In This Issue

See article, p. 591.

Little is known about the molecular characteristics of natural killer/T-cell lymphoma (NKTCL), a highly aggressive, lethal cancer for which there is no standard first-line treatment. Koo and colleagues performed exome sequencing on 4 NKTCL samples and identified missense mutations in exon 12 of Janus kinase 3 (JAK3) in 2 of the samples. The authors sequenced JAK3 in 61 additional NKTCLs and identified homozygous or heterozygous mutations in 23 of 65 (35.4%) samples, all within the Janus homology domain 2 (JH2) that negatively regulates JAK3 kinase activity. The proliferation of NKTCL cells harboring JAK3 mutations was no longer dependent on the cytokine interleukin-2 (IL-2), and JAK3 and its target STAT5 were constitutively phosphorylated in JAK3-mutant NKTCL cells. JAK3 knockdown blocked proliferation and JAK3/STAT5 phosphorylation in JAK3-mutant cells, and ectopic expression of mutant JAK3 in JAK3–wild-type NKTCL cells increased both JAK3/STAT5 phosphorylation and proliferation in the absence of IL-2. Together, these data indicate that gain-of-function mutations of JAK3 in NKTCL promote IL-2–independent activation of JAK3 signaling. The use of a pan-JAK inhibitor selectively reduced STAT5 phosphorylation and cell viability in a dose-dependent manner in NKTCL cells, further demonstrating that NKTCL cells are dependent on constitutive JAK3 signaling. These findings establish JAK3 as a potential therapeutic target in NKTCL and indicate that targeting the JAK/STAT pathway may be an effective strategy for the treatment of this disease.

See article, p. 598.

Chromosomal translocations that generate oncogenic fusion gene products commonly occur in human tumors. Recent evidence suggests that fusion products can also be created without a requirement for chromosomal rearrangement through the trans-splicing of 2 mRNA molecules. Zhang and colleagues identify cis-splicing as a third mechanism of gene fusion with the observation that transcription occurs across the boundary of 2 adjacent genes on chromosome 1q32, solute carrier family 45, member 3 (SLC45A3) and ETS-domain protein SRF accessory protein 1 (ELK4), in prostate cancers and other cancer cell lines. The authors scanned the entire region between SLC45A3 and ELK4 by Southern blotting to rule out genomic DNA rearrangements, and showed through the failure to generate chimeric human SLC45A3 and monkey ELK4 transcripts in vitro that trans-splicing did not occur. Instead, transcripts were detected that contained intergenic sequence, suggesting that transcription spans the SLC45A3–ELK4 boundary before SLC45A3 and ELK4 exons are spliced together. Furthermore, androgen treatment increased SLC45A3–ELK4 levels and decreased CCCTC-binding factor (CTCF) occupancy at insulator sequences between SLC45A3 and ELK4, implying that diminished insulator activity may lead to aberrant read-through. SLC45A3–ELK4, but not wild-type ELK4, was required for the proliferation of multiple prostate cancer cell lines, and high SLC45A3–ELK4 transcript levels correlated with disease progression in clinical samples. Together, these findings implicate SLC45A3–ELK4 in prostate cancer development and identify an additional mechanism by which gene fusions can occur.

See article, p. 608.

