Chapter 18: The Molecular Biology of Cancer

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Welcome, Deep Divers.

Today, we're diving into a topic that, well, unfortunately, touches nearly every one of us, directly or indirectly.

Cancer.

It's a heavy word, definitely.

Yeah, loaded with emotion.

But what if we could actually peel back the layers, understand what's really happening at the cellular level?

Uh -huh, right down to the molecules.

That's exactly our mission today.

We're aiming to cut through some of the complexity of cancer's molecular biology using Mark's basic medical biochemistry as our guide.

It's a great resource for this.

Right.

The goal is to give you a clear sort of foundational grasp of where cancer starts genetically, who the key molecular players are.

The oncogenes, the tumor suppressors.

Exactly.

And how our own cells can just go so incredibly rogue.

It's like uncovering the ultimate inside job.

That inside job idea is really spot on because, you know, cancer isn't just one thing.

No, it's a whole group of diseases.

A whole group.

But what ties them together is this uncontrolled cell growth, the ability to invade nearby tissues.

And that devastating ability to metastasize.

Right, where cells break off, travel, and start new tumors elsewhere.

And it all blows down to mutations.

Mutations in the genes that are supposed to be controlling everything.

Precisely.

Genes that manage cell growth, division, even cell death.

Cancer cells basically just,

they stop listening to the normal signals.

So it's a complete breakdown of the rules.

What are these key characteristics, these hallmarks that define a malignant cell?

How does it differ?

Well, the most obvious thing is that abnormal uncontrolled growth, they form tumors, right?

But not just any lump.

Malignant ones.

Malignant tumors that actively invade the healthy tissue around them.

And that ability to metastasize, to spread, that's really a defining feature of malignancy.

We've seen such stark examples of this.

Like Michael T., whose lung cancer spread to his brain.

Yeah, just months after surgery.

It shows how relentless that spread can be.

Or Calvin A.'s melanoma scare from a suspicious mole, you know, a carcinoma from cells.

It really highlights why noticing changes is so critical.

Absolutely.

And it's not just about growth.

Cancer cells break, like almost every rule of normal cell behavior.

Like what?

Okay, so normal cells have contact inhibition.

They grow until they touch each other, then they stop.

Orderly.

Right.

Polite society.

Cancer cells, they just ignore that.

They pile up, cross boundaries.

They also become resistant to signals telling them to stop growing.

So they're deaf to the dividing messages.

Totally deaf.

And another huge one, resistance to epoptosis.

Program cell death.

Exactly.

Even if a cell is badly damaged, normally it would trigger self -destruction.

Cancer cells, they find ways to dodge that.

Wow.

Keep going.

They also achieve immortalization.

They can basically divide forever, unlike normal cells that have a limited lifespan.

Endless proliferation.

And crucially for metastasis, they lose anchorage dependence.

They don't need to be attached to a surface to grow.

Which makes it easier for them to break free and travel.

Precisely.

Helps them spread through the body.

Okay, this molecular understanding didn't just pop up, right?

There was a huge revolution in genetic research.

Bishop and Varmus winning the Nobel Prize.

What was their big insight?

Yeah, that was absolutely game -changing.

Before them, a common idea was maybe cancer comes from some foreign invader, a totally new gene.

Something from outside.

Right.

But they showed it's not about alien genes.

Cancer fundamentally arises from mutations, often subtle ones, in our own genes.

The inside job again.

Exactly.

They identified proto -oncogenes.

These are normal healthy genes.

Think of them like a car's gas pedal.

They promote cell growth and survival when needed.

Okay, so proto -oncogenes are the normal gas pedal.

What happens when they mutate?

When they mutate, they become oncogenes.

The gas pedal essentially gets stuck down.

Always on.

Always on.

It's a gain -of -function mutation.

And what's really striking is that often a mutation in just one copy of that gene is enough to drive uncontrolled growth.

Just one bad copy.

What about the brakes?

Ah, the brakes.

Those are the tumor suppressor genes.

They're the guardians.

Their job is to inhibit growth, trigger cell death, if necessary, or repair damaged DNA.

So they protect us.

