Study of tumor growth pattern and histopathological characteristics of patient–derived acral melanoma xenografts

INTRODUCTION AND OBJECTIVES: Recent scientific advances have contributed to a better understanding of the biology of melanoma and the establishment of new therapeutic strategies. However, these advances have not contemplated acral melanoma (AM), a subtype with worse prognosis which is relatively more common non-European descent populations, such as those in Latin America, and for which experimental models are still lacking. Patient-derived xenograft (PDX) mouse models are a high-fidelity tool for Oncology research and have high predictive power for preclinical studies. These models accurately represent the histological and genetic characteristics and recapitulate the microenvironment of human tumors during in vivo passaging. Here, we report the characterization of PDX models of different phases of AM progression with respect to the histopathological characteristics and the dynamics of tumor growth during three serial passages in vivo. MATERIAL AND METHODS: Tumor fragments derived from the first generation (X1) of the PDX model AM025a, a local recurrence on the heel, and its respective lymph node metastasis, AM025b, were used to establish a new passage (X2) in immunocompromised (NSG) mice. These tumors were enzymatically digested and the obtained cells were subcutaneously implanted in NSG mice (X3, 2x106 cells/mice). The same procedure was performed for two more serial passages (X4 and X5). Tumor growth was measured 3 times a week. For histology analysis, paraffin-embedded tumor tissues from the PDX models (X1 to X5) and from the surgically-removed patient sample were analyzed by H&E staining and by immunohistochemistry for Ki-67, to evaluate cell proliferation, and human-specific Lamin A/C, to evaluate percentage of human cells. RESULTS AND CONCLUSION: Tumor growth analyses showed no consistent and significant differences across in vivo passaging. H&E staining demonstrated that AM-PDX maintained the morphological characteristics of the patient tumor. Importantly, an increase in Ki-67 staining especially in the late PDX passages (AM025a-X5; AM025b-X4) compared to the first ones (AM025a-X1 and X2; AM025b-X1) shows that in vivo passaging confers higher proliferative potential to the PDX models (p<0,01). Human-specific Lamin A/C staining showed preservation of the percentage of human cells during in vivo passaging (variation of ~1,2% between the X1-X5 AM025a passages and variation of ~0,6% between the X1-X5 AM025b passages). The results shown here are important for the characterization of acral melanoma PDX models and for their use in functional experiments in the future.