Detection of genomically-tagged cancer cells in different tissues at different stages of tumor development: lack of correlation with the formation of metastasis

Genetic detection of tumor cells in blood, lymphatic nodes or bone marrow using reverse transcription and polymerase chain reaction (PCR) is quite attractive because it allows the early diagnosis of cancer dissemination. Unfortunately, this type of detection strategy cannot be applied to solid parenchymas, because they usually share with tumor cells the mRNA markers. To avoid this impediment, we have developed an experimental model of cancer using cells with a genome-associated tag. DHD/K12-PROb cancer cells were stably transfected with pcDNA3.1CAT. Approximately 106 transfected cells (DHD-CAT cells) were injected subcutaneously into the chest of BD-IX rats. Animals were divided into 11 groups according to the time between injection of tumor cells and euthanasia. An additional ‘untagged group’ was injected with untransfected cells (DHD-Wild). Blood and tissues samples were collected after euthanasia. Macroscopic and microscopic analysis was done. To detect circulating tumor cells or their presence in peripheral organs, we performed PCR with nested primers to amplify chloramphenicol acetyl transferase-encoding (CAT-encoding) DNA sequences. The minimum number of cells that yielded detectable cells routinely was 2 in 106. No modification of cancer aggressiveness was observed in DHD-CAT cells. DHD-CAT cells were detected by PCR in lung from the 1st week after inoculation, in liver, spleen and kidney from the 3rd week and in the blood from the 5th week. All animals analyzed 12 weeks after injection showed lung metastases. Metastases in liver, spleen or kidney, either microscopic or macroscopic, were never detected. We have developed an experimental model of cancer based on genomic tagging of tumor cells that allows the detection of small numbers of cells in all organs and the blood. The presence of cancer cells in parenchymas detected with molecular technology does not correlate with the development of clinically relevant metastases.