Archived cancer biomarkers: Phosphoprotein panel identified from preserved prostate cancer tissue

Prostate cancer diagnosis

Prostate cancer is the most common cancer among men in many countries of the western world and is a major contributor to cancer deaths, despite the slow progression of the disease. Nevertheless, men can live for decades with prostate cancer and die from other causes and the outlook for patients is generally good. If discovered early, it can usually be cured.

And therein lies one of the major problems with prostate cancer. The conventional diagnostic test measures levels of the protein prostate-specific antigen (PSA) in the blood but it is stricken with problems.

The test suffers from a very high false positive rate, with some reports declaring that only 25-35% of men who have a biopsy due to elevated PSA levels turn out to have prostate cancer. The additional procedures incur unnecessary clinical costs and exert great anxiety on the patient and his family.

There is also a low but significant false negative rate with PSA levels in the normal range even though the cancer is present. Subsequent tests may detect the cancer in time for treatment.

Scientists are looking for ways to modify the PSA test in an effort to improve its diagnostic capability and to distinguish cancerous and benign growths as well as slow- and fast growing cancers.

But other research groups are looking at completely different solutions to prostate cancer diagnosis. One such team consists of Sarka Beranova-Giorgianni, Li Chen, Bin Fang and Francesco Giorgianni from the University of Tennessee Health Science Center, Memphis, and Jeffrey Gingrich from the Department of Urology at the University of Pittsburgh.

They have taken the view that establishing a panel of cancer biomarkers might be a better approach, since a number of novel individual candidate biomarkers reported to date have not yet been converted into clinical tests.

In a second diversion from the norm, they decided to concentrate on phosphoproteins because irregularities in protein phosphorylation have been linked to cancer progression. Limiting the target proteins in this way is not as severe as it seems since it is estimated that 30% of cellular proteins are phosphorylated.

Archived tissue for phosphoprotein biomarkers

The research team turned away from the normal types of prostate cancer cell line to generate proteins because "they only mimic the biology of prostate cancer." Instead, they focused on bulk prostate cancer specimens in order to detect changes that "are directly associated with disease progression in human."

Five specimens were taken from a tissue bank where they had been stored in liquid nitrogen following radical prostatectomy. The proteins were extracted before processing using a gel-free methodology to digest the whole proteome, enrich the resulting phosphopeptides and identify them by mass spectrometry searching a standard human protein sequence database. The appearance of peaks in the tandem mass spectra corresponding to the loss of phosphoric acid supported the identification of phosphopeptides.

Between 19-30 phosphopeptides were identified in each of the five specimens, corresponding to 16-23 phosphoproteins. The majority of the proteins were found in single specimens, which the researchers attributed to genetic and environmental variations and potential differences in the tissue storage times.

However, nearly 20% of the phosphopeptides were detected across at least 3 samples, indicating that a subset of phosphoproteins was preserved. This subset could form the basis of a protein panel for detecting prostate cancer.

Some of the proteins in the panel have been associated with cancer, including prostate cancer in previous studies. These include the filament proteins desmin, vimentin and caldesmon and several chaperones such as heat shock protein 27, also known as heat shock protein beta-1.

So, archived tissues appear to be sufficiently preserved to be used retrospectively for the identification of individual protein biomarkers for prostate cancer, although this conclusion needs to be validated on a far wider number of samples. A broader study should also lead to the establishment of a confirmed set of phosphoproteins to act as a biomarker panel.

Based on this pilot study, it is likely that preserved tissues can also be used directly for biomarker discovery in other types of cancer, giving more directly relevant results than with cancer cell lines.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.