Genetic changes in skin tumor progression: correlation between presence of a mutant ras gene and loss of heterozygosity on mouse chromosome 7

Ras/MAPK signalling intensity defines subclonal fitness in a mouse model of hepatocellular carcinoma

Quantitative differences in signal transduction are to date an understudied feature of tumour heterogeneity. The MAPK Erk pathway, which is activated in a large proportion of human tumours, is a prototypic example of distinct cell fates being driven by signal intensity. We have used primary hepatocyte precursors transformed with different dosages of an oncogenic form of Ras to model subclonal variations in MAPK signalling. Orthotopic allografts of Ras-transformed cells in immunocompromised mice gave rise to fast-growing aggressive tumours, both at the primary location and in the peritoneal cavity. Fluorescent labelling of cells expressing different oncogene levels, and consequently varying levels of MAPK Erk activation, highlighted the selection processes operating at the two sites of tumour growth. Indeed, significantly higher Ras expression was observed in primary as compared to secondary, metastatic sites, despite the apparent evolutionary trade-off of increased apoptotic death in the liver that correlated with high Ras dosage. Analysis of the immune tumour microenvironment at the two locations suggests that fast peritoneal tumour growth in the immunocompromised setting is abrogated in immunocompetent animals due to efficient antigen presentation by peritoneal dendritic cells. Furthermore, our data indicate that, in contrast to the metastatic-like outgrowth, strong MAPK signalling is required in the primary liver tumours to resist elimination by NK (natural killer) cells. Overall, this study describes a quantitative aspect of tumour heterogeneity and points to a potential vulnerability of a subtype of hepatocellular carcinoma as a function of MAPK Erk signalling intensity.

RAS oncogene signal strength regulates matrisomal gene expression and tumorigenicity of mouse keratinocytes

Environmental and molecular carcinogenesis are linked by the discovery that chemical carcinogen induced-mutations in the Hras or Kras genes drives tumor development in mouse skin. Importantly, enhanced expression or allele amplification of the mutant Ras gene contributes to selection of initiated cells, tumor persistence, and progression. To explore the consequences of Ras oncogene signal strength, primary keratinocytes were isolated and cultured from the LSL-HrasG12D and LSL-KrasG12D C57BL/6J mouse models and the mutant allele was activated by adeno-Cre recombinase. Keratinocytes expressing one (H) or two (HH) mutant alleles of HrasG12D, one KrasG12D allele (K), or one of each (HK) were studied. All combinations of activated Ras alleles stimulated proliferation and drove transformation marker expression, but only HH and HK formed tumors. HH, HK, and K sustained long-term keratinocyte growth in vitro, while H and WT could not. RNA-Seq yielded two distinct gene expression profiles; HH, HK, and K formed one cluster while H clustered with WT. Weak MAPK activation was seen in H keratinocytes but treatment with a BRAF inhibitor enhanced MAPK signaling and facilitated tumor formation. K keratinocytes became tumorigenic when they were isolated from mice where the LSL-KrasG12D allele was backcrossed from the C57BL/6 onto the FVB/N background. All tumorigenic keratinocytes but not the non-tumorigenic precursors shared a common remodeling of matrisomal gene expression that is associated with tumor formation. Thus, RAS oncogene signal strength determines cell-autonomous changes in initiated cells that are critical for their tumor-forming potential.

Enhanced BRAF engagement by NRAS mutants capable of promoting melanoma initiation

A distinct profile of NRAS mutants is observed in each tumor type. It is unclear whether these profiles are determined by mutagenic events or functional differences between NRAS oncoproteins. Here, we establish functional hallmarks of NRAS mutants enriched in human melanoma. We generate eight conditional, knock-in mouse models and show that rare melanoma mutants (NRAS G12D, G13D, G13R, Q61H, and Q61P) are poor drivers of spontaneous melanoma formation, whereas common melanoma mutants (NRAS Q61R, Q61K, or Q61L) induce rapid tumor onset with high penetrance. Molecular dynamics simulations, combined with cell-based protein-protein interaction studies, reveal that melanomagenic NRAS mutants form intramolecular contacts that enhance BRAF binding affinity, BRAF-CRAF heterodimer formation, and MAPK > ERK signaling. Along with the allelic series of conditional mouse models we describe, these results establish a mechanistic basis for the enrichment of specific NRAS mutants in human melanoma.

Ras Multimers on the Membrane: Many Ways for a Heart-to-Heart Conversation

Formation of Ras multimers, including dimers and nanoclusters, has emerged as an exciting, new front of research in the ‘old’ field of Ras biomedicine. With significant advances made in the past few years, we are beginning to understand the structure of Ras multimers and, albeit preliminary, mechanisms that regulate their formation in vitro and in cells. Here we aim to synthesize the knowledge accrued thus far on Ras multimers, particularly the presence of multiple globular (G-) domain interfaces, and discuss how membrane nanodomain composition and structure would influence Ras multimer formation. We end with some general thoughts on the potential implications of Ras multimers in basic and translational biology.

The Role of Wild-Type RAS in Oncogenic RAS Transformation

The RAS family of oncogenes (HRAS, NRAS, and KRAS) are among the most frequently mutated protein families in cancers. RAS-mutated tumors were originally thought to proliferate independently of upstream signaling inputs, but we now know that non-mutated wild-type (WT) RAS proteins play an important role in modulating downstream effector signaling and driving therapeutic resistance in RAS-mutated cancers. This modulation is complex as different WT RAS family members have opposing functions. The protein product of the WT RAS allele of the same isoform as mutated RAS is often tumor-suppressive and lost during tumor progression. In contrast, RTK-dependent activation of the WT RAS proteins from the two non-mutated WT RAS family members is tumor-promoting. Further, rebound activation of RTK–WT RAS signaling underlies therapeutic resistance to targeted therapeutics in RAS-mutated cancers. The contributions of WT RAS to proliferation and transformation in RAS-mutated cancer cells places renewed interest in upstream signaling molecules, including the phosphatase/adaptor SHP2 and the RasGEFs SOS1 and SOS2, as potential therapeutic targets in RAS-mutated cancers.

Nras Q61R/+ and Kras-/- cooperate to downregulate Rasgrp1 and promote lympho-myeloid leukemia in early T-cell precursors

Kras^−/−; Nras^Q61R/+ mice develop early onset of T-cell malignancy that recapitulates many biological and molecular features of human ETP-ALL.

We identify Rasgrp1 as a negative regulator of Ras/ERK signaling in oncogenic Nras-driven ETP-like leukemia.

