A Genetically Engineered Mouse Model of Sporadic Colorectal Cancer

The overall goal of this procedure is to achieve segmental adeno-cre infection of the colon in a genetically engineered mouse model. This method can help answer key questions in the field of colorectal cancer, or CRC, as it provides an excellent model for studying the biology and treatment of CRC. The main advantage of this technique is that any lab with small rodent surgery experience can use this model to produce highly reproducible and easily accessible colon-localized tumors.

Begin by placing an anesthetized mouse in the supine position and apply ointment to the animal's eyes. Tape the mouse in a secure position and then confirm the appropriate level of sedation by toe pinch. Make an approximately 15-millimeter midline incision in the skin of the lower abdomen.

Then, use forceps to grasp the abdominal wall musculature and carefully incise the muscle with scissors to open the abdominal cavity. After identifying the distal colon, carefully clinch the tissue with a delicate clamp approximately 15 millimeters proximal to the anus. Next, insert a flexible Teflon tube transanally carefully advancing the tube until it reaches the luminal occlusion.

Encannulate the tube with a 30-gauge cannula. Connect a standard one-milliliter syringe to the cannula and flush the colon with several milliliters of normal saline until all of the fecal matter has been evacuated. Once the distal colon is empty, replace the saline tube with a new Teflon tube and use a second clamp to occlude the colon approximately three millimeters distal to the first clamp.

Using a second one-milliliter syringe equipped with a 30-gauge cannula, carefully inject 50 to 80 microliters of 0.25%trypsin-EDTA into the clamped colon segment. The colon should inflate enough to break up the mucosal barrier without perforating the clamped tissue segment. After 10 minutes, remove the distal clamp.

Then, remove the trypsin tube and flush the distal colon with 500 microliters of normal saline. After inserting a new Teflon tube, clamp the colon again and inflate the tissue with 50 to 80 microliters of the adenoviral solution. After 30 minutes, remove the clamps and the tube and close the abdominal wall with 6-0 rapidly absorbable running sutures.

Close the skin with surgical wound clips and monitor the animal on a heating pad until it is fully recovered. To monitor tumor growth via colonoscopy, place the anesthetized tumor-bearing animal in the supine position on a heating pad and transanally insert the endoscope into the intestinal tract. Gently insufflate air under visual control to distend the colon.

Then, carefully push the scope forward until a mucosal lesion can be identified in the distal colon saving each endoscopic image as it is acquired for later evaluation. When all of the images have been obtained, slowly remove the endoscope and place the mouse on a 38-degree-Celsius heating pad until fully recovered. The tumors can be easily and repeatedly monitored via colonoscopy without major stress for the animals.

The disease manifestations are very similar to the human colorectal cancer with the animals first developing adenomas followed ultimately by adenocarcinoma development in the distal colon. Depending on the conditional genotype of the animals, the tumors also metastasize to the peritoneum and the liver. If a fluorescent cre-reporter allele is used, the tumor manifestations can be easily identified by fluorescent protein expression which depending on the reporter allele, may be bright enough to be visualized in daylight.

Histologically, 95%of the developing tumors are adenocarcinomas closely resembling human colorectal cancer and feature the entire spectrum of tumor development from non-invasive adenoma to adenocarcinoma invading the surrounding structures. Once mastered, this technique can be completed in 50 minutes if it's performed properly. While attempting this procedure, it is important to remember to give special attention to the vulnerability of the colon at all times.

Perforation inevitably leads to peritonitis and sepsis and requires a termination of the enema. Don't forget that working with adenovirus requires biosafety level two safety measures as the virus can be hazardous.