The EVI1 gene encodes for a transcription factor with two zinc finger domains and is transcriptionally activated in a subset of myeloid leukemias. In leukemia, the transcriptional activation of EVI1 usually results from chromosomal rearrangements. Besides leukemia, EVI1 has also been linked to solid tumors including breast cancer, lung cancer, ovarian cancer and colon cancer. The MDS1/EVI1 gene is encoded by the same locus as EVI1. While EVI1 functions as a transcription repressor, MDS1/EVI1 acts as a transcription activator. The fusion protein encoded by the AML1/MDS1/EVI1 chimeric gene, resulting from chromosomal translocations in a subset of chronic myeloid leukemia, exhibits a similar function to EVI1. EVI1 has been shown to regulate cell proliferation, differentiation and apoptosis, whereas the functions of MDS1/EVI1 and AML1/MDS1/EVI1 remain elusive. In this review, we summarize the genetic structures, biochemical properties and biological functions of these proteins in cancer.

Colorectal cancer is the second leading cause of cancer mortality in the United States. Although immune checkpoint blockade therapies including anti-PD-1/PD-L1 have been successful in treating a subset of patients with colorectal cancer, the response rates remain low. We have found that riluzole, a well-tolerated FDA-approved oral medicine for treating amyotrophic lateral sclerosis, increased intratumoral CD8+ T cells and suppressed tumor growth of colon cancer cells in syngeneic immune–competent mice. Riluzole-mediated tumor suppression was dependent on the presence of CD8+ T cells. Riluzole activates the cytosolic DNA sensing cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) pathway in colon cancer cells, resulting in increased expression of IFNβ and IFNβ-regulated genes including CXCL10. Inhibition of ataxia telangiectasia mutated (ATM), but not ATM-related, resulted in a synergistic increase in IFNβ expression, suggesting that riluzole induces ATM-mediated damage response that contributes to cGAS/STING activation. Depletion of cGAS or STING significantly attenuated riluzole-induced expression of IFNβ and CXCL10 as well as increase of intratumoral CD8+ T cells and suppression of tumor growth. These results indicate that riluzole-mediated tumor infiltration of CD8+ T cells and attenuation of tumor growth is dependent on tumor cell–intrinsic STING activation. To determine whether riluzole treatment primes the tumor microenvironment for immune checkpoint modulation, riluzole was combined with anti-PD-1 treatment. This combination showed greater efficacy than either single agent and strongly suppressed tumor growth in vivo. Taken together, our studies indicate that riluzole activates cGAS/STING-mediated innate immune responses, which might be exploited to sensitize colorectal tumors to anti-PD-1/PD-L1 therapies.

The cGAS/STING cytosolic DNA-sensing pathway plays a significant role in antitumor immunity. Expression of STING is tightly regulated and commonly reduced or defective in many types of cancer. We have identified SIX4 as a significant regulator of STING expression in colon cancer cells. We showed that knockout of SIX4 decreased STING expression at the mRNA and protein levels while ectopic expression of SIX4 increased STING expression. Depletion of SIX4 led to attenuated STING activation and downstream signaling. Reexpression of SIX4 or ectopic expression of STING in SIX4 knockout cells reversed the effect. Ectopic expression of SIX4 enhanced DMXAA and cGAMP-induced STING activation and downstream signaling. Importantly, decrease of SIX4 expression substantially decreased tumor infiltration of CD8+ T cells and reduced the efficacy of PD-1 antibodies to diminish tumor growth in immune competent mice in vivo. Finally, analysis of The Cancer Genome Atlas colon cancer dataset indicated that tumors with high SIX4 expression were significantly enriched in the Inflammatory Response pathway. SIX4 expression also correlated with expression of multiple IFN-stimulated genes, inflammatory cytokines, and CD8A. Taken together, our results implicate that SIX4 is a principal regulator of STING expression in colon cancer cells, providing an additional mechanism and genetic marker to predict effective immune checkpoint blockade therapy responses. Significance: Our studies demonstrate that SIX4 is an important regulator of STING expression, providing a genetic marker or a therapeutic target to predict or enhance immune checkpoint blockade therapy responses in colon cancer.

Purpose: Colorectal cancer is the second leading cause of cancer-related mortality in the United States. Chemotherapies based on 5-fluorouracil (5-FU), when combined with targeted agents, remain the standard of care for patients with metastatic or locally advanced disease. New treatment strategies are needed for patients with metastatic colorectal cancer with microsatellite stable disease. Preclinical studies have shown that riluzole, an oral medicine for amyotrophic lateral sclerosis, inhibits glutamate release and synergizes with 5-fluorouracil to reduce cell viability in colorectal cancer cell lines. Patients and Methods: In this single-arm, phase I trial of riluzole in combination with mFOLFOX6/bevacizumab for patients with metastatic colorectal cancer, the riluzole dose started at 50 mg twice daily, escalating to 100 mg twice daily or de-escalating to 50 mg once daily. Patients received riluzole for 16 weeks in combination with mFOLFOX6/bevacizumab for eight cycles. Patients then either continued mFOLFOX6/bevacizumab or switched therapies. Results: Twelve of the 14 patients enrolled were evaluable. All patients had previously received FOLFOX, and five patients (41.7%) showed disease resistance to it. Two patients obtained partial responses, nine had stable disease, and one had progressive disease. The objective response rate was 16.7%, and the disease control rate was 91.7%. The median duration of response was 4.9 months (95% confidence interval, 1.6–9.8). Median progression-free survival and overall survival were 4.89 and 12.98 months, respectively. Conclusions: Our study showed that riluzole plus mFOLFOX6/bevacizumab is well tolerated in patients with metastatic colorectal cancer and may have clinical activity in patients whose disease is resistant to FOLFOX.

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