An In Vivo Metastasis Model Using Genotype-Defined Tumor Organoids.

Recent cancer genome analyses have identified frequently mutated genes that are responsible for the development and malignant progression of cancers, including colorectal cancer (CRC). We previously constructed mouse models that carried major driver mutations of CRC, namely Apc, Kras, Tgfbr2, Trp53, and Fbxw7, in combinations. Comprehensive histological analyses of the models showed a link between mutation combinations and malignant phenotypes, such as invasion, epithelial-mesenchymal transition (EMT), and metastasis. The major cause of cancer-related death is metastasis, making it important to understand the mechanism underlying metastasis in order to develop novel therapeutic strategies. To this end, we have established intestinal tumor-derived organoids from different genotyped mice and generated liver metastasis models via transplantation of the organoids into the spleen. Through histological and imaging analyses of the transplantation models, we have determined that the combination of Apc, Kras, Tgfbr2, and Trp53 mutations promotes liver metastasis at a high incidence. We also demonstrated polyclonal metastasis of tumor cell clusters consisting of genetically and phenotypically distinct cells through our model analysis. These organoid transplantation models recapitulate human CRC metastasis, constituting a useful tool for basic and clinical cancer research as a preclinical model. We herein report the experimental protocols of the organoid culture and generation of metastasis models.

In Vitro and In Vivo Models for Metastatic Intestinal Tumors Using Genotype-Defined Organoids.

It has been established that the accumulation of driver gene mutations causes malignant progression of colorectal cancer (CRC) through positive selection and clonal expansion, similar to Darwin's evolution. Following this multistep tumorigenesis concept, we previously showed the specific mutation patterns for each process of malignant progression, including submucosal invasion, epithelial mesenchymal transition (EMT), intravasation, and metastasis, using genetically engineered mouse and organoid models. However, we also found that certain populations of cancer-derived organoid cells lost malignant characteristics of metastatic ability, although driver mutations were not impaired, and such subpopulations were eliminated from the tumor tissues by negative selection. These organoid model studies have contributed to our understanding of the cancer evolution mechanism. We herein report the in vitro and in vivo experimental protocols to investigate the survival, growth, and metastatic ability of intestinal tumor-derived organoids. The model system will be useful for basic research as well as the development of clinical strategies.

Genetic and nongenetic mechanisms for colorectal cancer evolution.

The stepwise accumulation of key driver mutations is responsible for the development and malignant progression of colorectal cancer in primary sites. Genetic mouse model studies have revealed combinations of driver gene mutations that induce phenotypic changes in tumors toward malignancy. However, cancer evolution is regulated by not only genetic alterations but also nongenetic mechanisms. For example, certain populations of metastatic cancer cells show a loss of malignant characteristics even after the accumulation of driver mutations, and such cells are eliminated in a negative selection manner. Furthermore, a polyclonal metastasis model has recently been proposed, in which cell clusters consisting of genetically heterogeneous cells break off from the primary site, disseminate to distant organs, and develop into heterogenous metastatic tumors. Such nongenetic mechanisms for malignant progression have been elucidated using genetically engineered mouse models as well as organoid transplantation experiments. In this review article, we discuss the role of genetic alterations in the malignant progression of primary intestinal tumors and nongenetic mechanisms for negative selection and polyclonal metastasis, which we learned from model studies.

Frequent loss of metastatic ability in subclones of Apc, Kras, Tgfbr2, and Trp53 mutant intestinal tumor organoids.

Cancer evolution is explained by the accumulation of driver mutations and subsequent positive selection by acquired growth advantages, like Darwin's evolution theory. However, whether the negative selection of cells that have lost malignant properties contributes to cancer progression has not yet been fully investigated. Using intestinal metastatic tumor-derived organoids carrying Apc, Kras, Tgfbr2, and Trp53 quadruple mutations, we demonstrate here that approximately 30% of subclones of the organoids show loss of metastatic ability to the liver while keeping the driver mutations and oncogenic pathways. Notably, highly metastatic subclones also showed a gradual loss of metastatic ability during further passages. Such non-metastatic subclones revealed significantly decreased survival and proliferation ability in Matrigel and collagen gel culture conditions, which may cause elimination from the tumor tissues in vivo. RNA sequencing indicated that stemness-related genes, including Lgr5 and Myb, were significantly downregulated in non-metastatic subclones as well as subclones that lost metastatic ability during additional passages. Furthermore, a CGH analysis showed that non-metastatic subclones were derived from a minor population of parental organoid cells. These results indicate that metastatic ability is continuously lost with decreased stem cell property in certain subpopulations of malignant tumors, and such subpopulations are eliminated by negative selection. Therefore, it is possible that cancer evolution is regulated not only by positive selection but also by negative selection. The mechanism underlying the loss of metastatic ability will be important for the future development of therapeutic strategies against metastasis.

