Predominant formation of heavily pigmented dermal melanocytomas resembling 'animal-type' melanomas in hepatocyte growth factor (C57BL/6 x C3H)F1 mice following neonatal UV irradiation.

Background: Transgenic mice expressing hepatocyte growth factor (HGF) develop cutaneous melanocytic tumors following neonatal UV exposure. Here, we examined the histologic spectrum of UV-induced melanocytic tumors in HGF mice on a pigmented (C57BL/6 x C3H/HeN)F(1) background.

Methods: Neonatally irradiated (4000 J/m(2)) mice were monitored for 43 weeks, and 31/34 (91%) animals developed a total of 163 melanocytic tumors.

Results: Of 54 primary tumors analyzed, most (49/54, 91%) demonstrated exclusively dermal collections of epithelioid cells with voluminous densely pigmented cytoplasm. Seven of these also demonstrated a population of spindled cells with mitoses. Several (3/54, 6%) tumors exhibited a junctional component with melanocytes present in the epidermis. Staining with PEP8 confirmed the presence of interfollicular melanocytes at the dermal-epidermal junction in neonatal skin.

Conclusions: In contrast to HGF animals on an albino (FVB) background, HGF animals on the pigmented (C57BL/6 x C3H/HeN)F(1) background do not develop classic radial growth phase melanoma but rather predominantly develop dermal melanocytomas resembling the 'animal-type' melanoma occasionally seen in humans. These results demonstrate the influence of genetic background on histologic pattern of UV-induced melanomas in mice.