In This Issue

See article, p. 512.

Pediatric glioblastomas are clinically and biologically distinct from adult glioblastomas and have recently been found to be characterized by mutations in H3F3A, the gene encoding the histone H3 variant H3.3. The mutations specifically affect lysine 27 (K27) or glycine 34 (G34), thus occurring at or near critical histone tail residues where methylation marks regulate gene expression. To gain mechanistic insight into how these mutations drive pediatric glioblastoma, Bjerke and colleagues analyzed pediatric glioblastoma gene expression signatures and observed significant differences between K27- and G34-mutant tumors. Hypothesizing that G34 mutations might affect trimethylation of the nearby residue K36 (H3K36me3), a histone modification associated with gene activation, the authors evaluated G34-mutant and H3F3A–wild-type pediatric glioblastoma cells and observed significant alterations in the genome-wide distribution of H3K36me3 in G34-mutant cells that were correlated with increased RNA polymerase II binding and gene expression. Strikingly, the most differentially and highly expressed gene was the oncogene MYCN, which has been shown to induce glioblastomas in mice when expressed in neural stem cells of the developing forebrain. Knockdown of MYCN reduced the viability of G34-mutant cells, as did knockdown or inhibition of CHK1 and Aurora kinase A, kinases that promote MYCN protein stability. These findings implicate epigenetic upregulation of MYCN as a driving event in H3.3 G34-mutant pediatric glioblastoma and suggest that targeting MYCN stability may benefit this patient subgroup.

See article, p. 520.

The oncogenic BRAF inhibitor vemurafenib (PLX4032) blocks MAPK signaling and improves survival in patients with BRAF-mutant melanoma. In contrast, BRAF-mutant colorectal carcinoma cells are relatively resistant to RAF inhibition due to relief of negative feedback and activation of EGF receptor (EGFR) signaling. Montero-Conde and colleagues found that BRAF-mutant thyroid cancers cells were also refractory to RAF and MAP/ERK kinase (MEK) inhibitors, which only transiently suppressed ERK activation in these cells. Reactivation of RAS signaling was associated with increased expression of receptor tyrosine kinases (RTK) including HER3; induction of HER3 transcription was mediated by decreased binding of the corepressors C-terminal binding protein 1 and 2 (CTBP1/CTBP2) to the HER3 promoter following RAF/MEK inhibition. Furthermore, PLX4032 triggered elevated phosphorylation of HER3 but not EGFR and formation of activated HER2/HER3 heterodimers specifically in BRAF-mutant thyroid cancer cells, resulting in stimulation of downstream MAPK and phosphoinositide 3-kinase (PI3K) signaling. This effect was dependent on autocrine production of the HER3 ligand neuregulin 1 (NRG1) in thyroid cancer cells, whereas NRG1 was not secreted by melanoma or colorectal cancer cells. Combined treatment with the HER2 inhibitor lapatinib but not inhibitors of other RTKs sensitized thyroid cancer cells to RAF/MEK blockade and inhibited the growth of murine thyroid tumors. These results identify a lineage-specific mechanism of primary MAPK inhibitor resistance and suggest a potential therapeutic strategy to overcome this resistance in patients with thyroid cancer.

See article, p. 534.

EGF receptor (EGFR) is one of the most commonly mutated genes in glioblastoma, but EGFR inhibitors have had limited clinical success. Resistance can arise through secondary EGFR mutations, mutations in downstream effectors, or mutations in compensatory pathways, but nongenetic resistance mechanisms may also exist. Akhavan and colleagues noted significant upregulation of platelet-derived growth factor β (PDGFRβ) without gene amplification in EGFR-mutant glioblastoma cells treated with the EGFR inhibitor erlotinib. Consistent with these in vitro findings, EGFR and PDGFRβ levels were inversely correlated in glioblastoma patient samples, and samples from patients treated with the EGFR inhibitor lapatinib showed evidence of reduced EGFR activation and increased PDGFRβ expression. Suppression of PDGFRβ was dependent on AKT/mTOR and MAPK signaling downstream of EGFR, and genetic or pharmacologic inhibition of either EGFR or these effector pathways led to transcriptional derepression of PDGFRβ. Notably, modulation of PDGFRβ activity had little effect on the growth of EGFR-mutant glioblastoma cells, but knockdown or inhibition of PDGFRβ significantly suppressed the growth of glioblastoma xenografts or patient-derived neurospheres when EGFR was inhibited, suggesting that EGFR inhibitor–mediated PDGFRβ activation switches glioblastoma cells from an EGFR-dependent state to a PDGFRβ-dependent state. These data indicate that receptor tyrosine kinase switching may represent a nongenetic resistance mechanism and suggest that combined inhibition of EGFR and PDGFRβ may be an effective therapeutic strategy in glioblastomas and other cancers with hyperactive EGFR signaling.

See article, p. 548.

