Chapter 25: Genetic Basis of Cancer

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Welcome to the Deep Dive.

We're the show that helps you cut through the noise,

distill really complex info, and bring you the core insights you need.

That's the goal.

And today, we're tackling a big one, a deep dive into the genetic basis of cancer.

Our mission really is to unpack a key chapter from Brooker's Genetics Analysis and Principles.

We want to pull out the essential concepts, you know, the mechanisms, the experiments, the case studies.

Yeah, and make that intricate world of cancer genetics a bit more accessible for you.

Exactly.

And it's, well, it's a fundamentally important topic because when we talk about cancer, it's not just a disease label, it's this fascinating,

sometimes terrifying example of our own genetics and cell processes going, well, going wrong.

Like a battle inside ourselves.

Pretty much a microscopic battleground.

By the end of this, you should have a much clearer view of what's actually happening at that genetic level.

Okay, let's dive in then.

Starting point,

cancer.

It's a disease in multicellular organisms.

And the key thing is uncontrolled cell division.

That's the hallmark.

Cells just dividing without stopping, without regulation.

And the impact is, well, it's huge.

It's the second leading cause of death globally.

That scale is just sobering.

It really is.

And the book calls it a genetic disease at the cellular level.

What does that mean exactly?

Because not all cancer is inherited, is it?

No, not at all.

That's a common misconception.

Only about, say, 10 % of cancers have a strong inherited link.

The vast majority, maybe 90%, are acquired.

Meaning they develop during a person's lifetime, often because of exposure to environmental carcinogens.

Think UV radiation from the sun, or chemicals in cigarette smoke.

These things act as mutagens.

They damage the DNA.

They promote genetic changes, mutations in our somatic cell.

That's our regular body cells, not sperm or egg cells.

And crucially, it's not just DNA sequence changes.

We also need to talk about epigenetic changes.

Ah, epigenetics.

Changes in how genes are used, not the genes themselves.

Exactly.

Modifications that affect gene expression without altering the DNA code.

That plays a big role too.

Okay, so diverse causes.

But are there common features?

The source mentions a few key characteristics.

Like, most cancers start from just one cell.

That's right.

They are typically clonal in origin.

One, the single cell goes rogue, and all the cancerous cells in a tumor are descendants of that initial one.

Like a clone army, as you said earlier.

And it's not instant.

It's a multi -step process.

Absolutely.

It doesn't just pop up fully formed.

It usually starts with benign changes, like pre -cancerous growths.

Then, over time, more and more genetic and epigenetic alterations accumulate, hit after hit.

And these hits eventually lead to malignant growth.

Yes.

And malignant means two key things.

First, the cells become invasive.

They break out of their original spot and invade surrounding healthy tissues.

And second.

They become metastatic.

They can detach, travel, often through the blood or lymph system, and start secondary tumors elsewhere in the body.

And that metastasis, that spreading, is what makes it so hard to treat, often.

It's very often the factor that dramatically impacts prognosis.

Yes, makes treatment incredibly challenging.

Okay, so zooming back into the genetics, the book highlights two main kinds of genes involved.

Opposing forces, almost.

You could think of it that way.

On one hand, you have oncogenes.

These are mutated genes that get turned up too high or become overly active.

They actively promote cancer growth, like a stuck accelerator pedal.

Accelerators.

Got it.

And the other type.

Tumor suppressor genes.

These are the good guys, normally.

Their job is to prevent cancer, usually by controlling cell division or repairing DNA damage.

So it's when you lose the function of these genes that cancer can take hold.

They're like the brakes failing.

Accelerators and brakes.

And the key difference is how the mutation affects them.

Oncogenes gain function, tumor suppressors lose function.

Exactly.

A gain of function mutation turns a normal gene, a proto -oncogene, into an oncogene, making it hyperactive or expressed inappropriately.

A loss of function mutation knocks out a tumor suppressor, removing a critical safeguard.

It's a really fundamental distinction.

All right, let's focus on the accelerators first, the oncogenes.

They come from normal proto -oncogenes through these gain of function mutations.

What does that actually look like?

Well, a gain of function mutation can do a few things.

One, it might cause the cell to make way too much of the protein the gene codes for, just an excess amount.

Two, it could change the protein's structure, making it permanently switched on, hyperactive.

Always on.

Right.

Or three, the protein might end up being made in a type where it just doesn't belong, causing problems there.

And these oncogenes often mess with cell division pathways, don't they?

Like the book mentions epidermal growth factor, EGF.

Normally that triggers division in a controlled way.

Precisely.

Normal growth factors like EGF bind to receptors and set off a very carefully regulated signaling cascade inside the cell, telling it when it's appropriate to divide.

Oncogenes often short -circuit this pathway.

The classic one is the RACE protein.

