Chapter 19: The Genetics of Cancer

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Welcome to The Deep Dive.

Today we're tackling a really big topic, cancer.

It impacts so many people.

It's the second leading cause of death in Western countries.

And, you know, the odds are something like one in three people will get a diagnosis eventually.

It's huge.

So our mission today is to really get into the weeds, you know, go beyond just knowing about cancer and try to unravel its genetic secrets.

We want to understand the specific molecular nuts and bolts that make this disease tick.

What's the core idea here?

Yeah, that's a good way to frame it because cancer looks so different, right?

Leukemia, breast cancer, colon cancer, they seem completely distinct.

But actually, when you drill down to the molecular level, there's a surprising amount of unity.

The really fundamental thing we've learned is that cancer is at its heart a genetic disease.

And crucially, most of the time, it's not something you inherit.

It mostly starts in our regular body cells, our somatic cells during life.

Okay, somatic cells, that's key.

So not necessarily passed down.

We're going to break down how like tiny changes in genes or even big changes in chromosomes and these things called epigenetic changes, changes in how genes are read.

And of course, environmental factors, how all this stuff comes together.

It's like a big complicated puzzle.

It's trying to unpack it.

Exactly.

At that molecular level, cancer is all about abnormal gene products, proteins that shouldn't be there or are there in the wrong amounts.

And these come from genes that are either mutated or just expressed weirdly.

All these faulty signals basically tell the cell to grow uncontrollably and spread where it shouldn't.

And like you said, maybe only five, 10 % of cancers involve inherited mutations.

Most happen spontaneously in somatic cells.

Right.

So no matter the type of cancer, if you look really closely at a cancer cell, there are two basic problems, right?

One, they just grow and divide like crazy, proliferation, no breaks.

And two, they lose the normal restraints.

They can spread, metastasize.

That's it.

Those are the two fundamental properties,

abnormal growth and the ability to invade and If a cell just loses growth control, you might get a benign tumor.

It's a lump.

It grows, but it stays put, often removable.

Okay.

So benign means it's contained.

Right.

But malignancy, that's when those cells gain the ability to metastasize, to break away, travel, start new tumors elsewhere.

That's what makes cancer so dangerous.

And getting from benign to malignant isn't just one step.

It takes several more genetic kits, multiple mutations.

That leads to something really fascinating.

This idea of clonal origin.

You can have a huge tumor, billions of cells spread all over.

But get this,

every single one of those cancer cells in the main tumor and any secondary ones, they all came from one original ancestor cell.

It's pretty amazing, isn't it?

They're all descendants of that single cell that first picked up those key cancer causing mutations.

And we have solid proof.

Like in Burkitt lymphoma, you can look at the chromosomes.

All the cancer cells in one patient showed the exact same unique break and rejoin points between, say, chromosome eight and chromosome 14.

Identical.

That proves they're all related.

Wow.

Okay.

So one ancestor cell.

And there's also this related idea, the cancer stem cell hypothesis.

It suggests maybe not all cells in a tumor can keep dividing forever and start new tumors.

Maybe it's just a small group, these cancer stem cells that have that self -renewal power, like the queen bee in a hive, maybe.

Interesting.

So a hierarchy within the tumor.

Okay.

This brings us to the next really big point.

Here's where it gets really interesting.

It's not just one mutation.

Cancer is a multi -step process.

Absolutely critical.

A single mutation just isn't enough to turn a normal cell cancerous.

Think about it.

We get mutations all the time.

Our cells divide billions of times.

If one hit was enough, we'd all have cancer constantly.

Right.

The statistics wouldn't work.

And it explains why cancer risk goes up so much with age, doesn't it?

It takes time to accumulate those multiple hits.

Exactly.

Age is a huge factor.

And also, think about carcinogen exposure.

After Hiroshima and Nagasaki, leukemia rates went up, but not immediately.

There was a lag, like five to eight years.

That delay suggests it took time for enough mutations to build up.

Okay.

