Lack of characteristic pigmentation and a wide range of clinical presentations account for the diagnostic challenge associated with amelanotic
malignant melanoma. Experimental studies of this important human
cancer have been hampered by the lack of an appropriate animal model. We previously described a transgenic mouse line (TG-3) that spontaneously develops pigmented cutaneous
melanoma. F1 crosses were generated with TG-3 and several albino strains, and backcrosses were then made with the albinos. In the present report, we describe the restricted development and characterization of cutaneous
amelanotic melanoma in these albino transgenic backcrosses. The incidence and behavior of
melanoma in these mice were monitored. A high incidence (80-100%) of spontaneous
amelanotic melanoma was observed in albino transgenic mice derived from backcrosses with A, AKR, FVB, and SJL strains. The lowest incidence (30%) was obtained in BALB/c-derived crosses. No
tumors were observed in non-transgenic mice. Immunohistochemical and western blot analyses using
antibodies against three melanocyte-specific markers of the
tyrosinase family of
proteins confirmed that the
tumors were composed of amelanotic melanocytes. Furthermore, the presence of numerous premelanosomes observed by electron microscopy further supported the melanocytic origin of these
tumors. Previous in vitro studies on human
melanoma have suggested that cutaneous
amelanotic melanoma was evolving from preexisting pigmented cutaneous
melanoma. However, our results indicate that it can occur directly, as evidenced by the appearance of cutaneous
amelanotic melanoma in the
tyrosinase-deficient albino mice. These mice represent a potentially valuable model for studying the mechanistic, diagnostic, and therapeutic features of this highly
malignant neoplasm.