Mitotic recombination effects homozygosity for NF1 germline mutations in neurofibromas

Pure populations of neurofibroma-derived Schwann cells bearing both NF1 mutated alleles (NF1-/-) have been isolated from different neurofibromas showing loss of heterozygosity of nearly the entire 17q chromosome. By comparing molecular and fluorescent in situ hybridization analysis of these cells, we demonstrate mitotic recombination is the mechanism underlying this type of loss of heterozygosity leading to reduction to homozygosity of NF1 germline mutation.

Schwann cells harbor the somatic NF1 mutation in neurofibromas: evidence of two different Schwann cell subpopulations

Neurofibromas are one of the most characteristic features of neurofibromatosis type 1 (NF1), an inherited autosomal-dominant neurogenetic disorder affecting 1 in 3500 individuals worldwide. These benign tumors mainly consist of Schwann cells (SCs) and fibroblasts. Recent evidence demonstrates that somatic mutations at the NF1 gene are found in neurofibromas, but it has not been demonstrated whether SCs, fibroblasts and/or both cell types bear a somatic loss of NF1. We recently established a cell culture system that allows selective expansion of human SCs from neurofibromas. We cultured pure populations of SCs and fibroblasts derived from 10 neurofibromas with characterized NF1 mutations and found that SCs but not fibroblasts harbored a somatic mutation at the NF1 locus in all studied tumors. Furthermore, by culturing neurofibroma-derived SCs under different in vitro conditions we were able to obtain two genetically distinct SC subpopulations: NF1(-/-) and NF1(+/-). These data strongly support the idea that NF1 mutations in SCs, but not in fibroblasts, correlate to neurofibroma formation and demonstrate that only a portion of SCs in neurofibromas have mutations in both NF1 alleles.

Long-term culture and characterization of human neurofibroma-derived Schwann cells

Neurofibromas are benign tumors arising from the peripheral nerve sheath and are a typical finding in neurofibromatosis type 1 (NF1). Schwann cells are the predominant cell type in neurofibromas and thus are supposed to play a major role in the pathogenesis of these tumors. It is not known, however, if NF1 mutations in Schwann cells result in an altered phenotype that subsequently leads to tumor formation. To characterize the biological properties of neurofibroma-derived Schwann cells we developed cell culture techniques that enabled us to isolate Schwann cells from neurofibromas and grow them in vitro for several weeks without significant fibroblast contamination. Neurofibroma-derived Schwann cells were characterized by altered morphology, heterogeneous growth behavior, and increased expression of the P0 antigen while several other features of normal human Schwann cells were retained. We conclude that neurofibroma-derived Schwann cells exhibit a distinct phenotype in vitro but that the observed abnormalities by themselves are insufficient to explain neurofibroma formation. Application of our improved culture conditions makes neurofibroma-derived Schwann cells readily available for further studies to define their role in tumorigenesis in neurofibromatosis type 1.