Diagnostic potential and limitation of imaging cancer cells in cytological samples using telomerase-specific replicative adenovirus

Cytological cancer screening that targets genetic or epigenetic abnormalities may be a viable alternative to morphological screening. Detecting cancer cells by specific genetic markers helps their easy detection in cytological samples. We recently established the telomerase-specific replication-selective adenovirus OBP-401, in which the human telomerase reverse transcriptase (hTERT) gene promoter has been inserted upstream of the E1 genes, and in which the green fluorescent protein (GFP) gene is driven by the CMV promoter. This virus selectively replicates only in telomerasepositive cells, expressing GFP, and therefore may be a tool for cancer screening. In the present study, we first confirmed that cytological samples can easily be infected with OBP-401, allowing visualization of GFP-positive cells under fluorescent microscopy 24 h after infection. After 32 cytological samples from patients with cervical, endometrial or ovarian cancers were infected with OBP-401, GFP signals were detected in 31 (96%) of the samples. However, some normal endometrial scrapings exhibited GFP-signals, possibly due to endometrial glandular cells with constitutive telomerase activity. The ability of OBP-401 to enrich cancer cells was then tested. Cytological samples containing cervical or endometrial cancer cells were infected with OBP-401, and GFP-positive cells were sorted by flow cytometry; DNA was extracted from the GFP-positive cells. Direct DNA sequencing or methylationspecific PCR identified cancer-derived mutations or hypermethylations of tumor suppressor genes more efficiently than analyses using crude cytological samples. Thus, OBP-401-based sorting of GFP-positive cells successfully enriched cancer cells, allowing efficient detection of genetic or epigenetic abnormalities in cytological samples.