Overview of Cancer: Study with Video Lessons, Practice Problems & Examples

Cancer is characterized by uncontrolled cell growth and division, known as proliferation, leading to tumor formation. It arises from multiple mutations affecting cell cycle regulation, apoptosis, and signal transduction pathways. Tumors can be benign, localized, and easily removed, or malignant, capable of metastasis, making them more dangerous. Causes include viral infections, epigenetic changes, and environmental factors like carcinogens. Understanding these mechanisms is crucial for developing targeted therapies, particularly against cancer stem cells, which contribute to tumor resilience and growth.

Cancer Characteristics

So cancer is a disease, we're all familiar with it. But essentially, the basic definition of cancer is that it is abnormal, or can be unregulated growth. It's a little bit more difficult to term, but abnormal cell growth and division is a key feature. So you're getting a ton of cell growth, cells are growing in size, and they're dividing. And, generally what we term this whole cell growth and division is called proliferation. In cancer, it's unregulated, it's uncontrolled; it is going all over the place, proliferation, which is cell growth and division. And if you get a lot of cell growth and division, eventually, that's going to develop into a tumor. But it's not only cell growth and division that is unregulated; it's also unregulated death. And so, cell death is called apoptosis, and that is a process that is this whole regulated pathway that tells cells when it's time for them to die. And unregulated and uncontrolled apoptosis also causes cancer, because you're getting these cell growths, you're getting these divisions, and nothing's telling them, "Oh, wait. You're growing too much. You're dividing too much. It's time to die." So it's this combination of growth, division, and unregulated death that allows for these tumors to grow. And, in order to affect cell growth, division, and death, it requires multiple mutations. A lot of people think, "Oh, cancer is one mutation, and that leads to cancer," but actually what makes cancer difficult to treat is that it's an accumulation of mutations. And those mutations are different for every cancer and for every individual. So we call cancer, you know, a disease, like, we say breast cancer. But everyone's breast cancer is different because they all have different mutations and different genes. There may be some common ones that we're familiar with, like the BRCA genes in breast cancer. Those are very commonly found in breast cancer cells, but each one of those, even if they do contain BRCA, also has to contain other mutations that can vary in different genes, in different positions, throughout the genome. So these multiple mutations exist. And because there's multiple mutations, we say that cancer cells are genetically unstable, and this is a really common term that you hear when describing cancer. And so what that means is that there's just a ton of mutations and even chromosomal aberrations, meaning that there's chromosomal breakage, chromosomal inversions, sort of transitions. All these different things that we've talked about in different chapters are commonly found in cancers. And so, we just sort of say, "You know, cancer cells have a lot of these; therefore, they're genetically unstable." Now there are two types of cancerous tumors. The first is benign. And these are still cancer. Right? This is still cancer. It's created a tumor. It has unregulated cell growth division and cell death. But the positive part of benign tumors is that they're proliferating abnormally, but they're contained to a single area. So if you have a benign tumor and it's causing problems, a surgeon can just go in, chop it out, and you may need a little bit of extra cancer therapy. But, generally, the prognosis or the survival rate of people with benign tumors is extremely high because they're confined to this one area, and you can just go and take them out. Whereas, malignant tumors are much more dangerous because they metastasize, which means they travel to other areas of the body. So they may have started out in the breast, but then they go to the brain. And that's very dangerous. So they may have started out in the kneecap, in the bone in the kneecap, but then they travel to the liver. And it's very difficult because you can't just take out, you know, your kneecap and then also your liver, and then also part of your brain, because that is very difficult. It's very harmful to your body. You can't just cut out one single benign tumor and expect that to work for a malignant tumor. So an example of this is if this is an organism, you have a benign tumor, it has very clear edges here, and a surgeon can just come, cut this region out, and then the cancer is mostly gone for the most part. Malignant tumors are much more difficult because if a surgeon comes in, it can cut this much out. Well, you're still left with these regions here that may travel and exist in other organs, or elsewhere throughout the body, and it can be very dangerous. Now tumorigenesis is a term you're going to see, and what that is is it's the development of the tumo

