In 1997, Caprioli et al detailed the novel application of MALDI-TOF MS directly on tissue for mapping of protein and peptide species [1]. Imaging Mass Spectrometry (IMS), is typically used to determine the distribution of proteins in fresh frozen tissue. In this respect, however, Tryptic Peptide Imaging has some advantages over imaging of intact proteins. These include access to higher molecular weight and hydrophobic proteins. Furthermore, peptide level analysis provides the possibility for identification by matching accurate m/z and in situ MS/MS to high quality LC-MS/MS data obtained through digestion of relevant laser dissected tissue. Finally, formalin-fixed paraffin embedded (FFPE) tissue can be analysed after antigen retrieval. FFPE archives have the additional advantage of providing large sample numbers complete with patient and disease specific annotations.
Here we present the latest developments within our group, including up-to-date methods for analysis of formalin-fixed tissue (e.g. tryptic peptide MALDI-IMS), a method for linking LC-MS/MS data to MALDI-IMS data using internal calibrants as well as the generation of the first data for a MALDI-IMS patient and disease specific tryptic peptide database. This database will serve as a reference for tissue micro-array MALDI-IMS experiments, with the ultimate aim of extracting clinical information such as therapy response and patient outcome. The potential for tryptic peptide imaging on FFPE tissue as a diagnostic tool for ovarian cancer will be discussed using FFPE tumour tissue-micro-arrays from patients responding versus patients not responding to chemotherapy.
[1] Caprioli RM, Farmer TB, Gile J. Molecular imaging of biological samples: localization of peptides and proteins using MALDI-TOF MS. Anal Chem. 1997 Dec 1;69(23):4751-60
[2] J.O.R. Gustafsson, J.S. Eddes, S. Meding, T. Koudelka, M.K. Oehler ,S.R. McColl, P. Hoffmann. Internal calibrants for high accuracy matching of tryptic peptide MALDI imaging features to peptides identified by LC-MS/MS. Journal of Proteomics, 2012 Aug 30; 75(16):5093-105