Early T-cell precursor acute lymphoblastic leukemia (ETP-ALL) is an aggressive subtype of T-cell ALL. Although genetic mutations hyperactivating cytokine receptor/Ras signaling are prevalent in ETP-ALL, it remains unknown how activated Ras signaling contributes to ETP-ALL. Here, we find that in addition to the frequent oncogenic RAS mutations, wild-type (WT) KRAS transcript level was significantly downregulated in human ETP-ALL cells. Similarly, loss of WT Kras in Nras^Q61R/+ mice promoted hyperactivation of extracellular signal-regulated kinase (ERK) signaling, thymocyte hyperproliferation, and expansion of the ETP compartment. Kras^−/−; Nras^Q61R/+ mice developed early onset of T-cell malignancy that recapitulates many biological and molecular features of human ETP-ALL. Mechanistically, RNA-sequencing analysis and quantitative proteomics study identified that Rasgrp1, a Ras guanine nucleotide exchange factor, was greatly downregulated in mouse and human ETP-ALL. Unexpectedly, hyperactivated Nras/ERK signaling suppressed Rasgrp1 expression and reduced Rasgrp1 level led to increased ERK signaling, thereby establishing a positive feedback loop to augment Nras/ERK signaling and promote cell proliferation. Corroborating our cell line data, Rasgrp1 haploinsufficiency induced Rasgrp1 downregulation and increased phosphorylated ERK level and ETP expansion in Nras^Q61R/+ mice. Our study identifies Rasgrp1 as a negative regulator of Ras/ERK signaling in oncogenic Nras-driven ETP-like leukemia.

Loss of wild type KRAS in KRASMUT lung adenocarcinoma is associated with cancer mortality and confers sensitivity to FASN inhibitors

OBJECTIVES

Wild type RAS (RAS^WT) suppresses the function of oncogenic RAS mutants (RAS^MUT) in laboratory models. Loss of RAS^WT, which we termed loss of heterozygosity (LOH) for any RAS (LAR) or LAKR in the context of KRAS (LOH at KRAS), is found in patients with RAS^MUT cancers. However, the incidence and prognostic significance of LAR has not been studied in modern patient cohorts. LAR or LAKR in RAS^MUT cancers is attractive as a potential biomarker for targeted therapy.

MATERIALS AND METHODS

We evaluated for associations between LAKR and cancer mortality in patients with KRAS^MUT lung adenocarcinoma (LUAD). We also evaluated for associations between LAKR and the metabolic state of cancer cell lines, given that KRAS has been shown to regulate fatty acid synthesis. In line with this, we investigated fatty acid synthase (FASN) inhibitors as potential therapies for KRAS^MUT LAKR, including combination strategies involving clinical KRAS^G12C and FASN inhibitors.

RESULTS

24 % of patients with KRAS^MUT LUAD showed LAKR. KRAS^MUT LAKR cases had a median survival of 16 vs. 30 months in KRAS^MUT non-LAKR (p =  0.017) and LAKR was independently associated with death in this cohort (p =  0.011). We also found that KRAS^MUT LUAD cell lines with LAKR contained elevated levels of FASN and fatty acids relative to non-LAKR cell lines. KRAS^MUT LUAD cells with LAKR showed higher sensitivity to treatment with FASN inhibitors than those without. FASN inhibitors such as TVB-3664 showed synergistic effects with the KRAS^G12C inhibitor MRTX849 in LUAD cells with KRAS^G12C and LAKR, including an in vivo trial using a xenograft model.

CONCLUSIONS

LAKR in KRAS^MUT cancers may represent an independent negative prognostic factor for patients with KRAS^MUT LUAD. It also predicts for response to treatment with FASN inhibitors. Prospective testing of combination therapies including KRAS^G12C and FASN inhibitors in patients with KRAS^G12C LAKR is warranted.

Analysis of RAS protein interactions in living cells reveals a mechanism for pan-RAS depletion by membrane-targeted RAS binders

HRAS, NRAS, and KRAS4A/KRAS4B comprise the RAS family of small GTPases that regulate signaling pathways controlling cell proliferation, differentiation, and survival. RAS pathway abnormalities cause developmental disorders and cancers. We found that KRAS4B colocalizes on the cell membrane with other RAS isoforms and a subset of prenylated small GTPase family members using a live-cell quantitative split luciferase complementation assay. RAS protein coclustering is mainly mediated by membrane association-facilitated interactions (MAFIs). Using the RAS-RBD (CRAF RAS binding domain) interaction as a model system, we showed that MAFI alone is not sufficient to induce RBD-mediated RAS inhibition. Surprisingly, we discovered that high-affinity membrane-targeted RAS binding proteins inhibit RAS activity and deplete RAS proteins through an autophagosome-lysosome-mediated degradation pathway. Our results provide a mechanism for regulating RAS activity and protein levels, a more detailed understanding of which should lead to therapeutic strategies for inhibiting and depleting oncogenic RAS proteins.

The genetic evolution of metastatic uveal melanoma

Uveal melanoma is a clinically distinct and particularly lethal subtype of melanoma originating from melanocytes in the eye. Here, we performed multi-region DNA sequencing of primary uveal melanomas and their matched metastases from 35 patients. We observed novel driver mutations and established the order in which these and known driver mutations undergo selection. Metastases had genomic alterations distinct from their primary tumors, and metastatic dissemination sometimes occurred early during the development of the primary tumor. Our study offers new insights into the genetics and evolution of this melanoma subtype, providing potential biomarkers for progression and therapy.

Multi-region sequencing of 35 primary uveal melanomas and their matched metastases yields new insights into the genetics and evolution of these tumors and provides potential biomarkers for progression and therapy.

Adaptive Immune Resistance Emerges from Tumor-Initiating Stem Cells

Our bodies are equipped with powerful immune surveillance to clear cancerous cells as they emerge. How tumor-initiating stem cells (tSCs) that form and propagate cancers equip themselves to overcome this barrier remains poorly understood. To tackle this problem, we designed a skin cancer model for squamous cell carcinoma (SCC) that can be effectively challenged by adoptive cytotoxic T cell transfer (ACT)-based immunotherapy. Using single-cell RNA sequencing (RNA-seq) and lineage tracing, we found that transforming growth factor β (TGF-β)-responding tSCs are superior at resisting ACT and form the root of tumor relapse. Probing mechanism, we discovered that during malignancy, tSCs selectively acquire CD80, a surface ligand previously identified on immune cells. Moreover, upon engaging cytotoxic T lymphocyte antigen-4 (CTLA4), CD80-expressing tSCs directly dampen cytotoxic T cell activity. Conversely, upon CTLA4- or TGF-β-blocking immunotherapies or Cd80 ablation, tSCs become vulnerable, diminishing tumor relapse after ACT treatment. Our findings place tSCs at the crux of how immune checkpoint pathways are activated.

A model for RAS mutation patterns in cancers: finding the sweet spot

The three RAS genes - HRAS, NRAS and KRAS - are collectively mutated in one-third of human cancers, where they act as prototypic oncogenes. Interestingly, there are rather distinct patterns to RAS mutations; the isoform mutated as well as the position and type of substitution vary between different cancers. As RAS genes are among the earliest, if not the first, genes mutated in a variety of cancers, understanding how these mutation patterns arise could inform on not only how cancer begins but also the factors influencing this event, which has implications for cancer prevention. To this end, we suggest that there is a narrow window or 'sweet spot' by which oncogenic RAS signalling can promote tumour initiation in normal cells. As a consequence, RAS mutation patterns in each normal cell are a product of the specific RAS isoform mutated, as well as the position of the mutation and type of substitution to achieve an ideal level of signalling.