Cancer Causes

Okay. So now let's talk about the causes of cancer. Cancer mutations can occur in many different ways. One that you may not even be familiar with is that viruses can cause cancer. A significant one that hopefully you're familiar with, because this vaccine came out during your lifetime, is HPV. HPV carries two genes called E6 and E7, and these very easily lead to cancer. They don't do it every time, but they can. Viruses can introduce different genes that can activate cancer-causing genes or interfere with the normal cell pathway that leads to cancer very easily. And so, I'm only mentioning HPV here, but there are lots of others, especially in liver cancer, HCV, hepatitis C, is another example. There's Kaposi's sarcoma, which is found in HIV and AIDS patients, but essentially, these different viruses can lead to cancer. But the good part about viruses leading to cancer is that you can have vaccines against viruses, and therefore, there is a vaccine against HPV. It's the only vaccine that currently exists that is a vaccine against cancer, and so it's super amazing. It's an incredible vaccine, and so, this will actually not only prevent you from getting HPV but can prevent overwhelmingly cervical cancer in women, but also head, neck, and throat cancer in men, which a lot of people don't know about. But viruses can very easily and often cause cancer.

Then a second way is epigenetic changes. Epigenetic changes are changes to the histone protein modifications that are found in the packaging of DNA, and we know that different modifications can cause genes to be overexpressed or genes to be underexpressed. Either way, this is resulting in gene misregulation, causing it not to be expressed correctly. And if a gene is overactivated or underactivated, it can affect regulatory genes that are controlling cell growth, division, and death, and that very easily can lead to cancer.

Viruses, epigenetic changes, and then finally, environmental substances. These are things like cigarette smoke, which we definitely know cause cancer. It can also be exposure to UV rays in the sun or tanning beds. It can be exposure to certain chemicals, like asbestos, but essentially all these different environmental substances that we encounter all the time, can very easily lead to a mutation or multiple mutations that can accumulate over time if you're exposed to various different ones, and those accumulation of mutations will lead to cancer because they'll affect cell growth, division, and death.

So those are some of the causes of cancer. Now, one way that these causes all sort of come together and really cause cancer is through misregulation of the cell cycle. Misregulation of the cell cycle is one way these mutations can affect a single cell. And so, essentially, misregulation of the cell cycle is obviously going to affect everything from growth, division, and death, which are the 3 main facets of cancer. And so how the cell cycle is regulated is actually through different proteins. Some of these proteins are called cyclins, and some of these proteins are called cyclin-dependent kinases. A kinase is going to add phosphate, and when phosphates are added to proteins, that can activate more proteins. The cell cycle has these different regions called checkpoints, and at different checkpoints, these proteins called cyclins, and the proteins that depend on cyclins, the cyclin-dependent kinases, have to be appropriately regulated at these different points to allow a cell cycle to continue. An example of some of these cell cycle checkpoints includes G1 to S. So remember the phases of the cell cycle is G1, then you have S, then you have G2, then you have M, and then you back to the interface. This is the cell cycle here. The G1 to S transition, in S, DNA replication occurs, and the cell does not want to replicate damaged DNA, so it stops here and it checks to make sure that the DNA is right. It's not damaged, and if it is damaged, it repairs it. An important protein that you'll read about that is important in this transition is retinoblastoma, and we'll talk about this protein a little bit more in later videos, but essentially, this is a transcription factor. And what do transcription factors do? They regulate gene expression. The retinoblastoma transcription factor is really important in the G1 to S transition, making sure that the DNA is appropriately repaired and is correct before replicating it. The second one is G2 to M, and this is making sure DNA replication has gone correctly. DNA was replicated here, and now the cell wants to stop before it divides and make sure something hasn't gone incredibly wrong with DNA replication, which makes sense. Right? If your DNA is replicated wrong, then that can induce a lot of different mutations that can very easily lead to cancer. Here's a cyclin called CDC2, and Cyclin B, and this is the cyclin, and these are important in these transitions. You don't necessarily need to know these, that's for cell biology for most of you, but, just know that cyclins and these cyclin-dependent kinases are here. So if we look at this sort of graph, the different phases, G1 phase, DNA replication happens here in the S phase, G2 phase, and mitosis. You can see that different cyclins activate at different times where cyclin E is going to really affect the G1 to S transition, where, you know, cyclin B, for instance, will really affect this G2 to M transition. And there are these different concentrations of cyclins that happen over time, and the different concentrations of cyclin affect the cyclin-dependent kinases that go on to add phosphates to different proteins in the G2 phase, and it's like, oh, wait, we need to do something else, let's start over. And it will, it will actually go back, it will start over, it will pause and make sure that everything is right before mitosis continues. Now, you can imagine if mutations happen in these regions, in these proteins that are regulating these sections, or any of the proteins that are going to have phosphate added to them by the kinases, then this whole system will get messed up and that is what allows mutations to accumulate over time to pass through mitosis because it says, oh, I have a mutation, but don't worry about it. We can move on. We don't need to check it and repair it. Oh, the DNA wasn't replicated correctly. Oh, don't worry about it. We'll move on. We don't have to correct it. Impacting these pathways is super important in creating this abnormal growth, proliferation, and death and preventing it from happening in normal cells, and that leads very easily to cancer. So, I know that's a mouthful, but with that as the overview of cancer, let's now move on.

