Precancerous esophagus
It became clear how there is a precancerous condition of the esophagus – Barrett's syndrome
Vyacheslav Kalinin, "Elements"
Fig. 1. Schematic representation of the epithelium at the junction of the esophagus (Oesophagus) and stomach (Stomach), in which epithelial cells of different types meet. The transitional epithelium is formed by basal (Basal layer) and luminal (Luminal layer) cells, which are characterized by different levels of expression of three marker proteins (p63, KRT5 and KRT7). Reflux caused by bile acids or a violation of the expression of some genes can cause the epithelium of the border region to turn into an abnormal one - similar to the intestinal epithelium – with inclusions of goblet cells (Goblet cell), and a growing structure called Barrett's esophagus (Barrett's oesophagus) is formed. A drawing from a popular synopsis to the article under discussion.
Scientists have managed to understand the mechanism of the occurrence of a precancerous condition at the junction of the esophagus and stomach – Barrett's syndrome. They were able to characterize in detail the epithelium in this border region and in laboratory mice showed that basal cells with a certain set of markers can become precursors of tissue similar to intestinal tissue, which replaces the normal epithelium of this area. Cells with similar properties are found at the border of the esophagus and stomach and in humans, so this study should help improve methods of early diagnosis and treatment of cancer.
It is well known that the earlier cancer is diagnosed and treated, the easier the treatment itself can be and the higher the chances of recovery. For some types of cancer, so–called precancerous conditions have been identified - changes in tissues that are not malignant in themselves, but increase the likelihood of developing cancer in these tissues. Such conditions are known so far for a relatively small number of cancers. Therefore, on the one hand, it is necessary to try to expand the list of correspondences between malignant tumors and precancerous conditions, and on the other hand, to find out the mechanisms of occurrence of these conditions in order to increase the effectiveness of cancer prevention.
One of the types of precancerous conditions is metaplasia, in which cells of one type are replaced by cells of another (usually reversible). Metaplasia often occurs at the boundaries between different types of epithelium and can in such cases give rise to carcinomas – malignant tumors of epithelial cells. The epithelium is the tissue lining the surface of the body (that is, roughly speaking, the skin), internal cavities and mucous membranes of organs. In Russian medicine , there are two main types of epithelium – multilayer flat epithelium (skin, mucous membranes, esophagus) and a single-layer cylindrical epithelium (stomach, intestines). The epithelium lining the organs undergoing severe stretching (for example, in the urinary system), the so-called transitional, is particularly distinguished. In Western medicine, a more detailed classification of the epithelium has been adopted (see Epithelium).
The most common and actively researched form of metaplasia is Barrett's esophagus (Barrett's syndrome). This is a dangerous complication of reflux disease – the throwing of stomach contents into the esophagus, in which the normally characteristic of the esophagus multilayer flat epithelium in the border area of the esophagus junction with the stomach is replaced by cylindrical (characteristic of the stomach) interspersed with goblet cells characteristic of the intestine – "unicellular glands" that secrete moisturizing mucus (see Fig. 1). The frequency of the syndrome with reflux, it is 10%, and in the general population – 1%. Over the past 40 years, the incidence of Barrett's esophagus has increased almost 8 times. It is considered a precancerous condition, since high-flying adenocarcinoma of the lower part of the esophagus occurs with it 10 times more often than in a normal esophagus.
Although it has been actively studied since the description of Barrett's syndrome in 1950, the key processes of its development remained unknown: how does the replacement of a flat epithelium with a cylindrical one (metaplasia) occur? from which cells does the newly formed cylindrical epithelium of the esophagus originate? how is the subsequent transformation into malignant neoplasms going?
Five main theories have been proposed to explain metaplasia (they are schematically shown in Fig. 2):
1) direct conversion of the squamous epithelium into a cylindrical one is possible – damage can cause the transformation of one type of tissue into another (Fig. 2, a);
2) the precursors of the cylindrical epithelium are stem cells circulating in the bloodstream, capable of differentiating into a cylindrical epithelium (Fig. 2, b);
3) the precursors of the cylindrical epithelium are cells of submucous (mucous) glands localized under the squamous epithelium (Fig. 2, c);
4) expansion into the border region of the cells of the cylindrical epithelium of the stomach is possible (Fig. 2, d);
5) the precursors of the cylindrical epithelium are residual embryonic cells localized near the border region (Fig. 2, e).
Fig. 2. Hypotheses of the origin of Barrett's esophagus, proposed earlier. a – transferentiation of the esophageal squamous epithelium; b – differentiation of bone marrow stem cells circulating in the blood; c – expansion of esophageal mucosal gland cells and their transformation into Barrett's epithelium; d – transformation of mucosal gland stem cells; e – expansion and differentiation of dormant residual embryonic cells of the esophagus/stomach border region. A drawing from the discussed article in Nature
But none of these theories has received rigorous experimental confirmation. And none of them explained the appearance of inclusions of goblet cells peculiar to the intestine (and not the esophagus or stomach).
A large team of scientists from Columbia University Medical Center and other scientific institutions in the USA and China conducted a comparative analysis of the expression of genes characteristic of the epithelium. They showed that in mice, the border epithelium, consisting of basal and luminal (facing the lumen of the esophagus) cells, is characterized by different expression of three markers in these cells. Two cytokeratins, Krt5 and Krt7, as well as a transcription regulation factor p63, are expressed in basal cells. Only Krt7 is expressed in the cells of the luminal layer. This distinguishes the border region from the above-located esophageal region, in which neither the basal cells nor the squamous epithelium express Krt7 (Fig. 1). In the epithelium of the stomach, none of these markers are expressed.
