Priority Report A Novel Mouse Model for De novo Melanoma Mayuko Y. Kumasaka 1 , Ichiro Yajima 1 , Khaled Hossain 1 , Machiko Iida 1 , Toyonori Tsuzuki 2 , Tamio Ohno 3 , Masahide Takahashi 4 , Masashi Yanagisawa 5 , and Masashi Kato 1 Abstract Nevus-associated melanomas arise from pre-existing benign lesions, but de novo melanomas can also develop in the absence of such lesions. Few studies have addressed the latter phenomenon because no animal models have been described in which melanomas clearly develop in a de novo manner. In this study, we have address this need in defining RFP-RET-transgenic mice (RET mice) as a mouse model for multi-step melanomagenesis that proceeds via tumor-free, benign, premalignant, and malignant stages. Melanomas from RET mice exhibited decreased expression levels of endothelin receptor B (Ednrb) com- pared with benign tumors. In RET mice that were heterozygous for Ednrb (Ednrb+/-;RET mice), >80% of the arising primary tumors were malignant. Life span after tumor development in the mice was signifi- cantly shorter than in RET mice. Lung metastasis after tumor development was significantly higher than in RET mice. The observed process of melanomagenesis in Ednrb+/-;RET mice, which proceeded without a pre-existing benign lesion, along with the emergent characteristics in the model after tumor develop- ment corresponded well with the formation of de novo melanoma in humans. Our findings define a novel transgenic mouse model for de novo melanoma and suggest that reduced expression of Ednrb might facilitate the development of de novo melanoma in humans. Cancer Res; 70(1); 24–9. ©2010 AACR. Introduction It has been shown histopathologically that there are two kinds of carcinogenesis in human cancers. One is multistep carcinogenesis that arises from a pre-existing benign lesion, and the other is de novo carcinogenesis that arises without a pre-existing lesion (1–4). At present, however, mechanisms for multistep carcinogenesis and de novo carcinogenesis are still largely unclear. To our knowledge, there are no melano- ma animal models in which tumors have been clearly shown to be de novo melanoma. Human melanomas develop from pre-existing benign lesions (multistep melanomagenesis) and in the absence of benign lesions (de novo melanomagenesis; refs. 5–7). Howev- er, there is very limited information about the biochemical mechanisms underlying multistep melanomagenesis and de novo melanomagenesis. This is because observation of the entire process for melanomagenesis in humans is impos- sible. Establishment of animal models that can be used for the study of both multistep melanomagenesis and de novo melanomagenesis may contribute to the elucidation of their pathogenetic differences. We previously established metallothionein-I/RFP-RET- transgenic mice of line 304/B6 (RET mice) that spontaneous- ly develop systemic skin melanosis, benign melanocytic tumors, and melanoma metastasizing to distant organs step- wise (Fig. 1A; ref. 8). In this study, we introduce RET mice with heterozygously deleted Ednrb [Ednrb(+/-);RET mice] as a novel mouse model for de novo melanoma. Materials and Methods Mice. RET mice (Fig. 3A), in which solitary or multiple pri- mary dome-shaped tumors macroscopically develop on the skin (head, neck, trunk, limbs, and tail) and eyes (9, 10), were used in this study. Endothelin receptor B (Ednrb)-heterozy- gously deleted RET mice [Ednrb(+/-);RET mice] were newly generated in this study by crossing RET mice and Ednrb- deficient mice (11). Location and shape of tumors in Ednrb (+/-);RET mice were similar to those in RET mice (Fig. 3A). We calculated tumor volumes by the integral method based on their radius and height with calculation of hemisphere volume after choosing the largest primary tumors in the case of mice with multiple tumors. Ednrb-homozygously deleted RET mice [Ednrb (-/-);RET mice] died of Hirschsprung dis- ease within a month. This study was approved by the Animal Cancer Research Authors' Affiliations: 1 Unit of the Environmental Health Sciences, Department of Biomedical Sciences, College of Life and Health Sciences, Chubu University, Kasugai, Aichi, Japan, 2 Department of Pathology, Nagoya Daini Red Cross Hospital, 3 Division of Experimental Animals, Center for Promotion of Medical Research and Education and 4 Department of Pathology, Nagoya University Graduate School of Medicine, Nagoya, Aichi, Japan; and 5 Department of Molecular Genetics, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Corresponding Author: Masashi Kato, Department of Biomedical Sciences, College of Life and Health Sciences, Building no. 50, 11F, Chu- bu University, 1200 Matsumoto-cho, Kasugai-shi, Aichi 487-8501, Japan. Phone: 81-568-51-7364; Fax: 81-568-51-9635; E-mail: katomasa@isc. chubu.ac.jp. doi: 10.1158/0008-5472.CAN-09-2838 ©2010 American Association for Cancer Research. Cancer Res; 70(1) January 1, 2010 24 Downloaded from http://aacrjournals.org/cancerres/article-pdf/70/1/24/2636381/24.pdf by guest on 19 June 2022