Widespread Selection for Oncogenic Mutant Allele Imbalance in Cancer

Driver mutations in oncogenes encode proteins with gain-of-function properties that enhance fitness. Heterozygous mutations are thus viewed as sufficient for tumorigenesis. We describe widespread oncogenic mutant allele imbalance in 13,448 prospectively characterized cancers. Imbalance was selected for through modest dosage increases of gain-of-fitness mutations. Negative selection targeted haplo-essential effectors of the spliceosome. Loss of the normal allele comprised a distinct class of imbalance driven by competitive fitness, which correlated with enhanced response to targeted therapies. In many cancers, an antecedent oncogenic mutation drove evolutionarily dependent allele-specific imbalance. In other instances, oncogenic mutations co-opted independent copy-number changes via the evolutionary process of exaptation. Oncogenic allele imbalance is a pervasive evolutionary innovation that enhances fitness and modulates sensitivity to targeted therapy.

Methionine 170 is an Environmentally Sensitive Membrane Anchor in the Disordered HVR of K-Ras4B

Ras protein colocalization at the plasma membrane is implicated in the activation of signaling cascades that promote cell growth, survival, and motility. However, the mechanisms that underpin Ras self-association remain unclear. We use molecular dynamics simulations to show how basic and hydrophobic components of the disordered C-terminal membrane tether of K-Ras4B combine to regulate its membrane interactions. Specifically, anionic lipids attract lysine residues to the membrane surface, thereby splitting the peptide population into two states that exchange on the microsecond time scale. These states differ in the membrane insertion of a methionine residue, which is influenced by local membrane composition. As a result, these states may impose context-dependent biases on the disposition of Ras' signaling domain, with possible implications for the accessibility of its effector binding surfaces. We investigate Ras' ability to nanocluster by fly-casting for patches of anionic lipids and find that while anionic lipids promote the intermolecular association of K-Ras4B membrane tethers, at short range this appears to be a passive process in which anionic lipids electrostatically screen these cationic peptides to mitigate their natural repulsion. Together with the sub-microsecond stability of interpeptide contacts, this result suggests that experimentally observed K-Ras4B nanoclustering is not driven by direct intermolecular contact of its membrane tethers.

EIF1AX and RAS Mutations Cooperate to Drive Thyroid Tumorigenesis through ATF4 and c-MYC

Translation initiation is orchestrated by the cap binding and 43S preinitiation complexes (PIC). Eukaryotic initiation factor 1A (EIF1A) is essential for recruitment of the ternary complex and for assembling the 43S PIC. Recurrent EIF1AX mutations in papillary thyroid cancers are mutually exclusive with other drivers, including RAS. EIF1AX mutations are enriched in advanced thyroid cancers, where they display a striking co-occurrence with RAS, which cooperates to induce tumorigenesis in mice and isogenic cell lines. The C-terminal EIF1AX-A113splice mutation is the most prevalent in advanced thyroid cancer. EIF1AX-A113splice variants stabilize the PIC and induce ATF4, a sensor of cellular stress, which is co-opted to suppress EIF2α phosphorylation, enabling a general increase in protein synthesis. RAS stabilizes c-MYC, an effect augmented by EIF1AX-A113splice. ATF4 and c-MYC induce expression of amino acid transporters and enhance sensitivity of mTOR to amino acid supply. These mutually reinforcing events generate therapeutic vulnerabilities to MEK, BRD4, and mTOR kinase inhibitors. SIGNIFICANCE: Mutations of EIF1AX, a component of the translation PIC, co-occur with RAS in advanced thyroid cancers and promote tumorigenesis. EIF1AX-A113splice drives an ATF4-induced dephosphorylation of EIF2α, resulting in increased protein synthesis. ATF4 also cooperates with c-MYC to sensitize mTOR to amino acid supply, thus generating vulnerability to mTOR kinase inhibitors. This article is highlighted in the In This Issue feature, p. 151.

KIT Suppresses BRAFV600E-Mutant Melanoma by Attenuating Oncogenic RAS/MAPK Signaling

The receptor tyrosine kinase KIT promotes survival and migration of melanocytes during development, and excessive KIT activity hyperactivates the RAS/MAPK pathway and can drive formation of melanomas, most notably of rare melanomas that occur on volar and mucosal surfaces of the skin. The much larger fraction of melanomas that occur on sun-exposed skin is driven primarily by BRAF- or NRAS-activating mutations, but these melanomas exhibit a surprising loss of KIT expression, which raises the question of whether loss of KIT in these tumors facilitates tumorigenesis. To address this question, we introduced a kit(lf) mutation into a strain of Tg(mitfa:BRAF^V600E); p53(lf) melanoma-prone zebrafish. Melanoma onset was accelerated in kit(lf); Tg(mitfa:BRAF^V600E); p53(lf) fish. Tumors from kit(lf) animals were more invasive and had higher RAS/MAPK pathway activation. KIT knockdown also increased RAS/MAPK pathway activation in a BRAF^V600E-mutant human melanoma cell line. We found that pathway stimulation upstream of BRAF^V600E could paradoxically reduce signaling downstream of BRAF^V600E, and wild-type BRAF was necessary for this effect, suggesting that its activation can dampen oncogenic BRAF^V600E signaling. In vivo, expression of wild-type BRAF delayed melanoma onset, but only in a kit-dependent manner. Together, these results suggest that KIT can activate signaling through wild-type RAF proteins, thus interfering with oncogenic BRAF^V600E-driven melanoma formation. Cancer Res; 77(21); 5820-30. ©2017 AACR.

RAS Proteins and Their Regulators in Human Disease

RAS proteins are binary switches, cycling between ON and OFF states during signal transduction. These switches are normally tightly controlled, but in RAS-related diseases, such as cancer, RASopathies, and many psychiatric disorders, mutations in the RAS genes or their regulators render RAS proteins persistently active. The structural basis of the switch and many of the pathways that RAS controls are well known, but the precise mechanisms by which RAS proteins function are less clear. All RAS biology occurs in membranes: a precise understanding of RAS' interaction with membranes is essential to understand RAS action and to intervene in RAS-driven diseases.

Doubling Down on Mutant RAS Can MEK or Break Leukemia

Targeting of the RAS pathway has long been a critical therapeutic challenge in oncology. Burgess et al. examine how the relative expression of mutant and wild-type KRAS modulates clonal fitness and sensitivity to MEK inhibitors in a model of Kras^G12D mutant acute myeloid leukemia and propose its use as a predictive biomarker.

Transposon mutagenesis identifies chromatin modifiers cooperating with Ras in thyroid tumorigenesis and detects ATXN7 as a cancer gene

Oncogenic RAS mutations are present in 15-30% of thyroid carcinomas. Endogenous expression of mutant Ras is insufficient to initiate thyroid tumorigenesis in murine models, indicating that additional genetic alterations are required. We used Sleeping Beauty (SB) transposon mutagenesis to identify events that cooperate with Hras^G12V in thyroid tumor development. Random genomic integration of SB transposons primarily generated loss-of-function events that significantly increased thyroid tumor penetrance in Tpo-Cre/homozygous FR-Hras^G12V mice. The thyroid tumors closely phenocopied the histological features of human RAS-driven, poorly differentiated thyroid cancers. Characterization of transposon insertion sites in the SB-induced tumors identified 45 recurrently mutated candidate cancer genes. These mutation profiles were remarkably concordant with mutated cancer genes identified in a large series of human poorly differentiated and anaplastic thyroid cancers screened by next-generation sequencing using the MSK-IMPACT panel of cancer genes, which we modified to include all SB candidates. The disrupted genes primarily clustered in chromatin remodeling functional nodes and in the PI3K pathway. ATXN7, a component of a multiprotein complex with histone acetylase activity, scored as a significant SB hit. It was recurrently mutated in advanced human cancers and significantly co-occurred with RAS or NF1 mutations. Expression of ATXN7 mutants cooperated with oncogenic RAS to induce thyroid cell proliferation, pointing to ATXN7 as a previously unrecognized cancer gene.

