Cancer is a disease that can involve any tissue in the human
body. Some cancers, of course, are more
common than others such as lung cancer, colon cancer, breast cancer in women
and prostate cancer in men. What all
cancers share in common is that the tissue cell involved undergoes a
transformation that imparts a selective advantage to that cell allowing it
divide outside the normal controls imposed upon that particular cell type. This process is known as clonal expansion.
It is now clearly understood that this selective advantage
is a direct result of somatic mutation of the affected cell’s genetic material
– DNA.
This causal relationship was first suspected in 1914 when it was
observed that chromosomal abnormalities were present in cancer cells. Note that this discovery was made even before
DNA was known to be substance responsible for carrying hereditary
information. Considerably later, it was
found that introducing DNA fragments from a malignant cell into a healthy one
led to transformation of the recipient cell into a cancerous one. Soon after this finding, oncogenes were
discovered. Oncogenes are the result of
mutations in a class of genes responsible for cell growth or its modulation as
is the case with tumor suppressor genes.
In nature, somatic mutations are common and are harmless for
the most part – they do not generally impact genes that are involved in
cellular process that would impart a selective advantage to the cell type
affected. These innocuous changes are
referred to as “passenger mutations.”
Those rare mutations that do provide a selective advantage to the
affected cell are referred to as “driver mutations.” Ii is this type of mutation chat can lead to
cancer – the production of a clone of cells that has the capacity to function
autonomously and thereby no longer constrained by the controls normally imposed
upon cells of a particular tissue.
Although this evolution of a cell from a normal state to one
that is cancerous has been understood in general terms, the underlying process
has not been amenable to discovery until the advent of high-throughput DNA
sequencing.
Utilizing this technology, the cancer-associated genes from
over 10,000 human subjects have been sequenced along with over 2500 complete genomes
from cancerous tissue samples. Dr. Inigo
Marincorena and associate from the Wellcome Trust Sanger Institute in
Cambridge, UK have reviewed some of the result obtained from this data.
From this extensive study, new cancer genes have been
discovered as well as new kinds of mutational events. Mutations occur as a result of a number of
possible factors. These include:
- DNA replication errors
- DNA damage that is subsequently incorrectly repaired by the normal DNA repair machinery or damage that is left unrepaired
- DNA damage that is a result of exogenous factors such as mutagens, ultraviolet light and ionizing radiation
- DNA damage that is a result of endogenous factors such as reactive oxygen species (free radicals), aldehydes or mitotic errors
- Viruses and endogenous retrotransposons that lead to insertions into the genome.
It seems that the frequency of mutations found in various
cancerous cell types vary depending upon the tissue involved. For example, pediatric brain tumors and various
types of leukemia have the lowest numbers of mutations whereas lung and skin
cancers show the highest rates.
Furthermore these mutation rates can be greatly enhance if there is
definitive loss of DNA repair pathways.
One of the conclusions drawn from the extensive genomic data
derived from these studies is that more than one driver mutation is required in
order for a clone originating from a transformed cell to successfully evolve
into the fully cancerous state with the capability of metastasis.
This report represents a brief and limited overview of the
extensive of this work and its significance.
It is fair to say, that future studies of this magnitude will shed
greater light on the complex processes that are involved in cellular transformation
and oncogenesis.