They do.

And for cancer to arise because of them, you usually need to lose the function of both copies of the gene, a loss of function in both alleles.

That's a key difference.

One hit for oncogenes, often two for tumor suppressors.

And you mentioned tumors is a simple wart, the same as malignant cancer.

Good question.

No.

Big difference.

A benign tumor, like a wart, grows slowly, stays put, usually harmless.

A malignant tumor or neoplasm grows rapidly, invades, infiltrates, destroys surrounding tissue.

And they need fuel for that growth.

Right.

They develop angiogenic potential.

They actually trick the body into growing new blood vessels for them to supply oxygen and nutrients.

That's critical for their spread.

Okay.

So we know genetic changes are the root cause.

How does our DNA actually get damaged in the first place?

What causes these mutations?

Well, DNA damage is an absolute prerequisite for cancer.

And our DNA faces constant threats.

First, there are chemical carcinogens.

Things in the environment.

Diet.

Exactly.

Compounds that, after our body metabolizes them, become reactive and can directly bind to DNA, altering the code.

Ironically, some chemotherapy drugs can also be carcinogenic.

Wow.

That's a tough reality.

Fixing one problem, potentially causing another.

It is.

Then you've got radiation and UV light.

Think sunlight.

Over 90 % of skin cancers.

Sun exposed areas.

Especially UVB rays.

Yes.

UVB is notorious for causing pyrimidine dimers.

Imagine two adjacent DNA letters getting fused together.

It distorts the helix badly.

Like welding bits of DNA.

Pretty much.

Our cells have repair crews like nucleotide excision repair, but too much sun overwhelms them.

That's relevant to Calvin A's melanoma scare.

Makes sense.

What else damages DNA?

Certain viruses can be culprits.

They might insert their own genes into ours or mess with how our genes are expressed.

And finally, simple replication errors.

Just mistakes when DNA copies itself.

Yeah, typos during copying.

If the proof and repair systems miss them, they become permanent mutations passed down to daughter cells.

Let's zoom in on how those proto -oncogenes, the gas pedals,

turn into oncogenes.

How does that gain of function happen?

Several ways.

One common way is point mutations, small changes in the DNA sequence, or mutations in the regulatory regions that control the gene.

So either the protein itself becomes hyperactive, or the cell just makes too much of it.

Exactly.

Like in Clark T's colon cancer polyp, there was a point mutation in the proto -oncogene.

Another way is translocation or transposition.

Where genes get moved around.

Right.

A piece of a chromosome carrying a proto -oncogene breaks off and sticks onto a different chromosome.

In its new spot, it might fall under the control of a really strong promoter.

Leading to overexpression.

Or inappropriate expression, yeah.

The classic example is the Philadelphia chromosome.

Ah, in CML, chronic myelogenous leukemia.

Precisely.

Manny W.

CML involves this.

It's a swap between chromosome 9 and 22.

This creates a fusion gene, BCR -ABL.

And what does that fusion do?

The BCR -ABL protein is a tyrosine kinase that's constantly active.

It's lost its normal off switch.

So it continuously signals for cells to proliferate, driving the leukemia.

Incredible how just misplacing a gene can cause so much damage.

It really is.

We also see gene amplification.

Making extra copies.

Instead of the normal two copies, the cell makes many, many copies of a proto -oncogene.

Think of the HER2 gene in some breast cancers.

Too much HER2 protein means too much growth signal.

Like having way too many gas pedals.

Exactly.

And as we touched on, oncogenic viruses can directly insert their own cancer -causing genes or strong promoters into our DNA.

So mutations are happening, oncogenes are activated.

But what about our defenses?

Those DNA repair enzymes, the tumor suppressors.

They're absolutely critical, our first line of defense.

You can actually think of the repair enzymes themselves as tumor suppressors.

Because they prevent the mutations that lead to cancer.

Exactly.

They fix errors before the cell copies its DNA, preventing mutations from becoming permanent.

If these repair systems are faulty, a loss of function mutation in the repair gene itself, then mutations across the entire genome start accumulating much, much faster.

And can people inherit faulty repair genes?

