BACKGROUND: The histologic boundary between actinic keratosis (AK), Bowen’s disease (BD), and cutaneous squamous cell carcinoma (cSCC) is not clear in many cases. Studying proteomic alterations between AK, BD and cSCC will aid us with molecular classification of the disease, understanding their progression behaviour and their clinical management.
METHOD: Formalin-fixed paraffin-embedded samples of normal skins (four cases, pooled) and AK, BD and cSCC (10 cases each) were acquired and subjected to laser microdissection followed by label-free mass spectrometry analysis. Differential abundance analysis and principle component analysis (PCA) followed by Ingenuity Pathway Analysis (IPA) were performed to investigate the role of differentially abundant proteins in the tumours’ pathophysiology. Our proteomics finding was subsequently validated using immunohistochemistry.
RESULTS: In total, 2145 unique proteins were identified across 34 samples. Label-free quantification analysis revealed cSCC had the highest number of differentially abundant proteins compared to the normal skin (63 proteins) followed by BD (57 proteins) and AK (46 proteins) (adjusted p-value <0.05). Interestingly, 36 of these proteins were common between the three tumour categories, while 16, 13 and 3 proteins were exclusive to cSCC, BD and AK category, respectively. Additionally, six proteins were differentially abundant in cSCC compared to the AK category. Matching our data to the oncogenic pathways in IPA, increased activation of upstream regulators including MTOR, STAT3 and WNT3A, and increased inhibition of CLDN7, CD28 and EFNA4 was predicted in cSCC compared to BD and AK. Immunohistochemistry confirmed the changes observed in our proteomic analysis.
CONCLUSION: While crucial proteome changes were identified between the three tumour types, our finding confirms the assumption that AKs and BDs are the precursor lesions of cSCCs.