KRAS Allelic Imbalance Enhances Fitness and Modulates MAP Kinase Dependence in Cancer

Investigating therapeutic "outliers" that show exceptional responses to anti-cancer treatment can uncover biomarkers of drug sensitivity. We performed preclinical trials investigating primary murine acute myeloid leukemias (AMLs) generated by retroviral insertional mutagenesis in Kras^G12D "knockin" mice with the MEK inhibitor PD0325901 (PD901). One outlier AML responded and exhibited intrinsic drug resistance at relapse. Loss of wild-type (WT) Kras enhanced the fitness of the dominant clone and rendered it sensitive to MEK inhibition. Similarly, human colorectal cancer cell lines with increased KRAS mutant allele frequency were more sensitive to MAP kinase inhibition, and CRISPR-Cas9-mediated replacement of WT KRAS with a mutant allele sensitized heterozygous mutant HCT116 cells to treatment. In a prospectively characterized cohort of patients with advanced cancer, 642 of 1,168 (55%) with KRAS mutations exhibited allelic imbalance. These studies demonstrate that serial genetic changes at the Kras/KRAS locus are frequent in cancer and modulate competitive fitness and MEK dependency.

Ras Dimer Formation as a New Signaling Mechanism and Potential Cancer Therapeutic Target

The K-, N-, and HRas small GTPases are key regulators of cell physiology and are frequently mutated in human cancers. Despite intensive research, previous efforts to target hyperactive Ras based on known mechanisms of Ras signaling have been met with little success. Several studies have provided compelling evidence for the existence and biological relevance of Ras dimers, establishing a new mechanism for regulating Ras activity in cells additionally to GTP-loading and membrane localization. Existing data also start to reveal how Ras proteins dimerize on the membrane. We propose a dimer model to describe Ras-mediated effector activation, which contrasts existing models of Ras signaling as a monomer or as a 5-8 membered multimer. We also discuss potential implications of this model in both basic and translational Ras biology.

The role of wild type RAS isoforms in cancer

Mutationally activated RAS proteins are critical oncogenic drivers in nearly 30% of all human cancers. As with mutant RAS, the role of wild type RAS proteins in oncogenesis, tumour maintenance and metastasis is context-dependent. Complexity is introduced by the existence of multiple RAS genes (HRAS, KRAS, NRAS) and protein "isoforms" (KRAS4A, KRAS4B), by the ever more complicated network of RAS signaling, and by the increasing identification of numerous genetic aberrations in cancers that do and do not harbour mutant RAS. Numerous mouse model carcinogenesis studies and examination of patient tumours reveal that, in RAS-mutant cancers, wild type RAS proteins are likely to serve as tumour suppressors when the mutant RAS is of the same isoform. This evidence is particularly robust in KRAS mutant cancers, which often display suppression or loss of wild type KRAS, but is not as strong for NRAS. In contrast, although not yet fully elucidated, the preponderance of evidence indicates that wild type RAS proteins play a tumour promoting role when the mutant RAS is of a different isoform. In non-RAS mutant cancers, wild type RAS is recognized as a mediator of oncogenic signaling due to chronic activation of upstream receptor tyrosine kinases that feed through RAS. Additionally, in the absence of mutant RAS, activation of wild type RAS may drive cancer upon the loss of negative RAS regulators such as NF1 GAP or SPRY proteins. Here we explore the current state of knowledge with respect to the roles of wild type RAS proteins in human cancers.

RAS isoforms and mutations in cancer at a glance

RAS proteins (KRAS4A, KRAS4B, NRAS and HRAS) function as GDP-GTP-regulated binary on-off switches, which regulate cytoplasmic signaling networks that control diverse normal cellular processes. Gain-of-function missense mutations in RAS genes are found in ∼25% of human cancers, prompting interest in identifying anti-RAS therapeutic strategies for cancer treatment. However, despite more than three decades of intense effort, no anti-RAS therapies have reached clinical application. Contributing to this failure has been an underestimation of the complexities of RAS. First, there is now appreciation that the four human RAS proteins are not functionally identical. Second, with >130 different missense mutations found in cancer, there is an emerging view that there are mutation-specific consequences on RAS structure, biochemistry and biology, and mutation-selective therapeutic strategies are needed. In this Cell Science at a Glance article and accompanying poster, we provide a snapshot of the differences between RAS isoforms and mutations, as well as the current status of anti-RAS drug-discovery efforts.

Loss of wild-type Kras promotes activation of all Ras isoforms in oncogenic Kras-induced leukemogenesis

Despite the well-established role of oncogenic RAS in promoting tumor formation, whether and how wild-type (WT) Ras inhibits tumorigenesis under physiological conditions remains controversial. Here, we show that in a fraction of endogenous oncogenic Kras-induced hematopoietic malignancies, including acute T-cell lymphoblastic leukemia/lymphoma (T-ALL) and myeloproliferative neoplasm (MPN), WT Kras expression is lost through epigenetic or genetic mechanisms. Using conditional Kras(G12D/-) mice, we find that WT Kras deficiency promotes oncogenic Kras-induced MPN, but not T-ALL, in a cell-autonomous manner. Loss of WT Kras rescues oncogenic Kras-mediated hematopoietic stem cell depletion and further enhances granulocyte-macrophage colony-stimulating factor signaling in myeloid cells expressing oncogenic Kras. Quantitative signaling studies reveal that oncogenic Kras but not oncogenic Nras leads to cross-activation of WT Ras, whereas loss of WT Kras further promotes the activation of all Ras isoforms. Our results demonstrate the tumor suppressor function of WT Kras in oncogenic Kras-induced leukemogenesis and elucidate its underlying cellular and signaling mechanisms.

NF2 Loss Promotes Oncogenic RAS-Induced Thyroid Cancers via YAP-Dependent Transactivation of RAS Proteins and Sensitizes Them to MEK Inhibition

Ch22q LOH is preferentially associated with RAS mutations in papillary and in poorly differentiated thyroid cancer (PDTC). The 22q tumor suppressor NF2, encoding merlin, is implicated in this interaction because of its frequent loss of function in human thyroid cancer cell lines. Nf2 deletion or Hras mutation is insufficient for transformation, whereas their combined disruption leads to murine PDTC with increased MAPK signaling. Merlin loss induces RAS signaling in part through inactivation of Hippo, which activates a YAP-TEAD transcriptional program. We find that the three RAS genes are themselves YAP-TEAD1 transcriptional targets, providing a novel mechanism of promotion of RAS-induced tumorigenesis. Moreover, pharmacologic disruption of YAP-TEAD with verteporfin blocks RAS transcription and signaling and inhibits cell growth. The increased MAPK output generated by NF2 loss in RAS-mutant cancers may inform therapeutic strategies, as it generates greater dependency on the MAPK pathway for viability.

