Tissue imaging desorption electrospray ionization mass spectrometry in disease state characterization

Allison L Dill, Purdue University

Abstract

Desorption electrospray ionization (DESI), a relatively new ambient ionization technique used in mass spectrometry (MS), allows for the direct analysis of samples in their native ambient environment and often without sample preparation. DESI-MS has been employed to analyze and image thin tissue sections of cancerous and normal tissue or tissues at different stages of disease. MS tissue molecular imaging has the potential to provide revolutionary histopathological information for the diagnosis and prognosis of cancer. This is due to the fact that imaging MS provides highly specific chemical information concurrent with molecular spatial distributions in animal tissue. Preliminary experiments carried out on canine bladder tissue yielded multiple potential disease marker glycerophospholipids and fatty acids. These potential markers were found at increased abundances in cancerous tissues when compared to normal tissues as revealed by DESI-MS ion images and statistical analysis. All DESI-MS results were validated by pathological examination of hematoxylin and eosin (H&E) stained serial tissue sections. These successful animal experiments led to examination of a larger sample set of human cancerous tissues with matched normal tissue including; human transitional cell carcinoma of the urinary bladder, two types of renal cell carcinomas (RCC), gliomas and prostate adenocarcinoma. DESI-MS images showing the spatial distributions of particular glycerophospholipids and free fatty acids were compared to pathologically examined serial tissue sections for diagnosis. In the study of human transitional cell carcinoma of the urinary bladder DESI-MS ion images allowed for a clear distinction to be made between cancerous and normal tissue based on lipid profiles. When examining two types of human RCC, DESI-MS ion images obtained from papillary RCC showed distinct absolute and relative intensities for particular molecular species in cancerous samples when compared to normal renal tissue. In contrast, the DESI-MS lipid profiles of clear cell RCC tissues could not be distinguished from normal renal tissue, highlighting the need for multivariate statistical analysis. While individual ions can be used to distinguish tumor from normal tissue, the use of multiple ions should improve the strength of the diagnosis, as accomplished through multivariate statistics to reduce the high-dimensional data acquired for each sample. Orthogonal projection to latent structures treated partial least square discriminate analysis (PLS-DA) was employed for visualization and classification of the tissue pairs using the full mass spectra as predictors. A clear separation of human bladder cancer and both types of RCC from adjacent normal tissue was achieved using PLS-DA. In addition PLS-DA successfully achieved a separation between papillary RCC, clear cell RCC and the pooled normal tissue samples. Imaging DESI-MS studies were expanded to the analysis and characterization of different grades of human gliomas, and the lipid profiles were used to differentiate between degrees of malignancy. Human prostate cancer was also studied by DESI-MS imaging. As revealed by DESI-MS ion images, a novel tumor marker, cholesterol sulfate, was discovered in all cancerous and pre-cancerous prostate tissues. To validate this biomarker, levels of cholesterol sulfate in serum and urine from prostate cancer patients will be quantitated using nano-electrospray ionization MS and correlated to detection and progression of the disease. In summary my research has focused on advancing DESI-MS imaging in the biomedical field. This is accomplished by employing the technique to molecularly characterize thin sections of cancerous tissue, providing diagnostic or prognostic disease information that will hopefully improve cancer management. Furthermore, since DESI-MS is an ambient technique requiring minimal sample preparation there is the future possibility, based on the studies performed here, of translating this research into in situ analysis of tissue samples to assist in intraoperative surgical planning.

Degree

Ph.D.

Advisors

Cooks, Purdue University.

Subject Area

Analytical chemistry|Biochemistry

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