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Comparison of matched formalin-fixed paraffin embedded and fresh frozen tissue reveals bias in proteomic profiles

ABSTRACT: Tissue biopsies are most commonly archived in a paraffin block following tissue fixation with formaldehyde (FFPE) or as fresh frozen tissue (FFT). While both methods preserve biological samples, little is known about how they affect the quantifiable proteome. We performed a ‘bottom-up’ proteomic analysis (N=20) of short and long-term archived FFPE surgical samples of human meningiomas and compared them to matched FFT specimens. FFT facilitated identification of a slightly higher number of proteins compared with matched FFPE specimens (5735 vs 5670 proteins, respectively (p < 0.05), regardless of archival time. However, marked differences in the proteome composition were apparent between FFPE and FFT specimens. Twenty-three percent of FFPE-derived peptides and 8% of FFT-derived peptides contained at least one chemical modification. Methylation and formylation were most prominent in FFPE-derives peptides (36% and 17% of modified FFPE peptides, respectively) while, most of phosphorylation and iron modifications appeared in FFT-derived peptides (p<0.001). A mean 14% (2.9) of peptides identified in FFPE contained at least one modified Lysine residue. Importantly, larger proteins were significantly overrepresented in FFT specimens, while FFPE specimens were enriched with smaller proteins. This work cautions against comparing results of proteomic studies derived from different archival methods.

INSTRUMENT(S): Q Exactive HF

ORGANISM(S): Homo Sapiens (human)

TISSUE(S): Brain

SUBMITTER: Yishai Levin

LAB HEAD: Tal Shahar

PROVIDER: PXD032108 | Pride | 2024-02-12

REPOSITORIES: Pride

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Project description:Patients with loss-of-function mutations of the von Hippel Lindau gene (“VHL-patients”) often develop clear cell renal cell carcinoma (ccRCC). Using archived, formalin-fixed, paraffin-embedded (FFPE) samples and a cohort of eight cases, we sought to determine global proteome alterations that distinguish ccRCC tissue from adjacent, non-malignant kidney tissue in VHL-patients. Our quantitative proteomic analysis clearly discriminated tumor and non-malignant tissue, yielding comparable proteome profiles for the eight ccRCC cases. Limma statistics was used to identify significantly dysregulated proteins in ccRCC. Functional classification of these proteins highlighted a decrease in proteins involved in the tricarboxylic acid cycle and an increase in proteins involved in glycolysis. This profile possibly represents a proteomic fingerprint of the “Warburg effect”, which is a molecular hallmark of ccRCC. Furthermore, we observed an increase in proteins involved in extracellular matrix organization. Of particular note were opposing alterations of Xaa-Pro Aminopeptidases-1 and -2 (XPNPEP-1 and -2): a strong decrease of XPNPEP-2 in ccRCC was accompanied by a strong increase of the related protease XPNPEP-1. In both cases, we corroborated the proteomic results by immunohistochemical analysis of ccRCC and adjacent, non-malignant kidney tissue of VHL patients. To functionally investigate the role of XPNPEP-1 in ccRCC, we performed small-hairpin RNA mediated XPNPEP-1 expression silencing in 786-O ccRCC cells harboring a mutated VHL gene. We found that XPNPEP-1 expression suppresses cellular proliferation and migration. These results suggest that XPNPEP-1 is rather an anti-target in VHL-driven ccRCC. Generally, we present one of the first proteomic profiling studies of ccRCC in VHL patients, comparing normal and tumor tissue, which are both deficient for the VHL tumor suppressor. Methodologically, our work further validates the robustness of using FFPE material for quantitative proteomics.

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Comparison of matched formalin-fixed paraffin embedded and fresh frozen tissue reveals bias in proteomic profiles

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