Unique Patterns of Heterogeneous Mismatch Repair Protein Expression in Colorectal Cancer Unveil Different Degrees of Tumor Mutational Burden and Distinct Tumor Microenvironment Features.

Mismatch repair (MMR) protein expression in colorectal cancer (CRC) cells is usually homogeneously retained or lost. Rare lesions may show a heterogeneous pattern of MMR protein expression. We evaluated MMR protein expression (MLH1, MSH2, MSH6, and PMS2) in 200 CRCs, identifying 3 groups with proficient MMR protein expression (MMRp), deficient MMR protein expression (MMRd), and heterogeneous MMR protein expression (MMRh). MMRh tumors were microdissected on the basis of the expression of the heterogeneous marker. DNA was extracted and subjected to targeted sequencing. RNA was purified from bulk tumors of all MMRh cases and in a control series of 15 MMRp and 10 MMRd CRCs and analyzed using the PanCancer IO 360 Panel (NanoString Technologies). Twenty-nine of the 200 cases (14.5%) were MMRd. Nine cases (4.5%) showed a heterogeneous pattern of MMR expression, with 6 tumors harboring concomitant loss of one of the other MMR proteins, thus featuring areas with double loss at immunohistochemistry (IHC) testing (MMRh double-loss cases). Four of the 6 MMRh double-loss cases were suitable for a separate sequence variant analysis of IHC double-negative and IHC single-negative components of the tumor. In all lesions, both components exhibited a high tumor mutation burden (TMB). Nevertheless, a significant increase in TMB in the double-negative components was observed (mean TMB: negative, 70 mut/Mb vs positive, 59 mut/Mb) because of a higher number of subclonal variants compared with the other component. Comparative gene expression analyses among MMRd, MMRp, and MMRh CRCs highlighted differential gene expression patterns and an increased number of tumor-infiltrating lymphocytes in MMRh lesions, which is also characterized by a substantial population of exhausted CD8+ lymphocytes. We describe a unique subgroup of CRCs showing heterogeneous expression of MMR proteins in a background of concomitant loss of one of the other markers.

Inactivation of DNA repair triggers neoantigen generation and impairs tumour growth.

Molecular alterations in genes involved in DNA mismatch repair (MMR) promote cancer initiation and foster tumour progression. Cancers deficient in MMR frequently show favourable prognosis and indolent progression. The functional basis of the clinical outcome of patients with tumours that are deficient in MMR is not clear. Here we genetically inactivate MutL homologue 1 (MLH1) in colorectal, breast and pancreatic mouse cancer cells. The growth of MMR-deficient cells was comparable to their proficient counterparts in vitro and on transplantation in immunocompromised mice. By contrast, MMR-deficient cancer cells grew poorly when transplanted in syngeneic mice. The inactivation of MMR increased the mutational burden and led to dynamic mutational profiles, which resulted in the persistent renewal of neoantigens in vitro and in vivo, whereas MMR-proficient cells exhibited stable mutational load and neoantigen profiles over time. Immune surveillance improved when cancer cells, in which MLH1 had been inactivated, accumulated neoantigens for several generations. When restricted to a clonal population, the dynamic generation of neoantigens driven by MMR further increased immune surveillance. Inactivation of MMR, driven by acquired resistance to the clinical agent temozolomide, increased mutational load, promoted continuous renewal of neoantigens in human colorectal cancers and triggered immune surveillance in mouse models. These results suggest that targeting DNA repair processes can increase the burden of neoantigens in tumour cells; this has the potential to be exploited in therapeutic approaches.

The atypical chemokine receptor ACKR2 drives pulmonary fibrosis by tuning influx of CCR2+ and CCR5+ IFNγ-producing γδT cells in mice.