Multiple steps, multiple mutations.

Are all mutations equal in this process?

Good question.

No, they're not.

Scientists talk about driver versus passenger mutations.

Driver mutations are the key ones.

Maybe only two to eight per tumor.

They give the real growth advantage.

They're pushing the cancer forward.

Okay.

The main culprits.

Right.

Then you have passenger mutations.

There can be tons of these.

They happen because the cell's DNA repair might be sloppy.

We'll get to that.

Initially, they don't really do anything for the cancer.

They're just along for the ride.

But sometimes the passenger mutation can become useful later.

Maybe it helps the cell resist chemotherapy.

Then it suddenly becomes a driver for that resistant subclone.

Ah, okay.

So the context matters.

Let's make this concrete.

Can we walk through an example like colorectal cancer?

Perfect example.

It often follows a pretty clear path.

You start with normal colon cells.

Step one might be losing a tumor suppressor gene called APC that causes a small benign polyp and adenoma.

Okay.

First hit, loss of APC.

What's next?

Next in one of those polyp cells, maybe there's a mutation in a gene called a KRAS.

KRAS is involved in growth signaling.

The mutation basically jams the signal in the on position, uncontrolled growth, so that small polyp grows into a bigger intermediate adenoma.

Right.

Second hit, KRAS mutation.

Still technically benign.

Mostly, yeah.

But then to get to a full -blown malignant cancer, a carcinoma, you need more hits.

Often involves losing function in other key genes like TP53.

We'll talk lots more about that one and others involved in cell cycle control, cell death.

And this whole process from normal cell time of lignancy, it can take like 40 years or even more.

40 years.

That really drives home the multi -step idea.

It's a long, slow accumulation.

So these accumulating mutations,

is there something fundamentally wrong with how cancer cells handle their DNA?

Yes, absolutely.

That's another core concept.

Genomic instability, sometimes called mutator phenotype.

It means the cancer cells have fundamental defects in their DNA repair systems.

They're just sloppy.

This jacks up the overall mutation rate across the whole genome.

So they make more mistakes and they're worse at fixing them.

Exactly.

And you see the results.

Lots of point mutations, chromosomes breaking and rejoining incorrectly, cells ending up with the wrong number of chromosomes.

It's a mess.

A classic example is the Philadelphia chromosome in chronic myelogenous leukemia, CML.

It's a specific swap between chromosome 9 and 22, creates a faulty fusion protein, BCRABL, that constantly tells the cell to divide, dries the leukemia.

Okay, so that instability fuels the fire, creating more mutations.

And you mentioned faulty DNA repair.

Are there inherited conditions that show this?

Yes, perfect examples.

Xeroderma pigmentosum or XP.

People with XP inherit defects in a specific DNA repair pathway called nucleotide excision repair.

This pathway normally fixes damage caused by UV light.

So XP patients are incredibly sensitive to sunlight, get severe burns and have a hugely increased risk of skin cancer, often very young.

Wow.

So the repair kit for sun damage is broken.

Pretty much.

Another one is HNPCC, hereditary non -polyposis colorectal cancer, also called Lynch syndrome.

Here, the inherited defect is in mismatched repair genes, the system that proofreads DNA after replication.

When that's faulty, mutations pile up really fast, especially in repetitive DNA sequences leading to colon and other cancers.

These conditions really highlight how vital DNA repair is.

Okay, so we have DNA sequence changes, mutations, we have genomic instability.

What about those epigenetic changes you mentioned earlier?

Right, epigenetics.

This is fascinating.

It's about changes in how genes are used or expressed, but without changing the actual DNA code itself.

Think of it like sticky notes on the DNA or how tightly the DNA is packed.

These two main types are DNA methylation chemical tags on the DNA and histone modifications changes to the proteins DNA wraps around.

And how does this go wrong in cancer?

Well, cancer cells have really messed up epigenetic patterns.

Often the overall genome has less methylation than normal, which can turn on genes that should be off.