Yes, definitely.

Think of BRCA1 and BRCA2.

Inherited mutations in these dramatically increase the risk of breast and ovarian cancer.

Because they're needed for DNA repair.

Yes.

They're crucial for fixing dangerous double -strand breaks in DNA.

Or take HNPCC, hereditary non -polyposis colorectal cancer.

Lynch syndrome.

Right.

That results from inherited mutations in genes responsible for mismatch repair fixing those DNA typos we talked about.

So, a faulty repair system doesn't directly cause cancer, but it lets all the other cancer -causing mutations pile up incredibly quickly.

It sort of accelerates the whole process.

What about those tiny RNAs, the microRNAs, where do they fit in?

Myrnaz.

They're fascinating.

These small RNA molecules fine -tune gene expression, and they can play both sides of the street in cancer.

How so?

Oncogenes and tumor suppressors.

Exactly.

If a microRNA that normally silences a growth -promoting protein gets lost or under -expressed, that protein's level goes up, acting like an oncogene.

Okay.

Loss of myrnaz leads to too much growth signal.

Right.

Conversely, if a microRNA that normally targets a growth -inhibitory protein gets over -expressed, it knocks down that inhibitor, takes the breaks off.

So the myrnaz itself acts like an oncogene in that context.

So they add another layer of complexity to gene regulation.

They really do.

It shows how delicate the balance is.

Speaking of delicate balances, let's talk cell cycle.

That tightly controlled rhythm of cell growth and division.

How does cancer mess that up?

Oh, it hijacks it completely.

Normal cell growth is governed by cyclins and cyclin -dependent kinases, CDKs.

They act like checkpoints.

Making sure everything is ready before the cell divides.

Precisely.

The G1S checkpoint is a big one.

The decision point for replicating DNA.

Key players here are CDK46, cyclin -D, the retinoblastoma protein.

I agree.

That sounds important.

It is.

And E2F transcription factors.

Basically, the cyclin -CDK conflicts is phosphorylate RB.

Think of RB as a break.

When it gets phosphorylated, it releases E2F.

And the E2S.

E2F then switches on the genes needed for DNA replication for S phase.

So phosphorylating RB takes the break off, allowing the cell cycle to proceed.

You got it.

And there are also inhibitors, like P21 and P16, that can step in and halt the cycle, often if DNA damage is detected.

But if the break itself, like RB, is broken?

That's exactly what happens in retinoblastoma.

Inheriting a faulty RB tumor suppressor gene means that break is gone from the start in retinal cells.

Leads directly to eye cancer.

Shows how critical that control point is.

And then there's the famous one, P53.

The guardian of the genome.

Why is it so central?

It's arguably the most important tumor suppressor.

It's mutated in something like over half of all humoral cancers.

Wow.

Over 50%.

What does it do?

It's a master regulator.

If DNA gets damaged, P53 can halt the cell cycle, giving time for repair.

It can activate repair enzymes directly.

It switches on P21, that cell cycle inhibitor we mentioned.

Putting the breaks on.

Yes.

And crucially, if the damage is too severe, irreparable, P53 triggers apoptosis.

It tells the cell to self -destruct, to prevent passing on mutations.

Exactly.

It prevents damaged cells from multiplying.

So inheriting a mutated P53, like in live from mania syndrome, is devastating.

People get multiple types of cancer at young ages because they lack this key guardian, and mutations just accumulate way faster.

So we have tumor suppressors controlling the cell cycle, like Rb and P53.

What about other types?

Affecting signaling, or how cells stick together?

Great point.

Tumor suppressors are diverse.

Take neurofibromin, the NF1 gene product.

It helps turn off RAS, another key signaling protein.

RAS is often mutated in cancer, right?

Very often.

It acts like a molecular switch for growth.

If you lose NF1, RAS stays stuck in the on position, driving growth.

That leads to neurofibromatosis.

Okay.

What about cell adhesion?

Proteins like ecadherin are crucial.

They act like cellular glue, holding tissues together.

So loss of ecadherin.

Can allow cells to detach, become invasive, metastasize.

