In This Issue

See article, p. 399.

Heterozygous germline mutations in the BRCA1 or BRCA2 tumor suppressors are associated with increased risk of breast and ovarian cancer, whereas biallelic BRCA2 mutations cause Fanconi anemia, which is characterized by pediatric cancer susceptibility and bone marrow failure. Complete loss of both BRCA1 alleles has not been reported, presumably because some BRCA1 function is required for embryogenesis. However, mice that express certain missense BRCA1 alleles are viable and exhibit cancer predisposition, suggesting that biallelic BRCA1 mutations might occur in humans. Domchek and colleagues identified a 28-year-old woman with ovarian carcinoma who harbored a known pathogenic BRCA1 mutation in trans with a BRCA1 variant of unknown significance (VUS), corresponding to a V1736A substitution in the BRCA1 protein. The woman exhibited congenital abnormalities and hypersensitivity to the DNA crosslinking chemotherapeutic carboplatin, features not characteristic of heterozygous BRCA1 mutation. The V1736A VUS was also carried by maternal relatives with breast and ovarian cancer and cosegregated with cancer in 9 additional pedigrees, and the V1736A protein showed decreased DNA damage response function, indicating that this variant is pathogenic. In addition, the allele encoding the V1736A variant was retained in all tumors examined, further suggesting that it is dysfunctional. Collectively, these findings document the first validated report of a biallelic deleterious BRCA1 mutation carrier with an associated phenotype of chemosensitivity and early-onset cancer. Moreover, they emphasize the advisability of using multifaceted approaches to investigate VUSs and have important implications for genetic testing.

See article, p. 406.

RNA interference (RNAi) is an endogenous mechanism whereby genes are silenced by siRNA-directed cleavage of mRNAs. Improvements in siRNA delivery are needed to allow RNAi to be harnessed for cancer therapy. Preclinical studies have indicated that lipid nanoparticles can effectively deliver multiple siRNAs to the liver, but it remains unclear whether this approach can work in humans. Tabernero and colleagues report the results of a phase I dose-escalation and expansion study that assessed the safety, antitumor activity, and pharmacodynamics of ALN-VSP, an siRNA-containing lipid nanoparticle, in patients with advanced solid tumors with liver involvement. ALN-VSP targets vascular endothelial growth factor A (VEGFA) and kinesin spindle protein (KSP), genes involved in tumor angiogenesis and proliferation, respectively. ALN-VSP was well tolerated, and of 37 evaluable patients, 4 patients experienced disease control, including 1 patient with metastatic endometrial cancer who had a complete response and 3 patients with stable disease (2 with metastatic renal cell carcinoma and 1 with metastatic pancreatic neuroendocrine tumor). Post-treatment tumor biopsies had detectable VEGFA and KSP siRNA, suggestive of intratumoral delivery, and showed increased VEGFA mRNA cleavage and decreased VEGFA and KSP mRNA levels compared with pre-treatment biopsy samples, indicative of an RNAi mechanism of action. These findings suggest that multiple genes can feasibly be silenced in human cancers with RNAi and provide support for the further clinical development of ALN-VSP in advanced solid tumors.

See article, p. 418.

Adoptive cell transfer (ACT) is an immunotherapy in which tumor-specific effector T cells are generated and expanded ex vivo and administered to patients. Although some durable clinical responses have been observed with ACT therapy, most responses are transient even though adoptively transferred cells persist in circulation. To determine how adoptively transferred cells and potentially tumor-responsive endogenous immune compartments influence antitumor activity, Ma and colleagues performed single-cell immune monitoring of peripheral blood T lymphocytes collected from 3 patients throughout an ACT therapy clinical trial using T cells engineered to recognize the melanoma-specific antigen MART-1. By coupling multicolor flow cytometry cell sorting with a single-cell barcode chip capable of quantifying 19 cytokines associated with different functions, the authors characterized the functional kinetics of 4 distinct T-cell phenotypes. Among the patients, similar levels of MART-1⁺ antigen-specific T cells remained in circulation after ACT therapy, but their functional behaviors varied greatly across patients and over time. The adoptively transferred cells exhibited strong but transient antitumor functions in all patients. However, endogenous tumor-antigen specific T cells with active tumor-killing functions were detected in the patients with slower relapses about a month into therapy. The patient with the fastest relapse exhibited only the transient antitumor functions from the engineered cells. Although few patients were studied, these results suggest that maintenance of antitumor T-cell functionality will be critical for successful ACT therapy.

See article, p. 430.