But paradoxically, the promoter regions of specific genes, especially tumor suppressor genes, often become hypermethylated.

They get silenced when they should be active.

Histone modifications are all out of rack too.

It affects gene expression across the board.

The really interesting part, epigenetic changes are potentially reversible in ways that DNA mutations aren't.

So it's a hot area for new drug development.

Reversible.

Yeah, that's hopeful.

Okay, let's shift gears slightly.

Another fundamental problem in all cancers is losing control of cell division,

right?

Proliferation.

Absolutely central.

Normal cells follow a strict cycle.

Grow, copy DNA, prepare, divide.

G1, S, G2, M phases.

Many specialized cells, like our nerve cells, exit the cycle and enter a quiet state called G0.

They're alive, working, but not dividing.

Cancer cells.

They often can't enter G0.

They're stuck in the cycle dividing relentlessly, maybe not faster, but they just don't stop when they should.

How do normal cells know when to stop or pause?

Through critical checkpoints.

Think of them like security gates in the cell cycle.

There's one before DNA copying, G1S, one before division, G2M, and one during division M.

They check, is the cell big enough?

Is the DNA damaged?

Are the chromosomes lined up properly?

If not, the cell cycle halts until things are fixed.

So who manages these checkpoints?

Key proteins called cyclins and cyclin -dependent kinases, CDKs.

Cyclins build up and disappear at specific points in the cycle.

When a cyclin is present, it partners with the CDK.

That cyclin -CDK pair then activates other proteins to push the cell past a checkpoint.

It's a very precise timing mechanism.

And in cancer,

this timing gets screwed up.

Exactly.

Mutations in the genes controlling checkpoints or in the cyclin or CDK genes themselves can break this regulation.

The cell might ignore DNA damage or divide before it's ready.

This leads directly to uncontrolled proliferation and more genomic instability.

So they divide recklessly.

What about dying?

Don't cells have a self -destruct mechanism for when things go really wrong?

They do.

It's called apoptosis or programmed cell death.

It's a crucial safety net.

If a cell suffers massive DNA damage or gets infected or is just not needed anymore, it can trigger its own clean destruction.

Like a cellular suicide program.

Pretty much.

And guess what?

Cancer cells are masters at evading apoptosis.

The genes that control this process are often mutated or silenced in cancer.

So even a cell with catastrophic DNA damage, which should self -destruct, survives and divides and passes on those errors.

It's like disabling the emergency break and the ejector seat.

Wow.

Okay.

So we've mentioned genes like APC, KRAS, checkpoint genes.

Can we categorize these cancer -related genes?

Yeah.

Broadly speaking, they fall into two main camps.

First, proto -oncuds.

These are normal genes usually involved in promoting cell growth and division in a controlled way, like the accelerator pedal.

But when they get mutated or overexpressed, they become oncogenes, cancer genes.

It's a gain of function.

They get stuck in the on state.

Only one copy needs to be altered.

Okay.

Proto -oncogenes become oncogenes.

Accelerator stuck on what's the other camp?

Tumor suppressor genes.

These are the breaks.

Their normal job is to restrain cell growth, monitor DNA, trigger repair, or initiate apoptosis if needed.

When these genes are mutated or lost, the cell loses that crucial control.

It's a loss of function.

And because you usually have two copies of each gene, you typically need to lose or inactivate both copies of a tumor suppressor gene to fully lose its function.

It's recessive at the cellular level.

Gain of function oncogenes, loss of function tumor suppressors.

Got it.

Can we look at examples?

You mentioned KRAS.

Is that a proto -oncogenes?

Yes.

KRAS is part of the RAS gene family.

These are classic proto -oncogenes.

They make proteins involved in signal pathways that tell the cell to grow.

Mutated RAS genes are found in over 30 % of all human cancers.

The mutant RAS protein is basically permanently switched on, constantly signaling divide, divide, divide.

30%.

Okay, that's huge.

What about a major tumor suppressor?