Chemokines coordinate lung inflammation and fibrosis by acting on chemokine receptors expressed on leukocytes and other cell types. Atypical chemokine receptors (ACKRs) bind, internalize, and degrade chemokines, tuning homeostasis and immune responses. ACKR2 recognizes and decreases the levels of inflammatory CC chemokines. The role of ACKR2 in fibrogenesis is unknown. The purpose of the study was to investigate the role of ACKR2 in the context of pulmonary fibrosis. The effects of ACKR2 expression and deficiency during inflammation and fibrosis were analyzed using a bleomycin-model of fibrosis, ACKR2-deficient mice, bone marrow chimeras, and antibody-mediated leukocyte depletion. ACKR2 was upregulated acutely in response to bleomycin and normalized over time. ACKR2-/- mice showed reduced lethality and lung fibrosis. Bone marrow chimeras showed that lethality and fibrosis depended on ACKR2 expression in pulmonary resident (nonhematopoietic) cells but not on leukocytes. ACKR2-/- mice exhibited decreased expression of tissue-remodeling genes, reduced leukocyte influx, pulmonary injury, and dysfunction. ACKR2-/- mice had early increased levels of CCL5, CCL12, CCL17, and IFNγ and an increased number of CCR2+ and CCR5+ IFNγ-producing γδT cells in the airways counterbalanced by low Th17-lymphocyte influx. There was reduced accumulation of IFNγ-producing γδT cells in CCR2-/- and CCR5-/- mice. Moreover, depletion of γδT cells worsened the clinical symptoms induced by bleomycin and reversed the phenotype of ACKR2-/- mice exposed to bleomycin. ACKR2 controls the CC chemokine expression that drives the influx of CCR2+ and CCR5+ IFNγ-producing γδT cells, tuning the Th17 response that mediated pulmonary fibrosis triggered by bleomycin instillation.

Non-canonical antigens are the largest fraction of peptides presented by MHC class I in mismatch repair deficient murine colorectal cancer.

BACKGROUND: Immunotherapy based on checkpoint inhibitors is highly effective in mismatch repair deficient (MMRd) colorectal cancer (CRC). These tumors carry a high number of mutations, which are predicted to translate into a wide array of neoepitopes; however, a systematic classification of the neoantigen repertoire in MMRd CRC is lacking. Mass spectrometry peptidomics has demonstrated the existence of MHC class I associated peptides (MAPs) originating from non-coding DNA regions. Based on these premises we investigated DNA genomic regions responsible for generating MMRd-induced peptides. METHODS: We exploited mouse CRC models in which the MMR gene Mlh1 was genetically inactivated. Isogenic cell lines CT26 Mlh1+/+ and Mlh1-/- were inoculated in immunocompromised and immunocompetent mice. Whole genome and RNA sequencing data were generated from samples obtained before and after injection in murine hosts. First, peptide databases were built from transcriptomes of isogenic cell lines. We then compiled a database of peptides lost after tumor cells injection in immunocompetent mice, likely due to immune editing. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) and matched next-generation sequencing databases were employed to identify the DNA regions from which the immune-targeted MAPs originated. Finally, we adopted in vitro T cell assays to verify whether MAP-specific T cells were part of the in vivo immune response against Mlh1-/- cells. RESULTS: Whole genome sequencing analyses revealed an unbalanced distribution of immune edited alterations across the genome in Mlh1-/- cells grown in immunocompetent mice. Specifically, untranslated (UTR) and coding regions exhibited the largest fraction of mutations leading to highly immunogenic peptides. Moreover, the integrated computational and LC-MS/MS analyses revealed that MAPs originate mainly from atypical translational events in both Mlh1+/+ and Mlh1-/- tumor cells. In addition, mutated MAPs-derived from UTRs and out-of-frame translation of coding regions-were highly enriched in Mlh1-/- cells. The MAPs trigger T-cell activation in mice primed with Mlh1-/- cells. CONCLUSIONS: Our results suggest that-in comparison to MMR proficient CRC-MMRd tumors generate a significantly higher number of non-canonical mutated peptides able to elicit T cell responses. These results reveal the importance of evaluating the diversity of neoepitope repertoire in MMRd tumors.

Dendritic cell-targeted therapy expands CD8 T cell responses to bona-fide neoantigens in lung tumors.

Cross-presentation by type 1 DCs (cDC1) is critical to induce and sustain antitumoral CD8 T cell responses to model antigens, in various tumor settings. However, the impact of cross-presenting cDC1 and the potential of DC-based therapies in tumors carrying varied levels of bona-fide neoantigens (neoAgs) remain unclear. Here we develop a hypermutated model of non-small cell lung cancer in female mice, encoding genuine MHC-I neoepitopes to study neoAgs-specific CD8 T cell responses in spontaneous settings and upon Flt3L + αCD40 (DC-therapy). We find that cDC1 are required to generate broad CD8 responses against a range of diverse neoAgs. DC-therapy promotes immunogenicity of weaker neoAgs and strongly inhibits the growth of high tumor-mutational burden (TMB) tumors. In contrast, low TMB tumors respond poorly to DC-therapy, generating mild CD8 T cell responses that are not sufficient to block progression. scRNA transcriptional analysis, immune profiling and functional assays unveil the changes induced by DC-therapy in lung tissues, which comprise accumulation of cDC1 with increased immunostimulatory properties and less exhausted effector CD8 T cells. We conclude that boosting cDC1 activity is critical to broaden the diversity of anti-tumoral CD8 T cell responses and to leverage neoAgs content for therapeutic advantage.