Wobbly hedgehog syndrome with disseminated histiocytic sarcoma and lateral ventricular meningioma in an African pygmy hedgehog.

An 8-mo-old male African pygmy hedgehog was anorectic and ataxic; physical examination revealed tetraparesis and a gangrenous left hindlimb. Analgesic and supportive care were administered, but the animal died 3 d after presentation. Postmortem examination revealed a histiocytic sarcoma in a mesenteric lymph node with metastasis to several organs, multifocal vacuolation in the cerebral and cerebellar white matter, and a meningioma in the left lateral ventricle. We diagnosed wobbly hedgehog syndrome (WHS) with disseminated histiocytic sarcoma and lateral ventricular meningioma. Ventricular meningioma, a rare neoplasm in veterinary and human patients, has not been reported previously in hedgehogs, to our knowledge. The neurologic signs in our case were probably caused by the WHS-related vacuolar lesions and are consistent with those of reported WHS cases. Duration of illness was shorter than is typical of WHS cases, which might be related to the disseminated histiocytic sarcoma. Clinical relevance of the lateral ventricular meningioma was not evident because the ventricular mass was localized and not invasive.

High drug efflux pump capacity and low DNA damage response induce doxorubicin resistance in canine hemangiosarcoma cell lines.

Canine hemangiosarcoma (HSA) is an aggressive malignant endothelial tumor in dogs and characterized by poor prognosis because of its high invasiveness, high metastatic potential, and poor responsiveness to anti-cancer drugs. Although doxorubicin-based chemotherapy is regularly conducted after surgical treatment, its effects on survival rates are limited. Acquisition of drug resistance is one of the causes of this problem, but the underlying mechanisms remain unclear. In the present study, we aimed to identify the drug-resistance mechanism in canine HSA by establishing doxorubicin-resistant (DR) HSA cell lines. HSA cell lines were exposed to doxorubicin at gradually increasing concentrations. When the cells were able to grow in the presence of a 16-fold higher doxorubicin concentration compared with the initial culture, they were designated DR-HSA cell lines. Characterization of these DR-HSA cell lines revealed higher drug efflux pump capacity compared with the parental cell lines. Furthermore, the DR-HSA cell lines did not show activation of the DNA damage response despite carrying high DNA damage burdens, meaning that apoptosis was not strongly induced. In conclusion, canine HSA cell lines acquired doxorubicin resistance by increasing their drug efflux pump capacity and decreasing the DNA damage response. This study provides useful findings to promote further research on the drug-resistance mechanisms in canine HSA.

Notch2 signal is required for the maintenance of canine hemangiosarcoma cancer stem cell-like cells.

BACKGROUND: Hemangiosarcoma (HSA) is a malignant tumor derived from endothelial cells which usually shows poor prognosis due to its high invasiveness, metastatic rate and severe hemorrhage from tumor ruptures. Since the pathogenesis of HSA is not yet complete, further understanding of its molecular basis is required. RESULTS: Here, we identified Notch2 signal as a key factor in maintaining canine HSA cancer stem cell (CSC)-like cells. We first cultured HSA cell lines in adherent serum-free condition and confirmed their CSC-like characteristics. Notch signal was upregulated in the CSC-like cells and Notch signal inhibition by a γ-secretase inhibitor significantly repressed their growth. Notch2, a Notch receptor, was highly expressed in the CSC-like cells. Constitutive activation of Notch2 increased clonogenicity and number of cells which were able to survive in serum-free condition. In contrast, inhibition of Notch2 activity showed opposite effects. These results suggest that Notch2 is an important factor for maintaining HSA CSC-like cells. Neoplastic cells in clinical cases also express Notch2 higher than endothelial cells in the normal blood vessels in the same slides. CONCLUSION: This study provides foundation for further stem cell research in HSA and can provide a way to develop effective treatments to CSCs of endothelial tumors.