An engineered zinc finger protein repressor of TrkA reduces nociception in a mouse model of bone cancer pain

Nerve growth factor (NGF), through its tyrosine kinase receptor (TrkA), plays an important role in nociception. Agents that broadly interfere with either NGF or TrkA function have been shown to have analgesic effects, illustrating the potential of targeting NGF/TrkA for pain management. However, successful therapies will likely require therapeutic molecules with superior specificity for NGF/TrkA, along with delivery methods that selectively target nociceptive neurons. Engineered zinc finger protein transcription factors (ZFP TFs) function at the DNA level, and can be made to target virtually any gene with high specificity. We have designed a ZFP TF that represses TrkA transcription and renders cells insensitive to NGF in culture. We then tested whether the TrkA repressor can impact nociceptive signaling in vivo when delivered by a non-replicating Herpes Simplex Virus (HSV) vector. Bone cancer pain was induced in mice by injecting NTCT 2472 osteolytic sarcoma cells into the medullary space of the right femur; one week later the ipsilateral lumbar dorsal root ganglia (DRG) were transduced by subcutaneous inoculation into the footpad with HSV vectors that express either the ZFP-TrkA-repressor, proenkephalin (positive control), or GFP (negative control). Pain behavior was assessed 1 and 2 weeks after vector inoculation using a 4-point spontaneous ambulatory pain score (SAPS), and mechanical allodynia (MA) was measured using Von Frey filaments. We found that tumor-bearing mice inoculated with the ZFP vector had significantly reduced SAPS and MA at both the 1- and 2-week time points compared to those inoculated with GFP or PBS. The analgesia achieved by the ZFP-TrkA-repressor was similar to that provided by the proenkephalin vector. These first proof-of-principle data highlight the potential for the engineered ZFP TFs technology in combination with neurotropic delivery via HSV in the treatment of chronic pain.