RUNX3 in oncogenic and anti-oncogenic signaling in gastrointestinal cancers

p53 Deficiency-Dependent Oncogenicity of Runx3

The RUNX transcription factors are frequently dysregulated in human cancers, suggesting their potential as attractive targets for drug treatment. However, all three transcription factors have been described as both tumor suppressors and oncogenes, indicating the need to determine their molecular mechanisms of action. Although RUNX3 has long been considered a tumor suppressor in human cancers, several recent studies have shown that RUNX3 is upregulated during the development or progression of various malignant tumors, suggesting it may act as a "conditional" oncogene. Resolving this paradox and understanding how a single gene can exhibit both oncogenic and tumor-suppressive properties is essential for successful drug targeting of RUNX. This review describes the evidence for the activities of RUNX3 in human cancer and proposes an explanation for the duality of RUNX3 involving the status of p53. In this model, p53 deficiency causes RUNX3 to become oncogenic, leading to aberrant upregulation of MYC.

RUNX Proteins as Epigenetic Modulators in Cancer

RUNX proteins are highly conserved in metazoans and perform critical functions during development. Dysregulation of RUNX proteins through various molecular mechanisms facilitates the development and progression of various cancers, where different RUNX proteins show tumor type-specific functions and regulate different aspects of tumorigenesis by cross-talking with different signaling pathways such as Wnt, TGF-β, and Hippo. Molecularly, they could serve as transcription factors (TFs) to activate their direct target genes or interact with many other TFs to modulate chromatin architecture globally. Here, we review the current knowledge on the functions and regulations of RUNX proteins in different cancer types and highlight their potential role as epigenetic modulators in cancer.

A Point Mutation R122C in RUNX3 Promotes the Expansion of Isthmus Stem Cells and Inhibits Their Differentiation in the Stomach

BACKGROUND & AIMS

RUNX transcription factors play pivotal roles in embryonic development and neoplasia. We previously identified the single missense mutation R122C in RUNX3 from human gastric cancer. However, how RUNX3^R122C mutation disrupts stem cell homeostasis and promotes gastric carcinogenesis remained unclear.

METHODS

To understand the oncogenic nature of this mutation in vivo, we generated the RUNX3^R122C knock-in mice. Stomach tissues were harvested, followed by histologic and immunofluorescence staining, organoid culture, flow cytometry to isolate gastric corpus isthmus and nonisthmus epithelial cells, and RNA extraction for transcriptomic analysis.

RESULTS

The corpus tissue of RUNX3^R122C/R122C homozygous mice showed a precancerous phenotype such as spasmolytic polypeptide-expressing metaplasia. We observed mucous neck cell hyperplasia; massive reduction of pit, parietal, and chief cell populations; as well as a dramatic increase in the number of rapidly proliferating isthmus stem/progenitor cells in the corpus of RUNX3^R122C/R122C mice. Transcriptomic analyses of the isolated epithelial cells showed that the cell-cycle-related MYC target gene signature was enriched in the corpus epithelial cells of RUNX3^R122C/R122C mice compared with the wild-type corpus. Mechanistically, RUNX3^R122C mutant protein disrupted the regulation of the restriction point where cells decide to enter either a proliferative or quiescent state, thereby driving stem cell expansion and limiting the ability of cells to terminally differentiate.

CONCLUSIONS

RUNX3^R122C missense mutation is associated with the continuous cycling of isthmus stem/progenitor cells, maturation arrest, and development of a precancerous state. This work highlights the importance of RUNX3 in the prevention of metaplasia and gastric cancer.

Histone demethylase Jumonji domain-containing 1A inhibits proliferation and progression of gastric cancer by upregulating runt-related transcription factor 3

The histone demethylase Jumonji domain‐containing 1A (JMJD1A) is overexpressed in multiple cancers and promotes cancer progression. However, the role and mechanism of JMJD1A in gastric cancer (GC) remains poorly understood. Here, we found that JMJD1A could suppress GC cell proliferation and xenograft tumor growth. Using RNA sequencing, we identified runt‐related transcription factor 3 (RUNX3) as a novel target gene of JMJD1A. Mechanistically, we identified that JMJD1A upregulated RUNX3 through co–activating Ets‐1 and reducing the H3K9me1/2 levels at the RUNX3 promoter in GC cells. Functionally, JMJD1A inhibits the growth of GC cells in vivo, which is partially dependent on RUNX3. Moreover, JMJD1A expression was decreased in GC and low expression of JMJD1A was correlated with an aggressive phenotype and a poor prognosis in patients with GC. Importantly, JMJD1A expression was positively associated with RUNX3 expression in GC samples. These studies indicated that JMJD1A upregulates RUNX3 expression via co–activation of transcription factor Ets‐1 to inhibit proliferation of GC cells. Our findings provide new insight into the mechanism by which JMJD1A regulates RUNX3 transcription and suggest that JMJD1A and/or RUNX3 may be used as a therapeutic intervention for GC.

