okay so welcome to this next video on uh the vocal steam model for colorectal carcinoma so so far what we have seen is uh that you start off with a cell a normal cell in the colonic epithelium this first cell is unlucky enough to suffer two loss of function mutations in the APC Gene that leads to it over dividing and creating a whole population of cells which are genetically identical to it and which all have a loss of function in both APC genes now within that population of cells all with two loss of function mutations
in APC uh you are get the next program to progress to the next level of uh the colorectal carcinoma progression uh you need to have one of those cells in that green population that is unlucky enough to suffer another mutation and this Next Mutation is a gain of function mutation in a k-ras one of the k-ras genes that then leads to that sound dividing even more rapidly than it did with the two loss of function mutations in APC so it produces a whole population of cells genetic identical to it and this is these pink cells
which all have two loss of function mutations in APC as well as a gain of function mutation in a single Keras Gene now the next that's at that stage it's known as an intermediate adenoma the next stage is for one of these um pink cells which has uh two loss of function mutations in its ABC genes and one gain of function mutation in its Keras Gene uh to get uh two loss of function mutations in somad 4 and then that leads to it growing even more rapidly it leads to it dividing even more rapidly still
and it produces a population of cells genetically identical to it which then have two loss of function mutations in ABC one gain of function mutation in k-ras and loss of function in both of the smad four genes okay so the next stage then one of these cells is going to get a mutation in the tumor suppressor peritone p53 so this is the final mutation and this takes to into cancer basically okay so let me explain the pathway by which p53 operates and we're going to specifically look at the pathway uh by which p53 controls uh
well tries to control cells which have had DNA damage so well how it's involved in the response to DNA damage so let's say a a cell's DNA is damaged maybe it gets a mutation or something bad happens to it basically okay so the archetype for example is a double strand break okay so let's say some x-rays are going to come in and they're going to cause the DNA to break down the middle here okay so the DNA is going to break into two portions all right so let's show this so the DNA has now broken
into two portions okay so this is a double strand break so this is an example of DNA damage but there are other examples so a double strand break is a specific example of um DNA damage that can be caused by x-rays double strand break okay so how does the cell respond to this this is bad basically you need to do something about this well basically there are certain proteins which become active when they recognize uh damage to DNA basically okay and two examples of proteins which become active upon recognizing DNA damage are Ataxia tilange ectasia
mutated so this is ataxia tilonjectasia mutated okay and uh often Ataxia theological mutated protein is abbreviated to ATM so this is one of these proteins that can that can recognize DNA double strand breaks and will become an active kinase enzyme when it uh when double strand breaks are recognized by it when it sees a double strand break it will become active another example of a protein which can be activated by double strand breaks or other DNA damages so this is just an example of DNA damage more generally we're just talking about DNA damage activating Downstream
pathways so another example of a protein which can be activated by DNA damage is ataxia tilonjectasia and rad free related peptide ataxia foreign related protein okay red free related protein right so this is usually um denoted a t r for ATAC City launch Octavia and then red free related protein so this is a t r right so when the DNA gets damaged an example which is this double stand break that I've shown you here these two proteins will be activated ATM and ATR will be activated and they are both serine threonine kinases so both of
these are Under the category of serine freeanine kinases okay so they phosphorylate serine and threonine residues now for our purposes they both do essentially the same thing so from now on I will relate I will refer to ATM slash ATR by which I mean either ATM or ATR I don't mean some sort of chimera of both I mean one or the other but they're both doing effectively the same thing okay so they are a steering freoning kinase which I will denote by uh this sort of archetypal enzyme with its active site here okay so basically
what they're going to do is they're going to be connective when they recognize some DNA damage such as this double strand break which we've drawn and they're going to now phosphorylate serine and freonium residues on other proteins and two proteins which are targets for cerium frionine kinase are the checkpoint kinase one enzyme okay so checkpoint um kinase one enzyme and checkpoint kinase II okay so checkpoint Kylie's one or checkpoint kinase two and these are often abbreviated to chk1 chk2 so the checkpoint kinase one and the checkpoint kinase two they're both also serine free and kinases
which are basically acted uh oh sorry are activated when they are phosphorylated by the ataxity law injection mutated slash ataxity large octasia red free related protein so these get phosphorylated the check one and the check two get phosphorylated and when they are violated they become active so so far what we've seen is that when DNA is damaged it activates the ataxity large octasia mutated protein or the ataxity large ectasia rad free related protein so ATM or ATR those then phosphorylate the checkpoint kinase one or the checkpoint kinase two now what does the checkpoint kinase one