SIGNIFICANCE

Intensification of mutant RAS signaling through copy-number imbalances is commonly associated with transformation. We show that NF2/merlin inactivation augments mutant RAS signaling by promoting YAP/TEAD-driven transcription of oncogenic and wild-type RAS, resulting in greater MAPK output and increased sensitivity to MEK inhibitors.

Soluble endoglin antagonizes Met signaling in spindle carcinoma cells

Increased levels of soluble endoglin (Sol-Eng) correlate with poor outcome in human cancer. We have previously shown that shedding of membrane endoglin, and concomitant release of Sol-Eng is a late event in chemical mouse skin carcinogenesis associated with the development of undifferentiated spindle cell carcinomas (SpCCs). In this report, we show that mouse skin SpCCs exhibit a high expression of hepatocyte growth factor (HGF) and an elevated ratio of its active tyrosine kinase receptor Met versus total Met levels. We have evaluated the effect of Sol-Eng in spindle carcinoma cells by transfection of a cDNA encoding most of the endoglin ectodomain or by using purified recombinant Sol-Eng. We found that Sol-Eng inhibited both mitogen-activated protein kinase (MAPK) activity and cell growth in vitro and in vivo. Sol-Eng also blocked MAPK activation by transforming growth factor-β1 (TGF-β1) and impaired both basal and HGF-induced activation of Met and downstream MAPK. Moreover, Sol-Eng strongly reduced basal and HGF-stimulated spindle cell migration and invasion. Both Sol-Eng and full-length endoglin were shown to interact with Met by coimmunoprecipitation experiments. However, full-length endoglin expressed at the plasma membrane of spindle carcinoma cells had no effect on Met signaling activity, and was unable to inhibit HGF-induced cell migration/invasion. These results point to a paradoxical suppressor role for Sol-Eng in carcinogenesis.

Drugging the undruggable RAS: Mission possible?

Despite more than three decades of intensive effort, no effective pharmacological inhibitors of the RAS oncoproteins have reached the clinic, prompting the widely held perception that RAS proteins are 'undruggable'. However, recent data from the laboratory and the clinic have renewed our hope for the development of RAS-inhibitory molecules. In this Review, we summarize the progress and the promise of five key approaches. Firstly, we focus on the prospects of using direct inhibitors of RAS. Secondly, we address the issue of whether blocking RAS membrane association is a viable approach. Thirdly, we assess the status of targeting RAS downstream effector signalling, which is arguably the most favourable current approach. Fourthly, we address whether the search for synthetic lethal interactors of mutant RAS still holds promise. Finally, RAS-mediated changes in cell metabolism have recently been described and we discuss whether these changes could be exploited for new therapeutic directions. We conclude with perspectives on how additional complexities, which are not yet fully understood, may affect each of these approaches.

Modeling cutaneous squamous carcinoma development in the mouse

Cutaneous squamous cell carcinoma (SCC) is one of the most common cancers in Caucasian populations and is associated with a significant risk of morbidity and mortality. The classic mouse model for studying SCC involves two-stage chemical carcinogenesis, which has been instrumental in the evolution of the concept of multistage carcinogenesis, as widely applied to both human and mouse cancers. Much is now known about the sequence of biological and genetic events that occur in this skin carcinogenesis model and the factors that can influence the course of tumor development, such as perturbations in the oncogene/tumor-suppressor signaling pathways involved, the nature of the target cell that acquires the first genetic hit, and the role of inflammation. Increasingly, studies of tumor-initiating cells, malignant progression, and metastasis in mouse skin cancer models will have the potential to inform future approaches to treatment and chemoprevention of human squamous malignancies.

Cellular reprogramming in skin cancer

Early primitive stem cells have long been viewed as the cancer cells of origin (tumor initiating target cells) due to their intrinsic features of self-renewal and longevity. However, emerging evidence suggests a surprising capacity for normal committed cells to function as reserve stem cells upon reprogramming as a consequence of tissue damage resulting in inflammation and wound healing. This results in an alternative concept positing that tumors may originate from differentiated cells that can re-acquire stem cell properties due to genetic or epigenetic reprogramming. It is likely that both models are correct, and that a continuum of potential cells of origin exists, ranging from early primitive stem cells to committed progenitor or even terminally differentiated cells. A combination of the nature of the target cell and the specific types of gene mutations introduced determine tumor cell lineage, as well as potential for malignant conversion. Evidence from mouse skin models of carcinogenesis suggests that initiated cells at different stages within a stem cell hierarchy have varying degrees of requirement for reprogramming (e.g. inflammation stimuli), depending on their degree of differentiation. This article will present evidence in favor of these concepts that has been developed from studies of several mouse models of skin carcinogenesis.

Wild-type H- and N-Ras promote mutant K-Ras-driven tumorigenesis by modulating the DNA damage response

Mutations in KRAS are prevalent in human cancers and universally predictive of resistance to anticancer therapeutics. Although it is widely accepted that acquisition of an activating mutation endows RAS genes with functional autonomy, recent studies suggest that the wild-type forms of Ras may contribute to mutant Ras-driven tumorigenesis. Here, we show that downregulation of wild-type H-Ras or N-Ras in mutant K-Ras cancer cells leads to hyperactivation of the Erk/p90RSK and PI3K/Akt pathways and, consequently, the phosphorylation of Chk1 at an inhibitory site, Ser 280. The resulting inhibition of ATR/Chk1 signaling abrogates the activation of the G2 DNA damage checkpoint and confers specific sensitization of mutant K-Ras cancer cells to DNA damage chemotherapeutic agents in vitro and in vivo.

Transformation by Hras(G12V) is consistently associated with mutant allele copy gains and is reversed by farnesyl transferase inhibition

RAS-driven malignancies remain a major therapeutic challenge. The two-stage 7,12-dimethylbenz(a)anthracene (DMBA)/12-o-tetradecanoylphorbol-13-acetate (TPA) model of mouse skin carcinogenesis has been used to study mechanisms of epithelial tumor development by oncogenic Hras. We used mice with a Hras^G12V knock-in allele to elucidate the early events after Hras activation, and to evaluate the therapeutic effectiveness of farnesyltransferase (FTI) inhibition. Treatment of Caggs-Cre/FR-Hras^G12V mice with TPA alone was sufficient to trigger papilloma development with shorter latency and a ~10-fold greater tumor burden than DMBA/TPA-treated WT controls. Hras^G12V allele copy number was increased in all papillomas induced by TPA. DMBA/TPA treatment of Hras^G12V knock-in mice induced an even greater incidence of papillomas, which either harbored Hras^G12V amplification, or developed a Hras^Q61L mutation in the second allele. Laser-capture microdissection of normal skin, hyperplastic skin and papillomas showed that amplification occurred only at the papilloma stage. HRAS mutant allelic imbalance was also observed in human cancer cell lines, consistent with a requirement for augmented oncogenic HRAS signaling for tumor development. The FTI SCH66336 blocks HRAS farnesylation and delocalizes it from the plasma membrane. NRAS and KRAS are not affected as they are alternatively prenylated. When tested in lines harboring HRAS, NRAS or KRAS mutations, SCH66336 delocalized, inhibited signaling and preferentially inhibited growth only of HRAS-mutant lines. Treatment with SCH66336 also induced near-complete regression of papillomas of TPA-treated Hras^G12V knock-in mice. These data suggest that farnesyl transferase inhibitors should be reevaluated as targeted agents for human HRAS-driven cancers, such as those of bladder, thyroid and other epithelial lineages.

