Georgia Tech | EPICS Lab

Biomarker Discovery in Ovarian Cancer Tumors by Laser Desorption Single Photon Ionization Mass Spectrometry

Ovarian cancer is the fifth leading cause of cancer death among women in the United States.¹ Therefore, it is important that new research be done to determine biomarkers for ovarian cancer and the effects of treatment among tissue with different histological characteristics. In a joint project between Dr. Facundo Fernandez and Dr. Thomas M. Orlando, work is being done using laser desorption single photon ionization mass spectrometry (LD/SPI MS) to study ovarian cancer serum samples provided by the Ovarian Cancer Institute of the School of Biology. Although different levels of many protein biomarkers, have been observed in tissue samples from patients with ovarian cancer versus those from the control population², little information is known about biomarkers and signaling molecules in the low mass range (<1KDa). and how they change with treatment. This lack of knowledge is due to inherent limitations in the techniques commonly used, such as matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). In order to bridge this gap in knowledge, LD/SPI MS will be used to study the distribution of low molecular weight biomarkers which may be up or down regulated in cells found to be cancerous. It is hoped that LD/SPI MS will lead to a better understanding of the molecular basis of ovarian cancer at its various stages, the discovery of new biomarkers in a mass range yet to be explored, and a clearer picture of how treatment affects the up or down regulation of low-mass biomarkers in ovarian cancer cells.

LD/SPI MS is a technique in which ions are created in a two step process. In the first step of LD/SPI MS, desorption of the analyte occurs using a 355nm laser. This causes the non-thermal transition of molecules in the sample into the gas phase. The target molecules, now in the gas phase, are then rapidly excited by a second pulse of vacuum ultra violet (VUV) photons (118 nm, 10.5 eV), causing the ionization of the analyte. Ions generated in this process are then analyzed in a time-of-flight mass spectrometer, which allows for the ions to separate spatially based on their mass to charge ratio m/z. In this way, a mass spectrum is produced.

As mentioned before, the most common technique for analyzing biological samples is matrix-assisted laser desorption ionization mass spectrometry (MALDI). MALDI MS employs a one step process to create ions. The sample to be studied is co-crystallized with a matrix, such as sinapinic acid, which absorbs in the ultra violet region. An ultra violet laser is then used to excite this matrix, which sublimates into the gas phase carrying the analytes. Once in the gas-phase, ions are formed by the transfer of a proton from the excited photoacidic matrix to the analytes. Although this technique has proven to be very powerful for proteins, oligosaccharides, polymers and nucleic acids, it suffers from the drawback that the matrix produces a very strong background which masks all signals below m/z 700.

LD/SPI has many advantages and offers complementary information to MALDI. Serum samples can be used in their natural, unmodified state without labeling. Since LD/SPI MS does not use a matrix, low masses can easily be measured, a feat not possible with MALDI. This allows for the detection of sugars, amino acids, small peptides, and cytotoxic compounds formed as result of treatment.

References

Ovarian Cancer. Basic Information. 5 Oct. 2006. Centers for Disease Control and Prevention. 3 Nov. 2006 http://www.cdc.gov/cancer/ovarian/basic_info/
E. F. Petricoin, A. M. Ardekani, B. A. Hitt, P. J. Levine, V. A. Fusaro, S. M. Steinberg, G. .B Mills, C. Simone, D. A. Fishman, E. C. Kohn and L. A. Liotta. Lancet, 2002. 359(9306) 572-7
Y. Chen, C. M. Sullards, T. T. Hoang, S. W. May, T. M. Orlando. Anal Chem, 2006. 78(24), 8386-8394.