The expression of prostate-specific membrane antigen in peripheral blood leukocytes

Purpose: Prostate-specific membrane antigen (PSM) and prostate-specific antigen (PSA) have been used as marker genes for detection of cancer cells in circulation of prostate cancer patients. However, PSA was recently found to be expressed in non-prostate cell lines and normal blood. To evaluate this phenomenon for PSM, we studied its mRNA expression in non-prostatic cells and cell lines, in blood from healthy donors and patients with prostate cancer. Materials and Methods: We studied PSM expression by a highly sensitive reverse transcription-polymerase chain reaction (RT-PCR) in peripheral blood of 24 patients with cancer of the prostate (CAP) and 13 healthy young male and female donors, in four non-prostatic cell lines and in isolated blood cells. The identity of the RT-PCR product was confirmed by sequencing. Contamination of the samples with cDNA or prostatic RNA was rigorously excluded by subjecting each sample to PCR reaction without RT-enzyme and by RT-PCR with PSA primers, respectively. Results: We found PSM mRNA expression in blood from 18 of 24 CAP patients and 12 of 13 healthy donors and in the leukocyte fraction of normal blood cells. PSM expression could not be detected in erythroblasts, platelets, K-562, U-937, HL-60 or Jurkat cell lines. Conclusions: Our results indicate that sensitive nested RT-PCR method detects PSM mRNA in the leukocyte fraction of normal blood. This ''background'' expression is probably caused by a leaky promoter of PSM. We conclude that it is necessary to develop quantitative RT-PCR assays to differentiate PSM mRNA expression derived from circulating cancer cells from background expression in blood cells.