NF1 deletions in S-100 protein-positive and negative cells of sporadic and neurofibromatosis 1 (NF1)-associated plexiform neurofibromas and malignant peripheral nerve sheath tumors

Abstract

Although plexiform neurofibroma (PN) is thought to represent a benign neoplasm with the potential for malignant transformation (malignant peripheral nerve sheath tumor; MPNST), its neoplastic nature has been difficult to prove due to cellular heterogeneity, which hampers standard molecular genetic analysis. Its mixed composition typically includes Schwann cells, fibroblasts, perineurial-like cells, and mast cells. Although NF1 loss of heterozygosity has been reported in subsets of PNs, it remains uncertain which cell type(s) harbor these alterations. Using a dual-color fluorescence in situ hybridization and immunohistochemistry technique, we studied NF1 gene status in S-100 protein-positive and -negative cell subpopulations in archival paraffin-embedded specimens from seven PNs, two atypical PNs, one cellular/atypical PN, and eight MPNSTs derived from 13 patients, seven of which had neurofibromatosis type 1 (NF1). NF1 loss was detected in four of seven PNs and one atypical PN, with deletions entirely restricted to S-100 protein-immunoreactive Schwann cells. In contrast, all eight MPNSTs harbored NF1 deletions, regardless of S-100 protein expression or NF1 clinical status. Our results suggest that the Schwann cell is the primary neoplastic component in PNs and that S-100 protein-negative cells in MPNST represent dedifferentiated Schwann cells, which harbor NF1 deletions in both NF1-associated and sporadic tumors.

Figure 1.

Representative examples of dual S-100 protein immunohistochemistry and NF1 FISH hybridization. A: Low-power image from a control schwannoma, demonstrating relatively diffuse S-100 protein immunoreactivity (red). As is typical of this antibody, some of the staining is cytoplasmic and some is nuclear. B: At higher magnification, two S-100 protein-positive cells demonstrate the normal disomic state, with two copies of NF1 (green signals) per nucleus. C: Two adjacent nuclei from a representative plexiform neurofibroma (case 957-A) are shown. The nucleus on the right demonstrates S-100 protein immunoreactivity and only a single NF1 signal, whereas the nucleus on the left is S-100 protein-negative with the normal disomic NF1 dosage. Hybridization counts from this case revealed one NF1 signal in 67% of S-100 protein-positive versus 27% of S-100 protein-negative nuclei. This is consistent with a gene deletion that is restricted to the S-100 protein-positive population of cells. D: This S-100 protein-negative region of an MPNST (case 566) demonstrated one NF1 signal in 93% of nuclei, consistent with deletion. S-100 protein-positive regions of the same tumor similarly showed evidence of deletion (not illustrated).