Background

UM is the most common primary intraocular tumor in adults, with an incidence rate varying from 5 to 10 cases per million in the world (Singh et al., 2011). Almost half of the patients develop metastasis within 15 years from initial diagnosis even after treatment or/and removal of primary tumor (Kujala et al., 2003; Weis et al., 2016). In over 4% of the patients, micrometastasis already exists at the time of diagnosis (Finger et al., 2005). The number might be underestimated because of limitations in detection of early UM. Current medical treatments such as enucleation, plaque brachytherapy, proton beam irradiation have been successful in removing or repressing early focal ocular UMs (Dogrusoz et al., 2017). But in general, UM is resistant to the standard chemotherapies and to date no effective systemic treatment is available for metastatic lesions (Goh and Layton, 2016; Carvajal et al., 2017). Although various advances have been made in UM treatment over decades, one-year survival rate after metastasis remains unchanged at 10-15% (Woodman, 2012).

UM arises from uveal melanocytes. Enriched with blood supply and lack of ocular lymph ducts, UM cells mainly spread to distant organs hematogenously. As a result, 95% of UM metastases have a predilection for the liver (Woodman, 2012). The mechanism underlying metastatic transformation of UM is still not clear. Since the liver metastasis is the leading cause of UM-related death (Collaborative Ocular Melanoma Study, 2011), current studies have been focused on the mechanism and molecular targeted prevention of tumor metastasis. Based on the metastatic proclivity, UMs are divided into two categories: class 1 and 2. Class 2 UMs are more inclined to metastasis with primitive stem cell-like gene expression pattern (Harbour and Chao, 2014). The activation of RB/P53, PI3K/AKT and MAPK signaling pathways leads to tumor overgrowth and anti-apoptosis (Coupland et al., 2013; Reichstein, 2017). 85% of primary and metastatic UMs are presented with gain-function mutations in either of two G-protein genes, GNAQ and GNA11 (Shoushtari and Carvajal, 2014). Loss of chromosome 3 or loss-function mutation in BAP1 gene indicates a poor prognosis and metastatic UM (Damato et al., 2011; van Essen et al., 2014). Transcription factors such as ID2, ZEB1 and TWIST1 are involved in UM growth and invasiveness (Chen et al., 2017), expression of PD-1 in UM cells avoids immune destruction by suppressing T cell, facilitating tumor dissemination, and resistance to chemotherapies (Komatsubara and Carvajal, 2017).

Appropriate animal models for UM are critical in understanding molecular mechanisms and evaluating therapeutic effectiveness. Mice are most commonly utilized for tumor models. No spontaneous UM was found in wild-type mice (Stei et al., 2016). Mutation in GNAQ gene could generate choroidal melanoma in mice, but the tumor exclusively metastasizes to the lung (Huang et al., 2015). To date, no genetic animal model mimicking the aforementioned biological and molecular features of human UM has been generated (Stei et al., 2016). By contrast, mouse intraocular xenograft tumor is a widely accepted UM animal model with an effective formation of primary ocular tumors and potential to metastasize to the liver. This article details a protocol to xenograft human UM cells in the vitreous cavity of nude mice, which develops primary tumors in the eye and metastases in the liver in a relatively short period of time.