The effects of hypoxic environments on tumor cells are well studied, but little is known about the impact of low oxygen concentrations on the tumor stroma. Previous work showed that tumor-associated vascular endothelial cells selectively express the T-cell activation marker CD137 under hypoxic conditions, which could possibly contribute to the anti-tumor immune responses elicited by treatment with an anti-CD137 agonist monoclonal antibody (mAb) in preclinical studies. Palazón and colleagues observed that, under hypoxic conditions, tumor-infiltrating CD4⁺ and CD8⁺ T lymphocytes also specifically expressed high levels of CD137 compared with T cells in other tissues or other tumor-infiltrating cells in a hypoxia-inducible factor 1α (HIF-1α)–dependent manner. Given that CD137 expression was largely restricted to the sites of tumors, the authors reasoned that intratumoral delivery of low doses of an anti-CD137 agonist mAb would be effective and minimize the possibility of liver inflammation previously observed following systemic anti-CD137 treatment. Low-dose, intratumoral injection of an anti-CD137 agonist mAb induced the regression of approximately 50% of established tumors and elicited systemic antitumor immunity without liver toxicity. Furthermore, treatment of tumor-bearing mice with a combination of low-dose intratumoral anti-CD137 mAb and systemic administration of an antibody blocking the immunoinhibitory factor programmed cell death 1 ligand 1 (PD-L1) synergistically induced tumor regression and prolonged survival. These findings therefore raise the intriguing possibility that the hypoxic tumor microenvironment creates targets that can be exploited by intratumoral and systemic immunotherapy.

See article, p. 624.

Bacterial strains such as Salmonella typhimurium specifically localize to tumors and elicit an anti-tumor immune response upon colonization. Because of its ability to thrive in the hypoxic, acidic conditions that frequently thwart delivery and efficacy of anti-cancer therapeutics, attenuated Salmonella may be useful as a tumor-specific vector for gene transfer or targeted delivery of biologics. To understand the genetic responses of Salmonella to tumor cells and rationally improve Salmonella-based therapeutic strategies, Flentie and colleagues performed an unbiased, large-scale screen for Salmonella genes that were upregulated by co-culture with human tumor cells. Briefly, S. typhimurium were electroporated with a luciferase-expressing, promoterless transposon such that bioluminescence was dependent on integration downstream of an active promoter. Five clones with statistically significant enhancement of bioluminescence upon tumor cell co-culture were identified that mapped to the Salmonella genes STM1787, STM1791, STM1793, adiY, and yohJ. The STM1787 promoter was most highly activated in response to an acidic pH in vitro and could induce luciferase expression upon direct injection into xenografted tumors, suggesting that preferential activation of the STM1787 promoter in response to the tumor microenvironment could be used to drive tumor-specific expression of a therapeutic transgene. Indeed, intratumoral injection of Salmonella expressing Shiga toxin 2, which encodes a cytotoxic inhibitor of protein synthesis, under the control of the STM1787 promoter induced significant tumor regression. Engineered Salmonella vectors that specifically accumulate and express transgenes in tumors may therefore represent a useful approach for targeted treatment.

See article, p. 638.

Approximately 20% to 30% of primary breast cancers harbor a chromosome 20q13 amplification that leads to zinc finger protein 217 (ZNF217) overexpression. Littlepage and colleagues showed that high ZNF217 expression is significantly associated with shorter disease-free and overall survival regardless of breast cancer subtype and can predict resistance to neoadjuvant chemotherapy. Overexpression of Znf217 in murine mammary epithelial cells led to increased motility and migration that was concurrent with actin cytoskeleton reorganization and upregulation of epithelial-mesenchymal transition markers. Znf217 overexpression also increased self-renewal capacity and induced a gene expression signature associated with a less differentiated progenitor cell-like phenotype. Consistent with these findings suggesting that ZNF217 promotes cellular phenotypes that contribute to breast cancer progression, Znf217 overexpression in a mouse breast cancer model significantly accelerated tumor formation and increased metastasis. Given the link between ZNF217 overexpression in patients with breast cancer and poor clinical outcome, the authors performed an in silico screen of a National Cancer Institute anticancer drug database to identify compounds that selectively inhibited the growth of cancer cells overexpressing ZNF217. Breast cancer cells expressing high levels of ZNF217 required lower concentrations of triciribine, a tricyclic nucleoside analog, to inhibit cell growth than cells with low ZNF217 expression. Importantly, triciribine blocked tumor growth and restored doxorubicin sensitivity of cells with high ZNF217 expression, suggesting that triciribine may be a useful component of therapy for high-risk breast cancer patients.

Note: In This Issue is written by Cancer Discovery Science Writers. Readers are encouraged to consult the original articles for full details.