and checkpoint kinase 2 then do well it phospholates the protein p53 and that activates it but we need to see why that activates it so what usually prevents the activity of p53 well basically in a normal cell that doesn't need p53 active because DNA damage hasn't occurred the cell is making p53 all the time so how come you don't get active p53 well basically the reason you don't get active p53 is that the moment the p53 has been made another protein comes and binds to it and this other protein is known as mdm2 which stands
for something like Mouse dependent minute two or something ridiculous like that um named because it was discovered in experimental animals okay um so um mdm2 comes and binds to p53 and that stops the p53 from functioning but more than that mdm2 then targets p53 for ubiquitination so basically the p53 molecule has a ubiquitting group added onto it so this here is a ubiquitting group and we'll have this in purple okay so here is our ubiquitting group stuck on the side of the p53 now things which get ubiquitin stuck on them get destroyed by the proteosome
so this is going to be destroyed by the proteasome over here all right so basically that is why p53 is not active in normal cells you make it to destroy it the moment it's made it gets bounded by mdm2 then it gets you bigger donated and then it goes through the proteosome and is destroyed it may seem quite wasteful but that's the way it's done so the way the checkpoint kind is one of the checkpoint kind is to stop this process from happening is when the p53 is made what they do is they phosphorylate it
so they stick a phosphate group onto it and once they've stuck a phosphate group onto it mdm2 cannot bind to it so the p53 gets to survive basically because mdm2 can't bind to it with this phosphate group on it now when the p53 when the p53 is then active what it does is it forms um tetramas so let me show this down here so it forms tetramas like so okay and these tetramas go into the nucleus and they will bind to promoter regions on genes and they will alter the transcription of the target genes basically
so p53 forms a tetramer which is then a transcription factor and that's how it Alters um cellular activity so let's say this is the DNA here and p53 has bound to the promoter region of sun Gene of the target genes okay so here's the DNA so let's say this is the gene here okay and this is the promoter region here and I've drawn the promotion region far bigger than the gene but then might okay so um we'll have the promoter region in Orange and we'll have the gene in um pink right so what's going to
happen is p53 test trimmers are going to bind to promoter regions that's going to increase the Affinity of um the promoter region for RNA polymerase RNA polymerase is going to come in here begin the transcription of this Gene and because you've found the p53 there you're going to get increased transcription of this Gene here so you increase the transcription of a bunch of genes now which genes do you increase the transcription of well firstly you're going to increase the transcription of genes which make proteins involved in DNA repair that seems logical you have had DNA
damage that's the whole reason the p53 got activated in the first place so you need to make proteins which are going to go and try and repair that DNA damage so DNA repair mechanisms will be activated and you'll make basically the tools for the job okay you also make the uh tumor well you make the tumor suppressor Gene uh active which is p21 so you make lots of p21 protein this as we've seen uh previously in uh the uh transforming growth factor uh beta pathway arrests the cell cycle arrests cell cycle so it stops the
cell dividing that is a sensible thing to do if you have got a cell which has damaged DNA you do not want it going through the division process firstly because it risks passing on mutations down to the Next Generation secondly um it also risks major Havoc what if you've got a double strand break what's going to happen at mitosis when these uh when the sister chromatid should be pulling apart if you've got a massive great double strand break things could go seriously wrong you could get chromosome or Havoc um okay so um arresting the cell
cycle is a good idea and basically if you have very high levels of p53 for a prolonged period of time that indicates that the DNA repair mechanisms are not working and that the DNA is just remaining damaged basically and if that's the case what it will do is it will transcribe Pro apoptotic proteins specifically it will transcribe puma and noxa noxa okay right so puma and noxa are two hypoptotic proteins which will lead to the intrinsic path we have apoptosis being activated now this is the healthy pathway of p53 this is the pathway of p53
that protects you from DNA damage from major deer genetic havoc in your cells what happens if p53 loses its function what happens if you get absolutely no functional p53 then basically this entire pathway doesn't work anymore you get DNA damage and you do not get DNA repair mechanisms active you do not get arresting of the cell cycle you do not get apoptosis you get complete loss of all the genetic protection so p53 is often given the grandiose title of the guardian of the genome and it really really is it's protecting the genome from Havoc basically
from terrible mutations basically okay so in the next video we'll talk about what's going to happen in these um late adenoma cells
