Histopathological and immunohistochemical characterization of a new murine experimental model for the study of Neurofibromatosis type 1 (NF1)

Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disorder characterized by loss-of-function mutations in the NF1 tumor suppressor gene in humans and occasionally reported in other species. NF1 patients are predisposed to the development of malignant tumors arising from Schwann cells. Improvement in molecular biology, neuroimaging, and the development of new animal models contributed to elucidate the pathogenesis of NF1 and to test therapeutic approaches. To this aim we developed and characterized a new murine model of NF1-related malignant tumors with interesting results in terms of reduced timing of onset and reproducibility of lesions if compared to existing models. Specifically, tamoxifen chow was administered to NF1flox/flox; p53flox/flox; PLP-Cre+ mice for NF1 and p53 gene specific ablation in Schwann cells. After nerve damage, sciatic nerve was monitored through ultrasound inspection performing 3D reconstruction. To evaluate the development of neoplasia along the damaged sciatic nerve and to follow the growth we established four different groups (n= 6 per group) and mice were humanely sacrificed at 15, 30, 45- and 60-days post-surgery (dps). Each group included 3 controls NF1flox/flox; p53flox/flox; PLP-Cre- (Cre-) and 3 NF1flox/flox; p53flox/flox; PLP-Cre+ (Cre+) mice. All mice underwent necropsy and the main organs were sampled and submitted for histopathological and immunohistochemical (IHC) investigations. Interestingly, the lesion was detectable in Cre- and Cre+ mice at 15 days, but at 30-45-60 days nerve enlargement was not present in Cre- mice. A growing trend was noticed in Cre+ mice at 30, 45 and 60dps in the damaged sciatic nerve whereas the contralateral one was unremarkable. Histopathology (H&E) of the sciatic nerves showed proliferation of Schwann cells and perineurial cells at 15dps in both Cre- and Cre+ groups and interpreted as a regenerative attempt after trauma. Similar changes, with lower severity were observed at 30dps in Cre- mice with complete regeneration of the nerve at 60dps. As expected, the Cre+ groups at 30, 45 and 60dps showed a constant growth of the sciatic lesion in time with haphazardly proliferating Schwann cells showing atypical features morphologically consistent with nerve sheath tumors (NSTs). IHC for S-100 confirmed the diagnosis of NSTs and IBA-1 showed the presence of numerous tumor-associated-macrophages (TAMs). In conclusion this newly proposed experimental model for NF1 is able to develop and reproduce NSTs that mirror a subset of tumors associated to human NF1 in a reduced timeframe (30dps) if compared to existing animal models. Furthermore, this murine NST model recapitulates tumor heterogeneity of human NF1 associated tumors as indicated by the high macrophage infiltration rate. As a perspective, this model can be of interest to study new pathogenetical features and to identify novel therapeutical approaches.