Ras-GTP regulation is not altered in cultured melanocytes with reduced levels of neurofibromin derived from patients with neurofibromatosis 1 (NF1).

As derivatives of the neural crest, epidermal melanocytes are supposed to be clinically affected by NF1 gene defects. The NF1 gene shares sequence homology with the p120 GTPase activating protein (p120-GAP) and neurofibromin has been shown to participate in Ras-regulation. By immunoprecipitation and Western blotting, neurofibromin was found to be expressed in melanocytes from the unaffected skin and café au lait macules of NF1 patients, but the intensity of the neurofibromin band was decreased compared to control cultures. The Ras-GTP/Ras-GDP ratios of NF1 derived melanocyte cultures were comparable to those derived from healthy donors. Furthermore, the total GAP-activity of cell lysates was not altered in NF1 melanocyte cultures compared to controls. However, lysates of proliferating melanocytes, both from NF1 patients and from healthy donors, showed an about 2-fold higher GAP-activity than poorly growing cells. Neurofibromin contributed approximately one third of total GAP-activity, in both control and NF1 melanocytes, indicating that it is not the major regulator of Ras in these cells. These results suggest that the function of neurofibromin in melanocytes is not limited to regulation of Ras activity.