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 se quence homology with the p120 GTPase activating protein (p120-GAP) and neurofibromin has been shown to participate in Pas-regulation. By imm unoprecipitation and Western blotting, neurofibromin was found to be e xpressed in melanocytes from the unaffected skin and cafe au lait macu les of NF1 patients, but the intensity of the neurofibromin band was d ecreased compared to control cultures. The Ras-GTP/Ras-GDP ratios of N F1 derived melanocyte cultures were comparable to those derived from h ealthy 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 poorl y growing cells. Neurofibromin contributed approximately one third of total GAP-activity, in both control and NF1 melanocytes, indicating th at it is not the major regulator of Pas in these cells, These results suggest that the function of neurofibromin in melanocytes is not limit ed to regulation of Ras activity.