Immunoractivity of recombinant squamous cell carcinoma antigen and leupin/SCCA-2: Implications for tumor marker detection

Objectives. Squamous cell carcinoma antigen (SCCA) is a member of the serpin superfamily, and has been used as a serological tumor marker for cervical squamous cell carcinomas. We have identified a closely related serpin gene, leupin (SCCA-2), which may be the fraction previously thought to be the acidic isoform of SCCA. The purpose of this study is to isolate the individual recombinant proteins, to examine their reactivity with current immunological detection methods, and to use a gene-specific method to examine their expression in the uterine cervix. Methods. We have expressed and purified recombinant forms of SCCA and leupin individually. The proteins were characterized with respect to their isoelectric points and their reactivity with the monoclonal antibody from the current tumor marker diagnostic immunoassay (IMx SCC). Reverse transcription polymerase chain reaction (RT-PCR) with gene-specific primers was used to examine expression of both genes. Results. Isoelectric focusing shows that leupin is the more acidic antigen with a determined pI for recombinant leupin (rLeupin) of 6.01, with rSCCA having a pI of 6.17. The IMx SCC monoclonal antibody recognized both rSCCA and rLeupin in immunoassays and immunoblots and both genes are expressed in normal cervix and in cervical carcinoma tissue. Conclusions. The findings from this study suggest that all previous clinical studies examining SCCA expression have used methodology that detects two gene products. The confirmation that leupin or SCCA-2 is the more acidic protein and that its expression is significantly elevated in cervical cancer suggests that this gene product may be the more important tumor marker. (C) 2000 Academic Press.