Which of the following is NOT a cause of cancer?

• A. A collection of DNA mutations
• B. Unregulated proliferation
• C. Unregulated apoptosis
• D. A single DNA mutation

Which of the following statements about cancer is false?

• A. Cancer stem cells exist to continue growing the tumor for some cancer types
• B. Cancers are clona
• C. Cancer is triggered only through external mutagens
• D. Cancer can be caused by epigenetic changes

Which of the following are enzymes are responsible for controlling the cell cycle?

• A. G1 cylases
• B. Cyclin-dependent kinases
• C. Bypass polymerases
• D. Ligases

Here’s what students ask on this topic:

What is the basic definition of cancer?

Cancer is defined as abnormal or unregulated cell growth and division, known as proliferation. This uncontrolled proliferation leads to the formation of tumors. Additionally, cancer involves unregulated cell death, or apoptosis, which normally helps to control cell growth. The combination of unregulated cell growth, division, and death results in the development of tumors. These tumors can be benign, meaning they are localized and easier to remove, or malignant, meaning they can metastasize and spread to other parts of the body, making them more dangerous.

What are the main causes of cancer?

Cancer can be caused by several factors, including viral infections, epigenetic changes, and environmental substances. Viruses like HPV and HCV can introduce genes that lead to cancer. Epigenetic changes involve modifications to histone proteins, affecting gene expression and potentially leading to cancer. Environmental factors such as cigarette smoke, UV rays, and chemicals like asbestos can also cause mutations that accumulate over time, leading to cancer. These factors often result in the misregulation of the cell cycle, affecting cell growth, division, and death.

What is the difference between benign and malignant tumors?

Benign tumors are localized and do not spread to other parts of the body. They are characterized by abnormal cell growth and division but are confined to a single area, making them easier to remove surgically. Malignant tumors, on the other hand, are more dangerous because they can metastasize, meaning they can travel to other areas of the body. This makes them harder to treat, as they can affect multiple organs and tissues, complicating surgical removal and treatment.

How do cancer stem cells contribute to tumor growth?

Cancer stem cells are a type of cell found in some tumors that can both self-renew and produce more tumor cells. These cells are significant because they can continue to proliferate even after the tumor has accumulated many mutations that would normally halt cell division. By continuously producing new stem cells and tumor cells, cancer stem cells help sustain tumor growth and contribute to the tumor's resilience. Targeting these cells is a focus of current cancer research to improve treatment outcomes.

What role do signal transduction pathways play in cancer development?

Signal transduction pathways are networks of proteins that control various cell activities, including gene expression. Mutations in these pathways can lead to the misregulation of hundreds or thousands of genes, affecting cell growth, division, and death. This misregulation can result in uncontrolled cell proliferation and the development of tumors. Because these pathways are crucial for normal cell function, mutations in them can easily lead to cancer, making them a key area of study for understanding and treating the disease.