You mentioned TP53.

Ah, TP53.

The big one.

It encodes the P53 protein, mutated in over half of all human cancers.

P53 is often called the guardian of the genome, and for good reason.

Guardian of the genome.

What does it guard against?

Damage.

Normally P53 protein levels are kept low, but if the cell senses stress, DNA damage is a big one.

P53 levels shoot up.

Activated P53 can do two main things.

It can pause the cell cycle, often by turning on another protein called P21, giving the cell time to repair the damage.

Or if the damage is just too bad, P53 triggers apoptosis.

It tells the cell to self -destruct.

It regulates like over 50 different genes to do this.

So it's a master and in cancer cells with mutated TP53.

That guardian is gone.

The cell doesn't pause the cycle when its DNA is damaged.

It doesn't trigger apoptosis when it should.

Damaged cells just keep dividing, accumulating more and more mutations.

That's why losing P53 often leads to very aggressive cancers with high mutation rates.

It also helps explain things like Lefraud -Menni syndrome.

That's a rare inherited condition where people inherit one faulty copy of TP53.

Because P53 is so central, losing it predisposes them to a whole range of different cancers.

Breast, bone, brain tumors, leukemias, often at young ages.

One broken guardian leaves many gates unguarded.

That makes sense.

Okay.

We've covered your messed up genes, bad repair, uncontrolled division, avoiding death.

What about spreading?

Metastasis?

How do they manage that?

Metastasis is complex, but it involves acquiring new dangerous abilities.

The cancer cells have to detach from the main tumor first, then they need to invade nearby tissues.

They often do this by ramping up production of enzymes, like metalloproteinases, that literally digest the surrounding matrix, the scaffolding between cells.

They chew their way out.

Sort of, yeah.

They also often lose or alter cell adhesion molecules at the things that normally make cells stick together.

E.

cadherin is a famous one.

Lower levels make it easier to break loose.

Then they have to get into the bloodstream or lymphatic system, survive the journey, get out some more else, and start growing again.

It's a tough obstacle course, actually.

Only a few cells succeed.

But enough succeed to make it incredibly dangerous.

Now, you said most cancer isn't inherited, but 5 -10 % is.

How does that usually work?

It often involves inheriting one faulty copy of a crucial tumor suppressor gene, like BRCA1 or APC.

You start life with one good copy and one bad copy in every cell.

That one good copy is usually enough for a while.

But if, in a particular cell, that remaining good copy gets mutated or lost, maybe through a random error or chromosome loss, then that cell has no functional copies left.

That's called loss of heterozygosity, or LOH.

And that cell has now lost its breaks and can start down the path to cancer.

Ah, so you inherit the first hit, making the second hit much more likely to cause trouble.

You mentioned APC that links to FAP, familial adenomatous polyposis, right?

Where people get loads of colon polyps.

Exactly.

They inherit one bad APC gene.

That makes polyp formation almost inevitable, often hundreds or thousands of them.

Each polyp is then a potential starting point for the later mutations, like KRAS and TP53, that lead to colon cancer.

And BRCA1 and BRCA2, linked to breast and ovarian cancer.

Yes.

Inheriting a faulty BRCA1 or BRCA2 gene significantly increases the lifetime risk of breast, ovarian, and some other cancers.

They're involved in DNA repair, specifically fixing double strand breaks.

Which brings up genetic testing.

If someone finds out they have a BRCA mutation, what does that mean for you?

It sounds incredibly difficult.

It is incredibly difficult.

A positive test presents agonizing choices.

Maybe prophylactic surgery, like removing breasts or ovaries to drastically cut risk.

Or taking preventative drugs, which have side effects.

There's the psychological burden, fear, anxiety, and it affects the whole family.

Do you tell relatives?

Who needs testing?

Privacy is a huge concern.

And tests aren't perfect.

False negatives or positives can happen.

A negative test doesn't eliminate risk entirely.

It's heavy stuff.

Definitely complex ethical and personal territory.

