No abstract

SummaryThe risk of developing factor VIII inhibitor antibodies in haemophilia A may relate both to factor VIII genotype and genes within the HLA complex known to influence immune response. We investigated a cohort of 176 patients with severe haemophilia A and with either high-level inhibitors (>10BU/ml) or with no history of an inhibitor, stratified according to the presence or absence of the factor VIII gene intron 22 inversion.HLA DRB1, DQA1 and DQB1 polymorphisms were determined by PCR. HLA frequencies from 137 United Kingdom controls were used for comparison. HLA phenotype frequency differences, expressed as odds ratios with 95% confidence intervals were as follows: HLA- DRB*1501, DQB 1*0602 and DQA1*0102 were all increased in frequency in patients with inhibitors, only DQA1*0102 reaching statistical significance (OR 2.7,1.2-5.9). These alleles form part of an established HLA haplotype. The frequencies of HLA-DRB 1*1501, DQB1*0602 and DQA1*0102 were particularly raised in patients with inhibitors and a factor VIII gene intron 22 inversion, although again only DQA1*0102 achieved significance (OR 3.1, 1.0-10.1). The frequency of DRB 1*01, DQB 1 *0501, DQA 1*0101 were also increased in inhibitor patients lacking the intron 22 inversion although this failed to achieve statistical significance. This data suggests that HLA class II profile constitutes a weak risk factor for developing inhibitor antibodies to factor VIII. This may be more pronounced in patients with an intron 22 inversion.

SummaryThe presence of the 20210A allele of the prothrombin (PT) gene has recently been shown to be a risk factor for venous thromboembolism. This is probably mediated through increased plasma prothrombin levels. The aim of this study was to compare the prevalence of the prothrombin 20210A allele in control subjects and in subjects with recognised thrombophilia and to establish whether the additional inheritance of the PT 20210A allele is associated with an increased risk of venous thromboembolism. 101 subjects with a history of venous thromboembolism and diagnosed as having either factor V Leiden (R506Q) or heritable deficiencies of protein C, protein S or antithrombin were studied. The prevalence of the PT 20210A allele in this group was compared with the results obtained for 150 control subjects. In addition, the relationships were examined between genetic status and the number of documented thromboembolic episodes, and between plasma prothrombin levels and possession of the PT 20210A allele. 8 (7.9%) of the 101 patients were also heterozygous for the PT 20210A allele. This compares with 0.7% in the control subjects (p = 0.005). After exclusion of patients on warfarin, the mean plasma prothrombin of 113 subjects without 20210A was 1.09 U/ml, as compared with 1.32 U/ml in 8 with the allele (p = 0.0002). Among the 101 patients with either factor V Leiden, protein S deficiency, protein C deficiency or antithrombin deficiency, the age adjusted mean (SD) number of venous thromboembolic episodes at diagnosis was 3.7 (1.5) in those with the PT 20210A allele, as compared with 1.9 (1.1) in those without (p = 0.0001). We have demonstrated that the prevalence of the PT 20210A allele is significantly greater in subjects with venous thrombosis and characterised heritable thrombophilia than in normal control subjects and that the additional inheritance of PT 20210A is associated with an increased risk of venous thromboembolism. We have also confirmed that plasma prothrombin levels are significantly greater in subjects possessing the PT 20210A compared with those who do not.

A large number of different mutations in the factor VIII (F8) gene have been identified as a cause of haemophilia A. This compilation lists known single base-pair substitutions, deletions and insertions in the F8 gene and reviews the status of the inversional events which account for a substantial proportion of mutations causing severe haemophilia A.

Recent findings have indicated the importance of factor V (FV) in causing resistance to activated protein C (APC) in a high proportion of patients with venous thrombosis. This prompted us to investigate whether resistance could be due to defective inactivation of FVa by APC. Consequently, we amplified a 3.2 kb fragment of the FV gene sequence encoding the heavy chain APC cleavage site. DNA analysis showed a guanine to adenine transition at nucleotide 1691 in all affected members of two families with inherited APC resistance associated with thrombosis and confirmed suspected homozygosity in two individuals. The mutation, in heterozygous form, was also found in approximately 3.5% of our normal population (n = 144) and correlated with low APC resistance. The high prevalence of this mutation suggests that it may be a major contributory factor in early thrombosis.