RACE is normally like a switch.

It cycles between an on -state when bound to GTP and an off -state when bound to GDP.

It relays growth signals.

But what's fascinating and quite scary is that very specific missense mutations, sometimes just a single DNA litter change in the RAS gene, can break its off -mechanism.

So it can't turn itself off anymore.

Exactly.

It gets stuck in the GTP -bound state.

And that means it's constantly telling the cell divide, divide, divide, even when it shouldn't be.

Wow.

So one tiny change and the signal is just stuck on goal.

That's the uncontrolled growth right there.

That's a major driver.

And there are several ways these proto -ontogenes can turn into oncogenes.

We mentioned missense mutations, like in RAS, a glycine changing to avilene, for instance, often linked to chemical carcinogens.

Then there's gene amplification.

The cell actually makes multiple extra copies of the proto -oncogene DNA.

More gene copies mean more protein.

Like photocopying the accelerator piddle design?

Sort of, yeah.

Leading to too much accelerator protein being made, we see this with genes like CYMIKE in some leukemias and MIKE in neuroblastomas, IRB2 in certain breast cancers.

What about that?

Chromosomal translocation.

This is when bits of different chromosomes break off and swap places.

Swapping pieces.

How does that cause cancer?

A really famous example is the Philadelphia chromosome in chronic myelogenous leukemia, or CML.

Part of chromosome 9 swaps with part of chromosome 22.

Okay.

This physically fuses two genes together, the BCR gene and the ABL gene.

Now, ABL normally encodes a protein called a tyrosine kinase, which is involved in growth signaling.

Like another switch?

Kind of, yeah.

But this fused BCRIBEL protein is abnormal.

It's hyperactive, constantly signaling for white blood cells to divide.

Because the BCR gene's promoter is active in those cells, you get lots of this faulty signal leading to leukemia.

But isn't there a drug for that now?

Glevec?

Absolutely.

That's the amazing part of this story.

Understanding this specific fusion protein allowed scientists to design Glevec.

It's a targeted therapy.

It fits perfectly into the BCRIBEL protein where ATP normally binds.

ATP is the energy molecule.

Right.

Without ATP, the kinase can't function.

It can't send the growth signal.

Glevec basically jams the on switch.

It's been incredibly successful for CML patients.

A real triumph of targeted therapy based on understanding the genetics.

That's incredible.

Any other ways oncogens arise?

One more key way is viral integration.

Some viruses, like avian leukosis virus, actually insert their genetic material into the host cell's DNA.

A virus putting its DNA into ours.

Yes.

And if it happens to land next to a proto -oncogene, like CMYC, strong promoter or enhancer sequences within the viral DNA, can accidentally crank up the expression of that proto -oncogene.

Turning it into an oncogene just by landing nearby.

Exactly.

It hijacks the control.

And didn't the very first cancer gene discovered involve a virus?

It did.

Way back in 1911, Peyton Rearus found the Cyor C gene in Rhea's sarcoma virus.

That was the first identified oncogene.

And even today, viruses are linked to maybe up to 15 % of human cancers.

Things like HPV, Epstein -Barr, Hepatitis B and C.

Okay.

Fascinating.

So that covers the accelerators.

Let's switch gears now to the brakes.

The tumor suppressor genes, how do they normally stop cancer and what goes wrong?

Right.

The brakes.

The classic model to understand how these work or fail is the two -hit model for retinoblastoma.

This was Alfred Knudsen's idea back in the 70s.

Retinoblastoma, that's the eye cancer in kids.

Yes.

Knudsen noticed two patterns.

Some kids inherited a predisposition, got tumors early, often in both eyes.

Others get it sporadically, later, usually just one eye.

So two different scenarios.

He proposed the two -hit idea.

In the inherited form, the child gets one faulty copy of the crucial herb gene in all their cells from a parent.

So they only need one more hit a mutation in the remaining good copy in a retinal cell for cancer to start.

One hit down, one to go.

Yeah.

Makes sense it would happen earlier.

Exactly.

But in the non -inherited form, a single retinal cell needs to suffer two rare independent mutations knocking out both copies of the verb gene, much less likely, hence the later onset and rarity.

And molecular studies proved him right.

And the RB protein itself, what does it do?

The RB protein acts as a crucial gatekeeper.

It normally binds to and inactivates a transcription factor called E2F.

E2F's job is to turn on genes needed for DNA replication and cell division.

So RB puts the brakes on E2F.

Precisely.

It holds E2F back, preventing the cell from moving into the division phase, specifically at the G1 checkpoint in the cell cycle.

But cells do need to divide sometimes.

How does RB let go?

In a healthy cell cycle, proteins called cyclins and cyclin -dependent kinases, CDKs, add phosphate groups to RB.

They phosphorylate it.