Yeah.

Okay, shifting from inherited risk to environmental factors.

What are carcinogens exactly?

Broadly, a carcinogen is anything in the environment.

A chemical, radiation, even some viruses that can damage DNA and lead to mutations in those key cancer genes, the proto -oncogenes, and tumor suppressors.

And these aren't just like industrial chemicals, right?

Some are natural.

Absolutely.

Aflatoxin, for example, is produced by mold on crops like peanuts and corn.

It's one of the most potent liver carcinogens known.

UV radiation from sunlight is a major cause of skin cancer.

Ionizing radiation x -rays, gamma rays, radon gas seeping into homes can damage DNA.

Even normal metabolism creates reactive oxygen species that damage DNA constantly.

Our cells repair most of it, but some slip through.

Chronic inflammation can also promote cancer.

Okay, so natural sources are significant.

What about human -made stuff?

Pollution, diet.

Yes, those contribute too.

Air pollution is linked to a significant chunk of lung cancers, maybe 15%.

Industrial chemicals are a concern, though estimating risk from low doses is hard.

Diet definitely plays a role.

High intake of red meat, animal fat, obesity itself, heavy alcohol use, these are all linked to increased risk for various cancers.

But if we're talking environmental carcinogens, we have to talk about tobacco smoke.

It's just

carcinogenic linked to at least 17 types of cancer responsible for the vast majority of lung cancer deaths.

What's in smoke that's so bad?

It's a toxic cocktail.

Over 4 ,000 chemicals identified, and at least 60 are known carcinogens.

Things like benzene, arsenic, formaldehyde.

The mutagenic load is staggering.

Someone smoking a pack a day might be accumulating over 150 new mutations in their lung cells every year.

Plus, it causes epigenetic changes too.

Wow.

150 mutations a year.

Yeah.

But here's the crucial positive message.

Quitting works.

Within about five years of quitting, the excess risk of many cancers starts to drop significantly.

It's never too late.

That's really important to hear.

Okay, one last category.

Viruses.

Yes, viruses are a major factor globally, causing maybe 12 % of all human cancers.

Second only to tobacco.

Some key examples.

Epstein -Barr virus is linked to Burkitt lymphoma and some other cancers.

Hepatitis B and C viruses are major causes of liver cancer worldwide.

And human papillomaviruses, HPV, are the main cause of cervical cancer plus other cancers.

That's why we have the HPV vaccine.

But the virus itself doesn't just cause cancer immediately, right?

Usually not.

Viral infection is often just one step.

It might disrupt cell cycle control or promote inflammation.

But you typically still need those other accumulating mutations in proto -oncogenes and tumor suppressors to get full -blown cancer.

It feeds into that multi -step process.

Okay.

We've covered so much ground.

From cancer's global impact down to specific genes like TP53 and KRAS.

We've seen it's fundamentally genetic starting from one rogue cell progressing through multiple steps over time, fueled by genomic instability, broken cell cycle controls, faulty apoptosis switches, with oncogenes pressing the accelerator and tumor suppressors losing their brakes, all influenced by inheritance, environment, viruses.

It's incredibly complex.

It really is.

Yet there are these unifying principles underneath all that complexity.

And thinking about all this driver mutations, passenger mutations, those reversible epigenetic changes,

our ability now to sequence tumors in detail,

it really leads to a provocative thought for the future, doesn't it?

How do we move beyond just broad -stroke treatments like chemo?

How can we develop truly precise personalized therapies based on the specific genetic and epigenetic landscape of each individual's cancer?

What are the challenges and the opportunities that opens up for research and for patients?

That's a fantastic question to leave our listeners with.

The move towards truly personalized cancer medicine.

It's happening, but lots more to figure out.

Thank you so much for walking us through all of that.

And thank you for joining us on this deep dive into the genetics of cancer.

We hope it's given you a clearer picture of this complex disease.

Keep learning.

Keep asking questions.

Until next time, stay well informed.