Cancer,
by the way, is a very misunderstood entity, a
disease that is many diseases. It is, in fact,
fairly valid to state that each cancer is unique
in its own way. I’m not just saying one type
of cancer is different from another– this is
undeniably true– but also, a given cancer is
different when it occurs in a different
individual. Even the same cancer, arising at
different times within the same individual, can
behave quite differently each time.
Ultimately, a cancer can trace its
history back to one, single, errant cell.
Each and every cell of our bodies is under strict
instructions regarding whether to grow and
divide, and whether to stay put or to migrate to
another part of the body. Mostly, these
instructions arise from the genetic instructions
(DNA) within the cell itself, but cells also
receive these instructions from neighboring
cells. They even receive instructions from cells
that are far away, via chemical messengers known
as hormones. For the most part, your cells are
good little "team players" that follow
the rules and never step out of line.
Unfortunately, on occasion, a cell will rebel. It
may divide when it is not supposed to, or it may
inappropriately set off for distant parts.
But this is a very anthropomorphic
explanation; cells don’t have minds of their
own, all they have are a set of
instructions– their DNA. In order for a cell
to "disobey orders", the orders
themselves must change. In other words, the DNA
must change. This is known as a mutation. Since
there are many separate fail safe mechanisms that
tend to keep a cell in line, it takes several
mutations for an ordinary cell to become a cell
that can (potentially) murder you.
This theory (which is very firmly rooted in
solid experimental evidence) goes by a variety of
names, one of which is the multistep theory
of carcinogenesis. Carcinogenesis means
"the origin of cancer." To switch
metaphors: to become a lethal disease, a cell
must take several steps down a pathway which
leads gradually to cancer.
Here’s an example: a cell’s DNA is
mutated so that the cell has a growth advantage
relative to its neighbors. The cell divides, and
the daughter cells inherit this growth advantage.
Before long, there are a larger number of cells
that all descended from this one cell. Next step:
one of these cells changes (again due to a DNA
mutation) so that its DNA repair mechanisms are
crippled. In other words, cells usually have a
form of surveillance for DNA damage, and
self-repair is standard operating practice. This
new cell thus has a growth advantage over its
neighbors (which it inherited from the very first
errant cell) and it has sloppy DNA
repair, and is thus much more likely to develop
further DNA mutations.
This nasty cell divides, its daughters
divide, and so forth. Before long, there is a
group of cells that can trace their lineage back
to this original "two mutation" cell.
These cells, of course, are not an identical
population– remember, they all harbor a
mutation that makes their DNA "error
prone." Many of these mutations are
irrelevant or "neutral," many more are
detrimental, but on occasion, one mutation may
occur in just one of these cells which gives this
"step 3" cell a new property. What kind
of property? Perhaps the ability to invade
adjacent tissues. Perhaps the ability to become
"unstuck" from its neighbors and float
away in the blood stream. Perhaps the ability to
stimulate new blood vessel formation, bringing
more oxygen and nutrients into the heart of
tumor... for, indeed, by this time our group of
nasty cells warrants the name "tumor."
Tumors, as you know, are classified as benign
and malignant. What’s the difference? A
benign tumor is usually one that grows locally,
sometimes to an enormous size, and may cause
problems by pressing against adjacent structures.
A benign tumor usually does not invade adjacent
tissues, nor does it spread to distant parts of
the body. A malignant tumor CAN invade adjacent
tissues, and it may also spread to distant parts
of the body (metastasis). Malignant
tumors are more likely to be lethal than benign
tumors, but size and location are critical. After
all, a benign brain tumor is still potentially
quite lethal, simply because of its location.
One last point: cancer cells, for all their
"macho" properties, are sick cells.
While some mutations may confer an advantage to a
group of cells, there is usually a price to pay.
Consider the second mutation in the example
above. Having an error-prone DNA repair system is
a BAD thing. These cells do not function as
efficiently as cells with undamaged DNA. This is,
in fact, the Achilles’ heel of cancer cells.
Radiation therapy and chemotherapy injure
cells– ALL cells– but normal
(noncancerous) cells can recover from the injury,
while cancer cells cannot. If it were not for
this intrinsic weakness of cancer cells,
radiation therapy and chemotherapy would never
work.
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