Diagnosis of Melanocytic Skin Tumors by MALDI Imaging Mass Spectrometry (MALDI IMS)
Alireza Sepehr, Erin Seeley, Anna Harris, Steven Tahan, Richard Caprioli. Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MA; Vanderbilt University, Nashville, TN
Background: Accurate detection of melanoma is an important factor in increasing survival rates of this fatal disease. Diagnosis of melanoma is still heavily based on histomorphology. Imaging mass spectrometry (IMS) combines the measurement capability of mass spectrometers with a surface sampling process that allows probing/mapping of the protein content of a sample. With MALDI, we used IMS for the discrimination of benign versus malignant melanocytic skin tumors.
Design: Formalin fixed, paraffin embedded (FFPE) tissue blocks of human melanocytic skin tumors were retrieved. Hematoxylin and eosin (H&E) and unstained sections were prepared. Digital microscope image of the H&E sections were annotated/superimposed to an image of the unstained section. Trypsin and matrix were spotted onto the unstained sections at the annotations. Custom geometry files were created to allow for the targeting of the specific areas on the tissue where trypsin and matrix have been applied and mass spectral profiles were collected using MALDI TOF MS.
A class prediction model was created using a support vector machine algorithm.
Results: 40 melanocytic skin tumors were selected, including 20 primary malignant melanomas (MM; mean age: 66 yrs) and 20 benign dermal nevi (DN; mean age: 54 yrs). We generated IMS data and profiled tissues using robotic matrix deposition techniques with a 200 μm resolution and identified 216 candidate peptide peaks (m/z range: 700-4500) which were preferentially over-expressed in MMs or in DNs (p < 0.05). When we used a genetic algorithm classification with 12 selected peptide peaks, we reached 91% spectral classification accuracy for MM and 100% for the DN class, with an overall spectral classification capability of 95% for the entire selected spots. Using a minimum of 5 X 200 μm spots per case, the accuracy in discrimination of benign versus malignant cases was 100%.
Conclusions: We compared the peptide expression profiles of benign versus malignant melanocytic skin tumors by MALDI IMS on FFPE tissues. With the minimum cutoff of 5 X 200 μm spots per case, we developed algorithms that discriminate these two entities with 100% accuracy.
Wednesday, March 21, 2012 1:00 PM
Poster Session VI # 102, Wednesday Afternoon