Familial Adenomatous Polyposis (FAP) Cell Models for Research

Familial Adenomatous Polyposis (FAP) is a rare inherited cancer predisposition syndrome characterized by the development of hundreds to thousands of colorectal adenomas, with a nearly 100% lifetime risk of colorectal cancer (CRC) if prophylactic colectomy is not performed. The incidence of FAP is estimated at 1 in 7,000 to 1 in 22,000 live births, accounting for less than 1% of all CRC cases (WHO). The average age of adenoma onset is 16 years, and CRC develops by age 40 if untreated. The 5-year survival for localized CRC is about 90%, but drops to 14% for distant metastases (NCI SEER). FAP also presents with extracolonic manifestations, including duodenal adenomas, desmoid tumors, and congenital hypertrophy of the retinal pigment epithelium (CHRPE). Research on FAP is crucial for understanding the early stages of colorectal carcinogenesis and for developing targeted prevention and therapeutic strategies.

FAP serves as an ideal model for studying the adenoma-to-carcinoma sequence, as the progression from benign polyps to malignant tumors is well-defined and driven by a single initiating genetic event: germline mutations in the APC gene. This monogenic cause allows researchers to dissect the molecular mechanisms of tumor initiation and progression in a controlled manner. FAP also provides a platform for testing chemopreventive agents and novel therapeutic interventions. Public datasets, such as those from TCGA and COSMIC, provide extensive genomic and transcriptomic data on FAP-associated tumors, enabling integrative analyses. Open questions include the role of modifier genes, the impact of specific APC mutation sites on phenotype, and the development of targeted therapies for desmoid tumors and duodenal cancer.

The primary pathway dysregulated in FAP is the Wnt/β-catenin signaling pathway, which is constitutively activated due to loss of APC function. Key steps include:

1. Loss of functional APC protein.

2. Accumulation of β-catenin in the cytoplasm.

3. Translocation of β-catenin to the nucleus.

4. Activation of TCF/LEF transcription factors.

5. Upregulation of target genes such as MYC, CCND1, and MMP7.

Additionally, the PI3K/AKT pathway is often activated through mutations in PIK3CA or loss of PTEN, contributing to cell survival and proliferation. The MAPK pathway (RAS-RAF-MEK-ERK) is also frequently activated via KRAS mutations, promoting cell growth. Defects in DNA mismatch repair (MMR) may occur as secondary events, leading to microsatellite instability in a subset of tumors.

Data from TCGA and COSMIC.

The major signaling networks deregulated in FAP include:

• • Wnt/β-catenin pathway: Central to FAP pathogenesis. Key nodes: APC, β-catenin (CTNNB1), AXIN, GSK3β, TCF/LEF.
• • PI3K/AKT/mTOR pathway: Often activated via PIK3CA mutations or PTEN loss. Key nodes: PI3K, AKT, mTOR, PTEN.
• • MAPK/ERK pathway: Frequently activated via KRAS or BRAF mutations. Key nodes: RAS, RAF, MEK, ERK.
• • TGF-β/SMAD pathway: Inactivated via SMAD4 mutations, leading to loss of growth inhibition. Key nodes: TGFBR, SMAD2/3, SMAD4.
• • p53 pathway: Inactivated via TP53 mutations, affecting cell cycle arrest and apoptosis. Key nodes: p53, MDM2, CDKN1A.

Organoids derived from FAP patient tissues recapitulate the adenoma-to-carcinoma progression and can be used for drug testing and personalized medicine approaches. They maintain the genetic heterogeneity of the original tumor and can be genetically manipulated using CRISPR.

• • Patient-derived xenografts (PDX): Implantation of FAP tumor tissue into immunodeficient mice, preserving tumor heterogeneity and allowing drug efficacy testing.
• • Genetically engineered mouse models (GEMM): The ApcMin/+ mouse is the most common model, carrying a truncating mutation in Apc, developing multiple intestinal adenomas. Other models include ApcΔ716 and Apc1638N.
• • Induced models: Use of Cre-lox systems to conditionally knock out Apc in the intestinal epithelium, allowing temporal and spatial control of tumor initiation.
• • Carcinogen-induced models: Treatment of mice with azoxymethane (AOM) combined with dextran sulfate sodium (DSS) induces colonic tumors, but these are not specific to FAP.

CRISPR-Cas9 technology enables the generation of isogenic cell lines with precise genetic modifications, such as knockout of APC or knock-in of specific mutations. These models are invaluable for studying the functional consequences of individual mutations in a controlled genetic background. For example, an APC knockout in a wild-type colorectal cell line can recapitulate the loss of APC function and activate Wnt signaling. Conversely, a knock-in of a common APC mutation (e.g., Q1338*) can be introduced into a cell line to study its specific effects. Commercially available, sequence-verified gene-edited cell models accelerate research by providing validated tools that save time and ensure reproducibility. These models are widely used for drug screening, target validation, and mechanistic studies.

Gene-edited cell lines are used to validate the role of specific genes in FAP pathogenesis. For instance, knocking out APC in a colorectal cell line leads to constitutive Wnt activation, confirming its tumor suppressor function. Similarly, introducing a KRAS G12V mutation into a cell line can demonstrate its oncogenic potential. CRISPR screens using pooled libraries can identify genes that are synthetic lethal with APC loss, providing potential therapeutic targets.

Isogenic cell line pairs, differing only in a specific genetic alteration, are ideal for drug screening. For example, an APC knockout cell line and its wild-type counterpart can be used to identify compounds that selectively kill APC-deficient cells. Gene-edited models can also be used to study drug resistance mechanisms by exposing cells to increasing concentrations of a drug and identifying acquired mutations.

CRISPR-based synthetic lethality screens can identify biomarkers that predict response to targeted therapies. For instance, loss of APC may confer sensitivity to inhibitors of β-catenin or its downstream effectors. Gene-edited cell lines can be used to validate candidate biomarkers and to develop companion diagnostics.