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Elevated Dnmt3a activity promotes polyposis in Apc(Min) mice by relaxing extracellular restraints on Wnt signaling
Michael S Samuel1, Hiromu Suzuki, Michael Buchert
1Ludwig Institute for Cancer Research, Royal Melbourne Hospital, Victoria, Australia.
Background & Aims:
Aberrant DNA methylation is a common early event in neoplasia, but it is unclear how this relates to dysregulation of DNA (cytosine-5) methyltransferases (Dnmts). Here we use knock-in transgenic mice to investigate the consequences of intestinal epithelium-specific overexpression of de novo Dnmt3a.
Methods:
A novel gene targeting strategy, based on the intestinal epithelium-specific, uniform expression of the A33 glycoprotein, is employed to restrict Dnmt3a overexpression in homozygous A33(Dnmt3a) mutant mice.
Results:
A33(Dnmt3a) mice infrequently develop spontaneous intestinal polyps. However, when genetically challenged, tumor multiplicity in A33(Dnmt3a);Apc(Min) compound mice is 3-fold higher than in Apc(Min) mice. Although we observe a requirement for spontaneous loss of heterozygosity of the adenomatous polyposis coli (Apc) gene to trigger tumorigenesis in Apc(Min) mice, lesions in A33(Dnmt3a);Apc(Min) mice frequently retain the wild-type Apc allele. However, epithelia from normal mucosa and polyps of A33(Dnmt3a);Apc(Min) mice show hypermethylation-mediated transcriptional silencing of the Wnt antagonists Sfrp5, and to a lesser extent, Sfrp1 and increased nuclear beta-catenin alongside activation of the Wnt-target gene Axin2/Conductin. Conversely, enforced Sfrp5 expression suppresses canonical Wnt-signaling more effectively in wild-type than in Apc(Min) cells.
Conclusions:
Aberrant activation of the canonical Wnt pathway, either by mono-allelic Apc loss or transcriptional silencing of Sfrp5 is largely insufficient to promote polyposis, but epistatic interactions between these genetic and epigenetic events enables initiation and promotion of disease. This mechanism is likely to play a role in human colorectal cancer, because we also show that elevated DNMT3A expression coincides with repressed SFRP5 and enhanced AXIN2/CONDUCTIN expression in paired patient biopsies.
Insights
Overexpressing DNA methyltransferase 3A (DNMT3A) in the intestine promotes colorectal cancer by silencing Wnt antagonists. This epigenetic dysregulation interacts with genetic mutations to drive tumor growth, mirroring findings in human colorectal cancer patients.
Area of Science:
- Epigenetics and Cancer Biology
- Gastrointestinal Oncology
- Molecular Mechanisms of Neoplasia
Background:
- Aberrant DNA methylation is an early hallmark of cancer.
- The role of DNA methyltransferases (Dnmts), specifically Dnmt3a, in intestinal neoplasia is not fully understood.
- Investigating Dnmt3a's impact on intestinal epithelium-specific gene expression and tumorigenesis is crucial.
Purpose of the Study:
- To investigate the consequences of intestinal epithelium-specific overexpression of de novo DNA methyltransferase 3A (Dnmt3a).
- To elucidate the relationship between Dnmt3a dysregulation and aberrant DNA methylation in cancer development.
- To explore the interplay between genetic and epigenetic events in colorectal tumorigenesis.
Main Methods:
- Utilized knock-in transgenic mice with intestinal epithelium-specific Dnmt3a overexpression, achieved via a novel A33 glycoprotein targeting strategy.
- Generated A33(Dnmt3a) and compound A33(Dnmt3a);Apc(Min) mutant mice to study spontaneous and genetically challenged tumorigenesis.
- Analyzed gene expression, DNA methylation patterns, and Wnt pathway activation in normal mucosa and tumor tissues.
Main Results:
- Dnmt3a overexpression alone rarely caused spontaneous polyps but significantly increased tumor multiplicity in Apc(Min) mice.
- Tumorigenesis in compound mice often retained the wild-type Apc allele, indicating a role beyond Apc loss.
- Hypermethylation-mediated silencing of Wnt antagonists (Sfrp5, Sfrp1) and activation of the Wnt pathway (increased beta-catenin, Axin2/Conductin) were observed in A33(Dnmt3a);Apc(Min) mice.
Conclusions:
- Aberrant Wnt pathway activation, driven by genetic (Apc loss) or epigenetic (Sfrp5 silencing) events, is insufficient alone for polyposis.
- Epistatic interactions between genetic and epigenetic events are critical for colorectal cancer initiation and progression.
- Elevated DNMT3A expression correlates with SFRP5 repression and AXIN2/CONDUCTIN activation in human colorectal cancer biopsies, suggesting conserved mechanisms.
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