In This Issue

See article, p. 1262.

The use of targeted therapeutics has improved patient outcomes in subgroups of patients with lung cancer. However, nearly half of lung adenocarcinomas have no identified actionable genomic alterations, limiting treatment options. To characterize potential therapeutic targets, Cheng and colleagues analyzed the tumor of a young never-smoker patient with lung adenocarcinoma. A hybridization-capture–based next-generation sequencing assay identified only one genomic alteration, amplification of the rapamycin insensitive companion of mTOR (RICTOR), which encodes a unique subunit of mTOR complex 2 (mTORC2) that promotes cell proliferation and survival. Treatment of this index patient with mTORC1/2 inhibitors led to tumor stabilization for over 18 months. Analysis of two independent cohorts of patients revealed that RICTOR is amplified in 8–13% of patients with lung cancer, and that, in 11% of cases, RICTOR amplification was the only potentially actionable target. RICTOR knockdown reduced the growth of RICTOR-amplified lung cancer cells, as well as cells that express RICTOR but lack amplification, both in vitro and in vivo. Conversely, overexpression of RICTOR transformed Ba/F3 cells to growth-factor independence, suggesting that RICTOR is an oncogene. Pharmacologic inhibition of mTORC1/2 led to reduced cell viability and was more effective in RICTOR-amplified lung cancer cells compared with nonamplified cells; dual mTORC1/2 inhibitors were more active than other inhibitors targeting the PI3K–AKT–mTOR pathway. Together, these findings suggest that mTORC1/2 inhibitors may be successful in treating patients with RICTOR-amplified lung carcinoma.

See article, p. 1271.

Esophagogastric cancers (EGC) are frequently characterized by amplification or overexpression of oncogenic receptor tyrosine kinase (RTK) genes, including HER2 and MET. MET inhibitors are effective in a subset of patients with MET-amplified EGC; however, the mechanisms underlying MET inhibitor resistance remain unclear. To address this question, Kwak and colleagues performed molecular profiling of tumor biopsies from patients with MET-amplified EGC who received treatment with small-molecule MET kinase inhibitors. Analysis of a patient who initially responded to MET inhibitor treatment identified emergence of a KRAS^G12D mutation as a mechanism of acquired resistance to MET blockade. Furthermore, analysis of patients who failed to respond to MET kinase inhibition revealed coamplification of the RTK genes HER2 and/or EGFR within the same tumor cells as a common driver of de novo resistance to MET blockade. Of note, although coamplification of HER2 and MET conferred resistance to single-agent MET or HER2 inhibition, dual MET/HER2 inhibitor treatment overcame this resistance in vitro and resulted in profound clinical response in a patient. In addition, retrospective analysis of tumor biopsies from patients with mixed responses to MET inhibition highlighted molecular heterogeneity in RTK gene amplification between primary tumor samples and distinct metastatic lesions as a mechanism of MET inhibitor treatment failure. These findings suggest that analysis of RTK amplification in all available tumor biopsies may be required for targeted therapy selection and that combined inhibition of multiple RTKs may improve clinical efficacy.

See article, p. 1282.

Immunotherapy using adoptive T cells expressing chimeric antigen receptors against CD19 (CART-19) has shown significant clinical activity in patients with B-cell acute lymphoblastic leukemia (B-ALL). However, relapses associated with loss of the CD19 epitope have been observed in a subset of patients. To gain insight into the mechanisms underlying CD19 epitope loss, Sotillo and colleagues analyzed paired pre–CART-19, CD19-positive and post–CART-19, CD19-negative relapsed pediatric B-ALL samples. Genetic alterations such as hemizygous deletion of the CD19 locus and mutations primarily affecting CD19 exon 2 were observed in some, but not all, of the relapsed tumors. Multiple alternatively spliced CD19 transcripts were also identified in relapsed but not pretreatment samples, including a splice variant in which CD19 exon 2 was skipped (Δex2) that could override deleterious frameshift mutations. The splicing factor serine/arginine-rich splicing factor 3 (SRSF3) was found to regulate the inclusion of CD19 exon 2, and decreased SRSF3 expression, which was observed in relapsed samples, increased the frequency of CD19 exon 2 skipping. CD19 Δex2 resulted in the expression of a functional truncated protein that provided a proliferation advantage and partially rescued the effects of CD19 loss. However, unlike full-length CD19, which localizes to the plasma membrane and is recognized by CART-19, CD19 Δex2 is largely cytosolic, and CD19 Δex2–expressing cells remained viable upon exposure to CART-19, suggesting that alternative splicing can lead to epitope loss and evasion from CAR T-cell therapy.

See article, p. 1296.

