Clonal origin of tumor cells in a plexiform neurofibroma with LOH in NF1 intron 38 and in dermal neurofibromas without LOH of the NF1 gene.

LOH at the NF1 locus was investigated in 38 neurofibromas of 26 NF1 patients. Only in one of these tumors LOH was observed. In this plexiform neurofibroma of a NF1 patient with a constitutional one base-pair insertion in NF1 exon 4b, a non-random X-inactivation pattern was found, strongly suggesting a clonal origin of the tumor cells. The analysis of X-inactivation patterns allowed the classification of some of the other neurofibromas with regard to the detectability of clonal LOH. In 3 of 6 neurofibromas without LOH amenable to this analysis, a comparable X-inactivation pattern was found in constitutional and neurofibroma derived DNA. A clonal LOH would not have been detected in these tumors. However, we observed a nonrandom pattern in 3 of the 6 neurofibromas, suggesting a clonal origin of the tumor cells. LOH was not detected in these tumors, but could, however, have occurred by mutational events below the level of large somatic deletions, loss of a whole chromosome 17 or somatic recombination.