Developing Novel Models for Assessing Fundamental Mechanisms of Neuroma

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Neuromas are painful tumors of nerve tissue that result from amputation or other nerve injury. Peripheral nerve injury is the most-impactful comorbidity associated with the most common types of battlefield trauma, and many of these cases eventually result in formation of a painful neuroma. This condition often requires additional invasive surgery for our Veterans, including resectioning of the injured nerve to remove the neuroma, which may form again. Diabetes, a chronic health problem that is a priority for VAMCs across the country, is another common cause of amputation. Neuroma pain (or “nerve pain” as it is commonly called in the amputee community) is also one of the major reasons patients reduce/stop use of assistive devices, including prosthetics. Further, presence of neuroma can be a disqualifying factor for some prosthetics and nerve interfaces. Although treatments are available, there is currently no clinical Standard of Care for prevention or treatment of neuroma because the available approaches are not sufficiently effective. Although essentially all unrepaired nerve injuries result in a neuroma, not all neuromas are painful. Unfortunately, there is no “safe window” for neuromas – pain can begin at any point after the nerve injury which induced the neuroma. Neuromas continue to grow and their structure evolves over time, likely introducing interactions that would not occur in normal tissue and developing emergent properties. These etiological factors suggest that, although sensory neurons are necessary for neuroma pain, it is the neuroma structure itself that may establish the physical relationships which lead to pain. Therefore, understanding what factors control formation of neuroma structure is necessary to develop approaches to prevent neuroma formation. Our plan, to be pursued in subsequent MERIT applications, will tests the hypothesis that it is regeneration of injured axons which controls neuroma formation, and that subsequent aberrant coupling of sympathetic or motor axons and/or immune cells with sensory axons is the major factor driving neuroma pain. To this end, this SPiRE proposal is designed to validate genetic models and experimental designs to enable us, and the field, to address fundamental questions which remain unanswered. This project will test the utility of a set of cell type- specific knockout and reporter mice for neuroma research. The premise for the proposed mouse lines is that neuroma formation is apparently completely prevented by global knockout of a gene required for axonal regeneration. The premise for the new experimental design is that no existing animal models reflect the clinical reality of the repeated nerve injury that occurs with neuroma resection and recurrence, despite the fact that the condition of repeated injury induces changes that differ from that of single injuries. This SPiRE project will provide the animal lines, visualization methods, and preliminary data vital for MERIT projects to determine mechanisms and test treatments.