Infectious cancer

It has long been known that the several causes of cancer are infectious. Typically a virus contains a number of oncogenes to enhance its own proliferation, and in an infection gone wrong (for both virus and host) a viral oncogene is incorporated into the host DNA, creating an uncontrollable tumour cell. One of the best examples of this is human papillomavirus (HPV), a virus which infects most sexually active adults and is responsible for nearly every case of cervical cancer worldwide (which is why all girls should be vaccinated before they become sexually active).

However these cases are not "infectious cancers", they are infectious diseases which are capable of causing cancer. True infectious cancers, where a cancer cell from one individual takes up residency in a second individual and grows into a new cancer, were unknown until recently. With the publication of a new study in PNAS we now have three examples of truly infectious cancers.

1. In the most recent study, researchers in Japan documented the tragic case of a 28 year old Japanese woman who gave birth to a healthy baby but within two months had been diagnosed with acute lymphoblastic leukemia and died. At 11 months of age the child also become ill and was diagnosed with acute lymphoblastic leukemia. Genetic analysis of the tumour cells in the baby demonstrated that the tumour cells were not from the child herself, but rather maternal leukemia cells that had crossed the placenta during pregnancy or childbirth and had taken up residency in their new host. With this information, retrospective analysis indicates that this is probably not a one-off event, and at least 17 other cases of mother-to-child transmission of cancer have probably occurred.

2. In addition to mother-to-child transmission of cancer, cancer can spread from one identical twin to another. Identical (mono-zygotic) twins have identical immune systems, preventing rejection of "transplanted" cells, unlike non-identical (di-zygotic) twins. Thus a tumour which develops before birth in one identical twin can be transferred in utero to the other identical twin, where it can grow without being rejected. In one improbable but highly informative case, a set of triplets were born where two babies were identical and the third was non-identical. A tumour had arisen in one of the identical twins in utero and had passed to both other foetuses, but had been rejected by the non-identical foetus and accepted by the identical foetus. Of course, with the advent of medical transplantation, transmission of infectious cancers is now no longer limited to the uterus. Transplantation of an organ containing a cancer into a new host can allow the original cancer to grow and spread, as transplantation patients are immunosuppressed to prevent rejection. There is also a single case of a cancer being transmitted from a surgeon who cut his hand during surgery to a patient who was not immunosuppressed.

3. In a medical mystery well known to Australians, the population of Tasmanian Devils has been crashing as a fatal facial tumour has been spreading across the population. The way the fatal tumours have spread steadily across Tasmania and sparing Devils on smaller islands first suggested a new infectious disease that causes cancer, similar to HPV in humans. However a suprising study demonstrated that the cancer was directly spreading from one Devil to the next after having spontaneously developed in a single individual. These scrappy little monsters attack each other on first sight, biting each other's faces. The cancer resides in the salivary glands and gets transmitted by facial bites to the new Devil. Unfortunately for Tasmanian Devils, a genetic bottleneck left all Devils so genetically similar that they are, for immunological purposes, all identical twins. This means that the cancer cells transmitted from one Devil to another through biting are able to grow and kill Devil after Devil. The cancer from a single individual has already killed 50% of all Devils, and it is possible that we will have to wait until the cancer burns out by killing all potential hosts before reintroducing the Devil from the protected island populations. As unlikely as this seems, another similar spread occurs in dogs, where a cancer that arose in a single individual wolf is being spread through sexual transmission from dog to dog around the world. This example also illustrates the point made about cancers being "immortal" - the original cancer event may have occured up to 2500 years ago, with the tumour moving from host to host for thousands of years without dying out.