TrkB Inhibits the BMP Signaling-Mediated Growth Inhibition of Cancer Cells

We have previously observed that tropomyosin receptor kinase B (TrkB) induces breast cancer metastasis by activating both the Janus kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3) and phosphatidylinositol-3-Kinase (PI3K)/AKT signaling pathways and inhibiting runt-related transcription factor 3 (RUNX3) and kelch-like ECH-associated protein 1 (KEAP1). These studies indicated that TrkB expression is crucial to the pathogenesis of breast cancer. However, how TrkB regulates bone morphogenetic protein (BMP) signaling and tumor suppression is largely unknown. Herein, we report that TrkB is a key regulator of BMP-mediated tumor suppression. TrkB enhances the metastatic potential of cancer cells by promoting cell anchorage-independent growth, migration, and suppressing BMP-2-mediated growth inhibition. TrkB inhibits the BMP-mediated activation of SMAD family member 1 (SMAD1) by promoting the formation of the TrkB/BMP type II receptor complex and suppresses RUNX3 by depleting BMP receptor I (BMPRI) expression. In addition, the knockdown of TrkB restored the tumor-inhibitory effect of BMP-2 via the activation of SMAD1. Moreover, the TrkB kinase activity was required for its effect on BMP signaling. Our study identified a unique role of TrkB in the regulation of BMP-mediated growth inhibition and BMP-2-induced RUNX3 expression.

RUNX3 as a Potential Predictor of Metastasis in Human Pancreatic Cancer

BACKGROUND/AIM

In genetically engineered murine models of pancreatic ductal adenocarcinomas (PDAC), high levels of Runx3 increase the metastatic potential of cancer cells. In this study we evaluated the role of Runx3 in human pancreatic cancer.

MATERIALS AND METHODS

Runx3 was retrospectively assessed by immunohistochemistry in seventy-eight tumor samples of patients who underwent surgical resection for PDCA and were followed at least for 24 months.

RESULTS

Thirty-two cases resulted completely negative for Runx3; forty-six showed highly variable expression. We established an optimal cut-off value of Runx3 in predicting distant metastasis equal to 0.04. The odds ratio (ORs) for development of distant metastases at multivariate analysis for patients having Runx3 ≥0.04 was 4.26 (p=0.043) and 4.68 (p=0.032) after adjusting for residual tumor and treatment, respectively; OR for development of metastases in multiple sites was 4.28 (p=0.025) for Runx3 ≥0.04.

CONCLUSION

Our results support the ability of Runx3 to contribute to the dissemination of human PDAC thus confirming the observations from murine models.

Epigenetic Contribution of High-Mobility Group A Proteins to Stem Cell Properties

High-mobility group A (HMGA) proteins have been examined to understand their participation as structural epigenetic chromatin factors that confer stem-like properties to embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and cancer stem cells (CSCs). The function of HMGA was evaluated in conjunction with that of other epigenetic factors such as histones and microRNAs (miRs), taking into consideration the posttranscriptional modifications (PTMs) of histones (acetylation and methylation) and DNA methylation. HMGA proteins were coordinated or associated with histone and DNA modification and the expression of the factors related to pluripotency. CSCs showed remarkable differences compared with ESCs and iPSCs.