Genetic and pharmacological modulation of DNA mismatch repair heterogeneous tumors promotes immune surveillance.

Patients affected by colorectal cancer (CRC) with DNA mismatch repair deficiency (MMRd), often respond to immune checkpoint blockade therapies, while those with mismatch repair-proficient (MMRp) tumors generally do not. Interestingly, a subset of MMRp CRCs contains variable fractions of MMRd cells, but it is unknown how their presence impacts immune surveillance. We asked whether modulation of the MMRd fraction in MMR heterogeneous tumors acts as an endogenous cancer vaccine by promoting immune surveillance. To test this hypothesis, we use isogenic MMRp (Mlh1+/+) and MMRd (Mlh1-/-) mouse CRC cells. MMRp/MMRd cells mixed at different ratios are injected in immunocompetent mice and tumor rejection is observed when at least 50% of cells are MMRd. To enrich the MMRd fraction, MMRp/MMRd tumors are treated with 6-thioguanine, which leads to tumor rejection. These results suggest that genetic and pharmacological modulation of the DNA mismatch repair machinery potentiate the immunogenicity of MMR heterogeneous tumors.

Cancer-promoting tumor-associated macrophages: new vistas and open questions.

Tumor-associated macrophages (TAMs) are key components of the tumor macroenvironment. Cancer- and host cell-derived signals generally drive the functions of TAMs towards an M2-like polarized, tumor-propelling mode; however, when appropriately re-educated. TAMs also have the potential to elicit tumor destructive reactions. Here, we discuss recent advances regarding the immunobiology of TAMs and highlight open questions including the mechanisms of their accumulation (recruitment versus proliferation), their diversity and how to best therapeutically target these cells.

Chemokines in cancer related inflammation.

Chemokines are key players of the cancer-related inflammation. Chemokine ligands and receptors are downstream of genetic events that cause neoplastic transformation and are abundantly expressed in chronic inflammatory conditions which predispose to cancer. Components of the chemokine system affect multiple pathways of tumor progression including: leukocyte recruitment, neo-angiogenesis, tumor cell proliferation and survival, invasion and metastasis. Evidence in pre-clinical and clinical settings suggests that the chemokine system represents a valuable target for the development of innovative therapeutic strategies.

Temozolomide Treatment Alters Mismatch Repair and Boosts Mutational Burden in Tumor and Blood of Colorectal Cancer Patients.

The majority of metastatic colorectal cancers (mCRC) are mismatch repair (MMR) proficient and unresponsive to immunotherapy, whereas MMR-deficient (MMRd) tumors often respond to immune-checkpoint blockade. We previously reported that the treatment of colorectal cancer preclinical models with temozolomide (TMZ) leads to MMR deficiency, increased tumor mutational burden (TMB), and sensitization to immunotherapy. To clinically translate these findings, we designed the ARETHUSA clinical trial whereby O6-methylguanine-DNA-methyltransferase (MGMT)-deficient, MMR-proficient, RAS-mutant mCRC patients received priming therapy with TMZ. Analysis of tissue biopsies and circulating tumor DNA (ctDNA) revealed the emergence of a distinct mutational signature and increased TMB after TMZ treatment. Multiple alterations in the nucleotide context favored by the TMZ signature emerged in MMR genes, and the p.T1219I MSH6 variant was detected in ctDNA and tissue of 94% (16/17) of the cases. A subset of patients whose tumors displayed the MSH6 mutation, the TMZ mutational signature, and increased TMB achieved disease stabilization upon pembrolizumab treatment. SIGNIFICANCE: MMR-proficient mCRCs are unresponsive to immunotherapy. We provide the proof of concept that inactivation of MMR genes can be achieved pharmacologically with TMZ and molecularly monitored in the tissue and blood of patients with mCRC. This strategy deserves additional evaluation in mCRC patients whose tumors are no longer responsive to standard-of-care treatments. See related commentary by Willis and Overman, p. 1612. This article is highlighted in the In This Issue feature, p. 1599.

Mechanisms of Immune Escape and Resistance to Checkpoint Inhibitor Therapies in Mismatch Repair Deficient Metastatic Colorectal Cancers.

