Colorectal MAC is one of the poorest prognostic types of histological diagnoses, and its pathogenesis is not well understood. In this study, in silico analysis using the TCGA database found that there is a significant difference in CNA of Chr20q between MAC and NMAC. Moreover, the high expression of Chr20q genes is specifically associated with low expression of MUC2, which is a key molecule that can be used to distinguish the two histological types. Thus, the genes on Chr20q were considered to be important for defining MAC. We further found 475 DEGs, and some of the Chr20q genes were considered to be hub genes to define MAC.
The amplification of Chr20q in CRCs has been well known for a long time. It is found in the majority of CRCs, suggesting that it plays an important role in the process of ACS.18 In particular, it is reported as a key driver of the change from adenoma to carcinoma.21,25−27 In addition, it was recently reported to be associated with MSI tumors and shows better prognosis.28 As such, various findings exist for tumors with Chr20q amplification. However, comparisons to tumors without amplification have not been well performed. In this study, we found that the absence of Chr20q amplification is a histologically important factor that distinguishes MAC from other types of cancer. Considering that MAC is generally associated with microsatellite stable (MSS) or poor prognosis, our finding is consistent with previous reports.
Searching for Chr20q genes in DEGs that are negatively correlated with MUC2 expression narrowed the list to 6 genes, which were further matched with protein–protein interaction analysis, leading to POFUT1 and PLAGL2 as significant genes. In vitro experiments suggested that these proteins regulate MUC2 expression in CRC cells. Furthermore, their expression was indeed lower in MAC. These two genes are reported to share transcription factor-binding sites and have been experimentally shown to drive cancer progression synergistically.24 Wnt signaling and Notch signaling, which involve these genes, are both known to be important for CRC development, cell differentiation and homeostasis. In the differentiation process from tissue stem cells to secretory cells, inactivation of both Wnt and Notch signaling is necessary.29 Although there is no clear evidence of whether there is a molecular correlation between cancer and normal tissue in terms of histological differentiation, it is interesting to note that there may be a common signaling pathway that promotes differentiation into the secretory cell lineage.
In understanding the difference between MAC and NMAC, it is often unclear whether the origin is different or whether it is caused by differentiation plasticity among cells. According to the cancer stem cell concept, although there is one cell of origin, it is also known that heterogeneity is formed by the establishment of a hierarchy.30 In gastric cancer, mucinous features are reported to be a differentiated form of poorly differentiated adenocarcinoma, and there is plasticity by regulating Wnt signaling.31 The results from our study suggested that Wnt and/or Notch signaling-related genes were identified as pivotal genes, which was considered to be consistent.
It is unclear whether high MUC2 expression itself acts as a promoter of tumor growth, as there are both positive and negative reports.32,33 Some believe that the cells expressing the mucin are more important than the mucin itself.34 From our in vitro experiment showing that MUC2 expression was controlled by POFUT1 and PLAGL2, we obtained a similar impression. In other words, the expression of MUC2 and the histological appearance of MAC are a consequence of Chr20q amplification and its associated protein interactions and signaling pathways are important and may be a potential therapeutic target.
The current study has not necessarily clarified the relationship between the amplification of the 20q genes and the mechanism of carcinogenesis. However, it can be suggested that changes at the chromosome level drive gene expression involved in cell differentiation and consequently define the histological type. It is still unclear how these factors affect treatment resistance and susceptibility, and further investigation is warranted.
In conclusion, genetic changes reflected by Chr20q amplification were suggested to be a key driver of MAC differentiation.