One might also argue that evolution itself has played some tricks on us because some of the
properties selected for may themselves be processes that cancer cells use to become invasive
and metastatic. Or to phrase it differently: Is cancer an inevitable result of a complex evolutionary
process that has advantages and disadvantages? Some of these processes might be the
following:
1. The mechanism of cell invasiveness that allows the implantation of the early embryo
into the uterine wall and the development of a placenta.
2. Cell motility that allows neural cells, for example, to migrate from the original neural
crest to form the nervous system.
3. The development of a large, complex genome of up to 40,000 genes that must be
replicated perfectly every time a cell divides.
4. The large number of cells in a human or higher mammal that must replicate and
differentiate nearly perfectly every time (some can be destroyed if they become abnormal).
5. The long life span of humans and higher mammals, increasing the chance for a
genetic ‘‘hit’’ to occur and lead a cell down a malignant path.
As we shall see in later chapters of this book, cancer cells take advantage of a number of these
events and processes. Other questions that arose at the gathering above from scientists not in the field of cancer were the following:
1. Is there a single trait or traits that all cancer cells have?
2. How many genetic ‘‘hits’’ does it take to make a cancer cell?
3. What kinds of genes are involved in these hits?
These questions are all dealt with in later chapters. Suffice it to say here that for a cell to
become cancerous or at least take the first steps to becoming cancerous, at least two genetic hits are required. One may be inherited and another accrued after birth or both may be accrued after birth (so-called somatic, or spontaneous, hits). The kinds of genes involved are oncogenes, which when activated lead to dysregulated cell proliferation, and tumor suppressor genes, which become inactivated or deleted, producing a loss of the cell’s checks and balances controlling cell proliferation and differentiation. The single most common, if not universal, trait that occurs in all cancers is genetic drift. Or the ability of cells to lose the stringent requirement for preciseDNA replication and to acquire the ability to undergo sequential progressive changes in their genome, through mutations, gene rearrangement, or gene deletion. This has sometimes been called the acquisition of a ‘‘mutator phenotype.’’
