So welcome where we are now going to talk in more detail about the role of mutations in tumor formation and evolution. In the introduction to cancer course, Dr. Sara Ammond talked about the genetics of cancer, and if you viewed that lecture, some of the information in this section should sound familiar. I've mentioned mutations a couple times in this lecture so far, and I will now formally define what a mutation is. Normal cells become cancer cells through mutations in a cell's DNA. A mutation is a permanent alteration in the DNA sequence that is different from what is found in most people, and that causes or increases the risk for a disease. If all humans had identical DNA, we would all be identical. So there must be some variation in the DNA sequence from person to person. Most of this variation is harmless and actually causes all of the differences that we see in one another, which is completely normal. But some genetic variation can be harmful and actually cause disease such as cancer. This type of harmful genetic variation is called a mutation. In cancer, a mutation has to be able to contribute to a growth advantage for a cell. In other words, if a mutation occurred that causes the cell to stop dividing, that cell would never become a cancer, however, if the mutation allows the cell to proceed trough the cell cycle unchecked and continue to divide, that cell is much more likely to become a tumor. Before we continue talking about different types of mutations, I want to remind you of the central dogma of molecular Biology which simply put is that DNA makes RNA which makes protein. DNA is often referred to as the genetic code because it contains all of the information that a cell needs to survive encoded with a net sequence of As, Ts, Cs and Gs. The synthesis of RNA molecules from DNA is called transcription, and this takes place in the nucleus where the DNA resides. RNA molecules are then transported out of the nucleus where they are used to assemble proteins in a process called translation. Proteins are the work horses of the cell. They perform almost every function that a cell needs to carry out to survive, move, divide, and perform any other duty it might have. Earlier, we mentioned cell cycle check point machinery and programmed cell death machinery. And this is all comprised of different kinds of proteins. Mutations, however, occur in the DNA sequence. And if a mutation occurs in the DNA, creating an altered sequence, then the wrong message is transcribed into RNA, which makes an altered protein. That protein can either lose or gain functions based on the type of mutation that occurred in the DNA. Different types of genes are encoded in the DNA. An Oncogene is the title given to a gene that promotes cancer when it obtains an activating mutation. That means that a mutation in this gene created a protein that gain a function and that function promoted uncontrolled cell division and cancer. A tumor suppressor on the other hand, protects against cancer. So if a DNA obtains a mutation that causes it's protein to loss function, and then its loss may promote uncontrolled cell division and cancer. So let's take a few moments to talk about the types of mutations that can actually occur in the DNA itself. There are three main types, point mutations, which can either be substitutions or indels, which we'll talk about, amplifications or translocations. And, we're going to talk about each one of these in the upcoming slides. So, a substitution mutation is a type of point mutation and it is simply the changing out of a single nucleotide with a different nucleotide that was there previously. And, a nucleotide is also known as a base, and that can be either an A, a T, a C, or a G. In the example given on the slide, I've given you a small section of the DNA sequence of the normal B-Raf gene, which is just one of the many genes that exist in our genome. In the normal B-Raf box you will see the DNA sequence GTG and the T is green, which encodes a green Val in the protein sequence. This protein has normal function. When a substitution mutation occurs you will see the mutated B-Raf box that the green tea was substituted with a red A so that sequence now reads GAG. The red A now encodes a red glue in the protein sequence, and this changes the protein's functions so that it now causes the cell to divide constantly, ignoring cell cycle checkpoints. This is actually an example of a very common mutation that takes place in many cancers. Insertions and deletions are also types of point mutations and they're collectively called indels because they cause similar types of changes in the DNA. An insertion is, as its name implies, when additional nuclear tides are inserted into the original DNA sequence that were not there previously. On the slide, this is represented by the ACTG sequence in the green box inserted into the original yellow sequence. A deletion is the removal of several nuclear tides from the original sequence shown on the slide as well. Indels can cause relatively large changes in the DNA sequence, and therefore the encoded protein's function. Amplification is another type of mutation, in which a sequence of DNA is copied multiple times. The DNA sequence can be an individual gene or an entire chromosome, and in cancer, amplification generally occurs with oncogenes. So more oncogene DNA means more of that protein is made, which then can push the cell into uncontrolled cell division. In this example on the slide, the HER2 gene has been amplified many times in these breast cancer cells. So the HER2 gene is an oncogene in breast cancer, and its amplification promotes cancer. And in the slide, you see that the red dots represents HER2 genes. The