A requirement for wild-type Ras isoforms in mutant KRas-driven signalling and transformation

The mutant forms of KRas, NRas and HRas drive the initiation and progression of a number of human cancers, but less is known about the role of WT (wild-type) Ras alleles and isoforms in cancer. We used zinc-finger nucleases targeting HRas and NRas to modify both alleles of these genes in the mutant KRas-driven Hec1A endometrial cancer cell line, which normally expresses WT copies of these genes. The disruption of either WT isoform of Ras compromised growth-factor-dependent signalling through the ERK (extracellular-signal-regulated kinase) pathway. In addition, the disruption of HRas hindered the activation of Akt and subsequent downstream signalling. This was associated with decreased proliferation, increased apoptosis and decreased anchorage-independent growth in the HRas-disrupted cells. However, xenograft tumour growth was not significantly affected by the disruption of either NRas or HRas. As expected, deleting the mutant allele of KRas abolished tumour growth, whereas deletion of the remaining WT copy of KRas increased the tumorigenic properties of these cells; deleting a single copy of either HRas or NRas did not mimic this effect. The present study demonstrates that the WT copies of HRas, NRas and KRas play unique roles in the context of mutant KRas-driven tumours.

Dominant role of oncogene dosage and absence of tumor suppressor activity in Nras-driven hematopoietic transformation

Biochemical properties of Ras oncoproteins and their transforming ability strongly support a dominant mechanism of action in tumorigenesis. However, genetic studies unexpectedly suggested that wild-type (WT) Ras exerts tumor suppressor activity. Expressing oncogenic Nras(G12D) in the hematopoietic compartment of mice induces an aggressive myeloproliferative neoplasm that is exacerbated in homozygous mutant animals. Here, we show that increased Nras(G12D) gene dosage, but not inactivation of WT Nras, underlies the aggressive in vivo behavior of Nras(G12D/G12D) hematopoietic cells. Modulating Nras(G12D) dosage had discrete effects on myeloid progenitor growth, signal transduction, and sensitivity to MAP-ERK kinase (MEK) inhibition. Furthermore, enforced WT N-Ras expression neither suppressed the growth of Nras-mutant cells nor inhibited myeloid transformation by exogenous Nras(G12D). Importantly, NRAS expression increased in human cancer cell lines with NRAS mutations. These data have therapeutic implications and support reconsidering the proposed tumor suppressor activity of WT Ras in other cancers.

SIGNIFICANCE

Understanding the mechanisms of Ras -induced transformation and adaptive cellular responses is fundamental. The observation that oncogenic Nras lacks tumor suppressor activity, whereas increased dosage strongly modulates cell growth and alters sensitivity to MEK inhibition, suggests new therapeutic opportunities in cancer.

Emerging roles for intersectin (ITSN) in regulating signaling and disease pathways

Intersectins (ITSNs) represent a family of multi-domain adaptor proteins that regulate endocytosis and cell signaling. ITSN genes are highly conserved and present in all metazoan genomes examined thus far. Lower eukaryotes have only one ITSN gene, whereas higher eukaryotes have two ITSN genes. ITSN was first identified as an endocytic scaffold protein, and numerous studies reveal a conserved role for ITSN in endocytosis. Subsequently, ITSNs were found to regulate multiple signaling pathways including receptor tyrosine kinases (RTKs), GTPases, and phosphatidylinositol 3-kinase Class 2beta (PI3KC2β). ITSN has also been implicated in diseases such as Down Syndrome (DS), Alzheimer Disease (AD), and other neurodegenerative disorders. This review summarizes the evolutionary conservation of ITSN, the latest research on the role of ITSN in endocytosis, the emerging roles of ITSN in regulating cell signaling pathways, and the involvement of ITSN in human diseases such as DS, AD, and cancer.

Actin polymerization negatively regulates p53 function by impairing its nuclear import in response to DNA damage

Actin, one of the most evolutionarily conservative proteins in eukaryotes, is distributed both in the cytoplasm and the nucleus, and its dynamics plays important roles in numerous cellular processes. Previous evidence has shown that actin interacts with p53 and this interaction increases in the process of p53 responding to DNA damage, but the physiological significance of their interaction remains elusive. Here, we show that DNA damage induces both actin polymerization and p53 accumulation. To further understand the implication of actin polymerization in p53 function, cells were treated with actin aggregation agent. We find that the protein level of p53 decrease. The change in p53 is a consequence of the polymeric actin anchoring p53 in the cytoplasm, thus impairing p53 nuclear import. Analysis of phosphorylation and ubiquitination of p53 reveals that actin polymerization promotes the p53 phosphorylation at Ser315 and reduces the stabilization of p53 by recruiting Aurora kinase A. Taken together, our results suggest that the actin polymerization serves as a negative modulator leading to the impairment of nuclear import and destabilization of p53. On the basis of our results, we propose that actin polymerization might be a factor participating in the process of orchestrating p53 function in response to DNA damage.

Inflammation and Hras signaling control epithelial-mesenchymal transition during skin tumor progression

Epithelial-mesenchymal transition (EMT) is thought to be an important, possibly essential, component of the process of tumor dissemination and metastasis. About 20%-30% of Hras mutant mouse skin carcinomas induced by chemical initiation/promotion protocols have undergone EMT. Reduced exposure to TPA-induced chronic inflammation causes a dramatic reduction in classical papillomas and squamous cell carcinomas (SCCs), but the mice still develop highly invasive carcinomas with EMT properties, reduced levels of Hras and Egfr signaling, and frequent Ink4/Arf deletions. Deletion of Hras from the mouse germline also leads to a strong reduction in squamous tumor development, but tumors now acquire activating Kras mutations and exhibit more aggressive metastatic properties. We propose that invasive carcinomas can arise by different genetic and biological routes dependent on exposure to chronic inflammation and possibly from different target cell populations within the skin. Our data have implications for the use of inhibitors of inflammation or of Ras/Egfr pathway signaling for prevention or treatment of invasive cancers.

Cellular hyperproliferation and cancer as evolutionary variables

Technological advances in biology have begun to dramatically change the way we think about evolution, development, health and disease. The ability to sequence the genomes of many individuals within a population, and across multiple species, has opened the door to the possibility of answering some long-standing and perplexing questions about our own genetic heritage. One such question revolves around the nature of cellular hyperproliferation. This cellular behavior is used to effect wound healing in most animals, as well as, in some animals, the regeneration of lost body parts. Yet at the same time, cellular hyperproliferation is the fundamental pathological condition responsible for cancers in humans. Here, I will discuss why microevolution, macroevolution and developmental biology all have to be taken into consideration when interpreting studies of both normal and malignant hyperproliferation. I will also illustrate how a synthesis of evolutionary sciences and developmental biology through the study of diverse model organisms can inform our understanding of both health and disease.