This changes RB's shape, making it release E2F.

E2F is then free to activate the necessary genes for division.

But if both copies of the mutated and nonfunctional, there's no working RB protein to hold E2F back.

So E2F is just always active?

Always active.

Constantly telling the cell to divide.

The brakes have completely failed.

That paints a clear picture.

Now, what about the other really famous tumor suppressor, P53, often called the guardian of the genome?

P53 is absolutely critical.

You could argue it's the most important one.

It primarily acts as a sensor for DNA damage, especially serious damage like double -strand breaks.

It detects problems.

When it detects damage, P53, which is also a transcription factor, swings into action to prevent that damaged cell from multiplying.

It has three main strategies.

Okay, what are they?

One, it can activate genes involved in DNA repair, trying to fix the damage.

Give the cell a chance to heal.

Exactly.

Two, it can induce cell cycle arrest.

It hits the pause button, often by turning on another gene called P21, which inhibits those cyclin -CDK complexes needed to progress through cell cycle.

This buys time for repair.

Makes sense.

And three, if the damage is just too severe, beyond repair, P53 triggers apoptosis programmed cell death.

It initiates a self -destruct sequence using enzymes called caspases.

Eliminates a dangerous cell entirely.

Right.

It's a crucial quality control mechanism, and it tells you how important P53 is that something like half of all human cancers have defects in the P53 gene.

Its loss removes a major burial to cancer development.

Wow, 50%.

So looking at RB and P53 and others, these tumor suppressors seem to fall into a couple of broad roles.

Yeah, you can group them.

Some, like HERB and another one called APC, important in colon cancer, are mainly negative regulators of cell division.

They directly slow things down.

The direct breaks.

And others are more about maintaining genome integrity.

Think P53, BRCA1, BRCA2, linked to breast and ovarian cancer, and various DNA repair enzymes.

Their job is to prevent mutations from happening or accumulating, or to ensure damaged cells don't survive.

So they're like the mechanics and quality control for the genome.

That's a good way to put it.

Checkpoint proteins that monitor the cell cycle, DNA repair crews.

Losing them makes a cell much more prone to accumulating other cancer -causing mutations in oncogenes or other tumor suppressors.

It destabilizes the whole system.

Okay, and how do these tumor suppressors actually get switched off?

Primarily two ways.

You can get direct mutations within the gene itself, maybe messing up the promoter so it isn't transcribed, or creating an early stop codon so the protein is truncated and useless.

A direct hit on the gene.

Or you can have chromosome loss.

Many cancers exhibit aneuploidy having the wrong number of chromosomes.

If a cell accidentally loses the chromosome that carries the only good copy of a tumor suppressor gene, well then that break is gone too.

Right, and all this reinforces that idea of cancer being a multi -hit process.

It's not usually one thing, but an accumulation of these problems over time.

Absolutely.

The progression from a normal cell to a fully malignant one typically involves multiple genetic changes piling up.

The book uses colorectal cancer as a great example of this stepwise progression.

Yeah, I saw that figure.

It starts with losing APC?

Often, yes.

Loss of the APC tumor suppressor leads to a benign polyp.

Then maybe you get an activating mutation in the razonka gene.

Then loss of another tumor suppressor called DCC.

Then, often later, loss of p53.

So step after step.

Exactly.

Each hit pushes the cells further along the path to malignancy.

And interestingly, while there's a common sequence, the exact order might vary.

It seems to be the total number of accumulated hits that's most critical for driving the progression to full -blown cancer.

Okay, that multi -hit idea is key.

But it brings up something you mentioned earlier.

Why are inherited cancers usually linked to tumor suppressors like BRCA or ERB and not typically oncogenes?

Oh, that's a really insightful question.

Remember the two -hit model?

Familial cancers, the 5 -10 % that run in families, almost always involve inheriting one faulty copy of a tumor suppressor gene.

So you start life one hit down.

Exactly.

Now think about inheriting an activated oncogene.

That single activated oncogene might be so detrimental to normal development that an embryo couldn't survive.

Or it might cause issues much broader than just a localized tumor later in life.

Ah, so inheriting a faulty break is bad, but inheriting a stuck accelerator might be immediately catastrophic for development.

That's the thinking.

Whereas inheriting one faulty tumor suppressor copy makes you highly susceptible, but you still need that second somatic hit in a specific cell type to lose the function entirely and initiate cancer.

This ties into the concept of loss of heterozygosity, or LOH.

LOH, explain that again.

Okay, so you inherit one good copy and one bad copy of a tumor suppressor, your heterozygous at that gene locus.

Cancer develops when a cell in your body suffers a mutation or loss affecting the good copy.

The second hit.

Right.

That cell then loses its heterozygosity, becoming effectively homozygous for the non -functional version.

That's LOH.

