Cancer cells have been programmed to revert to normal cellular functioning in a breakthrough that could lead to new treatments and even reverse tumour growth. For the first time, aggressive breast, lung and bladder cancer cells have been made harmless by restoring to them the self-regulating function that prevents them from multiplying incoherently and forming dangerous growths.Scientists at the Mayo Clinic, in Florida, in the US, said it was like applying the brakes to a speeding car.The technique has been tested only on human cells in the lab but researchers are hopeful that the technique will one day be used to target tumours so that cancers could be "switched off" without the need for harsh chemotherapy or surgery."We should be able to re-establish the brakes and restore normal cell function," said Professor Panos Anastasiadis, of the department for cancer biology at Mayo Clinic."Initial experiments with some aggressive types of cancer are very promising. This represents an unexpected new biology that provides the code, the software, for turning off cancer."Cells need to divide constantly to replace themselves. But in cancer the cells do not stop dividing, leading to huge cell reproduction and tumour growth.The scientists found that the glue that holds cells together is regulated by biological microprocessors called microRNAs. When everything is working normally, the microRNAs tell the cells to stop dividing when they have replicated sufficiently.They do this by triggering the production of a protein called PLEKHA7, which breaks cell bonds. But in cancer that process does not work.Scientists discovered that they can switch on cancer in cells by removing the microRNAs from cells and preventing them from producing the protein."We've now done this in human cells [taken from] very aggressive breast and bladder cancers," said Anastasiadis."Restoring either their PLEKHA7 levels or the microRNAs returns them to a benign state."Cancer experts said the research had solved a riddle that biologists had puzzled over for decades - why cells did not naturally prevent the proliferation of cancers...