Influence of Protein Tyrosine Phosphatase Polymorphisms on the Development of Antibodies to Platelet Factor 4 and the Risk of Heparin-Induced Thrombocytopenia.

Heparin-induced thrombocytopenia (HIT) results from an atypical immune response to platelet factor 4/heparin complexes (PF4/H), with rapid synthesis of platelet-activating IgG antibodies that activate platelets via FcgRIIa receptors. The reasons explaining why only a subset of patients treated with heparin develop IgG to PF4/H complexes, and why most patients who synthesize these antibodies do not develop HIT, have not been fully defined. The immune response in HIT involves both B and T cells, and protein tyrosine kinases (PTKs) and phosphatases (PTPs) are crucial for regulating antigen receptor-induced lymphocyte activation. Moreover, some PTPs such as CD148 and low-molecular-weight PTP (LMW-PTP) could also have a critical role in platelet activation. Dysregulation of the equilibrium between PTK and PTP function could therefore have pathologic consequences and influence the pathogenesis of HIT.

To investigate an association between polymorphisms affecting genes encoding 4 different PTPs i.e. CD45 (PTPRC), CD148 (PTPRJ), LYP (PTPN22) and LMW-PTP (ACP1) and the development of heparin-dependent antibodies to PF4 and HIT.

A cohort of 89 patients with definite HIT (positive PF4-specific ELISA and positive serotonin release assay) and two control groups were studied. The first control group (Ab^neg) consisted of 179 patients who had undergone cardiopulmonary bypass (CBP) with high doses of heparin and who did not develop Abs to PF4 post-operatively. The second control group (Ab^pos) consisted of 160 patients who had also undergone cardiac surgery with CPB and heparin, who had all developed significant levels of PF4-specific antibodies but without HIT. Genotypes of PTPRC 77C/G (rs17612648), PTPN22 1858C/T (rs2476601), PTPRJ 2965 C/G (rs4752904) and PTPRJ 1176 A/C (rs1566734) were studied by a PCR-HRM method using the LightCycler 480 (Roche). In addition, the ACP1 A, B, C alleles were defined by combining the analysis of T/C transition at codon 43 of exon 3 (rs11553742) and T/C transition at codon 41 of exon 4 (rs11553746).

The frequency of PTPRC 77G and PTPN22 1858T alleles was not different in HIT patients and controls, whether they had developed antibodies to PF4 or not. The third PTP gene analyzed was ACP1, in which three alleles (A, B and C) were previously associated with the synthesis of distinct active LMW-PTP isoforms exhibiting different catalytic properties. The percentage of subjects in our study carrying the AC, BB and BC genotypes was significantly higher in the HIT and the Ab^pos groups than in patients without antibodies to PF4 after CPB (Ab^neg). In addition, the ACP1 A allele was less frequent in patients with antibodies to PF4, whether they had developed HIT (25%) or not (27.5% in Ab^pos controls), than in Ab^neg subjects (37%). The AC, BB and BC genotypes (associated in Caucasians with the highest LMW-PTP enzyme activity) therefore appeared to increase the risk of antibody formation in heparin-treated patients (OR 1.8; 95% CI 1.2–2.6, p=0.004 after comparing Ab^pos + HIT vs. Ab^neg). We also evaluated 2 SNPs affecting PTPRJ encoding CD148. No significant difference was found concerning the 2965 C/G polymorphism, but the frequency of PTPRJ 1176 AC and CC genotypes was significantly lower in the HIT (17%) than in the Ab^neg and Ab^pos groups (35%, p=0.003 and 29.5%, p=0.041, respectively). The C allele therefore appeared to provide a significant protection from the risk of HIT (OR 0.52; 95%CI 0.29–0.94, p=0.041) in patients with antibodies to PF4.

Recent studies have demonstrated that CD148 is a positive regulator of platelet activation by maintaining a pool of active SFKs in platelets. This non-synonym PTPRJ 1176 A/C SNP is associated with a Q276P substitution inducing a torsional stress of a fibronectin domain that is critical for the activity of CD148 and may influence the pathogenic effects of HIT Abs. This study supports the hypothesis that PTPs such as LMW-PTP and CD148 influence the immune response to heparin and the risk of HIT in patients with antibodies to PF4.

No relevant conflicts of interest to declare.