In the next series of experiments, the authors performed a thin surgical operation on mice and made an anastomosis between the esophagus and duodenum (Fig. 3).
3. Diagram of the anastomosis between the duodenum and the esophagus of a mouse, as a result of which bile acids (red arrows) enter the esophagus. A drawing from the discussed article in Nature.
As a result, bile acids entered the esophagus, which allowed to simulate reflux.
18 weeks after the operation, the effect of bile acids on the border area led to the formation of cells of the "Barrett's esophagus", in which the characteristic CDX2 marker was expressed, and goblet-shaped cells appeared in this area. It is noteworthy that no such cells were observed in the part of the esophagus located above, despite the fact that it was also exposed to bile acids.
The scientists decided to exclude the possibility of replacement of the epithelium in the border region of the esophagus by migrating cells of neighboring tissues. To do this, using genetic engineering methods, they obtained mice in which the expression of the Krt7 gene in the epithelial cells of the border region was associated with the expression of the red fluorescent Tomato protein. Experiments have shown that Tomato, i.e. Krt7, is expressed in the cells of the cylindrical epithelium of the "Barrett's esophagus". And as already mentioned, the Krt7 gene is expressed in the epithelium of the border region, but is not expressed in the esophageal region located above and in the stomach. Consequently, the cells of the cylindrical epithelium of the "Barrett's esophagus" originate exclusively from the basal cells of the border region (Fig. 4).
Fig. 4. Precursors of basal cells (expressing genes p63 and KRT7) and luminal cells (KRT7+) of the epithelium of the esophagus/stomach border region in mice. Squamous epithelium is the squamous epithelium of the esophagus. Transitional epithelium is the epithelium of the border region of the esophagus. Cardia – stomach. A drawing from the discussed article in Nature
In conclusion, the authors decided to check how similar the structure of the border area between the esophagus of the stomach is in humans and in mice. They analyzed the expression of human epithelial markers and showed their similarity to mouse markers. Basal cells with expression of the p63, KRT5 and KRT7 genes were found in this region in humans, as well as luminal cells in which KRT7 was expressed, but there was no expression of the p63 gene. KRT7 was not expressed in basal cells belonging to the esophageal region located above (Fig. 5).
Fig. 5. In the border region of the esophagus and the human stomach, a specific transitional epithelium is localized, which grows in Barrett's esophagus. a – microscopic image of basal cell precursors (marked with arrows) and luminal epithelial cells, stained with hematoxylin and eosin. The length of the scale segment is 50 microns. b – basal epithelial cells of the border region express the genes p63, KRT5 and KRT7 (expression products are marked with triangles). Basal cells of the esophageal squamous epithelium do not express KRT7 (marked with arrows). Immunohistological staining with various fluorescent dyes. The length of the scale segment is 50 microns. c is a diagram of the structure of the epithelium of the border region of the human esophagus/stomach, notation as in Fig. 4. Drawing from the discussed article in Nature.
Two types of basal cells of the human esophagus (with markers p63+KRT7− and p63+KRT7+) were separated using flow cytometry. Three-dimensional organoid cultures were obtained from these cells in vitro and it turned out that organoids derived from p63+KRT7+ cells and originating from the border region of the esophagus are capable of forming epithelial cells similar to the intestinal epithelium. Organoids obtained from p63+KRT7−type cells taken from the above-located region did not possess such a property.
Thus, scientists were able to characterize the epithelium of the border region between the esophagus and stomach, which gives rise to a precancerous condition – Barrett's esophagus, using a model of laboratory mice, and also check that everything is arranged similarly in humans. It turned out that this epithelium is more sensitive to damaging factors than the epithelium of the higher regions of the esophagus or the epithelium of the stomach. The results obtained are best consistent with the previously proposed hypothesis of the origin of Barrett's esophagus about the direct transformation of the epithelial tissues of the border region: it is shown that the basal cells of the border epithelium can be precursors of an epithelium similar to the intestinal one, which includes goblet cells.
At the same time, the genetic markers of borderline epithelial cells identified during the study as such are unlikely to be the cause of the formation of Barrett's esophagus and the further development of cancer. This most likely involves some other factors - acid reflux, other chemical stimuli, hormonal disorders or viral infections.
Despite the remaining questions, the work under discussion gives a detailed picture of the formation of Barrett's esophagus. Since precancerous conditions and malignant tumors especially often occur in the border regions of the epithelium of different organs (uterus, esophagus, rectum), it is hoped that similar mechanisms work there. However, further studies will have to clarify this question, as well as whether the borderline epithelium is the only place where this metaplasia occurs and its further transformation into cancer. After all, it has long been shown that a structure similar to Barrett's esophagus can also occur in patients in whom the border region of the esophagus has been removed (S. R. Hamilton, J. H. Yardley, 1977. Regnative of cardiac type mucosa and acquisition of Barrett mucosa after esophagastrostomy). So the results obtained can contribute to the development of methods for the diagnosis, prevention and treatment of these conditions not only in relation to the esophagus, but also to other organs.
A source:
Ming Jiang et al., Transitional basal cells at the squamous-columnar junction generate Barrett's oesophagus // Nature, 2017.
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