SH2B3/LNK Loss of Function Promotes Atherosclerosis and Thrombosis

Human genome-wide association studies (GWAS) have revealed many novel genetic loci that are associated with coronary heart disease (CHD) but do not involve traditional risk factors. However, the relevant genes and mechanisms are largely unknown. One such locus resides in SH2B3/LNK, which is expressed in hematopoietic cells and suppresses thrombopoietin (TPO) signaling via its receptor (MPL). The common risk single nucleotide polymorphism (SNP) of SH2B3/LNK is associated with CHD, increased platelet and myeloid cell counts in peripheral blood and JAK2V617F positive myeloproliferative neoplasms.  

Analysis of human cord blood revealed that samples with TT risk SNP (R262W) in SH2B3/LNK was associated with expansion of hematopoietic stem cells (HSCs), increased MPL signaling in HSCs, increased megakaryopoiesis, and a paradoxical increased expression of LNK mRNA, indicating reduced LNK function but induction of LNK transcription downstream of increased MPL signaling. Since humans are hypercholesterolemic relative to mice, to assess the role of reduced LNK function in athero-thrombosis, we transplanted WT or Lnk^-/- bone marrow (BM) cells into irradiated WT or low-density lipoprotein receptor knock-out (Ldlr^-/-) recipient mice and fed the recipients with chow and western type diet (WTD), respectively. Although Lnk^-/- BM recipients showed similarly increased platelet counts on both diets, we found that platelet activation and aggregation, formation of platelet/leukocyte aggregates, atherosclerosis and arterial thrombosis were markedly increased by the combination of hyperlipidemia and hematopoietic LNK deficiency. These diet-genotype interactions mirrored an increase in mean platelet volume, protease-activated receptor-4 (PAR4) agonist-induced P-selectin exposure on platelets and platelet protein kinase C (PKC) activity, suggesting increased platelet granule secretion.  We also observed an increased number of bone marrow myeloid progenitor cells expressing elevated levels of the common-beta subunit (CBS) of the interleukin-3/granulocyte-macrophage colony-stimulating factor (IL-3/GM-CSF) receptor, which promotes leukocytosis and the formation of platelet/leukocyte aggregates in the bloodstream. The increased P-selectin exposure, platelet aggregation and PKC activity were largely reversed by high-density lipoprotein (HDL) or cyclodextrin, reagents that remove cholesterol from platelets. Consistently, cholesterol loading of platelets from the chow-fed mice led to increased P-selectin exposure and PKC activity upon PAR4 agonist stimulation but the increase was more pronounced in Lnk^-/- platelets, indicating a cholesterol-genotype interaction. LNK deficiency also resulted in defective MPL-mediated TPO internalization in platelets, leading to increased plasma TPO levels in mice on both diets. Elevated TPO levels and enhanced MPL signaling likely acted together to increase the pool of HSCs and megakaryopoiesis, leading to marked thrombocytosis.

These studies suggest that SH2B3/LNK loss of function variants lead to HSC expansion, overproduction of platelets, and in the setting of hypercholesterolemia, markedly increased myeloid cells, platelet activation and formation of platelet/leukocyte aggregates, leading to accelerated atherosclerosis and arterial thrombosis. Our studies point to the importance of treating dyslipidemia in the setting of platelet and myeloid cell overproduction, as observed in subjects with SH2B3/LNK risk alleles.