The CBX7 gene encodes a polycomb group protein that is known to be downregulated in many types of human cancers, although the role of this protein in carcinogenesis remains unclear. To shed light on this issue, we generated mice null for Cbx7. Mouse embryonic fibroblasts derived from these mice had a higher growth rate and reduced susceptibility to senescence compared with their WT counterparts. This was associated with upregulated expression of multiple cell cycle components, including cyclin E, which is known to play a key role in lung carcinogenesis in humans. Adult Cbx7-KO mice developed liver and lung adenomas and carcinomas. In in vivo and in vitro experiments, we demonstrated that CBX7 bound to the CCNE1 promoter in a complex that included HDAC2 and negatively regulated CCNE1 expression. Finally, we found that the lack of CBX7 protein expression in human lung carcinomas correlated with CCNE1 overexpression. These data suggest that CBX7 is a tumor suppressor and that its loss plays a key role in the pathogenesis of cancer.

Context: We have previously demonstrated that a set of micro-RNA (miRNA) is significantly down-regulated in anaplastic thyroid carcinomas with respect to normal thyroid tissues and to differentiated thyroid carcinomas. Objective: The objective was to evaluate the role of two of these down-regulated miRNA, miR-25 and miR-30d, in thyroid carcinogenesis. Design: miR-25 and miR-30d expression was restored in the ACT-1, 8505c, and FRO anaplastic thyroid cell lines, and their effects on cell proliferation, migration, and target expression were evaluated. Results: We report that miR-25 and miR-30d target the polycomb protein enhancer of zeste 2 (EZH2) that has oncogenic activity and is drastically up-regulated in anaplastic thyroid carcinomas but not in the differentiated ones. Ectopic expression of miR-25 and miR-30d inhibited proliferation and colony formation of anaplastic thyroid carcinoma cells by inducing G2/M-phase cell-cycle arrest. Finally, we found an inverse correlation between the expression of these miRNA and the EZH2 protein levels in anaplastic thyroid carcinomas, suggesting a critical role of these miRNA in regulating EZH2 expression also in vivo. Conclusion: The down-regulation of miR-25 and miR-30d could contribute to the process of thyroid cancer progression, leading to the development of anaplastic carcinomas targeting EZH2 mRNA.

High mobility group A (HMGA) overexpression plays a critical role in neoplastic transformation. To investigate whether HMGA acts by regulating the expression of microRNAs, we analyzed the microRNA expression profile of human breast adenocarcinoma cells (MCF7) transfected with the HMGA1 gene, which results in a highly malignant phenotype. Among the microRNAs induced by HMGA1, we focused on miR-181b, which was overexpressed in several malignant neoplasias including breast carcinomas. We show that miR-181b regulates CBX7 protein levels, which are down-regulated in cancer, and promotes cell cycle progression. We also demonstrate that CBX7, being negatively regulated by HMGA, is able to negatively regulate miR-181b expression. Finally, there was a direct correlation between HMGA1 and miR-181b expression and an inverse correlation between HMGA1 and CBX7 expression in human breast carcinomas. These data indicate the presence of a novel pathway involving HMGA1, miR-181b, and CBX7, which leads to breast cancer progression.

Chromobox protein homologue 7 (CBX7) is a chromobox family protein encoding a novel polycomb protein, the expression of which shows a progressive reduction, well related with the malignant grade of the thyroid neoplasias. Indeed, CBX7 protein levels decreased in an increasing percentage of cases going from benign adenomas to papillary, follicular, and anaplastic thyroid carcinomas. To elucidate the function of CBX7 in carcinogenesis, we searched for CBX7 interacting proteins by a proteomic analysis. By this approach, we identified several proteins. Among these proteins, we selected histone deacetylase 2 (HDAC2), which is well known to play a key role in neoplastic cell transformation and down-regulation of E-cadherin expression, the loss of which is a critical event in the epithelial-to-mesenchymal transition. We confirmed by coimmunoprecipitation that CBX7 physically interacts with the HDAC2 protein and is able to inhibit its activity. Then, we showed that both these proteins bind the E-cadherin promoter and that CBX7 up-regulates E-cadherin expression. Consistent with these data, we found a positive statistical correlation between CBX7 and E-cadherin expression in human thyroid carcinomas. Finally, we showed that the expression of CBX7 increases the acetylation status of the histones H3 and H4 on the E-cadherin promoter. Therefore, the ability of CBX7 to positively regulate E-cadherin expression by interacting with HDAC2 and inhibiting its activity on the E-cadherin promoter would account for the correlation between the loss of CBX7 expression and a highly malignant phenotype. [Cancer Res 2009;69(17):7079–87]