Polymorphisms in TNFA and CTLA-4 Genes and the Risk of Heparin Induced Thrombocytopenia.

Backgroud and objectives: The formation of antibodies (Abs) to heparin platelet factor 4 complexes (H/PF4) associated with heparin-induced thrombocytopenia (HIT) is a T-helper cell dependent event that involves antigen presenting cells (APC) and B-lymphocytes. Polymorphisms of the CTLA-4 (cytotoxic T lymphocyte antigen 4) gene have been described as a risk factor in several autoimmune diseases. In addition, TNFα is a major inflammatory cytokine with potent regulatory functions and polymorphisms in TNFA have also been associated with autoimmune antibody-mediated diseases. We therefore evaluated the possibility that an association between polymorphisms in CTLA-4 (−318 C/T and +49 A/G) or TNFA (−308 G/A) and the development of Abs to H/PF4 and HIT might exist.

Methods: Eighty-three patients identified as having developed definite HIT with significant levels of Abs to PVS/PF4 in ELISA (HAT 45, GTI, Brookfield, WI, USA) and positive serotonin release assay were studied (HIT group). Two control groups were studied: the Ab^neg group consisted of 83 patients who had undergone heart surgery with high doses of unfractionated heparin administered during cardiopulmonary bypass (CPB), and who were tested negative for Abs to PVS/PF4 at the 8^th post operative day. The Ab^pos group consisted of 58 patients who had also undergone CPB but had developed high levels of Abs to PVS/PF4 without significant change in the platelet count post-operatively. Three single nucleotide polymorphisms (SNPs), one in TNFA (−308G/A) and two in CTLA-4 (−318 C/T and +49A/G) were studied by conventional RFLP analysis as described (Astermark et al, Blood 2006 and Astermark et al, Thromb Haemost 2007).

Results: The CTLA-4 +49 A/G and −318 C/T genotypes and allele distributions were similar in the 3 groups of patients (Table). In contrast, the frequency of TNFA –308 G/G homozygotes was higher in the HIT group compared to patients without HIT whether they had developed PF4-specific Abs or not (p=0.035). Therefore, the A allele was less frequent in HIT patients (p=0.026, OR 0.49; CI95% 0.26–0.93) but there was no significant difference when comparing patients with and without PF4-dependent antibodies.

Conclusion: The TNFA –308 A allele appears to be protective regarding the risk of heparin-induced thrombocytopenia in patients having developed PF4-specific antibodies. A similar effect has been suggested in immune thrombocytopenic purpura (Foster et al, Brit J Haematol 2001) despite individuals with this allele have been identified as high TNFα producers. Therefore, the mechanisms involved for explaining this apparent protective effect of the TNFA −308A allele in patients at risk for HIT have to be identified.