CHARACTERIZATION OF PHEOCHROMOCYTOMAS IN A MOUSE STRAIN WITH A TARGETED DISRUPTIVE MUTATION OF THE NEUROFIBROMATOSIS GENE NF1

Patients with neurofibromatosis type 1 (NF1) show an increased frequen cy of pheochromocytomas. The NF1 gene encodes a GTPase-activating prot ein that controls the activity of ras proteins in intracellular signal ling. A mouse strain with a knockout mutation of Nf1, the murine count erpart of NF1, has recently been constructed. This mutation, designate d Nf1(n31), has been shown to be associated with the frequent developm ent of pheochromocytomas in heterozygous animals. Pheochromocytomas ar e extremely rare in wild-type mice. We have characterized the tumors t o assess their relevance as a model for human pheochromocytomas. The f requency of pheochromocytomas was determined in inbred compared to out bred mice carrying the Nf1(n31) mutation. Paraffin sections of pheochr omocytomas from seven mice were stained immunohistochemically for the catecholamine biosynthetic enzymes, tyrosine hydroxylase (TH) and phen ylethanolamine-N-methyltransferase (PNMT) to infer their profiles of c atecholamine synthesis, and for chromogranin A (CGA) to infer their co ntent of secretory granules. Cultured cells from a representative tumo r were studied in vitro to assess proliferation and neuronal different iation. Pheochromocytomas arose in approx 15% of Nf1(n31) mice with a mixed genetic background, but were absent in inbred mice. Approximatel y one-fourth of the tumors were bilateral. The tumors exhibited variab le morphology. All included cells that appeared well differentiated an d resembled normal chromaffin cells in that they expressed TH, PNMT, a nd CGA. Focal neuronal differentiation was also observed. In cell cult ure, the tumor cells ceased to proliferate and the majority underwent terminal differentiation into TH-positive cells with neuronal morpholo gy. The phenotype of pheochromocytomas in mice with the Nf1(n31) mutat ion resembles that of human pheochromocytomas, particularly with respe ct to their ability to produce epinephrine, as inferred from positive staining for PNMT. The tumors also resemble both normal and neoplastic human adrenal medulla with respect to their extensive differentiation into neuron-like cells in vitro. This change in phenotype may be rela ted to ras activation. Tufts University School of These neoplasms may be valuable both as models for the pathobiology of adrenal medullary n eoplasia, and as a source of epinephrine-producing pheochromocytoma ce ll lines, for which adequate models currently do not exist.