Interactions between wild-type and mutant Ras genes in lung and skin carcinogenesis

Ras oncogenes (Hras, Kras, and Nras) are important drivers of carcinogenesis. However, tumors with Ras mutations often show loss of the corresponding wildtype (WT) allele, suggesting that proto-oncogenic forms of Ras can function as a suppressor of carcinogenesis. In vitro studies also suggest that WT Ras proteins can suppress the tumorigenic properties of alternate mutant Ras family members, but in vivo evidence for these heterologous interactions is lacking. We have investigated the genetic interactions between different combinations of mutant and WT Ras alleles in vivo using carcinogen-induced lung and skin carcinogenesis in mice with targeted deletion of different Ras family members. The major suppressor effect of WT Kras is observed only in mutant Kras-driven lung carcinogenesis, where loss of one Kras allele led to increased tumor number and size. Deletion of one Hras allele dramatically reduced the number of skin papillomas with Hras mutations, consistent with Hras as the major target of mutation in these tumors. However, skin carcinoma numbers were very similar, suggesting that WT Hras functions as a suppressor of progression from papillomas to invasive squamous carcinomas. In the skin, the Kras proto-oncogene functions cooperatively with mutant Hras to promote papilloma development, although the effect is relatively small. In contrast, the Hras proto-oncogene attenuated the activity of mutant Kras in lung carcinogenesis. Interestingly, loss of Nras increased the number of mutant Kras-induced lung tumors but decreased the number of mutant Hras-induced skin papillomas. These results show that the strongest suppressor effects of WT Ras are only seen in the context of mutation of the cognate Ras protein, and only relatively weak effects are detected on tumor development induced by mutations in alternative family members. The data also underscore the complex and context-dependent nature of interactions between proto-oncogenic and oncogenic forms of different Ras family members during tumor development.

Using mice to unveil the genetics of cancer resistance

In the UK, four in ten people will develop some form of cancer during their lifetime, with an individual's relative risk depending on many factors, including age, lifestyle and genetic make-up. Much research has gone into identifying the genes that are mutated in tumorigenesis with the overwhelming majority of genetically-modified (GM) mice in cancer research showing accelerated tumorigenesis or recapitulating key aspects of the tumorigenic process. Yet if six out of ten people will not develop some form of cancer during their lifetime, together with the fact that some cancer patients experience spontaneous regression/remission, it suggests there are ways of 'resisting' cancer. Indeed, there are wildtype, spontaneously-arising mutants and GM mice that show some form of 'resistance' to cancer. Identification of mice with increased resistance to cancer is a novel aspect of cancer research that is important in terms of providing both chemopreventative and therapeutic options. In this review we describe the different mouse lines that display a 'cancer resistance' phenotype and discuss the molecular basis of their resistance.

Disruption of p16 and activation of Kras in pancreas increase ductal adenocarcinoma formation and metastasis in vivo

Inactivation of tumor suppressor gene p16/INK4A and oncogenic activation of KRAS occur in almost all pancreatic cancers. To better understand the roles of p16 in pancreatic tumorigenesis, we created a conditional p16 knockout mouse line (p16^flox/flox), in which p16 is specifically disrupted in a tissue-specific manner without affecting p19/ARF expression. p16^flox/flox; LSL-Kras^G12D; Pdx1-Cre mice developed the full spectrum of pancreatic intraepithelial neoplasia (mPanIN) lesions, pancreatic ductal adenocarcinoma (PDA), and metastases were observed in all the mice. Here we report a mouse model that simulates human pancreatic tumorigenesis at both genetic and histologic levels and is ideal for studies of metastasis. During the progression from primary tumors to metastases, the wild-type allele of Kras was progressively lost (loss of heterozygosity at Kras or LOH at Kras) in p16^flox/flox; LSL- Kras^G12D; Pdx1-Cre mice. These observations suggest a role for Kras beyond tumor initiation. In vitro assays performed with cancer cell lines derived from primary pancreatic tumors of these mice showed that cancer cells with LOH at Kras exhibited more aggressive phenotypes than those retained the wild-type Kras allele, indicating that LOH at Kras can provide cancer cells functional growth advantages and promote metastasis. Increased LOH at KRAS was also observed in progression of human pancreatic primary tumors to metastases, again supporting a role for the KRAS gene in cancer metastasis. This finding has potential translational implications- future KRAS target therapies may need to consider targeting oncogenic KRAS specifically without inhibiting wild-type KRAS function.

Progressive genomic instability in the FVB/Kras(LA2) mouse model of lung cancer

Alterations in DNA copy number contribute to the development and progression of cancers and are common in epithelial tumors. We have used array Comparative Genomic Hybridization (aCGH) to visualize DNA copy number alterations across the genomes of lung tumors in the Kras(LA2) model of lung cancer. Copy number gain involving the Kras locus, as focal amplification or whole chromosome gain, is the most common alteration in these tumors and with a prevalence that increased significantly with increasing tumor size. Furthermore, Kras amplification was the only major genomic event among the smallest lung tumors, suggesting that this alteration occurs early during the development of mutant Kras-driven lung cancers. Recurring gains and deletions of other chromosomes occur progressively more frequently among larger tumors. These results are in contrast to a previous aCGH analysis of lung tumors from Kras(LA2) mice on a mixed genetic background, in which relatively few DNA copy number alterations were observed regardless of tumor size. Our model features the Kras(LA2) allele on the inbred FVB/N mouse strain, and in this genetic background, there is a highly statistically significant increase in level of genomic instability with increasing tumor size. These data suggest that recurring DNA copy alterations are important for tumor progression in the Kras(LA2) model of lung cancer and that the requirement for these alterations may be dependent on the genetic background of the mouse strain.

Outgrowth of drug-resistant carcinomas expressing markers of tumor aggression after long-term TβRI/II kinase inhibition with LY2109761

TGF-β is produced excessively by many solid tumors and can drive malignant progression through multiple effects on the tumor cell and microenvironment. TGF-β signaling pathway inhibitors have shown efficacy in preclinical models of metastatic cancer. Here, we investigated the effect of systemic LY2109761, a TGF-β type I/II receptor (TβRI/TβRII) kinase inhibitor, in both a tumor allograft model and the mouse skin model of de novo chemically induced carcinogenesis in vivo. Systemic LY2109761 administration disrupted tumor vascular architecture and reduced myofibroblast differentiation of E4 skin carcinoma cells in a tumor allograft. In the 7,12-dimethyl-benzanthracene plus phorbol myristate acetate-induced skin chemical carcinogenesis model, acute dosing of established naive primary carcinomas with LY2109761 (100 mg/kg) every 8 hours for 10 days (100 mg/kg) diminished phospho-Smad2 (P-Smad2) levels and marginally decreased the expression of inflammatory and invasive markers. Sustained exposure to LY2109761 (100 mg/kg/d) throughout the tumor outgrowth phase had no effect on carcinoma latency or incidence. However, molecular analysis of resultant carcinomas by microarray gene expression, Western blotting, and immunohistochemistry suggests that long-term LY2109761 exposure leads to the outgrowth of carcinomas with elevated P-Smad2 levels that do not respond to drug. This is the first description of acquired resistance to a small-molecule inhibitor of the TβRI/TβRII kinase. Resultant carcinomas were more aggressive and inflammatory in nature, with delocalized E-cadherin and elevated expression of Il23a, laminin V, and matrix metalloproteinases. Therefore, TGF-β inhibitors might be clinically useful for applications requiring acute administration, but long-term patient exposure to such drugs should be undertaken with caution.

