Mass Spectrometric Discovery and Selective Reaction Monitoring (SRM) of Putative Protein Biomarker Candidates in First Trimester Trisomy 21 Maternal Serum

Abstract

The accurate diagnosis of Trisomy 21 requires invasive procedures that carry a risk of miscarriage. The current state-of-the-art maternal serum screening tests measure levels of PAPP-A, free bhCG, AFP, and uE3 in various combinations with a maximum sensitivity of 60-75% and a false positive rate of 5%. There is currently an unmet need for noninvasive screening tests with high selectivity that can detect pregnancies at risk, preferably within the first trimester. The aim of this study was to apply proteomics and mass spectrometry techniques for the discovery of new putative biomarkers for Trisomy 21 in first trimester maternal serum coupled with the immediate development of quantitative selective reaction monitoring (SRM) assays. The results of the novel workflow were 2-fold: (1) we identified a list of differentially expressed proteins in Trisomy 21 vs Normal samples, including PAPP-A, and (2) we developed a multiplexed, high-throughput SRM assay for verification of 12 new putative markers identified in the discovery experiments. To narrow down the initial large list of differentially expressed candidates resulting from the discovery experiments, we incorporated receiver operating characteristic (ROC) curve algorithms early in the data analysis process. We believe this approach provides a substantial advantage in sifting through the large and complex data typically obtained from discovery experiments. The workflow efficiently mined information derived from high-resolution LC-MS/MS discovery data for the seamless construction of rapid, targeted assays that were performed on unfractionated serum digests. The SRM assay lower limit of detection (LLOD) for the target peptides in a background of digested serum matrix was approximately 250-500 attomoles on column and the limit of accurate quantitation (LOQ) was approximately 1-5 femtomoles on column. The assay error as determined by coefficient of variation at LOQ and above ranged from 0 to 16%. The workflow developed in this study bridges the gap between proteomic biomarker discovery and translation into a clinical research environment. Specifically, for Trisomy 21, the described multiplexed SRM assay provides a vehicle for high-throughput verification of these, and potentially other, peptide candidates on larger sample cohorts.

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