Patient-derived Xenograft Modeling: A Technique to Generate Melanoma Mouse Models

All procedures involving animal models have been reviewed by the local institutional animal care committee and the JoVE veterinary review board.

1. Tumor tissue processing for mouse implantation

1. Surgical excision or surgical biopsy tissue processing
   1. Transfer the tissue to a sterile Petri dish and separate the tumor tissue from the surrounding normal tissue as much as possible.
   2. Remove necrotic tissue (usually identified as pale-whitish tissue located centrally within the tumor) from the remaining tumor as much as possible.
   3. Use a scalpel to subdivide an initial tumor chunk into approximately equal pieces (~3 mm x 3 mm) for surgical mouse implantation (Figure 1).
   4. Optionally, if enough tumor tissue is available, snap-freeze the tissue for downstream assays (RNA sequencing [RNASeq], whole exome sequencing [WES], etc.).
   5. Make a tumor slurry by mincing the tumor tissue using a cross blade technique with two scalpel blades. Mince the tumor chunks as finely as possible to form a slurry, which is now ready for surgical mouse implantation.
   6. Alternatively, if the tumor tissue is too hard for mechanical dissociation, use a digestion dissociation procedure to form a gel-like slurry and a single-cell suspension for implantation and/or injection.
      1. Mince the tumor chunks as finely as possible to form a slurry.
      2. Put the slurry in a 50 mL tube with cold Hank's balanced salt solution (HBSS)^-/- (without Ca⁺⁺ and Mg⁺⁺); then, centrifuge the pellet at 220 x g for 4 min at 4 °C.
      3. Resuspend the slurry in 10 mL of warmed fresh digest media (200 U/mL collagenase IV + 5 mM CaCl₂ + 50 U/mL DNase in HBSS^-/-) per 1g of tumor tissue.
      4. Place the tube in a 37 °C water bath for 20 min and mix vigorously every 5 min with a disposable pipette.
      5. Wash with up to 50 mL of HBSS^-/-; then, centrifuge at 220 x g for 4 min at 4 °C.
      6. Add 5 mL of prewarmed TEG (0.025% trypsin + 40 µg/mL ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid [EGTA] + 10 µg/mL polyvinyl alcohol [PVA]) per 1 g of tumor tissue, gently resuspend/shake, and place the tube at 37 °C for 2 min without mixing.
      7. Add at least 1 equal volume of cold staining media (1% bovine serum albumin [BSA] + 10 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid [HEPES] + 1x penicillin-streptomycin in L15 media) to quench the trypsin and centrifuge at 220 x g for 4 min at 4 °C.
      8. Resuspend the sample in 10 mL of staining media per 1g of tumor tissue and filter it through a 40 µm cell strainer to get a single-cell suspension for mouse injection (Figure 1).
      9. Slurry remaining on top of the cell strainer can also be collected for surgical mouse implantation.
2. Tumor implantation and injection in mice
   1. Implantation of surgical excision or surgical biopsy tissue
      NOTE: Ensure all surgical instruments are sterile by autoclaving or the use of pre-sterilized disposable instruments.
      1. Shave hair from the lower back of NSG 6-8 week male or female mice leaving an approximately 1.5 cm x 3 cm area with no hair. Anesthetize mice using isoflurane and confirm by gently squeezing the foot as a test of responsiveness. Use vet ointment on their eyes to prevent dryness.
      2. Place individual mice on a heat pad in the nose cone of the anesthesia machine, scrub the shaved area with chlorhexidine. Then, douse with 70% ethanol and allow it to evaporate.
      3. Prepare chunks or divide tumor slurry in a Petri dish into individual mounds for surgical implantation (i.e., into three equal mounds if to be implanted into 3 mice).
      4. Using the scalpel blade, make an incision of approximately 5 mm long on the center of the back of the mouse, take one pair of forceps and lift up the skin on the side of the incision opposite of the operator.
      5. Take the scissors into the other hand and separate the skin from the muscle layer by gently cutting the fascial membrane with small scissor cuts, thereby creating a "pocket" for the tumor tissue.
      6. Pick up one tumor chunk or one individual mound of tumor slurry tissue with the scalpel blade and gently place tissue into the created pocket.
      7. Administer 100 µL of artificial extracellular matrix on the tumor tissue mound in the pocket.
      8. Using two pairs of forceps, pull up the incision on both ends so that the wound edges come close together, and close the wound by applying one or two wound clips.
      9. Subcutaneously inject 1-5 mg/kg meloxicam as an analgesic in mice after surgery.
      10. Take the mouse out of the nose cone and place it back into its original cage; observe the mouse while waking up. Do not return to a cage until fully recovered.
      11. Remove wound clips after approximately 7 days. If healing is not complete after 7 days, leave the wound clip in for an additional one or two days.
          NOTE: If using a single cell suspension from surgical excision or surgical biopsy tissue processing, it will mix with artificial extracellular matrix (at 1:1 ratio) for mice injection.
   2. Injection of FNA or core biopsy tissue
      1. Place an NSG mouse on a steel grid rack, hold the mouse firmly by the tail, and gently pull the mouse back. It will grasp the grid with its front legs firmly. Alternatively, restrain the mouse in the non-dominant hand and let the flank area visible.
      2. Disinfect the skin of the flank with alcohol prep swabs. Slowly and steadily inject the contents of the syringe under the skin of the mouse.
      3. Pull out the needle and place the mouse back into its cage.
3. Monitor tumor growth
   1. Monitor mice once weekly to check for palpable tumors.
   2. Once tumors are at a measurable size (approximately 50 mm³), use a caliper to record tumor dimensions. Use the following formula to calculate tumor volumes: (width x width x length) / 2.
   3. Harvest tumor once the tumor volume reaches around 1.5 cm³ (approximately 4-10 weeks). The tumor is now called mouse passage 1 (MP1).