Immune checkpoint inhibitors (CPIs) represent an effective therapeutic strategy for several different types of solid tumors and are remarkably effective in mismatch repair deficient (MMRd) tumors, including colorectal cancer (CRC). The prevalent view is that the elevated and dynamic neoantigen burden associated with the mutator phenotype of MMRd fosters enhanced immune surveillance of these cancers. In addition, recent findings suggest that MMRd tumors have increased cytosolic DNA, which triggers the cGAS STING pathway, leading to interferon-mediated immune response. Unfortunately, approximately 30% of MMRd CRC exhibit primary resistance to CPIs, while a substantial fraction of tumors acquires resistance after an initial benefit. Profiling of clinical samples and preclinical studies suggests that alterations in the Wnt and the JAK-STAT signaling pathways are associated with refractoriness to CPIs. Intriguingly, mutations in the antigen presentation machinery, such as loss of MHC or Beta-2 microglobulin (B2M), are implicated in initial immune evasion but do not impair response to CPIs. In this review, we outline how understanding the mechanistic basis of immune evasion and CPI resistance in MMRd CRC provides the rationale for innovative strategies to increase the subset of patients benefiting from CPIs.

CD4 T Cell-Dependent Rejection of Beta-2 Microglobulin Null Mismatch Repair-Deficient Tumors.

Inactivation of beta-2 microglobulin (B2M) is considered a determinant of resistance to immune checkpoint inhibitors (ICPi) in melanoma and lung cancers. In contrast, B2M loss does not appear to affect response to ICPis in mismatch repair-deficient (MMRd) colorectal tumors where biallelic inactivation of B2M is frequently observed. We inactivated B2m in multiple murine MMRd cancer models. Although MMRd cells would not readily grow in immunocompetent mice, MMRd B2m null cells were tumorigenic and regressed when treated with anti-PD-1 and anti-CTLA4. The efficacy of ICPis against MMRd B2m null tumors did not require CD8+ T cells but relied on the presence of CD4+ T cells. Human tumors expressing low levels of B2M display increased intratumoral CD4+ T cells. We conclude that B2M inactivation does not blunt the efficacy of ICPi in MMRd tumors, and we identify a unique role for CD4+ T cells in tumor rejection. SIGNIFICANCE: B2M alterations, which impair antigen presentation, occur frequently in microsatellite-unstable colorectal cancers. Although in melanoma and lung cancers B2M loss is a mechanism of resistance to immune checkpoint blockade, we show that MMRd tumors respond to ICPis through CD4+ T-cell activation.This article is highlighted in the In This Issue feature, p. 1601.

T Cells Expressing Receptor Recombination/Revision Machinery Are Detected in the Tumor Microenvironment and Expanded in Genomically Over-unstable Models.

Tumors undergo dynamic immunoediting as part of a process that balances immunologic sensing of emerging neoantigens and evasion from immune responses. Tumor-infiltrating lymphocytes (TIL) comprise heterogeneous subsets of peripheral T cells characterized by diverse functional differentiation states and dependence on T-cell receptor (TCR) specificity gained through recombination events during their development. We hypothesized that within the tumor microenvironment (TME), an antigenic milieu and immunologic interface, tumor-infiltrating peripheral T cells could reexpress key elements of the TCR recombination machinery, namely, Rag1 and Rag2 recombinases and Tdt polymerase, as a potential mechanism involved in the revision of TCR specificity. Using two syngeneic invasive breast cancer transplantable models, 4T1 and TS/A, we observed that Rag1, Rag2, and Dntt in situ mRNA expression characterized rare tumor-infiltrating T cells. In situ expression of the transcripts was increased in coisogenic Mlh1-deficient tumors, characterized by genomic overinstability, and was also modulated by PD-1 immune-checkpoint blockade. Through immunolocalization and mRNA hybridization analyses, we detected the presence of rare TDT+RAG1/2+ cells populating primary tumors and draining lymph nodes in human invasive breast cancer. Analysis of harmonized single-cell RNA-sequencing data sets of human cancers identified a very small fraction of tumor-associated T cells, characterized by the expression of recombination/revision machinery transcripts, which on pseudotemporal ordering corresponded to differentiated effector T cells. We offer thought-provoking evidence of a TIL microniche marked by rare transcripts involved in TCR shaping.

High-dose vitamin C enhances cancer immunotherapy.

