Extracellular Tyrosinase mRNA within Apoptotic Bodies Is Protected from Degradation in Human Serum

Abstract

The amplification of tyrosinase mRNA by reverse transcription-PCR (RT-PCR) from peripheral blood has frequently been used as a marker for the presence of circulating tumor cells in melanoma patients (1). Recently, tyrosinase mRNA was also demonstrated to be detectable in serum and plasma samples from melanoma patients, indicating that tumor-specific mRNA is present in an extracellular form in the bloodstream (2)(3). This has been confirmed by other investigators demonstrating the presence of extracellular telomerase mRNA in the serum of breast cancer patients (4). It has been speculated that the presence of tumor-associated mRNA in serum and plasma is possibly related to tumor cell necrosis and release of transcripts into the circulation. Furthermore, there is evidence to suggest that mRNA may be actively secreted by tumor cells in vivo (5)(6). It is not yet clear, however, how serum and plasma mRNA is protected from RNases. Several findings have indicated that extracellular RNA might be protected from serum RNases by proteins or proteolipid complexes (6)(7)(8). Very recently, it was demonstrated that DNA and RNA are packaged separately into apoptotic bodies, producing two types of apoptotic bodies, one type containing RNA and no DNA and the other type containing DNA and no RNA (9). This finding suggests that extracellular mRNA may circulate within apoptotic bodies, which may decrease their susceptibility to nucleases. The present study was therefore conducted to analyze the serum stability of tyrosinase mRNA associated with apoptotic bodies in comparison with free tyrosinase transcripts.