletion removed 2761 base pairs, affecting exon 5 and flanking introns, with the deletion ends contained within the left components of two Alu elements. It is likely, therefore, that the deletion arose by homologous recombination between the two Alu elements.

An antithrombin III mutation database was collated and published in L99L by a group of investigators working on the molecular basis of antithrombin III deficiency (1). Soon after, under the auspices of the ISTH SSC, an Antithrombin III Working Party was formed of those involved in the preparation of the database, with the instruction to report to the "Thrombin and its Inhibitors" SSC on the developments in this and related areas. This document is one outcome of the work of the Antithrombin III Working Party and is a partial report of the deliberations of the "Thrombin and its Inhibitors" SSC Meeting held in Munich, July 1992. Other items discussed at this meeting included the nomenclature of the plasma coagulation inhibitors. Three alternative names were considered for this inhibitor, antithrombin III, antithrombin and thrombin inhibitor I. No unanimous view emerged regarding the name, other than the rejection of the term thrombin inhibitor I. For this report, the historical name antithrombin III will be used, despite the preference for anti. thrombin by the majority of the authors of this database. This is in deference to the journal, Thrombosis and Haemostasis, pending any final decision of the SSC regarding nomenclature. The intention behind the production and updating of the antithrombin III database has been to provide a readily accessible and up-to-date source of known mutations of antithrombin III. The complex effects of some mutations on structure/function relationships of the protein can only be indicated. For more information on this and on possible mechanisms involved in gene mutation (see below for brief consideration of mutations involving CpG dinucleotides), the reader is referred to the original papers and to several reviews in this area (2-6).