Vitamin C (VitC) is known to directly impair cancer cell growth in preclinical models, but there is little clinical evidence on its antitumoral efficacy. In addition, whether and how VitC modulates anticancer immune responses is mostly unknown. Here, we show that a fully competent immune system is required to maximize the antiproliferative effect of VitC in breast, colorectal, melanoma, and pancreatic murine tumors. High-dose VitC modulates infiltration of the tumor microenvironment by cells of the immune system and delays cancer growth in a T cell-dependent manner. VitC not only enhances the cytotoxic activity of adoptively transferred CD8 T cells but also cooperates with immune checkpoint therapy (ICT) in several cancer types. Combination of VitC and ICT can be curative in models of mismatch repair-deficient tumors with high mutational burden. This work provides a rationale for clinical trials combining ICT with high doses of VitC.

RaLP, a new member of the Src homology and collagen family, regulates cell migration and tumor growth of metastatic melanomas.

The Src homology and collagen (Src) family of adaptor proteins comprises six Shc-like proteins encoded by three loci in mammals (Shc, Rai, and Sli). Shc-like proteins are tyrosine kinase substrates, which regulate diverse signaling pathways and cellular functions, including Ras and proliferation (p52/p46Shc), phosphatidylinositol 3-kinase and survival (p54Rai), and mitochondrial permeability transition and apoptosis (p66Shc). Here, we report the identification, cloning, and sequence characterization of a new member of the Shc family that we termed RaLP. RaLP encodes a 69-kDa protein characterized by the CH2-PTB-CH1-SH2 modularity, typical of the Shc protein family, and expressed, among adult tissues, only in melanomas. Analysis of RaLP expression during the melanoma progression revealed low expression in normal melanocytes and benign nevi, whereas high levels of RaLP protein were found at the transition from radial growth phase to vertical growth phase and metastatic melanomas, when tumor cells acquire migratory competence and invasive potential. Notably, silencing of RaLP expression in metastatic melanomas by RNA interference reduced tumorigenesis in vivo. Analysis of RaLP in melanoma signal transduction pathways revealed that (a) when ectopically expressed in RaLP-negative melanocytes and nonmetastatic melanoma cells, it functions as a substrate of activated insulin-like growth factor-1 and epidermal growth factor receptors and increases Ras/mitogen-activated protein kinase (MAPK) signaling and cell migration, whereas (b) its silencing in RaLP-positive melanoma cells abrogates cell migration in vitro, without affecting MAPK signaling, suggesting that RaLP activates both Ras-dependent and Ras-independent migratory pathways in melanomas. These findings indicate that RaLP is a specific marker of metastatic melanomas, a critical determinant in the acquisition of the migratory phenotype by melanoma cells, and a potential target for novel anti-melanoma therapeutic strategies.

Evolving neoantigen profiles in colorectal cancers with DNA repair defects.

BACKGROUND: Neoantigens that arise as a consequence of tumor-specific mutations can be recognized by T lymphocytes leading to effective immune surveillance. In colorectal cancer (CRC) and other tumor types, a high number of neoantigens is associated with patient response to immune therapies. The molecular processes governing the generation of neoantigens and their turnover in cancer cells are poorly understood. We exploited CRC as a model system to understand how alterations in DNA repair pathways modulate neoantigen profiles over time. METHODS: We performed whole exome sequencing (WES) and RNA sequencing (RNAseq) in CRC cell lines, in vitro and in vivo, and in CRC patient-derived xenografts (PDXs) to track longitudinally genomic profiles, clonal evolution, mutational signatures, and predicted neoantigens. RESULTS: The majority of CRC models showed remarkably stable mutational and neoantigen profiles; however, those carrying defects in DNA repair genes continuously diversified. Rapidly evolving and evolutionary stable CRCs displayed characteristic genomic signatures and transcriptional profiles. Downregulation of molecules implicated in antigen presentation occurred selectively in highly mutated and rapidly evolving CRC. CONCLUSIONS: These results indicate that CRCs carrying alterations in DNA repair pathways display dynamic neoantigen patterns that fluctuate over time. We define CRC subsets characterized by slow and fast evolvability and link this phenotype to downregulation of antigen-presenting cellular mechanisms. Longitudinal monitoring of the neoantigen landscape could be relevant in the context of precision medicine.

Cytokines as a key component of cancer-related inflammation.

Inflammatory conditions in some tissues increase the risk of cancer. Cytokines and chemokines are components of an intensive dialog promoting angiogenesis, metastasis, subversion of adaptive immunity and changing response to hormones and to chemotherapeutic agents. Cytokines involved in cancer-related inflammation represent a target for innovative diagnostic and therapeutic strategies, and a future challenge for scientists and clinicians.

Inactivation of DNA repair-prospects for boosting cancer immune surveillance.

