Somatic NF1 mutational spectrum in benign neurofibromas: mRNA splice defects are common among point mutations

Neurofibromas, benign tumors that originate from the peripheral nerve sheat h, are a hallmark of neurofibromatosis type 1 (NF1). Although loss of heter ozygosity (LOH) is a common phenomenon in this neoplasia, it only accounts for part of the somatic NF1 mutations found. Somatic point mutations or the presence of "two hits" in the NF1 gene have only been reported for a few n eurofibromas. The large size of the NF1 gene together with the multicellula r composition of these tumors has greatly hampered their molecular characte rization. Here, we present the somatic NF1 mutational analysis of the whole set of neurofibromas studied by our group and consisting in 126 tumors der ived from 32 NF1 patients. We report the identification of 45 independent s omatic NF1 mutations, 20 of which are reported for the first time. Differen t types of point mutations together with LOH affecting the NF1 gene and its surrounding region or extending along the 17q arm have been found. Among p oint mutations, those affecting the correct splicing of the NF1 gene are co mmon, coinciding with results reported on germline NF1 mutations, In most c ases, we have been able to confirm that both copies of the NF1 gene are ina ctivated. We have also found that both somatic and germline mutations can b e expressed at the RNA level in the neoplastic cells. Furthermore, we have observed that the study of more than one tumor derived from the same patien t is useful for the identification of the germline mutation. Finally, we ha ve noticed that the culture of neurofibromas and their fibroblast clearance facilitates LOH detection in cases in which it is difficult to determine.