We see mutations in the ecadherin gene, CDH1, in some aggressive stomach cancers, for instance.

Makes sense.

Enables spread.

And then there's APC, adenomatous polyposis coli.

Another tumor suppressor, critical in colon cancer.

Like NFAP, familial adenomatous polyposis, relevant to Clark T.

Exactly.

APC normally helps control levels of another protein called beta -catenin.

Beta -catenin promotes proliferation.

If APC is mutated and non -functional, beta -catenin levels go up, driving uncontrolled growth in the colon lining, leading to polyps and eventually cancer.

So many different ways for the controls to fail.

Let's pivot to apoptosis again, that self -destruct sequence.

You said it's a key defense.

Absolutely critical.

Apoptosis is programmed cell death.

It's neat, orderly, the cell shrinks, DNA condenses, it breaks into little membrane -bound blobs.

Apoptotic bodies.

Right, which are then cleaned up by immune cells, macrophages.

No mess, no inflammation.

It's essential for getting rid of cells that are damaged beyond repair, potentially cancerous ones.

So failure to undergo apoptosis is a major win for a cancer cell.

A huge win.

It lets them survive despite having DNA damage or other abnormalities.

How does this self -destruction actually work?

What are the molecular triggers in executioners?

It's a cascade.

It can be triggered by external signals binding to death receptors on the cell surface, or by internal stress signals like severe DNA damage or problems with the mitochondria.

And the executioners.

The key players are enzymes called paspers.

Think of them as the demolition crew.

There are initiator caspases like caspase 8 or 9 that get activated first.

And they activate?

They activate the executioner, caspases 3, 6, and 7.

These guys go to work shopping up essential proteins throughout the cell, dismantling it from the inside out.

You mentioned mitochondria.

What's their role?

They're central to the intrinsic pathway.

Stress signals can cause mitochondria to release a protein called cytochrome c into the cytoplasm.

Isn't cytochrome c normally involved in energy production?

It is inside the mitochondria, but outside in the cytoplasm, it binds to another protein, apath 1.

Together they form the apoptosome.

And the apoptosome does what?

It activates initiator caspase 9, kicks off the caspase cascade from within the cell.

And regulating all this, the BCL2 family.

Yes, they are the crucial decision makers at the mitochondrial level.

It's a whole family of proteins.

Some are pro -death, some are anti -death.

A balancing act.

Exactly.

Anti -optotic ones, like BCL2 itself, can block cytochrome c release, protecting the cell.

Pro -apoptotic ones, like BACs or BAC, promote its release, pushing the cell towards death.

The ratio, the interactions between these proteins, determine whether the mitochondria spills its guts, so to speak.

So given how vital apoptosis is, how do cancer cells manage to evade it?

They must be pretty good at it.

They are incredibly cunning.

One major way is through mutations in those oncogenes we discussed.

Many oncogenic signaling pathways actually suppress apoptosis.

For example, growth factor signaling pathways, often hyperactive in cancer due to mutations like in RAS, can lead to the phosphorylation and inactivation of pro -apoptotic proteins, like members of the BCL2 family.

So the live signal overrides the die signal.

The same pathways driving growth also help the cell survive.

Precisely.

And another really common mechanism is simply over -expressing anti -epoptotic proteins like BCL2.

Making too much of the survival protein.

Exactly.

We see this in cancers like follicular lymphoma and even in Manny W's CML.

Too much BCL2 blocks the mitochondrial pathway, preventing cell death even when it should occur.

This allows damaged cells to persist and accumulate more mutations.

It's also linked to chemotherapy resistance.

Okay, so pulling this all together, cancer isn't just one mutation, it's a step -by -step process, right?

Multiple hits.

That's fundamentally important to grasp.

Cancer rarely, if ever, happens because of a single genetic hit.

Epidemiologists reckon it takes, on average, maybe four to seven critical mutations to turn a normal cell fully cancerous.

And it happens over time.

Over years, often decades.

It works through clonal expansion.

One cell gets a mutation, giving it a slight edge, maybe it divides a bit faster.

Creating a small clone of slightly abnormal cells.

Right.

Now you have a bigger pool of cells where a second mutation can occur.

