Science Focus (Issue 23)

Cancer – a disease notorious for its deadliness. Till now, be it chemotherapy or immunotherapy, it still has no perfect cures. A sad truth indeed, but have you ever thought about cancers in other animals, like mice and elephants? Make a guess on the likelihood of these animals getting cancer – the answer may come as a surprise. By Lambert Leung 梁卓霖 F i r s t , how exact l y d o e s c a n c e r develop? In general, it can be caused by mutations i n p roto - oncogenes o r t umo r suppressor genes, both of which function to regulate normal cell growth and division. There are multiple cell cycle checkpoints to ensure that the genome i s proper l y repl icated. Tumor suppressor genes may kick in to repair damaged DNA, arrest cell cycle or induce apoptosis (cell suicide) when DNA replication goes wrong. However, when these genes are mutated, the cell may gain the ability to escape from the protective mechanisms and divide in an uncontrolled manner, forming a tumor. J udg i ng f r om t h i s mechan i sm, i t i s reasonable to deduce that larger organisms, which clear ly have more cel l s, are more prone to developing cancer because cell division has obviously occurred many more times in those creatures. Random mutations (mistakes in replication) may take place in every round of cel l division, so the chance of larger and older individuals having cel l s accumulat ing enough of those deadly mutations should be higher. However, just as organisms wi th 1,000 t imes more cel l s than humans don’t have an increased risk of developing cancer, we are not more cancer-prone than mice [1]. Such a lack of cor relation between body size and risk of