An early biomarker and potential therapeutic target of RUNX 3 hypermethylation in breast cancer, a system review and meta-analysis

Runt-related transcription factor 3 (RUNX3) methylation plays an important role in the carcinogenesis of breast cancer (BC). However, the association between RUNX3 hypermethylation and significance of BC remains under investigation. The purpose of this study is to perform a meta-analysis and literature review to evaluate the clinicopathological significance of RUNX3 hypermethylation in BC. A comprehensive literature search was performed in Medline, Web of Science, EMBASE, Cochrane Library Database, CNKI and Google scholar. A total of 10 studies and 747 patients were included for the meta-analysis. Pooled odds ratios (ORs) with corresponding confidence intervals (CIs) were evaluated and summarized respectively. RUNX3 hypermethylation was significantly correlated with the risk of ductal carcinoma in situ (DCIS) and invasive ductal carcinoma (IDC), OR was 50.37, p < 0.00001 and 22.66, p < 0.00001 respectively. Interestingly, the frequency of RUNX3 hypermethylation increased in estrogen receptor (ER) positive BC, OR was 12.12, p = 0.005. High RUNX3 mRNA expression was strongly associated with better relapse-free survival (RFS) in BC patients. In summary, RUNX3 methylation could be a promising early biomarker for the diagnosis of BC. High RUNX3 mRNA expression is correlated to better RFS in BC patients. RUNX3 could be a potential therapeutic target for the development of personalized therapy.

Epigenetic roles in the malignant transformation of gastric mucosal cells

Gastric carcinogenesis occurs when gastric epithelial cells transition through the initial, immortal, premalignant, and malignant stages of transformation. Epigenetic regulations contribute to this multistep process. Due to the critical role of epigenetic modifications , these changes are highly likely to be of clinical use in the future as new biomarkers and therapeutic targets for the early detection and treatment of cancers. Here, we summarize the recent findings on how epigenetic modifications, including DNA methylation, histone modifications, and non-coding RNAs, regulate gastric carcinogenesis, and we discuss potential new strategies for the diagnosis and treatments of gastric cancer. The strategies may be helpful in the further understanding of epigenetic regulation in human diseases.

The clinicopathological significance of RUNX3 hypermethylation and mRNA expression in human breast cancer, a meta-analysis

Aberrant promoter methylation of RUNX3 has been reported in several tumors including human breast cancer (BC). However, the association between RUNX3 hypermethylation and incidence of BC remains elusive. In this study, a detailed literature search was performed in Medline and Google Scholar for related research publications. Analysis of pooled data were executed. Odds ratios with corresponding confidence intervals were determined and summarized, respectively. Finally, 13 studies were identified for the meta-analysis. Analysis of the pooled data showed that RUNX3 hypermethylation was significantly higher in both ductal carcinoma in situ and invasive ductal carcinoma (IDC) than in normal breast tissues. In addition, RUNX3 methylation was significantly higher in IDC than in benign tumor. However, RUNX3 methylation was not significantly higher in IDC than in ductal carcinoma in situ. We also determined that RUNX3 hypermethylation was significantly higher in ER positive BC than in ER negative BC. In addition, high RUNX3 mRNA expression was found to be correlated with better overall survival and relapse-free survival for all BC patients. Our results strongly support that RUNX3 hypermethylation may play an important role in BC incidence. RUNX3 methylation is a valuable early biomarker for the diagnosis of BC. Further large-scale studies will provide more insight into the role of RUNX3 hypermethylation in the carcinogenesis and clinical diagnosis of BC patients.

The RUNX family: developmental regulators in cancer

RUNX proteins belong to a family of metazoan transcription factors that serve as master regulators of development. They are frequently deregulated in human cancers, indicating a prominent and, at times, paradoxical role in cancer pathogenesis. The contextual cues that direct RUNX function represent a fast-growing field in cancer research and could provide insights that are applicable to early cancer detection and treatment. This Review describes how RUNX proteins communicate with key signalling pathways during the multistep progression to malignancy; in particular, we highlight the emerging partnership of RUNX with p53 in cancer suppression.

The role of miR-148a in gastric cancer

PURPOSE

Gastric cancer is one of the most common malignant diseases worldwide, although much progress has been achieved in recent years, the early diagnosis and treatment for gastric cancer are not yet satisfactory and, thus the prognosis is still poor. MicroRNAs (miRNAs) can regulate a variety of physiological and developmental processes, it has been revealed that many miRNAs contribute the initiation and progression of various cancers. MiR-148a is one of the most important miRNAs in gastric cancer, and the aim of this paper is to provide an overview of various roles of miR-148a in gastric cancer.

METHODS AND RESULTS

We searched studies in electronic databases. MiR-148a was down-regulated in gastric cancer tissues and cell lines, which was resulted from the hypermethylation in its promoter region. Furthermore, miR-148a could regulate several different target genes and pathways involving tumor proliferation, invasion and metastasis.