Crizotinib, a small-molecule ALK inhibitor, improves survival of patients with ALK-positive non–small cell lung carcinoma (NSCLC), but secondary ALK mutations or activation of compensatory signaling pathways ultimately lead to crizotinib resistance. Given that inhibition of the chaperone protein heat shock protein 90 (HSP90) destabilizes ALK fusion proteins as well as other proteins thought to contribute to crizotinib resistance, Sang and colleagues evaluated the antitumor activity of the next-generation HSP90 inhibitor ganetespib in models of ALK-positive NSCLC. Ganetespib was more cytotoxic to ALK-positive NSCLC cell lines than crizotinib, and more potently inhibited tumor growth and prolonged overall survival in mice bearing ALK-positive NSCLC xenografts. The combination of ganetespib and crizotinib was also more effective in vitro than either drug alone. In addition to inducing the degradation of ALK fusion proteins, ganetespib treatment also led to EGF receptor and MET destabilization and inactivation of their downstream effectors. Consistent with an ability to simultaneously disrupt multiple oncogenic pathways, ganetespib retained potent cytotoxic activity against crizotinib-resistant cells, including those with secondary ALK mutations, and a patient with crizotinib-resistant ALK-positive NSCLC experienced significant tumor shrinkage after a single cycle of ganetespib treatment. Additionally, in cells harboring RET or ROS1 gene fusions, which also occur in NSCLC, ganetespib similarly induced cell death, fusion kinase destabilization, and suppression of downstream signaling. Ganetespib may therefore represent a strategy for targeting ALK and overcoming ALK inhibitor resistance as well as a potential therapeutic option for NSCLC.

See article, p. 444.

Expression of mutant KRAS drives tumor formation and is critical for cancer cell survival in non-small cell lung cancer (NSCLC). However, direct inhibition of activated KRAS has proven difficult, emphasizing the need to identify downstream signaling nodes that are required for NSCLC maintenance. To identify vulnerabilities in KRAS-mutant tumors, Konstantinidou and colleagues used a mouse model of high-grade lung adenocarcinoma induced by KRAS^G12D expression in the context of cyclin-dependent kinase inhibitor 2A (Cdkn2A, also known as Ink4a/Arf) deficiency, which is commonly associated with KRAS mutation in NSCLC. These tumors exhibited sustained ERK1/2 signaling and increased activation of the small GTPase RHOA compared with control tumors. RHOA was specifically required for the viability and growth of KRAS-mutant, INK4A/ARF-null cells; this prosurvival effect was mediated primarily via activation of the RHOA effector focal adhesion kinase (FAK), as FAK depletion triggered apoptosis in KRAS-mutant NSCLC cells deficient in INK4A/ARF or TP53. Consistent with a critical role for this pathway in NSCLC, elevated activation of RHOA and FAK was correlated with the presence of mutations in KRAS and INK4A/ARF or TP53 in primary human NSCLC samples. Moreover, pharmacologic FAK blockade stimulated apoptosis and tumor regression and prolonged survival only in Kras-mutant, Ink4a/Arf-deficient mice without stimulating compensatory AKT upregulation. These findings define the RHOA-FAK pathway as a genotype-specific vulnerability in aggressive KRAS-mutant lung cancer and suggest that FAK inhibitors may be an effective therapeutic strategy in this subset of NSCLC.

See article, p. 458.

NRAS mutations occur frequently in cutaneous melanoma and in giant congenital melanocytic nevi (CMN), which are associated with increased risk of a rare form of primary melanoma in the central nervous system (CNS), but it is unclear whether oncogenic NRAS alone can induce melanoma. Pedersen and colleagues generated mice expressing NRAS^G12D from the endogenous Nras locus at physiologic levels in melanocytes. Mutant NRAS expression in either adult or embryonic melanocytes resulted in skin hyperpigmentation and dermal lesions resembling human blue nevi, but did not cause cutaneous melanoma. However, in addition to these dermal lesions, early embryonic NRAS^G12D expression induced primary melanoma of the CNS, resulting in progressive neurologic symptoms and reduced survival. Hyperproliferative pigmented melanocytic lesions were detected in the leptomeninges of early postnatal mice; these lesions progressed to invasive and highly mitotic malignant tumors that stained positive for melanocyte markers and were sensitive to treatment with MAP/ERK kinase (MEK) inhibitors. Strikingly, these tumors recapitulated the cardinal clinical features of primary CNS melanoma in 2 children whose tumors harbored NRAS mutations, including 1 child with neurocutaneous melanosis with a CMN in the absence of cutaneous melanoma. In this child, the same somatic NRAS mutation was present in the CNS melanoma and the CMN. These results suggest that early embryonic acquisition of somatic mutations in NRAS in the melanocyte lineage enhances the risk of CNS melanoma, and they establish a mouse model for this rare but fatal disease.

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