The Cancer Genome Project is an attempt to bring
the human genome to the understanding of how cancers
develop.
All our bodies are composed of cells.
The human body has about 100 million million cells within it,
and usually those cells behave in a very
ordered fashion.
They observe certain rules, they
divide when they're told to divide, they're
quiescent when they're told to remain quiescent, they stay in a particular
position within their tissue,
they don't move out of that. Occasionally, however,
a single cell of those 100 million million cells
decides to behave in a very different way.
That cell keeps on dividing
when all its signals around it tell it to stop dividing.
That cell ignores
its colleagues around it and pushes them out of the way.
That cell stops
observing the rules of the tissue within which it is located
and begins to move out of its normal position,
invading into the tissues around it and sometimes
entering the bloodstream and becoming a metastasis,
depositing in another tissue of the body.
The reason that the cell
behaves in this rogue fashion, is because it has acquired
within its genome, within its DNA,
a number of abnormalities that cause it to behave as a cancer cell.
Of course all these 100 million million cells in the human body
have got a copy of the human genome, they've got two copies,
a maternal and paternal copy. And throughout life
all those copies of the genome,
in those 100 million million cells, are acquiring
abnormalities, so-called somatic mutations.
These are mutations that are just present in the cell, they're not
transmitted from parents to offspring, they're just constrained to that individual
cell
and its progeny.
And those mutations occur in every cell of the body, normal
and abnormal, for a number of different reasons. They occur
because although we have an extraordinary mechanism
for replicating DNA, for those three thousand million letters of code
to be replicated each time a cell divides, hardly any mistakes are made,
despite that extraordinary mechanism, the occasional mistake is made.
Basically every time a cell divides
possibly one letter of code out of the three thousand million is replicated
wrongly.
So that's one source of mistakes one source of somatic mutations.
However in addition to that, all these
100 million million cells in the body are being exposed to
a number of different onslaughts. There is radiation,
of various sorts, which is hitting us the whole time, natural radiation;
we are a bag of chemicals, chemicals that we generate ourselves,
but chemicals that we also ingest or are exposed to,
and those also can bind
to DNA and can cause mistakes
in replication of DNA to be more common.
And finally, we have this mechanism, these
processes that allow DNA to be replicated
in this highly faithful way, this high fidelity mode,
but occasionally those mechanisms in a particular cell may breakdown
and then the DNA of that cell begins to acquire
mutations, somatic mutations rather more commonly
then other cells.
So, you put all that together, every cell in the body
is acquiring mutations
throughout a lifetime, and as we get older we acquire more and more of them.
As it turns out,
most of those mutations are
completely innocuous, they make no difference at all. They make no difference to
the cell itself,
and certainly they make no difference to us as
multicellular organisms. They're basically silent, they sit there,
we live with them.
Occasionally, however, a mutation takes place
in a particular type of gene, which
in an operative way we now call a cancer gene. What that mutation
does
it changes that particular gene, and of course the protein that it makes,
such that now that abnormal protein
is driving the cell to behave
in that unusual rogue fashion
that we call a cancer.