Enzymatically amplified mass tags for tissue mass spectrometry imaging

Tissue mass spectrometry imaging (MSI) is a rapidly developing technology which promises to provide biomarker molecular information within tissue context, which is an unmet medical need in the era of personalized medicine. However, challenges associated with tissue specimens as well as the MSI technical limitations have hindered the practical applications of this technology. We report here a mass tag based MSI method that combines the strength of signal amplification by immuno-enzymatic reactions with the superior detection characteristics of mass spectrometry to enable matrix-free MSI of protein biomarkers in formalin fixed paraffin embedded (FFPE) tissues. The method involves binding of the target protein with a primary antibody with high affinity and specificity, followed by binding with a secondary antibody-enzyme conjugate. Enzyme substrates suitable for mass spectrometry detection are locally deposited at the site of the target through enzymatically catalyzed transformation. The precipitates thus serve as mass tags to be detected in mass spectrometry to represent the target biomolecule in tissue. Two enzymes and various substrates have been identified and successfully used to demonstrate the feasibility of this novel MSI method to image protein targets in FFPE tissue samples.