One of the main features of neurofibromatosis type 1 (NF1) is benign 
neurofibromas, 10-20% of which become transformed into malignant peripheral 
nerve sheath tumors (MPNSTs). The molecular basis of NF1 tumorigenesis is, 
however, still unclear. Ninety-one tumors from 31 NF1 patients were screened for 
gross changes in the NF1 gene using microsatellite/restriction fragment length 
polymorphism (RFLP) markers; loss of heterozygosity (LOH) was found in 17 out of 
91 (19%) tumors (including two out of seven MPNSTs). Denaturing high performance 
liquid chromatography (DHPLC) was then used to screen 43 LOH-negative and 10 
LOH-positive tumors for NF1 microlesions at both RNA and DNA levels. Thirteen 
germline and 12 somatic mutations were identified, of which three germline 
(IVS7-2A>G, 3731delT, 6117delG) and eight somatic (1888delG, 4374-4375delCC, 
R2129S, 2088delG, 2341del18, IVS27b-5C>T, 4083insT, Q519P) were novel. A mosaic 
mutation (R2429X) was also identified in a neurofibroma by DHPLC analysis and 
cloning/sequencing. The observed somatic and germline mutational spectra were 
similar in terms of mutation type, relative frequency of occurrence, and 
putative underlying mechanisms of mutagenesis. Tumors lacking mutations were 
screened for NF1 gene promoter hypermethylation but none were found. 
Microsatellite instability (MSI) analysis revealed MSI in five out of 11 MPNSTs 
as compared to none out of 70 neurofibromas (p=1.8 x 10(-5)). The screening of 
seven MPNSTs for subtle mutations in the CDKN2A and TP53 genes proved negative, 
although the screening of 11 MPNSTs detected LOH involving either the TP53 or 
the CDKN2A gene in a total of four tumors. These findings are consistent with 
the view that NF1 tumorigenesis is a complex multistep process involving a 
variety of different types of genetic defect at multiple loci.