Etiology of Genetic Alterations in Cancer

00:00 Hello and welcome back. In this talk today we're going to talk about how genetic alterations occur. We've talked about the effects on tumor suppressor genes, we talked about the effects on oncogenes, we've talked about the effects on apoptotic genes, etc. Wanted to understand how these alterations actually occur. This is where we are on our larger road map. After this, it's just about tumor-host interactions, but for now the etiology of genetic alterations. So, there are inherited germline mutations. You can have a nucleotide that's not supposed to be there, that now either codes for a stop code on or codes for a protein with an inserted amino acid that isn't supposed to be there. You can also have acquired changes in the DNA. This can occur through chemical carcinogens that can be both environmental as well as host derived. When I talk about host-derived chemical carcinogens, I'm talking about things like reactive oxygen species or RAS. Radiation. And we're all walking around in radiation all the time, but we can also get increased exogenous radiation sources that will drive DNA breaks. And, we can have infections usually viral, but other forms of infections can also drive genetic alterations. Okay, let's look at each of these quickly in turn. So, the APC gene, the adenomatous polyposis coli gene, is a tumor suppressor gene. If we lose the normal activity of that gene because of a mutation, and actually 2 mutations because it usually takes 2, then we have lost the break on replicative potential. There is a small number of people in the population that contain a mutation in the APC gene which readily inactivates it so they can no longer act as a tumor suppressor. That first germline hit, okay you get the carrier around in all your somatic cells but if you require a second hit stochastically now you're going to be at risk of developing malignancy in the colon, for example. Talk about chemical carcinogens. So mesothelioma, this is an example of a lung encased in a big rind of white tumor. That's a mesothelio-malignant proliferation otherwise known as mesothelioma. And that is classically driven by the presence of asbestos. What we're looking at is a high powered micrograph of a lung that has significant asbestos exposure.

02:39 The kind of brown rod-shaped structures are called ferruginous bodies or asbestos bodies, and these are single filaments of asbestos that have been encrusted through the activity of macrophages that tried to ingest them to encrust it with protein and iron therefore they have kind of a rusty look to them. Those particles when they accumulate in the mesothelium can drive activation of the macrophages because they are not able to fully ingest them and they clearly don't have asbestase to be able to degrade them, but the activation of the macrophages will drive now the production of a variety of inflammatory mediators that in the mesothelial surface over the course of 20 to 30 years will lead to a malignancy.

03:25 This is just the histology of what a mesothelioma looks like or one form of it. So, this is driven through a chemical asbestos causing macrophage activation, but you can also have direct chemical carcinogenesis. Chemicals that are mutagens that cause DNA base replacement so that we accumulate a variety of mutations in the genome. Let's talk about other forms of kind of chemical exposure. So this is, on the left hand panel, a Barrett's esophagus.

04:03 We are staring down the burrow with an endoscope in the esophagus and we're looking at the gastroesophageal junction and that kind of reddish material in the middle and the arrows are all pointing around it indicates areas where we have abnormal metaplasia of the normal stratified squamous epithelium of the esophagus because of chronic gastric reflex of acid. The epithelium in that location has converted to either an intestinal or gastric epithelium and we're looking at that zone here at the very very top in the middle panel you can see a little bit of stratified squamous epithelium, but everything below that represents glandular epithelium as part of the Barrett's metaplasia. And in fact, as we go down near the bottom, we are beginning to see invasion so in that setting of metaplasia and the inflammation associated with that and the recurrent injury in that location, we have accumulated enough mutations to develop an adenocarcinoma of the esophagus. And grossly, this is how that could appear. So, at the upper part of this panel is the normal esophagus and then that raised kind of fungating lesion below it represents gastroesophageal adenocarcinoma arising in the setting of a Barrett's. So another example how a chemical exposure without directly causing mutations but by causing inflammation in metaplasia can drive malignancy. This one, this slide is an example of malignancy associated with radiation exposure. What I'm showing you on the left is kind of the skeleton, the honking carcass of the turnover reactor which had a meltdown several decades ago and has turned the surrounding area into a no man's land in terms of the residual radiation fallout. But, there is a significant spike in the surrounding communities of patients, people in that area being exposed to the radiation who developed recurrently over and over, in many people, thyroid cancer. And it's just showing you a thyroid in the lower right hand panel that has a heterogenous kind of white tan lesion in the middle, that's the carcinoma, and it's a follicular carcinoma as shown in the histology above due to radiation causing DNA breakage. And then let's talk in a quick bit more detail about viral carcinogenesis. So normal cervical epithelium here, shown on the left, it's a stratified squamous, non-keratinizing epithelium with a basal layer that progresses through maturation up to a squamous epithelium that is then sloughed off. With human papillomavirus infection, we can get loss of normal cell cycle regulation as I'll show you in subsequent slides.

07:06 So, with that HPV, we turn off the breaks, the normal breaks to cellular proliferation and we start to get increased numbers of cells that are proliferating. So instead of having just the basal layer of proliferate, now maybe the bottom 3rd is showing proliferation. The other thing that happens with the viral protein is they are expressed in the HPV infection are things that will cause genetic instability. As this progresses through more and more proliferation, more and more accumulation of defects or mutations, we are getting a higher grade intraepithelial neoplasm. These are graded as CIN1 which is in the 2nd panel, CIN2 which is the 3rd panel. And finally we get the carcinoma in situ, CIN3 where the entire thickness of this is composed of cells that have not only had an increased proliferative capacity but are also showing DNA instability, increased mutations that will eventually allow them to invade and metastasize. So this is now a malignant tumor, it's confined to the basal membrane and it hasn't yet learned the trick, hasn't acquired the necessary proteases to break down the basement membrane but that's carcinoma in situ.