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Cancer is a creepy and mysterious thing.
In the process of trying to understand it, to get better at killing
it, we discovered a biological paradox
that remains unsolved to this day.
Large animals seem to be immune to cancer.
Which doesn't make any sense. The bigger a being,
the more cancer it should have.
To understand why, we first need to take a look at
the nature of cancer itself.
[Kurzgesagt Theme]
Our cells are protein robots made out
of hundreds of millions of parts.
Guided only by chemical reactions, they create
and dismantle structures, sustain a
metabolism to gain energy, or make almost
perfect copies of themselves.
We call these complex chemical reactions pathways.
They are biochemical networks
upon networks, intertwined and stacked
on top of each other.
Most of them can barely be comprehended by a
single human mind, and yet they function perfectly.
Until... they don't.
With billions
of trillions of reactions, happening in thousands of networks
over many years, the question is not if
something will go wrong, but when.
Tiny mistakes add up, until the grandiose
machinery gets corrupted. To prevent this from
getting out of hand, our cells have kill-switches that
make them commit suicide.
But these kill-switches are not infallible.
If they fail, a cell can turn into a cancer cell.
Most of them are slain by the immune system very quickly.
But this is a numbers game. Given enough time, a cell will
accrue enough mistakes, slip by unnoticed, and
begin making more of itself. All
animals have to deal with this problem.
In general, the cells of different animals
are the same size. The cells of a mouse
aren't smaller than yours. It just has fewer cells in total
and a shorter lifespan.
Fewer cells and a short life means a
lower chance of things going wrong, or cells
mutating. Or, at least, it should mean that.
Humans live about fifty times longer, and have one
thousand times more cells than mice. Yet the rate of
cancer is basically the same in humans and
in mice. Even weirder, blue whales,
with about three thousand times more cells than humans
don't seem to get cancer at all, really.
This is Peto's Paradox- the baffling
realisation that large animals have much, much
less cancer than they should.
Scientists think there are two main ways of explaining
the paradox: evolution, and hypertumours.
Solution one: evolve, or become a blob
of cancer.
As multicellular beings developed six hundred million
years ago, animals became bigger, and bigger.
Which added more and more cells, and hence, more
and more chances that cells could be corrupted.
So, the collective had to invest in better and better
cancer defenses. The ones that did not died
out. But cancer doesn't just happen- it's a
process that involves many individual mistakes
and mutations in several specific genes
within the same cell.
These genes are called proto-oncogenes, and when they mutate, it's bad news.
For example, with the right mutation, a cell
would lose its ability to kill itself.
Another mutation, and it will develop the ability to hide.
Another, and it will send out calls for resources.
Another one, and it will multiply quickly.
These oncogenes have an antagonist, though.
Tumour suppressor genes.
They prevent these critical mutations from happening
or order the cell to kill itself
if they decide it's beyond repair.
It turns out that large animals have an increased number of them.
Because of this, elephant cells require more mutations
than mice cells to develop a tumour. They are not immune-
but more resilient. This adaption
probably comes with a cost in some form, but researchers
still aren't sure what it is.
Maybe tumour suppressors make elephants age quicker
later in life, or slow down how quickly injuries heal.
We don't know yet.
But the solution to the paradox may actually be
something different: hypertumours.
Solution two:
Hypertumours. Yes, really.
Hypertumours are named after hyperparasites: the
parasites of parasites. Hypertumours
are the tumours of tumours.
Cancer can be thought of as a breakdown in cooperation.
Normally, cells work together to form structures like
organs, tissue, or elements of the immune system.
But cancer cells are selfish, and only work for
their own short-term benefit. If they're successful,
they form tumours- huge cancer collectives that
can be very hard to kill.
Making a tumour is hard work, though.
Millions or billions of cancer cells multiply rapidly,
which requires a lot of resources and energy.
The amount of nutrients they can steal from the body
becomes the limiting factor for growth.
So the tumour cells trick the body to build new blood
vessels directly to the tumour, to feed
the thing killing it.
And here, the nature of cancer cells may become
their own undoing.
Cancer cells are inherently unstable, and so
they can continue to mutate- some of them faster
than their buddies. If they do this for a while,
at some point, one of the copies of the copies of the original cancer cell
might suddenly think of itself as an individual again, and
stop cooperating. Which means, just like the body,
the original tumour suddenly becomes an enemy,
fighting for the same scarce nutrients and resources.
So, the newly mutated cells can create a hypertumour.
Instead of helping, they cut off the blood supply to
their former buddies, which will starve and
kill the original cancer cells.
Cancer is killing cancer.
This process can repeat over and over,
and this may prevent cancer from becoming a problem
for a large organism. It is possible
that large have more of these hypertumours than we
realise. They might just not become big enough to notice.
Which makes sense. A two-gram tumour
is 10% of a mouse's body weight, while it's less than
0.002% of a human.
And 0.000002%
of a blue whale.
All three tumours require the same number of cell divisions,
and have the same number of cells. So an old blue whale
might be filled with tiny cancers, and just not care.
There are other proposed solutions to Peto's Paradox,
such as different metabolic rates, or different cellular
architecture. But right now, we just don't know.
Scientists are working on the problem.
Figuring out how large animals are so resilient
to one of the most deadly diseases we know could open the path
to new therapies and treatments.
Cancer has always been a challenge. Today,
we are finally beginning to understand it,
and by doing so, one day, we might finally
overcome it.
This video was sponsored by... you!
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and because you care.
Thank you for watching.
[End Credits]


なぜシロナガスクジラは癌にかからないのか - ピートのパラドックス (Why Blue Whales Don't Get Cancer - Peto's Paradox)

107 タグ追加 保存
Peggie Fang 2020 年 3 月 12 日 に公開
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