CONCLUSION

MiR-148a may serve as a novel biomarker for the diagnosis and as a new therapeutic target in gastric cancer.

RUNX3 expression is associated with sensitivity to pheophorbide a-based photodynamic therapy in keloids

Runt-related transcription factor 3 (RUNX3) has recently been reported to be a possible predictor of sensitivity of cancer cells for photodynamic therapy (PDT), a promising therapeutic modality for keloids. In this study, we aimed to elucidate the implications of RUNX3 for keloid pathogenesis and sensitivity to pheophorbide a-based PDT (Pa-PDT). RUNX3 and proliferating cell nuclear antigen (PCNA) expression were examined in 6 normal skin samples and 32 keloid tissue samples by immunohistochemistry. We found that RUNX3 expression was detected more often in keloid tissues than in dermis of normal skin. In keloid tissues, RUNX3 expression was significantly increased in patients presenting with symptoms of pain or pruritus, and was also significantly related to PCNA expression. The therapeutic effect of Pa-PDT was comparatively investigated in keloid fibroblasts (KFs) with and without RUNX3 expression. Significant differences were found after Pa-PDT between KFs with and without RUNX3 expression in cell viability, proliferative ability, type I collagen expression, generation of reactive oxygen species (ROS), and apoptotic cell death. In addition, RUNX3 expression was significantly decreased after Pa-PDT in KFs, and KFs with downregulation of RUNX3 showed significantly increased cell viability after Pa-PDT. Pa-PDT may be a potential therapeutic modality for keloids, and RUNX3, as a possible contributor to keloid pathogenesis, may improve sensitivity to Pa-PDT in KFs.

RUNX3 regulates vimentin expression via miR-30a during epithelial-mesenchymal transition in gastric cancer cells

Runt-related transcription factor 3 (RUNX3) is a putative tumour suppressor via regulating the expression of a series of target genes. Clinical studies demonstrated that loss of RUNX3 expression is associated with gastric cancer progression and poor prognosis, but the underlying mechanism is not entirely clear. Accumulating evidence shows that the epithelial–mesenchymal transition (EMT) plays an important role in cancer relapse and metastasis. Therefore, we addressed whether RUNX3 has a role in the EMT in gastric cancer. Knockdown of RUNX3 promoted cell invasion and increased the protein expression of the mesenchymal marker vimentin in human gastric cancer cells. Overexpression of RUNX3 suppressed cell invasion and decreased the protein expression of vimentin in the cells and inhibited gastric cancer cells colonization in nude mice. Furthermore, overexpression of RUNX3 increased the expression of microRNA-30a (miR-30a), and miR-30a directly targeted the 3′ untranslated region of vimentin and decreased its protein level. miR-30a inhibitor abrogated RUNX3-mediated inhibition of cell invasion and downregulation of vimentin. Thus, RUNX3 suppressed gastric cancer cell invasion and vimentin expression by activating miR-30a. In gastric cancer patients, levels of RUNX3 were positively correlated with miR-30a and negatively associated with the levels of vimentin. Collectively, our data suggest a novel molecular mechanism for the tumour suppressor activity of RUNX3. Effective therapy targeting the RUNX3 pathway may help control gastric cancer cell invasion and metastasis by inhibiting the EMT.

Loss of runt-related transcription factor 3 induces gemcitabine resistance in pancreatic cancer

BACKGROUND & AIM

Runt-related transcription factor 3 (RUNX3) is a tumor suppressor gene that is expressed in gastric and other cancers including pancreatic cancer. However, the precise function of RUNX3 in pancreatic cancer has not been fully elucidated. In this study, we aimed to determine the effect of decreased RUNX3 expression in pancreatic cancer.

METHODS

This study included 36 patients with primary pancreatic cancer, who had undergone pancreaticoduodenectomy. All patients were treated with 1000 mg/m2 gemcitabine after the surgery. The pancreatic cancer cell lines PANC-1, MIAPaCa-2, BxPC-3, SUIT-2, and KLM-1 were used for immunoblotting analysis of RUNX3 and multidrug resistance protein (MRP) expressions. Ectopic RUNX3 expression was achieved by cDNA transfection of the cells, and small interfering RNA (siRNA) against RUNX3 was used to knock down endogenous RUNX3. Cell growth in the presence of gemcitabine was assessed using the MTT assay.

