Description

Xenopus tropicalis is an excellent model organism for studies on vertebrate development and regeneration (Horb et al., 2019) and is also useful for the study of tumor formation (Van Nieuwenhuysen et al., 2015; Naert et al., 2016). Spontaneously occurring neoplasia in amphibians have been reported, such as in X. laevis, Rana pipiens, and Andria japonicus (McKinnell et al., 1968; Meyer-Rochow et al., 1991; Kawasumi et al., 2012). In X. laevis, one of 4,000 frogs (a 2.5–3-year-old female), which were maintained in artificial outdoor ponds, displayed renal adenocarcinoma without other developmental disorders, indicating that the tumor formation was quite rare in X. laevis (Meyer-Rochow et al., 1991). In contrast to X. laevis, 3.66 % of R. pipiens in the wild heterogeneous populations had renal adenocarcinoma (McKinnell et al., 1968), which was experimentally induced by herpesvirus (Granoff, 1973). Currently, there are no reports of spontaneous neoplasia in X. tropicalis, although recently developed genome editing technologies such as TALEN and CRISPR/Cas9 offer opportunities to induce neoplasia by simple disruption of tumor suppressor genes (Van Nieuwenhuysen et al., 2015; Naert et al., 2016), which would allow modeling of human cancer in this species. The spontaneously occurring neoplasia in X. tropicalis could provide excellent opportunities to understand how the genetic background of this species influences the neoplasia phenotypes in combination with disruption experiments of tumor suppressor genes.

The National Bioresource Project (NBRP) of X. tropicalis in Japan has successfully developed the four highly inbred wild-type strains, Nigerian A (NA), Nigerian H (NH, previously named Yasuda), Nigerian BH (NBH, previously named Golden), and Ivory Coast (IC) (Igawa et al., 2015). These strains are available for international research communities upon request. We found spontaneous neoplasia formation in the frog stocks at the NBRP (Figure 1). Analysis of frogs from seven colonies with different genetic backgrounds and chronological ages (16, 7, 6, or 5 years) identified neoplasias formed in various parts of the body, including the dorsal side, ventral side, dorsal head, dorsally on limbs, and eye (Figure 1D). Almost all frogs had a single site of neoplasia; just one frog from the NBH-VI-11 colony had multiple sites of neoplasia. The neoplasias of X. tropicalis could be divided into two types: black stone-like nodules (Figure 1A, white arrow) and white-red nodules (Figure 1A, white arrowhead). Most of the frogs were harboring only the black stone-like nodules, although a few frogs were harboring both types of nodules simultaneously (Figure 1A). Previous studies have reported that the histological analysis of amphibian and reptile neoplasias revealed the renal mass in A. japonicus exhibiting trabecular pattern and the epidermal mass in the Furcifer pardalis consisting of concentric keratin material (Kawasumi et al., 2012; Meyer et al., 2019). Histological analysis of X. tropicalis neoplasia using hematoxylin–eosin (HE) staining showed that melanocytes were enriched in the black stone-like nodules and that a layered structure was present (Figure 1B, white arrow). In contrast, cyst-like structures and blood vessels were present in the white-red nodules (Figure 1C, white arrowheads and black arrows, respectively). We then measured the neoplasia size (Figure 1E). Two frogs from the NA-XIV-1 colony had a large neoplasia, over 17 mm at the large end, over 15 mm at the small end, and over 12 mm in height (Figure 1E). The average length of the neoplasia was 7.3 mm at the large end, 4.1 mm at the small ends, and 3.5 mm in height. Therefore, it will be of interest if the frequency and size of neoplasia vary among the inbred strains on disruption of tumor suppressor genes via genome editing.