The role of haematological traits in risk of ischaemic stroke and its subtypes

Abstract

Thrombosis and platelet activation play a central role in stroke pathogenesis, and antiplatelet and anticoagulant therapies are central to stroke prevention. However, whether haematological traits contribute equally to all ischaemic stroke subtypes is uncertain. Furthermore, identification of associations with new traits may offer novel treatment opportunities. The aim of this research was to ascertain causal relationships between a wide range of haematological traits and ischaemic stroke and its subtypes. We obtained summary statistics from 27 published genome-wide association studies of haematological traits involving over 375 000 individuals, and genetic associations with stroke from the MEGASTROKE Consortium (n = 67 000 stroke cases). Using two-sample Mendelian randomization we analysed the association of genetically elevated levels of 36 blood cell traits (platelets, mature/immature red cells, and myeloid/lymphoid/compound white cells) and 49 haemostasis traits (including clotting cascade factors and markers of platelet function) with risk of developing ischaemic (AIS), cardioembolic (CES), large artery (LAS), and small vessel stroke (SVS). Several factors on the intrinsic clotting pathway were significantly associated (P < 3.85 × 10-4) with CES and LAS, but not with SVS (e.g. reduced factor VIII activity with AIS/CES/LAS; raised factor VIII antigen with AIS/CES; and increased factor XI activity with AIS/CES). On the common pathway, increased gamma (γ') fibrinogen was significantly associated with AIS/CES. Furthermore, elevated plateletcrit was significantly associated with AIS/CES, eosinophil percentage of white cells with LAS, and thrombin-activatable fibrinolysis inhibitor activation peptide antigen with AIS. We also conducted a follow-up analysis in UK Biobank, which showed that amongst individuals with atrial fibrillation, those with genetically lower levels of factor XI are at reduced risk of AIS compared to those with normal levels of factor XI. These results implicate components of the intrinsic and common pathways of the clotting cascade, as well as several other haematological traits, in the pathogenesis of CES and possibly LAS, but not SVS. The lack of associations with SVS suggests thrombosis may be less important for this stroke subtype. Plateletcrit and factor XI are potentially tractable new targets for secondary prevention of ischaemic stroke, while factor VIII and γ' fibrinogen require further population-based studies to ascertain their possible aetiological roles.

Keywords: Mendelian randomization; clotting cascade; genetics; haematology; stroke.

Figure 1

Overview of haematological pathways and targets for stroke treatment and prevention. arrow = agonize; B = basophils; E = eosinophils; L = lymphocytes; M = monocytes; N = neutrophils; perpendicular symbol = antagonize; R = reticulocytes; scissors = proteolytic cleavage. Traits significantly associated with one or more stroke subtypes are shown with a red border. Created with BioRender (https://biorender.com/).

Figure 2

Mendelian randomization results showing causal estimates for association of blood cell traits with stroke and its subtypes. Mendelian randomization results are grouped by Reactome pathways according to blood cell traits: (i) platelets; (ii) mature red cells; (iii) immature red cells; (iv) myeloid white cells; (v) lymphoid white cells; and (vi) compound white cells. Refer to Supplementary Table 1 for a description of each trait. Colours show magnitude and direction of P-value of association for estimate of causal effect using inverse-variance weighted Mendelian randomization approach. Asterisks indicate the significance of the P-value for the most significant association (either European or transancestral population): *P < 0.05; ^**P < 1 × 10⁻³; ^***P < 3.85 × 10⁻⁴.

Figure 3

Mendelian randomization results showing causal estimates for association of haemostasis traits with stroke and its subtypes. Mendelian randomization results are grouped by Reactome pathways according to haemostasis traits: (i) extrinsic pathway of fibrin clot formation (tissue factor activation); (ii) intrinsic pathway of fibrin clot formation (contact activation); (iii) common pathway of fibrin clot formation; (iv) dissolution of fibrin clot; (v) platelet adhesion to exposed collagen; (vi) platelet activation, signalling, and aggregation; and (vii) cell surface interactions at the vascular wall. Refer to Supplementary Table 1 for a description of each trait. Colours show magnitude and direction of P-value of association for estimate of causal effect using inverse-variance weighted Mendelian randomization approach. Asterisks also indicate the significance of the P-value for the most significant association (either European or transancestral population): *P < 0.05; ^**P < 1 × 10⁻³; ^***P < 3.85 × 10⁻⁴.

Figure 4

Genetic associations with haematological traits and stroke subtypes with significant causal estimates. The associations of each genetic variant associated with haematological traits with significant (P < 3.85 × 10⁻⁴) or marginally significant (P < 1 × 10⁻³) causal estimates are plotted against their association with selected stroke subtypes. Circles represent the associated change in levels of the trait and corresponding increased risk of stroke for each variant. The horizontal and vertical lines through each circle represent the corresponding 95% CIs for the genetic associations. Associations were oriented to the effect allele of each trait. Coloured lines show the slope (causal estimate) of the trait on stroke obtained using a variety of different Mendelian randomization (MR) approaches. Trait/outcome/population: (A) PCT/CES/European; (B) NEUT%GRAN/LAS/transancestral; (C) EO%/LAS/transancestral; (D) factor VIII activity/CES/European; (E) factor VIII antigen/CES/European; (F) factor XI activity/AIS/European; (G) γ′ fibrinogen/CES/transancestral; (H) protein C activity/CES/transancestral population; and (I) TAFI-AP:Ag/AIS/transancestral. See Supplementary Table 1 for a description of each trait.

Figure 5

Role of genetic variants associated with haematological traits in mediating association of atrial fibrillation with risk of ischaemic stroke. Cumulative probability of ischaemic stroke in individuals with normal and genetically lowered levels of factor XI (FXI) activity (based on rs710446 alleles in KNG1 locus) in UK Biobank participants (A) without atrial fibrillation and (B) with atrial fibrillation.