No phenotypes in Ttc22 knockout mice

Despite previous reports linking TTC22 to colon adenocarcinoma metastasis in humans, this study demonstrates that CRISPR-Cas9-mediated inactivation of the Ttc22 gene in C57BL/6N mice results in no observable phenotypic abnormalities, including normal development, fertility, and tumor incidence.

The Gist

The Big Question: What happens if we remove a specific "instruction manual" from a mouse?

Imagine the DNA inside a mouse is like a massive, 20,000-page instruction manual for building and running a living creature. One specific page in that manual is called TTC22.

Scientists already knew that in humans, this "TTC22 page" seems to be missing or broken in many cases of colon cancer. In fact, previous studies suggested that when this page is working, it actually helps cancer spread. This led scientists to a big question: "If we delete this page from a mouse's manual, will the mouse get sick? Will it be immune to cancer? Or will it just be a normal, healthy mouse?"

The Experiment: Building the "TTC22-Free" Mouse

To find out, the scientists used a high-tech pair of molecular scissors (called CRISPR-Cas9) to cut out a specific chunk of the TTC22 gene in mice. They targeted the part of the gene that acts like the "engine" of the protein (specifically exons 2 and 3).

Think of it like taking the engine out of a car. If you take the engine out, the car shouldn't run, right? The scientists confirmed that in these new mice, the TTC22 protein was completely gone. The "engine" was definitely missing.

The Results: The "Ghost" in the Machine

Here is where the story gets surprising. The scientists expected that removing this gene would cause major problems, perhaps making the mice sick, stunted, or immune to cancer.

Instead, they found nothing.

• The "Ghost" Analogy: Imagine you take a very important-looking tool out of a Swiss Army Knife, expecting the knife to fall apart. Instead, you find that the knife works perfectly fine. The other tools just picked up the slack.
• Growth and Health: The mice without the TTC22 gene grew up just like normal mice. They weighed the same, looked the same, and had the same number of babies as the normal mice.
• The Cancer Test: The scientists then tried to give these mice colon cancer using a chemical cocktail (AOM/DSS). Usually, this makes mice develop tumors.
  • The Result: The mice without the TTC22 gene got cancer at the exact same rate and with the same severity as the normal mice. The missing gene didn't protect them, nor did it make them super-sick.

Why Did This Happen? (The "Backup Plan")

If the gene is gone, why didn't the mouse suffer? The scientists looked at the mouse's cells and found a clue: The cells had a backup plan.

• The Orchestra Analogy: Imagine TTC22 was a specific violinist in an orchestra. The scientists expected that if they kicked the violinist out, the music would sound terrible. Instead, they found that the other musicians (other genes) immediately stepped up, played louder, and adjusted their notes to fill the empty spot. The music (the mouse's biology) sounded exactly the same as before.
• The "Adaptive Response": The study showed that when TTC22 was missing, the mouse's cells turned on a "panic button" that activated hundreds of other genes to help the cell cope. It's like a house losing a power line; the neighbors immediately ran over with extension cords to keep the lights on.

The Takeaway

This paper teaches us a valuable lesson about biology: Nature is incredibly robust.

Even though TTC22 seemed important in human cancer studies, in a living mouse, it turns out to be dispensable. The mouse has so many "safety nets" and "backup systems" that removing one specific part doesn't break the whole machine.

In short: The scientists tried to break the mouse by removing a gene they thought was crucial. The mouse didn't even notice. It just found a way to keep humming along, proving that biology is full of redundant, backup systems that keep us alive even when parts of our genetic manual go missing.

Technical Summary

1. Problem Statement

The study addresses a discrepancy between previous human molecular findings and the lack of in vivo validation.

• Background: The human TTC22 gene encodes a protein with seven tetratricopeptide repeats (TPRs) involved in protein-protein interactions. Previous studies indicated that TTC22 is downregulated in human colon adenocarcinoma (COAD) and that its upregulation promotes metastasis via an m6A-mediated mechanism involving the TTC22-RPL4-WTAP-SNAI1 axis.
• The Gap: While human data suggested a critical role for TTC22 in colon cancer progression and metabolism (linked to obesity and lipid profiles), the physiological necessity of Ttc22 in a whole organism was unknown.
• Objective: To determine if the inactivation of Ttc22 affects mouse development, fertility, metabolism, and susceptibility to chemically induced colon cancer (COAD).

2. Methodology

The researchers utilized a genetically engineered mouse model and a comprehensive suite of phenotypic and molecular assays.

