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Other interesting candidates
Several other interesting candidates were identified, both shedding light on the pathophysiology of SCI and with the potential to act as useful biomarkers.
Gelsolin binds to and remodels actin by both severing and capping the protein; it is present in both cytoplasm and plasma. Increased plasma levels have been identified as a poor prognostic marker in ischemic stroke.40 Further significant actions in the context of SCI include a role in promoting platelet formation and activation.41 In our genetic analysis, a genetic variant in the gelsolin gene showed a trend towards being associated with SCI.
Retinol binding protein-4 is a lipocalin transporting vita- min A from the liver to other tissues, including the brain; plasma levels have been found to be elevated in athero- sclerosis and also ischemic stroke.42,43
Pigment epithelium-derived factor is multifunctional with anti-angiogenic properties, secreted by different tis- sues including adipocytes. It is potentially significant in the development of SCI because of its ability to induce insulin resistance, and inflammation and proliferation of muscle cells.44 It is harder to know the significance of some other proteins, including the lower levels of immunoglob- ulin chains and platelet basic protein in SCI, although immunodeficiency has been associated with primary and recurrent stroke in the general pediatric population.45
Several plausible biomarkers for SCI were identified in the targeted plasma analysis, including von Willebrand factor C domain, although none of these reached statisti- cal significance when corrected for multiple assays. Similarly, none of the candidate proteins identified by pro- teomics showed a convincing genetic association with SCI in a separate cohort of 359 patients with SCA, although the pathophysiology of SCIs seen in the adults used in this part of the study may differ from that in the children used in the proteomic part. This may also reflect the relatively small size of this study, or that the changes seen in pro- teins of those with SCIs are secondary to other patholog- ical events rather than inherited quantitative trait loci.
The strength of our study is that it used an unbiased
proteomics approach to identify proteins involved in the pathogenesis of SCI, together with a combination of tar- geted plasma analysis and genetic association studies. Our findings suggest that proinflammatory and prothrombotic states contribute to the development of SCI, with possible abnormalities in lipoproteins. Previous studies to identify biomarkers for SCI in SCD have been summarized by Lance et al.35 They identified nine studies measuring a range of different candidates, although no studies used an unbiased proteomic approach. In total, 17 potential candi- dates were identified in this review, again with a predom- inance of factors suggesting coagulation activation and increased inflammation.35 None of the identified candi- dates were the same as our candidates, although one study found increased levels of thrombospondin-1 in chil- dren with SCIs and SCA.34
Although our study is relatively small, in keeping with proteomic discovery approaches, some significant factors have emerged. We confirmed the importance of hyperten- sion as a risk factor for SCI, together with support for higher HbF levels being protective, suggesting that anti- hypertensives and hydroxyurea may be of benefit. The utility of individual biomarkers, such as gelsolin or α-2- antiplasmin, to act as clinically useful biomarkers in screening for SCI needs to be confirmed in larger, prospec- tive studies. Previous studies have shown increased inflammation and coagulation in children with SCD,46,47 possibly linked to small blood vessel disease,48 and our study suggests that this is more marked in children who also have SCIs. This relatively prothrombotic and pro- inflammatory state associated with SCIs suggests that it may be useful to study anti-inflammatory and antiplatelet agents in clinical trials of SCI prevention. Ongoing clinical trials of antiplatelet agents,49 statins50 and canakinumab are particularly relevant in this context.
Acknowledgements
The authors would like to thank the patients involved in this study and the Stroke Association for funding this research (Grant TSA 2012/06).
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