3. Genetics of esophageal cancer

When food is being swallowed, the upper sphincter relaxes, allowing food to enter the pipe. Peristaltic contractions of the esophageal muscle push the food down through the lower sphincter into the stomach. Besides controlling the amount of swallowed food going down into the stomach, the lower esophageal sphincter also works like a dam sitting in between the esophagus and the stomach to prevent the stomach contents to back up into the esophagus. When this muscular structure does not hold well, gastroesophageal reflux disease (GERD) occurs, in which case stomach acid mixed with duodenal content flows back into the esophagus. If this happens frequently enough, it leads to esophagitis, and then to Barret's esophagus, a premalignant metaplasia of the esophageal lining changing from stratified squamous epithelium to simple columnar epithelium. Compared to normal people, individuals with Barret's esophagus can have as high as a 400-fold increased risk to develop esophageal cancer [3].

Esophageal cancer is the ninth most common malignancy in the world. Most of the cases are either squamous cell carcinoma (ESCC) or adenocarcinoma (EAC). The former is the predominant one, accounting for ~90% of the cases. ESCC occurs in the squamous cell lining of the middle section of the esophagus and is more often found in Asia and Africa. China alone is responsible for more than 50% of the patient population. EAC, on the other hand, takes place in the cuboidal cells of the esophageal glands near the gastroesophageal junction and has been growing rapidly in western countries in recent years. Both types of esophageal cancer happen more often in males than in females, and the overall ratios of male to female are approximately 2.5 for ESCC and 4.4 for EAC. At the first look, the incidence of esophageal cancer seems to be geographic-related, as ESCC is more seen in Asia and Africa while EAC more common in Europe and North America. However, if we analyze the data further, we notice that the issue is actually more ethnic rather than geographic. Take a look at the cases in the United States, where ESCC incidence is found 4.8 times higher in Asian- and African Americans than in Caucasians, while EAC is just the opposite, 5 times higher in Caucasians than in other Americans [2]. Apparently, these two diseases selectively adhere to certain races of people regardless of where they live. This notion is also supported by the data from China, where ESCC patient population is 77 times greater than that of EAC [4]. Apparently, after a long history of sharing residential resources, each ethnic group has formed its unique life habits. For this reason, they

As far as we know today, smoking is the No. 1 risk factor for ESCC, particularly when it is in conjunction with drinking. A study found that ESCC incidence increased 12-fold in males and 19-fold in females in the population who use tobacco and alcohol together, compared to those who have one of the hobbies alone [5]. This connection can be easily seen in China, where the tobacco consumption is the highest in the world, higher than all other developing countries combined [6]. The Chinese also consume a lot of alcohol, particularly in northern and central provinces, where the ESCC incidence can reach 0.8% of the local residential population [1]. Here is the east end of the so-called "esophageal cancer belt." This association is also reflected by the data on the American males of Asian and African origins, who tend to smoke and drink abreast, thus making up for 90% of the ESCC patient population in the United States [7].

2. Epidemiology of esophageal cancer

2 Esophageal Cancer and Beyond

tend to develop common health problems.

In addition to the factors associated to life habits, there are also genetic elements contributing to esophageal cancer development. The genomic analysis reveals distinct profiles between ESCC and EAC. ESCC is more similar to squamous cell carcinoma of the head and neck than to EAC, while the latter has more resemblance to gastric adenocarcinoma.

ESCC is believed to develop from basal cell hyperplasia and dysplasia. During this process, the main mutation pattern is C to A substitution, which is commonly found in smokers [9]. The most frequently mutated genes include TP53 (p53, tumor suppression transcription factor), CDKN2A (p16, cyclin-dependent kinase inhibitor), KDM6A (histone demethylase), KMT2D (lysine methyltransferase), and RB1 (retinoblastoma-associated protein). On the other hand, some genes are highly expressed, such as CCND1 (cyclin D1), TP63 (tumor protein), SOX2 (sex-determining region Y), MYC (c-myc), FGFR1 (fibroblast growth factor receptor), TNFAIP3 (tumor necrosis factor-induced protein), and CHN (chimerin) [10]. Table 1 lists the top five genes frequently mutated and the top five highly expressed.

EAC, on the other hand, is generally believed to originate from Barret's esophagus, an esophageal metaplasia in response to chronic acid reflux. During this process, esophageal epithelium transforms from a multilayer of squamous epithelial cells to a single layer of intestinal columnar epithelial cells, or like the metaphor used by Ahrens et al.: "turning skyscrapers into townhouses" [14]. The natural question is where the columnar cells come from. There are four theories currently to explain the origin of esophageal columnar cells: (1) true transdifferentiation of the esophageal squamous cells, (2) trans-commitment of the esophageal stem cells, (3) colonization and subsequent trans-commitment of bone marrow stem cells, and (4) replacement by gastric columnar epithelial cells.

