Allelic Loss of the NF1 Gene in NF1-Associated Plexiform Neurofibromas

Abstract

Neurofibromatosis 1 (NF1) is an autosomal dominant disorder with a complex variety of clinical symptoms. Genetic alteration of the NF1 gene on 17q11.2 is the cause of this disease. Neurofibromas of the peripheral nervous system are one main manifestation. A variant of neurofibroma is the plexiform neurofibroma which can be found in about 30% of NF1-patients, often causing severe clinical symptoms. In this study, we examined 14 such tumors from 10 NF1-patients for allele loss of the NF1 gene (LOH: loss of heterozygosity) using four intragenic polymorphic markers. Loss of heterozygosity was found in eight tumors from five patients, and suspected in one additional tumor from another patient. This finding suggests that loss of the second allele, and thus inactivation of both alleles of the NF1 gene, is associated with the development of plexiform neurofibromas. The 14 plexiform neufibromas were also examined for mutation in the TP53 gene by screening exons 5 through 8 using temperature gradient gel electrophoresis. No mutation was found in any of the tumors.

Introduction

Neurofibromatosis 1 (NF1) is a common autosomal dominant disorder with an incidence of one in 4000. This neurocutaneous disorder is associated with a variety of benign and malignant lesions (café-au-lait spots, neurofibromas, malignant peripheral nerve sheath tumors, pheochromocytomas, and pilocytic astrocytomas) 1, 2, 3. Plexiform neurofibromas—a variant of neurofibroma—are diagnosed in about 30% of patients [4]. A plexiform neurofibroma frequently originates from subcutaneous or visceral peripheral nerves and may cause a fusiform appearance in the affected nerve. Plexiform neurofibromas have the potential for malignant progression into malignant peripheral nerve sheath tumors; therefore, they have a position between benign and malignant NF1-related tumors. Plexiform neurofibromas are located in the chest, abdomen, or pelvis, and are commonly detected as a paraspinal mass that involves multiple spinal levels. They may also appear as anterior mediastinal masses, sciatic nerve lesions with pelvic extension, perirectal plexiform, and uterine tumors, all leading to severe clinical complications [5]. In contrast, dermal neurofibromas in NF1 are characterized by their superficial location and occurrence in large numbers. These tumors originate from terminal nerve branches in the skin and are not associated with major peripheral nerve trunks.

NF1 is caused by genetic alterations of the NF1 gene located on 17q11.2 6, 7, 8. This gene encompasses 335 kb of genomic DNA and consists of 60 exons which give rise to an 11–13-kb transcript. The product of the NF1 gene, neurofibromin, is considered a tumor suppressor in the sense that inactivation of both NF1 alleles would lead to tumorigenesis [9]. Allelic loss of chromosome 17, including the NF1 locus, has been found in NF1-related malignant tumors 10, 11. Recently, using intragenic markers in addition to conventional flanking markers, allelic loss of the NF1 gene was found in eight of 22 [12] and in 15 of 60 [13] benign neurofibromas. Furthermore, in one dermal neurofibroma [14] and one plexiform neurofibroma [15], genetic alterations have been identified on both alleles of the NF1 gene. These results supported the hypothesis that NF1 is a tumor suppressor gene. Rasmussen et al. [16] has recently reported NF1 allelic loss in seven of 14 plexiform neurofibromas at the Neurofibromatosis Symposium; however, the detailed result has not yet been published.

In this study, we examined NF1 allelic loss in 14 clinically and neuroradiologically ascertained plexiform neurofibromas removed from 10 NF1 patients. Using temperature gradient gel electrophoresis (TGGE), we also examined these tumors for mutations in the TP53 gene by screening exons 5–8 of the gene.