Mass Spec Imaging Develops Tissue Samples as Resource for Proteomic Biomarker Discovery

This blog post covers market trends in proteomics research - a field recently profiled by Kalorama Information withProteomics Research Markets: Instruments (Mass Spectrometry, Liquid Chromatography, Electrophoresis), Disease Areas (Oncology, Cardio/Blood, Neurology, Other) Market Share and Trends. The report offers unique insight into a mass spectrometry- and analytical separation-centric life sciences market, including market shares by geography, instrument type, and vendor.

Mass spectrometry provides translational research and the life sciences with unparalleled analytical sensitivity and specificity, yet most sample preparation techniques limit contextual insights available to researchers. Molecular analyte and putative biomarker concentrations in tissue cannot be determined following sample homogenization. Also, tissue sample preparation makes experimental and assay reproducibility more difficult and complicates discovery. While the most powerful tool in proteomic and other non-nucleic acid biomarker discovery and validation, mass spectrometry (mass spec; MS) through its traditional techniques has struggled to elucidate molecular distribution in tissue related to disease state. In recent years, mass spec imaging, including MALDI and DESI imaging, has seen significant application in proteomic biomarker discovery with its distinct advantages in its preservation of spatial proteomic data and potential to improve assay reproducibility.

Sample preparation is a tedious front-end process for virtually any MS analysis and presents a significant bottleneck in proteomics where differential analysis is used to compare protein expression and abundance between samples in increasingly greater volumes. Proteolytic digestion and other sample preparation procedures remove significant analytical context related to the spatial distribution and concentration of proteins within a single sample. Mass spec imaging avoids traditional tissue sample preparation. The relatively “non-destructive” techniques (all samples are eventually destroyed in a mass spectrometer) use soft ionization sources to desorb (release from sample surface) and ionize sample surface molecules for MS analysis. Imaging is accomplished by rastering or systematically scanning the sample surface at predefined coordinates with targeted mass-to-charge (m/z values) detected, recorded and displayed by position on a multi-dimensional basis.

Mass spec imaging techniques, including matrix-assisted laser desorption ionization (MALDI) imaging (MALDI-IMS) and desorption electrospray ionization (DESI) imaging are unrivaled in their effectiveness in studying molecular surface analytes. Tissue samples have seen increased significance as a resource in biomarker discovery activities otherwise preoccupied with plasma and serum. While a prevalent target of biomarker research, proteins from blood-derived samples have a very wide dynamic range of concentration (very high- and low-abundance proteins with the former obscuring the discovery of the latter) and are found in highly complex populations. Tissue proteins are found in significantly lower dynamic ranges of concentration and crucially may contain more putative biomarkers for the disease state under study, particularly in the case of cancers.

MALDI imaging of tissue was introduced in 1997 and has since been refined into a productive tool for cancer biomarker discovery. The mass spec imaging technique requires the tissue section sample to be coated with a low molecular weight matrix. During MS analysis, a laser beam is directed at the matrix-sample surface; significant energy is absorbed in vaporizing the matrix to create an ionization cloud while also preserving the integrity of sample proteins. MALDI imaging is commonly performed on a time-of-flight (TOF) mass spectrometer due to the platform’s advantages over quadrupoles in ease-of-use, fast repetition rate, and mass range for discovery proteomics. Higher resolution MS platforms such as FT-ICR and hybrid ion trap-orbitraps have also been used to perform MALDI imaging as part of thorough tissue peptide mapping.

In one study of the application of MALDI imaging in biomarker discovery, a Bruker ultrafleXtreme MALDI-TOF/TOF with smartbeam imaging laser technology was used for the differential analysis and detection of overexpressed proteins between HER2-positive and negative breast cancer tissue samples. The subsequent identification of overexpressed proteins was accomplished with an ion trap MS and led to the effective discovery of CRIP1 biomarker in the samples. Researchers noted the novelty of the MALDI imaging approach to discover proteins otherwise difficult to separate by electrophoresis or hidden in shotgun proteomic processes that are biased towards higher-mass proteins. Other recent studies have used MALDI imaging to associate several discovered biomarkers (including CRIP1) with gastric cancer survival prognosis, forms of ovarian cancer, and to develop a novel grading system for papillary non-invasive bladder cancer. Biomarkers discovered and advanced through MALDI imaging still face high attrition if moved to validation and subsequent assay development.

Less represented in proteomic biomarker discovery, DESI imaging targets an ion beam at a sample surface under ambient conditions (atmospheric pressure). Unlike MALDI, the technique does not require a matrix and has been promoted as a means of minimizing sample preparation in mass spec imaging. In June 2014, Waters secured exclusive rights to Prosolia’s DESI technology. Similarly, Waters will pair mass spec imaging technology with its TOF MS systems. In July 2014, Waters acquired MediMass Ltd.’s Rapid Evaporative Ionization Mass Spectrometry (REIMS) for direct tissue analysis and an enabling technology for intra-operative, real-time mass spec imaging during surgery.

Kalorama Information offers interested readers titles related to mass spec imaging including Proteomics Markets for Research and IVD Applications and Mass Spectrometry in Clinical Applications.