The emergence of drug resistance depends on the ability of the genome of cancer cells to constantly mutate and evolve under selective pressures. The generation of new mutations is accelerated when genes involved in DNA repair pathways are altered. Notably, although the emergence of new mutations fosters drug resistance, new variants can nevertheless become novel antigens that promote immune surveillance and even restrict cancer growth.

The Clinical Impact of the Genomic Landscape of Mismatch Repair-Deficient Cancers.

The mismatch repair (MMR) system which detects and corrects base mismatches and insertions and deletions that occur during DNA synthesis is deregulated in approximately 20% of human cancers. MMR-deficient tumors have peculiar properties, including early-onset metastatic potential but generally favorable prognosis, and remarkable response to immune therapy. The functional basis of these atypical clinical features has recently started to be elucidated. Here, we discuss how the biological and clinical features of MMR-deficient tumors might be traced back to their ability to continuously produce new somatic mutations, leading to increased levels of neoantigens, which in turn stimulate immune surveillance. SIGNIFICANCE: Tumors carrying defects in DNA MMR accumulate high levels of mutations, a feature linked to rapid tumor progression and acquisition of drug resistance but also favorable prognosis and response to immune-checkpoint blockade. We discuss how the genomic landscape of MMR-deficient tumors affects their biological and clinical behaviors.

Parallel Evaluation of Circulating Tumor DNA and Circulating Tumor Cells in Metastatic Colorectal Cancer.

BACKGROUND: Tissue biopsy is the gold standard for tumor genotyping, but it is an invasive procedure providing a single snapshot into tumor heterogeneity. Liquid biopsy approaches, encompassing the analysis of circulating tumor DNA (ctDNA) or circulating tumor cells (CTCs), have been proposed as an alternative, with the potential of providing a comprehensive portrait of the tumor molecular landscape. In metastatic colorectal cancer (mCRC), both CTCs and ctDNA analysis have been investigated, but comparative analyses are limited. METHODS: We collected blood samples from 20 consecutive patients with mCRC with at least 1 of the following inclusion criteria: high tumor burden (> 1 metastasis), intact colonic primary tumor, disease progression at the time of sampling, ≤ 2 cycles of cytotoxic chemotherapy of current treatment course, and time between last chemotherapy cycle ≥ 4 weeks. RESULTS: Nineteen of 20 samples displayed the appropriate quality for CTC analysis. CTCs could be isolated in 7 (36.8%) of 19 evaluable patients. The median number of CTCs was 0 (range, 0-73). In 2 patients, we isolated > 1 CTC, and in five, we found 1 CTC. We retrieved ctDNA in all samples, with a median amount of 732,573 GE/mL (range, 174,774-174,078,615 GE/mL). Concordance between ctDNA and tissue for RAS, BRAF, and ERBB2 alterations was found in 11 (84.6%) of 13 cases. CONCLUSIONS: In this cohort, we show that ctDNA was detectable in all cases, whereas CTCs were detectable in one-third of the cases. ctDNA analysis was achieved with a smaller amount of blood sampling and allowed molecular characterization. Our data indicate that ctDNA is a readily available candidate for clinical application in mCRC.

SHP2 is required for growth of KRAS-mutant non-small-cell lung cancer in vivo.

RAS mutations are frequent in human cancer, especially in pancreatic, colorectal and non-small-cell lung cancers (NSCLCs)1-3. Inhibition of the RAS oncoproteins has proven difficult4, and attempts to target downstream effectors5-7 have been hampered by the activation of compensatory resistance mechanisms8. It is also well established that KRAS-mutant tumors are insensitive to inhibition of upstream growth factor receptor signaling. Thus, epidermal growth factor receptor antibody therapy is only effective in KRAS wild-type colon cancers9,10. Consistently, inhibition of SHP2 (also known as PTPN11), which links receptor tyrosine kinase signaling to the RAS-RAF-MEK-ERK pathway11,12, was shown to be ineffective in KRAS-mutant or BRAF-mutant cancer cell lines13. Our data also indicate that SHP2 inhibition in KRAS-mutant NSCLC cells under normal cell culture conditions has little effect. By contrast, SHP2 inhibition under growth factor-limiting conditions in vitro results in a senescence response. In vivo, inhibition of SHP2 in KRAS-mutant NSCLC also provokes a senescence response, which is exacerbated by MEK inhibition. Our data identify SHP2 inhibition as an unexpected vulnerability of KRAS-mutant NSCLC cells that remains undetected in cell culture and can be exploited therapeutically.