Network analysis of skin tumor progression identifies a rewired genetic architecture affecting inflammation and tumor susceptibility

BACKGROUND

Germline polymorphisms can influence gene expression networks in normal mammalian tissues and can affect disease susceptibility. We and others have shown that analysis of this genetic architecture can identify single genes and whole pathways that influence complex traits, including inflammation and cancer susceptibility. Whether germline variants affect gene expression in tumors that have undergone somatic alterations, and the extent to which these variants influence tumor progression, is unknown.

RESULTS

Using an integrated linkage and genomic analysis of a mouse model of skin cancer that produces both benign tumors and malignant carcinomas, we document major changes in germline control of gene expression during skin tumor development resulting from cell selection, somatic genetic events, and changes in the tumor microenvironment. The number of significant expression quantitative trait loci (eQTL) is progressively reduced in benign and malignant skin tumors when compared to normal skin. However, novel tumor-specific eQTL are detected for several genes associated with tumor susceptibility, including IL18 (Il18), Granzyme E (Gzme), Sprouty homolog 2 (Spry2), and Mitogen-activated protein kinase kinase 4 (Map2k4).

CONCLUSIONS

We conclude that the genetic architecture is substantially altered in tumors, and that eQTL analysis of tumors can identify host factors that influence the tumor microenvironment, mitogen-activated protein (MAP) kinase signaling, and cancer susceptibility.

Stage-specific sensitivity to p53 restoration during lung cancer progression

Tumourigenesis is a multistep process that results from the sequential accumulation of mutations in key oncogene and tumour suppressor pathways. Personalized cancer therapy that is based on targeting these underlying genetic abnormalities presupposes that sustained inactivation of tumour suppressors and activation of oncogenes is essential in advanced cancers. Mutations in the p53 tumour-suppressor pathway are common in human cancer and significant efforts toward pharmaceutical reactivation of defective p53 pathways are underway1–3. Here we show that restoration of p53 in established murine lung tumours leads to significant but incomplete tumour cell loss specifically in malignant adenocarcinomas but not in adenomas. We define amplification of MAPK signaling as a critical determinant of malignant progression and also a stimulator of Arf tumour-suppressor expression. The response to p53 restoration in this context is critically dependent on the expression of Arf. We propose that p53 not only limits malignant progression by suppressing the acquisition of alterations that lead to tumour progression, but also, in the context of p53 restoration, responds to increased oncogenic signaling to mediate tumor regression. Our observations also underscore that the p53 pathway is not engaged by low levels of oncogene activity that are sufficient for early stages of lung tumour development. These data suggest that restoration of pathways important in tumour progression, as opposed to initiation, may lead to incomplete tumour regression due to the stage-heterogeneity of tumour cell populations.

The TGF-beta co-receptor endoglin modulates the expression and transforming potential of H-Ras

Endoglin is a coreceptor for transforming growth factor-β (TGF-β) that acts as a suppressor of malignancy during mouse skin carcinogenesis. Because in this model system H-Ras activation drives tumor initiation and progression, we have assessed the effects of endoglin on the expression of H-Ras in transformed keratinocytes. We found that TGF-β1 increases the expression of H-Ras at both messenger RNA and protein levels. The TGF-β1-induced H-Ras promoter transactivation was Smad4 independent but mediated by the activation of the TGF-β type I receptor ALK5 and the Ras-mitogen-activated protein kinase (MAPK) pathway. Endoglin attenuated stimulation by TGF-β1 of both MAPK signaling activity and H-Ras gene expression. Moreover, endoglin inhibited the Ras/MAPK pathway in transformed epidermal cells containing an H-Ras oncogene, as evidenced by the levels of Ras-guanosine triphosphate, phospho-MAPK kinase (MEK) and phospho-extracellular signal-regulated kinase (ERK) as well as the expression of c-fos, a MAPK downstream target gene. Interestingly, in spindle carcinoma cells, that have a hyperactivated Ras/MAPK pathway, endoglin inhibited ERK phosphorylation without affecting MEK or Ras activity. The mechanism for this effect is unknown but strongly depends on the endoglin extracellular domain. Because the MAPK pathway is a downstream mediator of the transforming potential of Ras, the effect of endoglin on the oncogenic function of H-Ras was assessed. Endoglin inhibited the transforming capacity of H-Ras(Q61K) and H-Ras(G12V) oncogenes in a NIH3T3 focus formation assay. The ability to interfere with the expression and oncogenic potential of H-Ras provides a new face of the suppressor role exhibited by endoglin in H-Ras-driven carcinogenesis.

Toward a genetics of cancer resistance

Two of three humans never get cancer. Even the majority of heavy smokers remain cancer free. Is this a matter of chance, or are there cancer-resistant genotypes? Based on the evidence discussed, it would appear that evolution has favored a limited number of relatively common resistance genes that may nip incipient cancerous foci in the bud, i.e., to stop them at their inception. It is further suggested that resistance genes may act at the level of tissue organization in a dominant fashion.

RasGRP1 overexpression in the epidermis of transgenic mice contributes to tumor progression during multistage skin carcinogenesis

RasGRP1 is a guanine nucleotide exchange factor for Ras, activated in response to the second messenger diacylglycerol and its ultrapotent analogues, the phorbol esters. We have previously shown that RasGRP1 is expressed in mouse epidermal keratinocytes and that transgenic mice overexpressing RasGRP1 in the epidermis under the keratin 5 promoter (K5.RasGRP1) are prone to developing spontaneous papillomas and squamous cell carcinomas, suggesting a role for RasGRP1 in skin tumorigenesis. Here, we examined the response of the K5.RasGRP1 mice to multistage skin carcinogenesis, using 7,12-dimethylbenz(a)anthracene as carcinogen and 12-O-tetradecanoylphorbol-13-acetate (TPA) as tumor promoter. We found that whereas tumor multiplicity did not differ between transgenic and wild-type groups, the transgenic tumors were significantly larger than those observed in the wild-type mice (wild-type, 4.58 +/- 0.25 mm; transgenic, 9.83 +/- 1.05 mm). Histologic analysis further revealed that squamous cell carcinomas generated in the transgenic mice were less differentiated and more invasive than the wild-type tumors. Additionally, 30% of the transgenic mice developed tumors in the absence of initiation, suggesting that RasGRP1 overexpression could partially substitute for the initiation step induced by dimethylbenz(a)anthracene. In primary keratinocytes isolated from K5.RasGRP1 mice, TPA stimulation induced higher levels of Ras activation compared with the levels measured in the wild-type cells, indicating that constitutive overexpression of RasGRP1 in epidermal cells leads to elevated biochemical activation of endogenous Ras in response to TPA. The present data suggests that RasGRP1 participates in skin carcinogenesis via biochemical activation of endogenous wild-type Ras and predisposes to malignant progression in cooperation with Ras oncogenic signals.