RESULTS

Patients with RUNX3-positive and RUNX3-negative pancreatic cancer had a median survival of 1006 and 643 days, respectively. Exogenous RUNX3 expression reduced the expression of MRP1, MRP2, and MRP5 in endogenous RUNX3-negative cells, whereas RUNX3 siRNA increased the expressions of these genes in endogenous RUNX3-positive cells. Exogenous RUNX3 expression decreased gemcitabine IC50 in RUNX3-negative cells.

CONCLUSION

Loss of RUNX3 expression contributes to gemcitabine resistance by inducing MRP expression, thereby resulting in poor patient survival.

MicroRNA-148a can regulate runt-related transcription factor 3 gene expression via modulation of DNA methyltransferase 1 in gastric cancer

Underexpression of the gene runt-related transcription factor 3 (RUNX3), an important tumor suppressor, is known to contribute to gastric cancer progression. However, the mechanism underlying aberrant RUNX3 expression has not been fully elucidated. We investigated the role of microRNA-148a (miR-148a) and DNA methyltransferases (DNMTs) in RUNX3 promoter methylation and gene expression. RUNX3 mRNA, RUNX3 protein, and methylation levels were assayed in human gastric cancer tissues and matched normal tissues, and AGS and BGC-823 cells by real-time reverse transcription PCR, Western blot, and methylation-specific PCR, respectively. A correlation between RUNX3 mRNA levels and that of miR-148a was also investigated in gastric cancer tissues. We found that RUNX3 mRNA levels were significantly downregulated in gastric cancer tissues compared with their matched normal tissues, and were closely associated with miR-148a expression. After treatment of human gastric cancer AGS and BGC-823 cells with the DNA methylation inhibitor 5-aza-2'-deoxycytidine, a significant increase in RUNX3 mRNA, RUNX3 protein, and the non-methylated form of the RUNX3 promoter were observed relative to untreated cells. Enforced expression of miR-148a, which can modulate DNMT1 and DNMT3B, also increased the expression of RUNX3 in gastric cancer cells. Knockdown of DNMT1 was associated with increased levels of RUNX3 mRNA and RUNX3 protein, while knockdown of DNMT3B did not have any effect on these in BGC-823 cells. Our results show that miR-148a may regulate RUNX3 expression through modulation of DNMT1-dependent DNA methylation in gastric cancer and highlight a miRNA-epigenetics regulation mechanism of gene expression.

Genome-wide analysis of DNA methylation in tongue squamous cell carcinoma

Tongue squamous cell carcinoma (TSCC) is one of the most common types of oral cancer; however, its molecular mechanisms remain unclear. In this study, methylated DNA immunoprecipitation (MeDIP) coupled with methylation microarray analysis was performed to screen for aberrantly methylated genes in adjacent normal control and TSCC tissues from 9 patients. Roche NimbleGen Human DNA Methylation 385K Promoter Plus CpG Island Arrays were used to detect 28,226 CpG sites. A total of 1,269 hypermethylated CpG sites covering 330 genes and 1,385 hypomethylated CpG sites covering 321 genes were found in TSCC tissue, compared to the adjacent normal tissue. Furthermore, we chose three candidate genes (FBLN1, ITIH5 and RUNX3) and validated the DNA methylation status by methylation-specific PCR (MS-PCR) and the mRNA expression levels by reverse transcription PCR (RT-PCR). In TSCC tissue, FBLN1 and ITIH5 were shown to be hypermethylated and their expression was found to be decreased, and RUNX3 was shown to be hypomethylated, however, its mRNA expression was found to be increased. In addition, another three genes (BCL2L14, CDCP1 and DIRAS3) were tested by RT-PCR. In TSCC tissue, BCL2L14 and CDCP1 expressions were markedly upregulated, and DIRAS3 expression was significantly downregulated. Our data demonstrated that aberrant DNA methylation is observed in TSCC tissue and plays an important role in the tumorigenesis, development and progression of TSCC.

Relationship between promoter methylation of the Runx3 and Rassf1a genes and Dnmt1 expression in gastric cancer

AIM: To analyze the promoter methylation of the human runt-related transcription factor 3 (Runx3) and ras-association domain family 1a (Rassf1a) genes and Dnmt1 protein expression in gastric cancer and to analyze their relationship with gastric carcinogenesis.