Pancreatic neuroendocrine tumors (PanNET) have been studied using the RIP1-Tag2 (RT2) genetically engineered mouse model, in which PanNETs are induced through SV40 T-antigen–mediated inactivation of the tumor suppressors Trp53 and Rb in insulin-producing islet β cells. However, it is unclear how well these tumors represent the human disease. To address this question, Sadanandam and colleagues compared the mRNA and miRNA transcriptomes of PanNET tumors from the mouse model and human patients. Similar to mouse PanNETs, human PanNETs could be classified into molecular subtypes including well-differentiated non-metastatic islet/insulinoma tumors (IT) and poorly differentiated metastasis-like primary (MLP) tumors, which were associated with liver metastases. In addition, a third intermediate subtype unique to human PanNETs and frequently characterized by multiple endocrine neoplasia 1 (MEN1) mutations, termed MEN1-like tumors, was also discovered. The tumor subtypes exhibited distinct mutation associations, including a high proportion of DAXX/ATRX mutations in MEN1-like and MLP tumors. Notably, MLP tumors were enriched for expression of genes associated with early pancreas development, whereas the IT subtype exhibited increased expression of genes associated with mature β-cells, suggesting different cellular origins. Furthermore, the IT and MLP subtypes were characterized by differences in pyruvate metabolic profiles. Together, these findings define distinct PanNET subtypes, provide insight into the heterogeneity of PanNETs, and indicate that the RT2 mouse model faithfully recapitulates the features of two of the three human PanNET subtypes.

See article, p. 1314.

Epigenetic regulators play an important role in driving cancer-related transcriptional programming and represent potential therapeutic targets. The polycomb repressor complex 1 component ring finger protein 2 (RNF2) was recently identified as a candidate proinvasive and prognostic factor in metastatic melanoma. Rai and colleagues found that RNF2 overexpression in melanoma cell lines enhanced invasion and spontaneous distant metastasis, as well as proliferation and primary tumor formation. In line with this finding, depletion of RNF2 in melanoma cells with high RNF2 levels reduced invasion and metastatic seeding to distant organs. Analysis of patient-derived samples confirmed that RNF2 expression increased with disease progression and that RNF2 amplification/overexpression correlated with poor survival. Mechanistically, expression of a catalytically inactive RNF2 mutant inhibited metastasis, but retained the ability to drive proliferation and tumor initiation, suggesting catalytic-dependent and catalytic-independent functions of RNF2. The pro-metastatic functions of RNF2 were linked to inhibition of the TGFβ regulator latent TGFβ-binding protein 2 (LTBP2) via RNF2-mediated histone H2A lysine 119 monoubiquitination (H2AK119ub) of the LTBP2 promoter. In contrast, RNF2 oncogenicity was associated with recruitment of the transcriptional activators UTX and p300 to the promoters of pro-proliferative genes, such as cyclin D2 (CCND2), which are poised for activation and harbor the repression-associated H3K27 trimethylation mark prior to RNF2-induced upregulation. Furthermore, MEK-mediated phosphorylation of RNF2 at serine 41 was required to promote UTX and p300 chromatin recruitment and CCND2 expression. Together, these results define the distinct biologic functions of RNF2 in melanoma tumorigenesis and progression.

See article, p. 1328.

Childhood B-cell precursor acute lymphoblastic leukemia (pB-ALL) is the most common pediatric cancer. Germline heterozygous mutations in paired box 5 (PAX5), which encodes an essential transcription factor in B-cell development, confer pB-ALL risk with incomplete penetrance, but the mechanisms that promote progression of preleukemic cells are unclear. In addition, delayed postnatal exposure to common infections has long been thought to cause childhood leukemia; however, evidence to support this hypothesis is lacking. Martín-Lorenzo, Hauer, Vicente-Dueñas, and colleagues demonstrated that, in a Pax5^+/− mouse model, 22% of mice exposed to common pathogens developed pB-ALL with a pathology similar to the human disease, whereas those kept in a specific pathogen-free environment did not, indicating that delayed exposure to infection can induce pB-ALL in mice with inherited susceptibility due to reduced PAX5 activity. Pax5^+/− mice had fewer mature B cells and increased B-cell precursors, indicating a defect in B-cell differentiation, which requires IL7/IL7 receptor signaling and activation of JAK3 and STAT5. Whole-exome sequencing identified recurrent nonsynonymous somatic Jak3 mutations in mouse tumors, but not in healthy bone marrow, suggesting that selection for de novo Jak3 mutations in Pax5^+/− cells drives leukemia with a short latency. These mutations activated JAK3 signaling and enabled IL7-independent outgrowth of Pax5^+/− pro-B cells. Consistent with these findings, Jak3-mutant, Pax5^+/− tumors were sensitive to JAK1/3 inhibition. Together, these results indicate that, in genetically susceptible mice, infection exposure can lead to pB-ALL, with secondary Jak3 mutations driving transformation.

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