20 AACR CANCER PROGRESS REPORT 2017
exposures or lifestyle factors (see sidebar on Sources of
Genetic Mutations, p. 19).
Not all mutations acquired by a cell lead to cancer. In
fact, the identity, order, and speed at which a cell acquires
mutations determine whether a cancer will develop and, if
a cancer does develop, the length of time it takes to happen.
The progressive nature of cancer provides distinct sites for
medical intervention to prevent cancer, detect it early, or
treat progressive disease. In general, the further a cancer
has progressed, the harder it is to stop the chain of events
that leads to the emergence of metastatic disease, which
is the cause of most deaths from solid tumors.
In addition to genetic mutations, changes in the physical
structure of DNA caused by modification of the DNA
and the proteins associated with it, termed epigenetic
modifications, are frequently detected in cancer cells
(see sidebar on Genetic and Epigenetic Control of Cell
Function). Epigenetic modifications regulate how and
when our genes are turned “on” or “off” and can be made
by specialized proteins that “add” or “erase” unique
chemical modifications on DNA and/or histones ( 26).
In contrast to genetic mutations, epigenetic changes are
often reversible, providing an attractive opportunity for
therapeutic intervention. Our understanding of the role
of epigenetics in cancer is, however, still incomplete, and
continued research is needed to reveal the real therapeutic
potential of the cancer epigenome.
Cancer is primarily caused by the disruption of normal
cellular functions through genetic and epigenetic changes.
The genetic material of a cell comprises strings of four deoxyribonucleic acid (DNA)
units called bases.
DNA bases are organized into genes. The order, or sequence, of the bases provides the
code used by the cell to produce the various proteins it needs to function.
The entirety of a person’s DNA is called the genome. Almost every cell in the body contains
a copy of the genome. The genome is packaged together with proteins known as histones
into structures called chromosomes.
Special chemical marks, called epigenetic marks, on the DNA and histones together
determine whether a gene is accessible for reading. The sum of these chemical marks
across the entire genome is called the epigenome.
The accessible genes within each cell are read to
produce the proteins that ultimately define the function
of the cell and the tissue in which the cell resides.
Genetic and Epigenetic Control of Cell Function
Adapted from ( 1)