Targeted inhibition of downstream effector pathways such as MAP/ERK kinase (MEK) and phosphoinositide 3-kinase (PI3K) has been suggested as an alternative therapeutic strategy to suppress oncogenic RAS activity in tumors such as non–small cell lung cancer (NSCLC). To identify additional targets that are specifically essential for KRAS-mutant NSCLC cell survival, Molina-Arcas and colleagues performed a small-molecule inhibitor screen on a panel of wild-type and KRAS-mutant cell lines. Although PI3K pathway inhibitors did not show genotype-specific toxicity, KRAS-mutant cells were preferentially sensitive to inhibitors of RAF, MEK, and insulin-like growth factor 1 receptor (IGF1R). IGF1R inhibition selectively suppressed AKT activation in KRAS-mutant cells, indicative of increased dependence on this pathway for survival. Consistent with this idea, basal IGF1R signaling was elevated and promoted AKT activation in these cells via interaction of insulin receptor substrate (IRS) proteins with the p85α regulatory PI3K subunit. KRAS expression was also required for MEK and PI3K signaling in mutant KRAS-expressing cells, and the induction of PI3K by oncogenic RAS was dependent on IGF1R activity. Intriguingly, combined MEK and IGF1R inhibition more potently blocked mTOR complex 1 (mTORC1) signaling, resulting in a greater reduction in cell viability and enhanced suppression of lung tumor growth in vivo compared with single-agent treatment. These results indicate that cooperative input from receptor tyrosine kinases and activated KRAS directly stimulates prosurvival PI3K signaling and suggest a therapeutic approach to induce synthetic lethality in KRAS-mutant NSCLC.

See article, p. 564.

Genomic analyses have provided insight into mutations that drive acute lymphoblastic leukemia (T-ALL), but of the recurring genetic abnormalities identified, few are candidates for targeted therapy. To uncover potentially targetable pathway dependencies in T-ALL, Sanda and colleagues performed a tyrosine kinome siRNA screen in a primary T-ALL sample and an shRNA dropout screen in several T-ALL cell lines, both of which showed that the Janus kinase (JAK) family member tyrosine kinase 2 (TYK2) was specifically required for T-ALL cell viability. Further analysis of additional cell lines and patient samples expanded ex vivo revealed that 88% of T-ALL cell lines and 62.5% of T-ALL patient samples were dependent on TYK2. The TYK2 downstream effector STAT1 was also required for TYK2-dependent T-ALL cell survival, and knockdown of either TYK2 or STAT1 specifically reduced expression of the antiapoptotic protein BCL2 in TYK2-dependent T-ALL cells. Activation of this TYK2–STAT1–BCL2 pathway occurred in some T-ALL cell lines through mutational activation of TYK2 and by upstream autocrine activation of the interleukin-10 receptor in others. TYK2-dependent T-ALL cell lines were sensitive to pan-JAK family inhibitor tool compounds, but not clinically available JAK-specific inhibitors that do not effectively target TYK2. Collectively, these findings suggest that TYK2 may represent a therapeutic target in T-ALL and provide a rationale for the development of TYK2-specific inhibitors as a potential T-ALL therapeutic strategy.

See article, p. 578.

Metastatic primary tumors colonize distant organs via systemic cytokine secretion and the recruitment of bone marrow-derived cells, including CD11b⁺Gr1⁺ myeloid cells, to generate a premetastatic microenvironment. However, it is not known whether tumors with poor metastatic potential generate metastasis-suppressive niches. Catena and colleagues found that poorly metastatic tumors retained the ability to recruit CD11b⁺Gr1⁺ myeloid cells to the lung microenvironment but that these Gr1⁺ myeloid cells were specifically induced to express thrombospondin-1 (TSP-1), a secreted antitumorigenic protein. TSP-1 induction was mediated by the activity of a protein secreted by poorly metastatic cells, prosaposin (PSAP), which acts systemically to reprogram CD11b⁺Gr1⁺ cells into metastasis-inhibitory cells. Deficiency of TSP-1 expression in the bone marrow increased tumor cell proliferation and augmented metastatic outgrowth, whereas reconstitution of TSP-1 expression in Gr1⁺ bone marrow cells prevented metastasis formation, suggesting that stimulation of TSP-1 activity may establish a growth-suppressive microenvironment. Consistent with this idea, a 5–amino acid peptide from the saposin A region of PSAP was identified to harbor TSP-1–stimulating activity, and systemic administration of this peptide was sufficient to stimulate TSP-1 expression in Gr1⁺ cells in the lung parenchyma and to inhibit lung metastasis. Furthermore, increased PSAP expression in primary tumors was correlated with prolonged survival in patients with prostate cancer. These findings define a mechanism by which nonmetastatic tumors modulate distant microenvironments to actively suppress metastasis and identify a therapeutic approach to limit the metastatic spread of advanced tumors.

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