Letter by Krishnan et al Regarding Article, “Platelet Expression Profiling and Clinical Validation of Myeloid-Related Protein-14 as a Novel Determinant of Cardiovascular Events”
To the Editor:
The article by Healy et al1 raises interesting possibilities in coronary artery disease by suggesting a new candidate biomarker for stable coronary artery disease and postulating a novel source for myeloid-related protein-14 (MRP-14). In our opinion, however, there are some factors that may need to be considered before platelets can be determined to be a source of MRP-14.
Patients with ST-elevation myocardial infarction in the study1 had significantly raised leukocyte counts, a factor that was not included in the multivariate model. Because leukocytes are a well-known source of MRP-14, this could explain the raised plasma levels of this protein in the patient group. Epidemiological studies suggest that leukocyte count is an independent predictor of coronary events in asymptomatic individuals. It would, therefore, be expected that leukocyte counts would be raised in the patient cohort in the validation study, significantly affecting the level of MRP-14 in plasma.
Although leukocyte contamination of the platelet preparations was monitored by analysis of CD45+ events by flow cytometry, this method may not provide the sensitivity to accurately detect contamination by leukocytes. In addition, this method may fail to detect leukocyte microparticles that may lack the CD45 antigen but that would be isolated with the platelets and could contribute to the mRNA pool. Given the 12 500-fold relative abundance of mRNA in leukocytes compared with platelets2 only, tiny levels of leukocyte contamination could account for the presence of MRP-14 in the platelet transcriptome.
The authors suggest that previous work in platelet transcriptome profiling has demonstrated the presence of MRP-14 in platelets.3 However, the reference cited states that the signal for the specific mRNA on microarray analysis was in the negative range. Further microarray hybridization analysis of >34 000 genes in platelets using different glass slide microarray systems suggests that platelets do not express the MRP-14 gene.4 Detailed analyses of platelet proteome have not indicated the presence of MRP-14.5
There is no doubt that the identification of raised plasma levels of MRP-14 in stable coronary artery disease would open new avenues in early risk stratification. However, whether this gene is expressed in platelets should be considered with caution.
Healy AM, Pickard MD, Pradhan AD, Wang Y, Chen Z, Croce K, Sakuma M, Shi C, Zago AC, Garasic J, Damokosh AI, Dowie TL, Poisson L, Lillie J, Libby P, Ridker PM, Simon DI. Platelet expression profiling and clinical validation of myeloid-related protein-14 as a novel determinant of cardiovascular events. Circulation. 2006; 113: 2278–2284.