final type of mutation we'll discuss is translocation, which is when two pieces of two chromosomes switch places. This is unusual and can create new genes that and proteins that did not previously exist. In this example on the slide, the end of chromosome 9 switches places with the end of chromosome 22. The able gene is normally located on chromosome nine and it normally does not cause cancer. Similarly, the BCR gene is located on chromosome 22 and also does not cause cancer. But when a translocation occurs a new gene called fusion gene is formed. The fusion gene called bcr-abl, promotes cell division and causes a specific type of cancer of white blood cells called Chronic Myeloid Leukemia, or CML. It's important to stop here and talk about changes in DNA in general. People can sometimes incorrectly think that all DNA changes cause cancer. But most of the time, a change in the DNA is either so obviously detrimental to the cell that it dies. And remember back when we talked about the cell cycle checkpoint in section A? Or the DNA is harmless enough that nothing ever even happens to the cell, no tumor ever forms. The mutations that we've been talking about here are a problem for the cell, because not only does the cell survive in the presence of the change in DNA, but it thrives and continues to divide and this is what can cause cancer. So let's continue our discussion of mutations. Besides the specific types of changes to the DNA sequence that can occur, there are also some broad classes of mutations called Hereditary and Somatic. A Hereditary mutation is present in every cell of the body, because it was present in the egg or sperm from a parent. And those cells form every cell in the body of a person born with that mutation. So just to try to clarify, hereditary mutation can be a point mutation, amplification or translocation, which we just finished talking about. The other class of mutations is somatic. And these occur in some point in a person's life, typically in a single cell, such as a skin cell or column cell or any other type of cell. These mutations can not be passed down to children, and they typically arise through a mistake in DNA replication step of the cell cycle or through carcinogens, such as a UV light or cigarette smoke. An important theory about the number of mutations required to develop cancer is called the two hit hypothesis. And this just simply states that both copies of DNA need to be mutated to become cancer causing. As a reminder, humans have 23 pairs of chromosomes, or 46 chromosomes, total. For each gene located on a chromosome, there is a copy of that gene on its paired chromosome. So there are really two of every gene, with exception of genes on the x and y chromosome. If one gene gets mutated, such as with a hereditary mutation, or a synaptic mutation, cancer is not caused. Hereditary mutations can be pass down to a child without that child ever having cancer. But over time, if the second copy of the gene becomes mutated somatically, then cancer can arise. Similarly, if a somatic mutation occurs in a person, cancer is not caused. But if another somatic mutation occurs in that gene's copy, then that can cause cancer. To reiterate, the two hit hypothesis states that both copies of a gene must be mutated to cause cancer. To continue this line of thought, there is also evidence to suggest that mutations in a single gene are not sufficient to cause full blown cancer. Rather, multiple mutations in multiple genes that accumulate over time are needed to cause a benign tumor to become a malignant cancerous tumor. For example, on this slide, we see that the APC mutation in a normal colon produces early adenoma, which is not actually cancer, but are relatively small benign tumors in the colon. Additional mutations in Ras, or loss of the chromosome 18q, can cause early adenomas to progress to late adenomas, which are more symptomatic. And further mutations in other genes, such as Smad4, p53, PI3K or TGF Beta, can cause these benign tumors to become actual cancer. It's important to note that once a cells DNA has been mutated and begins dividing uncontrollably, additional mutations are easier to gain over time. And this property is called genetic instability and this is a hallmark of cancer. Another way to show it is with this diagram that we started the lecture with, which shows a single cell gaining a mutation, and in this case, turning brown. Over time the brown cell gains other mutations and turns blue, and again over time, the blue cell gains another mutation and turns purple. And each color change represents the gain of another malignant property. Mutation can also cause cancer cells to change into different versions or clones of cancer cells. A mutation in one gene may cause a cell to behave in one way while a mutation in another gene might cause it to behave in another way. This tumor heterogeneity, which it's called, also frustrates oncologists which are trying to rid cancer patients of cancer, because some cancer clones are killed by chemotherapy while other clones are resistant and, therefore, survive in the patient. Also, clonal expansion my happen, meaning that a particular clone may gain certain genetic characteristics that allow it to outgrow other clones and become the dominant cell type in that tumor. This can also happen after a cancer patient is treated with chemotherapy, which kills off many of the other clones but leaves some alive. And the ones that survive can grow out after the treatment is over. So that concludes our section on mutations and the evolution of cancers as a result of mutations. And our next section we'll talk about neoangiogenesis, which is the formation of new blood vessels.
