In This Issue

See article, p. 791.

Kinase inhibitors have become accepted treatments for the 40% to 50% of metastatic melanomas harboring BRAF^V600E or KIT exon 11 and 13 mutations. To identify other potential targets in patients lacking these mutations, Dahlman and colleagues performed whole-genome sequencing on an aggressive, BRAF^V600E-negative, KIT-negative metastatic melanoma and matched blood. Interestingly, a BRAF^L597R mutation was identified that affects the same exon (exon 15) as the V600E mutation. Screening of BRAF exon 15 in an additional 49 melanomas revealed 2 other tumors with L597 mutations (L597Q and L597S), a third with a K601E mutation, and a fourth with a D594N mutation, indicating that as many as 8% of melanomas classified clinically as “BRAF wild-type” may actually harbor other less common BRAF mutations. To determine whether these BRAF mutants were functionally similar to BRAF^V600E, which causes constitutive MEK/ERK pathway activation, the authors ectopically expressed the L597Q, L597R, L597S, and K601E mutations in 293H cells and observed that all mutants led to elevated phospho-MEK and phospho-ERK levels. Furthermore, phospho-ERK was significantly decreased by a MEK inhibitor, suggesting that patients with metastatic melanoma harboring these BRAF mutations may also benefit from MEK inhibitor therapy. Indeed, a patient with BRAF^L597S-mutant metastatic melanoma who was enrolled in a phase I trial of an allosteric MEK inhibitor experienced a partial radiographic response and was progression-free for more than 24 weeks. These findings suggest that expanded BRAF mutational testing may benefit additional patients with metastatic melanoma.

See article, p. 798.

Small cell lung cancer (SCLC) is a highly lethal malignancy with a distinct metastatic potential and treatment response from the more common non–small cell lung cancer (NSCLC). Common genetic abnormalities identified in SCLC to date have not been amenable to targeted therapy, and clinical outcomes remain poor. Byers and colleagues analyzed the levels of 193 total and phosphorylated proteins in a large panel of SCLC and NSCLC cell lines to identify potential SCLC therapeutic targets. Compared with NSCLC cells, SCLC cells had significantly higher levels of the DNA repair protein poly (ADP-ribose) polymerase (PARP1). At the mRNA level, SCLC cells had the highest median PARP1 expression of any solid tumor cells tested, and PARP1 protein levels were significantly higher in primary tumor samples from patients with SCLC or other neuroendocrine lung cancers than those from patients with NSCLC. SCLC cell lines were also highly sensitive to small-molecule PARP inhibitors, which reduced DNA repair protein levels and therefore possibly exacerbated underlying DNA repair defects in SCLC cells. Combining PARP1 inhibition with double-strand break-inducing chemotherapeutic agents commonly used in SCLC therapy led to greater decreases in SCLC cell viability than either treatment alone, further suggesting that PARP inhibitors may sensitize SCLC cells to DNA damage. These preclinical data provide support for targeting PARP1 in SCLC and suggest that PARP inhibitors may have efficacy in SCLC in combination with standard chemotherapy.

See article, p. 812.

Identifying additional factors required for DNA replication might reveal potential vulnerabilities of rapidly proliferating cancer cells that are dependent on increased DNA synthesis. Mazurek and colleagues hypothesized that an inability to stably maintain a plasmid with only one origin of replication would distinguish cells lacking an essential DNA replication factor. An shRNA screen of a library of poorly characterized G₁ and S-phase genes identified DEAD (Asp-Glu-Ala-Asp) box helicase 5 (DDX5), encoding an ATP-dependent RNA helicase, as a significant determinant of plasmid stability. The authors observed that only cancer cell lines overexpressing DDX5 or with amplification of the DDX5 gene were sensitive to DDX5 knockdown, which reduced cell proliferation in association with slowed progression through S-phase, decreased levels of known DNA replication factors, and impaired DNA replication pre-initiation complex assembly. In these DDX5-amplified or overexpressing cells, DDX5 physically interacted with E2F1, a key regulator of S-phase gene expression, and recruited RNA polymerase II to the promoters of E2F target genes, indicating that DDX5 overexpression indirectly supports DNA synthesis in a subset of cancer cells by upregulating the expression of DNA replication factor genes. Importantly, an analysis of 2 independent breast cancer databases revealed that the DDX5 locus was amplified in 25% to 28% of samples, suggesting that an acquired role of DDX5 in DNA replication may be a common oncogenic mechanism. Inhibition of DDX5 may thus be an effective therapeutic strategy in the subset of cancers that are dependent on DDX5 activity.

See article, p. 826.

High mammographic density is a strong risk factor for breast cancer characterized histologically by reduced adipocyte differentiation and enhanced extracellular matrix deposition, which are also observed in tumor-associated desmoplastic stroma. To investigate the molecular mechanisms underlying these features, DeFilippis and colleagues examined fibroblasts derived from high and low mammographic density breast tissue. High-density associated fibroblasts (HDAF) exhibited impaired adipocyte differentiation and increased secretion of desmoplastic matrix proteins, thus recapitulating both aspects of the high mammographic density phenotype. Gene expression profiling identified the transmembrane receptor CD36 as the most significantly downregulated gene in HDAFs; CD36 expression was also decreased in invasive ductal carcinoma (IDC) samples compared with normal tissue, suggesting a functional role for this protein in regulating the shared desmoplastic phenotypes. Indeed, CD36 expression in normal fibroblasts was necessary and sufficient to modulate fat and matrix protein accumulation in vitro and CD36 deficiency resulted in decreased fat content and augmented matrix production in vivo. Reduced CD36 expression was observed in multiple stromal cell types in both nonmalignant high mammographic density tissue and in breast tumors, suggesting that widespread CD36 repression may precede and promote tumor formation. Furthermore, CD36 expression was inversely correlated with tumor grade and size in IDC. These results demonstrate that CD36 repression generates a proactive microenvironment that increases cancer risk and facilitates tumor initiation and progression, and suggest that CD36 modulation may be therapeutically useful.

See article, p. 840.

Mesenchymal stem cells (MSC) recruited to tumor-associated stroma modulate carcinoma cell behavior to facilitate epithelial–mesenchymal transition (EMT) and tumor progression. Induction of EMT is associated with acquisition of a stem cell–like phenotype that correlates with enhanced tumor-initiating capacity. To elucidate the mechanisms that generate this favorable tumor microenvironment, Li and colleagues investigated the interactions between tumor cells and MSCs. Secretion of interleukin (IL)-1 by colon carcinoma cells was necessary and sufficient to stimulate expression of the COX-2 enzyme and subsequent production of prostaglandin E₂ (PGE₂) by cocultured MSCs. Autocrine PGE₂ signaling in turn cooperated with carcinoma cell–derived IL-1 to induce the expression of other cytokines, including IL-6 and IL-8, in MSCs. Treatment of carcinoma cells with PGE₂ and these cytokines resulted in decreased expression of the epithelial marker E-cadherin and increased protein levels of mesenchymal markers, indicative of EMT, and promoted tumor cell invasion. Furthermore, this EMT program was associated with an increased frequency of aldehyde dehydrogenase (ALDH)–expressing cancer stem cells (CSC) and augmented tumor initiation, both of which were dependent on PGE₂ production by COX-2. Additionally, PGE₂-mediated paracrine stimulation of tumor cells triggered nuclear localization and activation of β-catenin via AKT, which contributed to the induction of a CSC phenotype. Importantly, IL-1 and COX-2 expression were elevated in aggressive human colon and breast carcinomas, suggesting that inhibition of this signaling pathway may be useful in treating advanced tumors.

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