The first theory is supported by the fact that the esophagus derives from the columnar epithelial cells initially during embryonic development and later is replaced by squamous


Table 1. The top five of the most commonly mutated genes () and the top five of the most highly expressed genes (+) in ESCC.

accumulation of gene mutations. EAC is one of cancers with a high rate of gene mutation. In 2013, a study conducted by Dulak et al. performed whole-genome sequence analysis on 149 pairs of EAC versus normal tissue samples and identified a total of 17,383 mutations in 8331 genes in EAC specimens, including 16,516 non-silent mutations and 1954 insertion–deletion-null mutations [24]. Of these genes, 26 were significantly mutated. As seen in ESCC, TP53 and CDKN2A were on top of the list. One of the differences, however, is A to C base transversion that is more common in EAC while C to A is more common in ESCC. The second theory involves a massive chromosomal instability due to the inactivation of p53 and p16. Loss of TP53 has been shown to increase the possibility of malignance by 16-fold [21]. Without functional p53, aneuploidy develops, which increases the pace of genome doubling. CDKN2A is the gene coding for p16, a cyclin-dependent kinase inhibitor that can sequester MDM2 and thereby prevent p53 being degraded. Inactivation of CDKN2A can be interpreted as a reinforcement to the elimination of p53. Table 2 lists the top five mutated genes and top five overexpressed genes associated

Table 2. The top five of the most commonly mutated genes () and the top five of the most highly expressed genes (+) in EAC.

Gene Protein Function Up (+)/down ()

response, cell death

CDKN2A p16 Stabilizing p53 — [22] SMAD4 Smad 4 Tumor suppression — [23]

DOCK2 Dedicator of cytokinesis 2 Cell migration — [24]

receptor

GATA6 GATA binding protein 6 Cell differentiation in gut + [26] VEGFA Vascular endothelial growth factor A Angiogenesis + [25] CCNE1 Cyclin E1 Cell cycle progression + [9]

transformation, apoptosis

TP53 p53 Tumor suppression, cellular stress

MYC c-myc Cell cycle progression, cell

ERBB2 Her2 Stabilizing EGF binding to its

SYNE1 Spectrin repeat containing nuclear envelope protein 1

in EAC

http://dx.doi.org/10.5772/intechopen.77995

Introductory Chapter: Esophagus and Esophageal Cancer

Organelle movement — [24]

— [21]

+ [25]

+ [9]

References

5

Based on multiple genetic analyses performed by several independent groups [9, 27–29], TP53 always appeared to be the most frequently mutated gene in both ESCC and EAC. Not just esophageal malignancy, 50–60% of human cancers that have been studied so far contain homozygous mutations in TP53 [30]. That is almost to say, p53 has to be disabled in order to

with EAC.

4. The guardian of the genome: p53

turn a normal cell into a cancerous cell. Why is p53 so important?

epithelium [15]. Therefore, the differentiated esophageal squamous epithelial cells might be able to transform back to columnar cells. Does it sound possible? However, in vitro study demonstrated that acid and/or bile treatment upregulated CDX-2 expression in normal esophageal epithelial cells (Het1A), which led to intestinal phenotype [16]. The esophageal epithelium is maintained by a distinct group of p63-expressing stem cells beneath the mucosa under the influence of a specific cue within the organ. When the esophagus is insulted by acidic refluxate repeatedly, the progenitor cells lose p63 expression and are misled to differentiate into columnar instead of squamous epithelial cells. This is where the second theory stands [17]. Does it sound reasonable? More evidence is needed. The third theory seems pretty strong, because it is supported by experimental evidence. In 2008, Sarosi et al. successfully transplanted female rats that had been surgically induced by reflux esophagitis with bone marrow from male rats and later identified Y-chromosome in half of the esophageal epithelial cell population, indicating a colonization of bone marrow stem cells in esophageal epithelium [18]. However, Aikou et al. conducted similar experiment using mice and could not confirm bone marrow-derived metaplastic esophageal epithelial cells [19]. Nevertheless, both the second and third theories recognize the important contribution from stem cells during metaplastic transformation. The only difference is the origin of progenitor cells. If bone marrow stem cells could be misguided to differentiate into columnar phenotype where squamous cells are supposed to be, why could not the local stem cells? The fourth theory has also found experimental evidence in human. In 2014, Lavery et al. showed that labeled gastric cells residing in the middle of esophageal glands had undergone metaplastic transformation, suggesting that esophageal columnar epithelial cells could be from the migration of gastric cardiac columnar epithelial cells [20].

The next question is how esophageal metaplasia turns into EAC. There are two main theories currently. The first theory thinks that EAC develops from Barret's esophagus through a stepwise


Table 2. The top five of the most commonly mutated genes () and the top five of the most highly expressed genes (+) in EAC.

accumulation of gene mutations. EAC is one of cancers with a high rate of gene mutation. In 2013, a study conducted by Dulak et al. performed whole-genome sequence analysis on 149 pairs of EAC versus normal tissue samples and identified a total of 17,383 mutations in 8331 genes in EAC specimens, including 16,516 non-silent mutations and 1954 insertion–deletion-null mutations [24]. Of these genes, 26 were significantly mutated. As seen in ESCC, TP53 and CDKN2A were on top of the list. One of the differences, however, is A to C base transversion that is more common in EAC while C to A is more common in ESCC. The second theory involves a massive chromosomal instability due to the inactivation of p53 and p16. Loss of TP53 has been shown to increase the possibility of malignance by 16-fold [21]. Without functional p53, aneuploidy develops, which increases the pace of genome doubling. CDKN2A is the gene coding for p16, a cyclin-dependent kinase inhibitor that can sequester MDM2 and thereby prevent p53 being degraded. Inactivation of CDKN2A can be interpreted as a reinforcement to the elimination of p53. Table 2 lists the top five mutated genes and top five overexpressed genes associated with EAC.
