The role and function of the Ras-related protein TC21 in Neurofibromatosis type 1
Neurofibromatosis type 1 is a common autosomal dominant disorder affecting 1in 3500 individuals worldwide. Neurofibromin, the protein mutated in NF1 disease, is a GTPase activating protein (GAP) for Ras proteins, inactivating the Ras proteins H-Ras, N-Ras, K-Ras, M-Ras, R-Ras, and TC21. Missense mutations in the GAP related domain of neurofibromin cause NF1 disease, indicating that increased Ras activity is likely critical for disease pathogenesis. Loss of NF1 in Schwann cells causes formation of benign tumors known as neurofibromas. These tumors can become malignant forming malignant peripheral nerve sheath tumors (MPNSTs), which are a major source of morbidity for NF1 patients. TC21 is a member of the R-Ras family of small Ras GTPases. TC21 is an oncogene able to transform epithelial and fibroblast cell lines, and it is also capable of inducing tumors in vivo. PI3K is the predominant effector of TC21 transforming activity, and studies support the idea that not all features of NF1 mutant cells can be ascribed to the activation of the classical Ras proteins (e.g. H-, N-, K-Ras). We hypothesize that the effects of neurofibromin mutation that are unrelated to classical Ras-GTP may be explained by activation of the non-classical Ras protein TC21. The signaling pathway of TC21 is unclear. We activated all Ras proteins in vivo by deletion of Nf1 and using these mice along with mice deficient for TC21-/-, we examined the role and function of TC21 in development and tumorigenesis. Additionally, we used NF1-/- human MPNST cell lines with an acute loss of TC21 by shRNA to examine tumor growth in xenograft mice. We found that TC21 loss delayed benign neurofibroma formation in Nf1fl/fl;DhhCre mice. Nf1 loss increased mRNA encoding the cytokine transforming growth factor-beta (TGF-beta) and rendered Schwann cell progenitors insensitive to TGF-beta; these phenotypes could be rescued by TC21 and were mediated through TGF-beta receptors. Conversely, growth of Nf1;Trp53 brain tumors and NF1-/- MPNST sarcomas were accelerated by TC21 loss. MPNST from Nf1;Trp53 mice and NF1-/- MPNST xenografts had increased levels of TGF-beta mRNA and protein and blocking TGF-beta decreased sarcoma size induced by shTC21. The results are important because TGF-beta acts as a tumor suppressor in numerous types of benign tumors and is also known for its oncogenic role in cellular transformation and tumor progression. Our data elucidates TGF-beta as a therapeutic target in NF1 malignancy.
Molecular Biology, NF1, Ras, TC21, TGF-beta, neurofibroma, sarcoma
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