If that second mutation gives a further advantage, that clone expands.

And so on.

Each hit makes the cells more abnormal, more aggressive.

That really explains the long latency periods we see, doesn't it?

Like Michael T.

smoking for 40 years.

Exactly.

That 20 -year lag between peak smoking rates and peak lung cancer rates is a population -level view of this multi -hit process playing out.

It takes time for those mutations to accumulate.

And inheriting one bad gene gives you a head start.

A definite head start.

If you inherit one mutated copy of a tumor suppressor, like ARB or APC, like in Clark T's FAP history,

every single one of your cells in that tissue already has the first hit.

You only need one more hit in the other copy, plus subsequent mutations, to get cancer going.

It dramatically increases risk and often leads to earlier onset.

Which brings us back to the idea that cancer isn't one disease.

Each tumor is potentially unique molecularly.

Absolutely.

Even two lung cancers in two different people might have different driving mutations.

That's why personalized precision medicine is the goal.

Like Matnib Gleevec for CML.

A perfect example.

That drug was specifically designed to inhibit the BCR -ABL fusion protein, the unique driver in Manny W's CML.

It targets the cancer cells specifically.

Right.

It shuts down their growth signal and triggers a poptosis, ideally leaving normal cells unharmed because they don't have BCR -ABL.

That kind of targeted approach, figuring out the specific mutations driving your tumor using genomics, proteomics,

that's the future.

And looking ahead, are there other clever strategies emerging?

You mentioned PRP inhibitors earlier.

Yes.

PRP -1 inhibitors are a really smart strategy, especially for breast cancers caused by inherited BRCA -1 or BRCA -2 mutations.

How does that work again?

It exploits a weakness.

It exploits a specific weakness, yeah.

It's called synthetic lethality.

Remember, BRCA -12 are needed for repairing double -strand DNA breaks.

PRP -1 is mainly involved in repairing single -strand breaks.

Okay, two different repair jobs.

So in a cancer cell that's already missing functional BRCA -1 or 2, its ability to fix double -strand breaks is crippled.

If you then use a drug to inhibit PRP -1… You block the single -strand break repair too.

Exactly.

Now, those unrepaired single -strand breaks often get converted into double -strand breaks when the cell tries to replicate its DNA.

Uh -oh.

And the cell, lacking BRCA function, can't fix them properly.

It might try using a different, very error -prone pathway called NHEJ, but ultimately the massive DNA damage becomes overwhelming and the cancer cell dies.

But normal cells are okay.

Normal cells with their working BRCA genes can still repair those double -strand breaks caused by PRP inhibition, so they survive.

It's beautifully targeted at the cancer cell's specific genetic defect.

That's incredibly clever, exploiting the cancer's own vulnerabilities.

What an amazing, if complex, picture we've painted.

It really is complex.

So to wrap up this deep dive, cancer is fundamentally a disease of uncontrolled cell growth.

Driven by an accumulation of genetic mutations over time.

Right.

We have oncogenes gain -of -function mutations, like stuck accelerators.

And tumor suppressor genes loss -of -function mutations, like failed breaks, including faulty DNA repair systems.

And cancer cells develop these elaborate ways to cheat death,

basically, evading normal controls like apoptosis.

But understanding these intricate molecular mechanisms is truly opening doors.

Yeah, paving the way for more targeted personalized therapies, moving away from just blasting everything.

Exactly.

Towards treatments tailored to the specific molecular fingerprint of each person's cancer.

So thinking about the future.

As we get even better at mapping the unique genetic landscape of every single tumor, the potential for precision medicine seems immense.

Tailoring treatments?

Maybe even preventing cancer based on risk.

The complexity is still staggering, but the progress is undeniable.

It really makes you wonder, doesn't it?

What completely new insights into these pathways are just waiting to be discovered?

What breakthroughs are just around the corner that could offer real hope?

That's the ongoing quest.

Well, thank you for joining us on this deep dive into the molecular world of cancer.

We really hope this sparks your curiosity to keep exploring how biochemistry shapes our health.

Thanks for listening.

Until next time, keep digging.