The research in Garrett’s lab involves the application and development of mass spectrometry for clinical research covering many areas such as cancer, the microbiome, rare diseases, and autoimmune disorders. This diversity in research is enabled because of the flexibility that mass spectrometry offers. A significant area of research focuses on the development of metabolomics and lipidomics. The metabolome encompasses the entire set of small molecules that may be found in a biological specimen. It consists of intermediates, conjugates, hormones, and even exogenous metabolites from food or the environment that have an impact on health (the exposome). Metabolomics refers to the measurement of these metabolites typically involving mass spectrometry. In Global metabolomics, the aim is to measure as many of these metabolites as possible, often in excess of 2000, in a single injection, while targeted metabolomics focuses on a smaller subset of metabolites. The result when comparing multiple samples from a control group to multiple samples from the experimental group (e.g. disease or treatment vs healthy) in a global metabolomics study is a pattern of metabolites that represent differences reflective of the disorder. These patterns are often elucidated by the use of multivariate statistics such as principal components analysis (PCA), multivariate longitudinal data analysis, and dimension reduction. An example of this can be observed in the analysis of urine samples from patients with prostate cancer compared to healthy and patients with either prostatitis or BPH where we utilized PCA to first identify clustering and then future statistical approaches to identify the key metabolites involved. (fig 1). The differences, or biomarkers, then define the characteristics of the disease in that subset of samples. Current metabolomic studies rely on the combination of liquid chromatography to high-resolution mass spectrometry (LC-HRMS), which does provide excellent metabolite coverage. This approach uses the power of exact mass measurements coupled with the separation power of chromatography.
Garrett’s lab also pushes the envelope in technology to develop faster approaches to analyzing the metabolome. The paper spray is an example of one of these methods (Fig 2). The paper spray is an ambient ionization technique that utilizes a triangular piece of cotton paper to store the sample. When ready for analysis, the triangular paper wedge is placed in front of the instrument, a small amount of solvent is added, and then a high voltage is applied. The combination of solvent, high voltage, and the triangular shape produces an electrospray in which we can readily measure the small molecules present in a biological sample using mass spectrometry. With this approach, the measurement time is reduced to 30 s, so a study of 100 samples would take 50 min while using LC-MS, as we did above in urine analysis for prostate cancer, it would take 33 hours (20 min/sample). A distinct advantage of paper spray is also the elimination of carryover effects that are often seen in LC-MS studies that utilize an autosampler for column injection. This is especially important for clinical-based samples that can have a high degree of concentration variability between subjects.
Finally, my lab is also interested in the development of imaging mass spectrometry (IMS). This is a technique similar in many ways to traditional histology except we have the ability to measure many different compounds from the same sample instead of only a few select species. Imaging tissue sections is a key field in mass spectrometry, filling a necessary gap in tissue metabolomics. This is due to the ability to profile a large variety of small molecules directly from sectioned tissue and to correlate differences in those specific ions to diseases or treatments. Critical to this comparison is the role of tandem mass spectrometry (MS/MS or MSn) in the detection and identification of known and unknown compounds desorbed from the tissue. The field of IMS pertains to the direct analysis of surfaces, primarily tissue sections, using a focused ion beam (secondary ion mass spectrometry or SIMS) or, more commonly, a focused laser beam with a highly absorbing matrix (matrix-assisted laser desorption/ionization, MALDI). Mass spectra are collected in a discrete x and y pattern across the tissue, with step sizes typically equal to the laser spot size (10-100 mm). Each spot that the laser interrogates is a called a mass pixel, with dimensions related to the spot size of the laser and the step size (Fig 3). A key feature of IMS is molecular specificity without the need for staining or labeling approaches that are targeted for specific compounds, as would be the case for traditional radiolabeling in autoradiography of drug distribution.
