Serrated Colorectal Cancer: The Road Less Travelled?

Abstract

Studies of colorectal cancer (CRC) originating through the conventional adenoma-carcinoma sequence have provided insight into the molecular mechanisms controlling its initiation and progression. Less is known about the alternative 'serrated' pathway, which has been associated with BRAF mutation and microsatellite instability. Recent transcriptomics approaches to classify human CRC revealed that mesenchymal and/or desmoplastic features combined with an immunosuppressive microenvironment are key determinants of CRC with the poorest prognosis. Importantly, these aggressive CRCs harbor the characteristics of serrated tumors, suggesting that initiation through this alternative pathway determines how aggressive the CRC becomes. Here, we review recent evidence on how serrated carcinogenesis contributes to the subtype of CRC with the poorest prognosis.

Keywords: atypical PKC; colorectal cancer; immune checkpoint therapy; mesenchymal; microenvironment; serrated.

Figure 1.. Molecular characterization of two types of serrated CRC: classical and mesenchymal

Typical molecular features of the two types (“Classical (left)” vs “Mesenchymal (right)”) of serrated CRC proposed in this review based on the previously reported evidence. CIMP, CpG island methylation phenotype; CRC, colorectal cancer; MSI-H, microsatellite instability-high; MSI-H, microsatellite instability-low; MSS, microsatellite stable.

Figure 2.. The roles of the aPKCs is intestinal homeostasis and cancer

(A) The atypical protein kinases C (aPKCs) are part of the PKC family. Classification of the PKCs by subfamilies (atypical, conventional, novel). The protein names of the different members are indicated. Schematic showing structural domain organization characteristic of each PKC subfamily. The aPKCs contain a unique PB1 (Phox and Bem 1) and they do not harbor a C2 domain, which binds calcium. The C1 domain, which is critical for diacylglycerol binding, is not functional in the aPKCs.

(B and C) Schematic representation of the functional role of the aPKCs in intestinal homeostasis and cancer. PKCλ/ι and PKCζ are both expressed in intestinal epithelial cells (grey cells). PKCλ/ι is highly expressed in Paneth cells (violet cells in the crypt) and PKCζ in intestinal stem cells (green cells in the crypt). (B) PKCλ/ι regulates Paneth cell differentiation through inhibition of EZH2 stability and intestinal cell death via JNK. PKCλ/ι is a tumor suppressor in CRC through the inhibition of ERK and YAP. It also regulates the IFN response. (C) PKCζ maintains intestinal stemness and is a versatile tumor suppressor by inhibiting miR200, PHGDH, β-Catenin and YAP to limit EMT, cell survival, stem cell function and cell growth, respectively.

Figure 3.. A simplified model of the molecular evolution of each colorectal cancer subtype

Pathways depicted are based largely on evidence from mouse models. The conventional pathway is initiated by inactivation of the tumor suppressor APC in normal colonic epithelium which results in the formation of conventional-type adenoma, further followed by the additional sequential mutations of oncogenes or tumor suppressor genes to progress to adenocarcinoma (adenoma-carcinoma sequence). This type of CRCs generally displays chromosomal instability (CIN) with immune cold tumor microenvironment (TME). Oncogenic mutation in BRAF (less frequently in KRAS) leads to the development of serrated precursors such as microvesicular HPs and SSA/Ps. Hypermethylation in the CpG island promoter regions (CpG island methylation phenotype; CIMP) of tumor suppressor genes such as p16^INK4a results in the silencing of these genes and allow a complete malignant transformation of these serrated lesions (classical serrated pathway). Epigenetic silencing of MLH1 in these tumor cells leads to the development of MSI-H CRCs with enhanced tumor mutational burden, which invokes a strong immune response in TME. Simultaneous loss of both PKCλ/ι and PKCζ results in the rapid development of serrated benign to invasive carcinoma lesions. These tumors display the highly activated mesenchyme and EMT/CAF signatures accompanied by a highly immunosuppressive TME. Loss of CDX2 in combination with alteration in other tumor suppressors (such as APC), also display the mesenchymal/stromal tumor phenotype. The loss of aPKCs or the hyperproduction of TGFβ in the TME has been proposed to skew the serrated precursors from the classical to the mesenchymal subtype.

Figure 4.. Immune backgrounds and potential therapeutic strategy for classical and mesenchymal serrated CRC

(A) Tumor microenvironment (TME) of classical serrated CRC. This type of serrated tumor displays dense CD8⁺ T cell infiltrate (immunogenic), which is counterbalanced by the expression of checkpoint inhibitors, such as programmed cell death protein 1 ligand 1 (PD-L1). For this type of serrated CRC, the immune checkpoint inhibition therapy (ICI) inhibits the PD-1/PD-L1 interaction to reactivate the CD8 T cells, leading to tumor cell killing. (B) In the TME of mesenchymal serrated CRC, tumor cells interact with stromal cells including cancer-associated fibroblasts (CAFs) and suppressive immune cells, such as myeloid-derived suppressor cells (MDSCs) and regulatory T (Treg) cells, to exclude cytotoxic CD8⁺ T cells. ICI combined with TGFβ pathway inhibitors is proposed to suppress tumorigenesis in this serrated CRC subtype.