HOW DOES CANCER DEVELOP?
In order to understand how cancer develops, it is necessary to include a little background on basic cell biology. The cell is the basic building block that makes up all living things. The human body, in common with all animals from the smallest such as yeast to the largest blue whale, is composed of cells. Some animals – yeast, for example – are made of single cells; others, we are included, are made of many different sorts of cells – blood, bone, brain, kidney, and so on. All cells in an organism have their own carefully controlled life cycle. Cancer occurs when the control of this cycle goes wrong, leading to the unregulated growth of a group of cells which can spread and damage other structures in the body. Here it will be focused on how cancer develops and also on some of the underlying biology needed to understand this. I will also illustrate how an understanding of the causes can be used to define treatment strategies.
The key component of the cell for understanding cancer is the nucleus, which holds the DNA that contains the genetic code. Figure 1 shows a diagram of a DNA molecule. Cancer is caused fundamentally by damage to the DNA leading to abnormal, unregulated growth of cells.
Remarkably, although different cells may differ markedly in their appearance and function (for example, nerve cells, muscle cells, and blood cells), all the cells in a given organism share the same DNA code. DNA is clustered into long strands called chromosomes. There are 23 pairs in each human cell. Within each chromosome, the DNA is arranged in genes, each one coding for a single protein. We can think about genes and chromosomes as being like a library of books, with each of the 23 chromosomes an individual volume and each of the 21,000 genes a page of instructions in that volume. It is easy to see conceptually how damage to a page of instructions can lead to alterations in the properties of a cell. Here it will be run through how these different structures work and interact, and how they can go wrong to lead to the development of cancer.
Everyone starts life as a single fertilized egg that develops first into a ball of identical cells and then, progressively, grows, organizes, and develops into a complete complex individual. The process by which cells develop from this initial group into highly specialized subtypes is one of the most incredibly complex processes in nature and yet is happening constantly all around us and within us. This clearly requires an intricate network of checks and balances. It requires that cells communicate with their neighbours to ensure that the right development path is followed at the right time. It requires that cells no longer required are deleted and eliminated with the minimum of disruption (a process called apoptosis, from the Greek word meaning ‘a falling off of petals’). As organs develop, they must grow their own blood supplies and maintain them in response to damage. It requires that organ systems communicate with each other, for example, nerves connecting with the muscles they control. Endocrine (hormone) glands are coordinated to produce their products in cycles (for example, the ovaries) or in response to stress (the adrenal glands). This is achieved by genes being switched on and off in a coordinated fashion as individual organ systems grow and develop. Once the growth process is complete and the animal is formed, tissues must be maintained, damage repaired, and general housekeeping kept ticking over – nutrients supplied and processed, waste products eliminated, and so on. The more one thinks about the mind-blowing complexity of all these tasks, the remarkable thing is that the processes run so reliably for so many years in most people and that cancer – essentially, unregulated cell division – does not occur more frequently than it does.