METHODS: Methylation status of the Runx3 and Rassf1a genes in 68 gastric carcinoma tissues and 68 paired surgical marginal normal gastric tissues was detected using methylation-specific PCR. Real-time RT-PCR was used to detect the expression of Runx3, Rassf1a, and Dnmt1 mRNAs, and immunohistochemistry was used to detect the expression of RUNX3, RASSF1a, and DNMT1 proteins in the above samples.

RESULTS: The positive rates of promoter methylation of the Runx3 and Rassf1a genes were significantly higher in gastric cancer than in normal tissue (45.59% vs 10.29%; 64.70% vs 7.35%; both P < 0.0001). The positive rates of Runx3 and Rassf1a mRNA expression in gastric cancer were significantly lower than those in normal tissue (36.76% vs 100%; 27.94% vs 97.06%; both P < 0.0001), while that of Dnmt1 mRNA expression was higher compared to normal tissue (80.88% vs 17.65%, P < 0.0001). The expression patterns of RUNX3, RASSF1a, and DNMT1 proteins were consistent with those of Runx3, Rassf1a, and Dnmt1 mRNAs. The negative rates of Runx3 and Rassf1a mRNA expression in gastric cancer were significantly higher in the methylation group than in the non-methylation group (72.09% vs 0%; 85.71% vs 2.94%; both P < 0.0001). There was a negative correlation between the expression of RUNX3 and RASSF1a proteins and that of DNMT1 protein in gastric cancer tissue (r = -0.627, P < 0.0001; r = - 0.477, P < 0.0001).

CONCLUSION: The promoter methylation of the Runx3 and Rassf1a genes and high expression of Dnmt1 may be associated with the occurrence of gastric cancer.

Global methylation profiling for risk prediction of prostate cancer

PURPOSE

The aim of this study was to investigate the promoter hypermethylation as diagnostic markers to detect malignant prostate cells and as prognostic markers to predict the clinical recurrence of prostate cancer.

EXPERIMENTAL DESIGN

DNA was isolated from prostate cancer and normal adjacent tissues. After bisulfite conversion, methylation of 14,495 genes was evaluated using the Methylation27 microarrays in 238 prostate tissues. We analyzed methylation profiles in four different groups: (i) tumor (n = 198) versus matched normal tissues (n = 40), (ii) recurrence (n = 123) versus nonrecurrence (n = 75), (iii) clinical recurrence (n = 80) versus biochemical recurrence (n = 43), and (iv) systemic recurrence (n = 36) versus local recurrence (n = 44). Group 1, 2, 3, and 4 genes signifying biomarkers for diagnosis, prediction of recurrence, clinical recurrence, and systemic progression were determined. Univariate and multivariate analyses were conducted to predict risk of recurrence. We validated the methylation of genes in 20 independent tissues representing each group by pyrosequencing.

RESULTS

Microarray analysis revealed significant methylation of genes in four different groups of prostate cancer tissues. The sensitivity and specificity of methylation for 25 genes from 1, 2, and 4 groups and 7 from group 3 were shown. Validation of genes by pyrosequencing from group 1 (GSTP1, HIF3A, HAAO, and RARβ), group 2 (CRIP1, FLNC, RASGRF2, RUNX3, and HS3ST2), group 3 (PHLDA3, RASGRF2, and TNFRSF10D), and group 4 (BCL11B, POU3F3, and RASGRF2) confirmed the microarray results.

CONCLUSIONS

Our study provides a global assessment of DNA methylation in prostate cancer and identifies the significance of genes as diagnostic and progression biomarkers of prostate cancer.

Tumor suppressor function of RUNX3 in breast cancer

Emerging evidence indicates that RUNX3 is a tumor suppressor in breast cancer. RUNX3 is frequently inactivated in human breast cancer cell lines and cancer samples by hemizygous deletion of the Runx3 gene, hypermethylation of the Runx3 promoter, or cytoplasmic sequestration of RUNX3 protein. Inactivation of RUNX3 is associated with the initiation and progression of breast cancer. Female Runx3(+/-) mice spontaneously develop ductal carcinoma, and overexpression of RUNX3 inhibits the proliferation, tumorigenic potential, and invasiveness of breast cancer cells. This review is intended to summarize these findings and discuss the tumor suppressor function of RUNX3 in breast cancer.