Harnessing Id1 as a Biomarker in a Plasmid Reporter System for Cervical Cancer

Introduction: Ten-year cervical cancer (CC) incidence in the U.S. has remained stable despite screening advancements such as HPV testing, suggesting that new CC screening technologies are needed to address stagnant rates. We proposed a diagnostic plasmid for screening that induces expression of a reporter enzyme (secreted embryonic alkaline phosphatase, SEAP) through the control of the cancer-specific inhibitor of differentiation 1 (Id1) promoter sequence. The resulting plasmid (pId1-SEAP) was used to transfect CC cells in vitro and characterize reporter SEAP production based on Id1 expression. Methods and Results: Western Blot and immunohistochemistry were used to establish Id1 expression in cell models and human tissues. Timed transfections in various conditions were used to correlate Id1 and SEAP expression. CC cell lines (HeLa 3.0±0.13, SiHa 2.9±0.27, CaSki 3.1±0.19, all P<0.0001) expressed increased normalized baseline Id1 compared to non-cancer 3T3 fibroblasts (1.0±0.0). In a microarray of CC tissues, normal samples had mean Id1 staining value of 3E4±3E4, while early- and late-stage cancers had increased mean Id1 staining (3E5±1E5 P<0.0001 and 2E5±1E5 P=0.0002, respectively). HeLa and SiHa lines produced increased normalized SEAP amounts with pId1-SEAP (0.63±0.25 and 0.50±0.10, P<0.05) compared to 3T3 cells with pId1-SEAP (0.16±0.058). Id1 and SEAP expression were strongly correlated (Pearson's r=0.99, P=0.012). As few as 12,500 pId1-SEAP plasmid-exposed HeLa cells co-cultured with naïve cells resulted in an increased SEAP value (3E4±3E3 P=0.004) compared to background level (1E4±4E2). Conclusions and Future Directions: pId1-SEAP can transfect CC cells to produce SEAP proportionally to endogenous Id1 expression, demonstrating its potential for future diagnostic tests for CC. These data support further studies to characterize pId1-SEAP function in 3-D tissue models and in vivo animal models, towards the development of a home-based approach to screen for cervical cancer using Id1 expression as a biomarker.