The more we learn about cancer, the more we realise how complex it really is.
Take breast cancer. Millions of dollars are raised every year for research into the disease. But are we any closer to a cure? As it turns out, to think there is a single "cure" for breast cancer, or indeed any type of cancer, is to misunderstand this highly complex disease.
Breast cancer is a major killer worldwide, accounting for almost a quarter of all invasive cancers in women. In Asia, breast cancer is the most common cancer diagnosed in women and typically the number two killer after lung cancer. The number of cases diagnosed each year has climbed steeply over the past few years, mainly due to greater awareness and increased screening, but also because of changes in lifestyle. Men also get breast cancer, although it is much rarer than in women. Is breast cancer one disease or many?
Cancer is caused by the loss of control over cell division, leading to excessive growth of cells in a given part of the body. The breast is made up of ducts and lobes, both of which can be sites of cancerous growth, also known as carcinoma. Risk factors include age, treatment with hormones such as oestrogen and progesterone, as well as alcohol consumption.
An important risk factor is genetics, because mutations in genes can be inherited from one's parents. Between 5 and 10 per cent of women have a mother or sister who has had breast cancer, and having a family history of breast cancer greatly increases a person's risk of developing the disease.
Well-known examples of inheritable genetic mutations that increase the risk of breast cancer are in the BRCA1 and BRCA2 genes. These genes are known as tumour suppressors because they encode the cellular components that repair damaged DNA and help prevent uncontrolled cell growth. Genetic testing using a blood sample can screen for harmful mutations in these genes, allowing for surveillance and early intervention if necessary. Recently, an international consortium of scientists published the most comprehensive analysis yet of breast cancer. Using five different cutting-edge technologies and 825 breast tumour samples, the researchers found that the cancers could be divided into four main classes: luminal A, luminal B, HER2-enriched and basal-like. Importantly, these results provide genetic evidence that the four types of breast cancer should be approached as separate diseases to determine optimal treatment.
The two luminal types of breast cancer arise from the cells lining the milk ducts and which express receptors for the oestrogen hormone. Luminal breast cancers are currently all treated in the same manner. The results of this study shows that the two types should be treated differently.
HER2-enriched cancers feature an over-abundance of a growth factor protein known as the EGF receptor. This type of cancer can be treated with Herceptin, a monoclonal antibody that interferes with the receptor to prevent the uncontrolled growth of tumour cells. Interestingly, the fourth type of breast cancer, basal-like, was found to be more similar to ovarian cancer than the other breast cancer types. This breakthrough raises the possibility of using ovarian cancer treatments to tackle basal-like breast cancer.
One promising candidate, a new type of drug treatment for ovarian cancer known as PARP inhibitors, is well tolerated in general with relatively few side effects. In comparison, anthracyclines, a drug treatment currently used for breast cancer, have been associated with heart damage and leukaemia. As you can imagine, the prospect of taking a drug that could be just as efficacious but with fewer side effects is very appealing.
As explained by Dr Matthew Ellis, chair of oncology at Washington University and co-leader of the team: "With this study, we're one giant step closer to understanding the genetic origins of the four major subtypes of breast cancer. Now, we can investigate which drugs work best for patients based on the genetic profiles of their tumours."
Clearly, breast cancer comes in many shapes and sizes and there is no single magic-bullet cure that can be developed. In fact, it is more helpful to think of breast cancer as a family of different diseases with their own characteristics.
David Tan is a research scientist at the Institute of Medical Biology in Singapore. He holds a PhD in stem cell biology from the University of Cambridge