Jan 8 2010
They can emerge even years after the successful treatment of a lung-, breast- or skin cancer tumour: Metastases suddenly begin to grown in the brain. These are of course extremely dangerous for the individual in question; they cannot be removed, and growth can only be delayed at best.
With an advanced microscopy technique, researchers headed by Frank Winkler of the Ludwig-Maximilians University (LMU) Munich have followed the different stages of metastases formation in real time. In the course of their work, they also found the processes that lead to a "dead end" for the cancer cells, meaning that no metastases form.
The scientists believe that their results, which are published in Nature Medicine (online, 20 December 2009), serve as promising candidates for future treatment techniques. Up to one in four cancer patients develop metastases in the brain, often long after the original tumour has been successfully treated. In such cases the prognosis can be expected to worsen dramatically. Usually it is the metastases, not the original tumours themselves, that are the cause of death in cancer patients.
They occur with particular frequency following lung-, breast- and skin cancer, and are extremely difficult to treat. In general, the growth of metastases of the brain can only be delayed, but not halted completely. Patients suffer from headaches and nausea, as well as neurological symptoms such as paralysis and speechlessness. "Unfortunately, we see more brain metastases today than in the past," says Frank Winkler, Head of the Neurooncology Research Group at the LMU’s Neurological Clinic. "Improvements in the treatment of malignancy have enhanced survival times. But this also means that more patients are at risk of developing brain metastases."
New application for two-photon microscopy
Exactly how metastases emerge has been somewhat unclear to date. Alongside Professor Jochen Herms of the LMU’s Centre for Neuropathology, and Prion Research, as well as scientists not far away at the Max-Planck Institute for Neurobiology in Martinsried, Winkler and his collaborators Yvonne Kienast and Louisa von Baumgarten have for the first time been able to follow the fate of single cancer cells in real time, over periods of weeks and months, up to the development of large brain metastases. This was possible thanks to a new technique in two-photon microscopy, which permits visualisation to the smallest detail of the deepest regions of the living brain. "Essentially, we were able to monitor the stages of metastasis formation live", said Yvonne Kienast.
Different fluorescent markers, injected directly by the team, were used to label blood vessels (in green) and tumour cells (red). Over some weeks, the scientists then observed four steps that are essential for the formation of a brain metastasis. "First, the tumour cells within the blood must become trapped at a fork in the vascular plexus. "In contrast to previous reports, intravascular growth is not sufficient to induce a metastasis", says Winkler. "We observed that such cells must then escape into the surrounding tissue by passing through tiny gaps between the cells of the vessel wall, before a third step, whereby they stick to the outer surface of the vessel." Micrometastases consisting of four to fifty cells can than develop.
Cancer cells rest for years, then come to life
It is the fourth step that gives the crucial signal for the development of a clinically relevant metastasis. This occurs when neighbouring micrometastases fuse and new blood vessels grow into the resulting mass. This angiogenesis, as it is known, ensures that the tumour has a constant supply of nutrients, allowing it to grow unchecked. The experiment also revealed the "dead ends" that must be avoided by the developing metastases. "Each one of the steps can go awry. Cells may not get out of the circulation, may fail to adhere to the outer vessel wall or be unable to induce angiogenesis," says Winkler. In the absence of angiogenesis, even cells that had attached to the outer vessel wall and proliferated strongly at first eventually died.
As Winkler and his colleagues confirmed, many cancer cells can remain in a resting state for long periods, before suddenly coming to life once again. "This is why metastases often appear years after successful therapy of the original tumour", says the doctor. But even in this standby mode, direct contact with a blood vessel is essential for the survival of tumour cells. These results are set to improve patient care. Winkler’s team have already shown that the anti-cancer drug Avastin blocks the step of angiogenesis, so that further growth in the micrometastases is permanently inhibited. "We now want to test other types of cancer drugs for their effects on the single steps of metastasis formation", says Winkler. "Thanks to these insights, it may be possible to discover new substances that allow us to treat existing metastases effectively, or even prevent them from developing at all."