And that's when the cell loses the protective function entirely and cancer can begin.

So the predisposition is inherited dominantly, but cancer development at the cell level is recessive, requires loss of both good copies.

That clarifies a lot.

Okay, we've covered direct DNA changes, mutations, deletions, translocations, but the book also stresses epigenetics.

How does that layer fit in?

Right.

Epigenetics.

These are changes to the DNA structure or its packaging proteins that affect gene activity, but without changing the actual ATCG sequence.

And these changes can be inherited when cells divide.

It's like adding sticky notes or highlighting to the DNA instruction manual, changing how it's read.

And this goes wrong in cancer too.

Oh, absolutely.

It's increasingly recognized as a major player.

There are three main types of epigenetic modifications often messed up in cancer cells.

Okay, what are they?

First, DNA methylation.

This involves adding small chemical tags, methyl groups directly onto the DNA, usually at specific sites called CPG islands, often found near gene promoters.

And in cancer.

We often see hypermethylation, too much methylation, specifically at the promoters of tumor suppressor genes.

This abnormal methylation can effectively silence those genes, turning them off inappropriately.

Another way to lose your breaks.

Silencing the breaks epigenetically.

What's the second type?

Covalent modification of histones.

Histones are the proteins DNA wraps around, like spools for thread.

Chemical tags can be added to histone tails, things like acetylation, methylation, phosphorylation.

Modifying the schools.

Exactly.

And these modifications change how tightly the DNA is wound.

Looser DNA is generally easier to read, tighter DNA is harder.

Abnormal histone modifications in cancer can wrongly activate oncogenes or silence tumor suppressors.

And the third.

Chromatin remodeling.

This involves physically moving those histone spools, nucleosomes, around.

Changing their position can expose or hide genes,

again influencing expression.

Cancer cells often have widespread abnormalities in nucleosome positioning.

So methylation, histone tags, moving nucleosomes, all ways to change gene activity without mutation.

What causes these epigenetic patterns to go haywire in the first place?

Good question.

Two main culprits.

First, you can actually have mutations in the genes that encode the epigenetic machinery.

So a genetic mutation breaks the epigenetic tools.

Precisely.

For instance, a mutation in a gene for a DNA methyltransferase enzyme, which adds methyl groups, or a histone modifying enzyme can lead to widespread epigenetic chaos and contribute to cancers like acute myeloid leukemia, a genetic root causing epigenetic problems.

Okay, makes sense.

What's the other cause?

Environmental agents.

We talked about carcinogens causing DNA mutations.

Well, some of those and other agents too can also be epimutagenic.

They mess with the epigenetic director.

Yes.

Things like components of tobacco smoke, polycyclic aromatic hydrocarbons, benzene, certain metals like cadmium and nickel, arsenic, even some endocrine disruptors.

They can directly interfere with the enzymes that add or remove epigenetic marks or trigger signaling pathways that lead to abnormal epigenetic patterns.

So the environment can hit both the genes and the epigenes.

Exactly.

It's complex interplay.

So what does this epigenetic angle mean practically?

Does it offer new ways to think about treatment?

It absolutely does.

If cancer involves epigenetic silencing of tumor suppressors, maybe we can reverse that silencing.

Like turning the brakes back on.

That's the hope.

And there's active research and even some approved drugs targeting these epigenetic changes.

For example, drugs called DNA methyltransferase inhibitors like 5 -azacetidine and decidabine are used for some leukemias.

How do they work?

They essentially prevent the addition of those silencing methyl groups or help remove existing ones.

The goal is to reactivate those improperly silenced tumor suppressor genes, letting them do their job again.

There's also work on histone modification inhibitors.

It's a really exciting area of cancer therapy development.

What an absolutely incredible complex picture.

We've gone from the basic idea of uncontrolled division through oncogenes as accelerators, tumor suppressors as brakes, the multi -hit process, and now this whole layer of epigenetic control.

It really highlights how many different ways the intricate regulation of our cells can be disrupted to lead to cancer.

It's a disease of accumulated errors at multiple levels, genetic and epigenetic.

It really is.

And it leaves me thinking,

well, considering how tightly lengthed the genetic and epigenetic factors are, this complex interplay,

what do you think the next big breakthroughs in diagnosis or treatment might look like, emerging from understanding these layers?

That's the multi -billion dollar question, isn't it?

Maybe more sophisticated combination therapies targeting both genetic drivers and epigenetic vulnerabilities, or perhaps using epigenetic markers for much earlier diagnosis.

It's wide open.

Definitely something for all of us to think about.

Thank you for joining us on this deep dive into the genetics and epigenetics of cancer.

We really hope unpacking this helps you feel a bit more empowered by understanding the science behind it.

Hope it was useful.

Thank you truly for being part of the Last Minute Lecture family.