• Mouse Model Generation:
  • Used a commercially available C57BL/6N mouse model where exons 2 and 3 of the Ttc22 gene (encoding the TPR4 domain) were deleted via CRISPR-Cas9.
  • Generated three genotypes: Wild-type (Ttc22+/+), Heterozygous (Ttc22+/-), and Homozygous Knockout (Ttc22-/-).
  • Validated the deletion via PCR sequencing and RNA sequencing (RNA-seq).
• Protein Validation:
  • Developed a rabbit polyclonal antibody against mouse Ttc22 to confirm protein loss.
  • Performed Western blotting on colon tissues and overexpressed cell lines (HEK293T) to verify that the antibody detects both wild-type and TPR-deleted variants, confirming that the knockout resulted in a complete absence of the protein in Ttc22-/- mice.
• Phenotypic Assessments:
  • Development & Growth: Monitored body weight weekly from 4 to 31 weeks.
  • Metabolism: Fed female mice a high-fat diet (45% calories from fat) to test susceptibility to obesity-related metabolic changes.
  • Fertility: Conducted mating tests between Ttc22-/- pairs vs. wild-type pairs to assess litter size and reproductive success.
  • Spontaneous Tumors: Observed mice up to 40+ weeks for spontaneous tumor incidence.
• Cancer Susceptibility Model (AOM/DSS):
  • Induced colon cancer in male mice using the Azoxymethane (AOM) and Dextran Sodium Sulfate (DSS) protocol (three cycles).
  • Evaluated tumor incidence, size, and histology.
• Molecular Analysis:
  • RNA Sequencing: Performed on colon tissues of 58-week-old mice to identify differentially expressed genes (DEGs).
  • m6A Quantification: Measured total RNA m6A levels using ELISA and LC-MS/MS to test if Ttc22 loss affected the previously reported m6A modification pathway.

3. Key Results

• Successful Knockout: The CRISPR-Cas9 deletion of exons 2 and 3 completely abolished Ttc22 mRNA and protein expression in the colon tissues of homozygous (Ttc22-/-) mice.
• No Physiological Phenotypes:
  • Growth & Development: No significant differences were observed in body weight, growth curves, or physical appearance between Ttc22-/-, Ttc22+/-, and wild-type mice, regardless of diet (regular vs. high-fat).
  • Fertility: Ttc22-/- mice exhibited normal fertility and litter sizes comparable to wild-type controls.
  • Spontaneous Tumors: No difference in the incidence or latency of spontaneous tumors (lymphoma, sebaceous carcinoma, basal carcinoma) was found across genotypes up to 40 weeks of age.
• No Effect on Colon Carcinogenesis:
  • In the AOM/DSS-induced COAD model, the incidence, severity, and histological characteristics of colon tumors were statistically similar between Ttc22-/-, Ttc22+/-, and wild-type mice.
  • Two Ttc22-/- mice died during the experiment, but this did not skew the overall tumor burden analysis significantly.
• Molecular Adaptation:
  • m6A Levels: Contrary to previous in vitro findings, total RNA m6A levels in the colon were unchanged in Ttc22-/- mice.
  • Transcriptomic Changes: RNA-seq revealed 528 differentially expressed genes (303 upregulated, 225 downregulated). Gene Ontology (GO) analysis showed upregulation of genes related to protein folding, stress response, and host defense against microorganisms.

4. Key Contributions

• Validation of Gene Redundancy: The study provides robust in vivo evidence that Ttc22 is dispensable for mouse development, fertility, and metabolism under physiological conditions.
• Decoupling Human and Mouse Phenotypes: It highlights that while TTC22 acts as an oncogene in human colon cancer cell lines, its total loss in a whole mouse model does not recapitulate the expected cancer susceptibility, suggesting species-specific mechanisms or compensatory pathways.
• Identification of Compensatory Mechanisms: The upregulation of stress-response and protein-folding genes in the absence of Ttc22 suggests that the organism activates a broad adaptive response to compensate for the loss of this chaperone-like protein.
• Methodological Rigor: The study confirms the complete loss of protein expression using a custom antibody and validates the knockout at both the genomic and transcriptomic levels before concluding the lack of phenotype.

5. Significance

• Re-evaluation of TTC22 Function: The findings challenge the assumption that TTC22 is a critical driver of colon cancer in vivo. It suggests that the pro-metastatic role observed in human cell lines may be context-dependent or compensated for by other scaffold proteins in a living organism.
• Implications for Gene Function Studies: The paper underscores the importance of in vivo validation. A gene can be essential in specific cellular contexts (e.g., metastasis in a monolayer) but dispensable in a complex organism due to genetic robustness and redundancy.
• Future Directions: The study suggests that future research into TTC22 should focus on identifying the redundant scaffold proteins that compensate for its loss or investigating specific environmental stresses (beyond AOM/DSS) that might unmask a phenotype. It also cautions against relying solely on in vitro data for therapeutic targeting without considering compensatory mechanisms.