Hypoxia drives CD39-dependent suppressor function in exhausted T cells to limit antitumor immunity.

CD8+ T cells are critical for elimination of cancer cells. Factors within the tumor microenvironment (TME) can drive these cells to a hypofunctional state known as exhaustion. The most terminally exhausted T (tTex) cells are resistant to checkpoint blockade immunotherapy and might instead limit immunotherapeutic efficacy. Here we show that intratumoral CD8+ tTex cells possess transcriptional features of CD4+Foxp3+ regulatory T cells and are similarly capable of directly suppressing T cell proliferation ex vivo. tTex cell suppression requires CD39, which generates immunosuppressive adenosine. Restricted deletion of CD39 in endogenous CD8+ T cells resulted in slowed tumor progression, improved immunotherapy responsiveness and enhanced infiltration of transferred tumor-specific T cells. CD39 is induced on tTex cells by tumor hypoxia, thus mitigation of hypoxia limits tTex suppression. Together, these data suggest tTex cells are an important regulatory population in cancer and strategies to limit their generation, reprogram their immunosuppressive state or remove them from the TME might potentiate immunotherapy.

Mitochondrial stress induced by continuous stimulation under hypoxia rapidly drives T cell exhaustion.

Cancer and chronic infections induce T cell exhaustion, a hypofunctional fate carrying distinct epigenetic, transcriptomic and metabolic characteristics. However, drivers of exhaustion remain poorly understood. As intratumoral exhausted T cells experience severe hypoxia, we hypothesized that metabolic stress alters their responses to other signals, specifically, persistent antigenic stimulation. In vitro, although CD8+ T cells experiencing continuous stimulation or hypoxia alone differentiated into functional effectors, the combination rapidly drove T cell dysfunction consistent with exhaustion. Continuous stimulation promoted Blimp-1-mediated repression of PGC-1α-dependent mitochondrial reprogramming, rendering cells poorly responsive to hypoxia. Loss of mitochondrial function generated intolerable levels of reactive oxygen species (ROS), sufficient to promote exhausted-like states, in part through phosphatase inhibition and the consequent activity of nuclear factor of activated T cells. Reducing T cell-intrinsic ROS and lowering tumor hypoxia limited T cell exhaustion, synergizing with immunotherapy. Thus, immunologic and metabolic signaling are intrinsically linked: through mitigation of metabolic stress, T cell differentiation can be altered to promote more functional cellular fates.

Oncolytic Viruses Engineered to Enforce Leptin Expression Reprogram Tumor-Infiltrating T Cell Metabolism and Promote Tumor Clearance.

Immunotherapy can reinvigorate dormant responses to cancer, but response rates remain low. Oncolytic viruses, which replicate in cancer cells, induce tumor lysis and immune priming, but their immune consequences are unclear. We profiled the infiltrate of aggressive melanomas induced by oncolytic Vaccinia virus using RNA sequencing and found substantial remodeling of the tumor microenvironment, dominated by effector T cell influx. However, responses to oncolytic viruses were incomplete due to metabolic insufficiencies induced by the tumor microenvironment. We identified the adipokine leptin as a potent metabolic reprogramming agent that supported antitumor responses. Leptin metabolically reprogrammed T cells in vitro, and melanoma cells expressing leptin were immunologically controlled in mice. Engineering oncolytic viruses to express leptin in tumor cells induced complete responses in tumor-bearing mice and supported memory development in the tumor infiltrate. Thus, leptin can provide metabolic support to tumor immunity, and oncolytic viruses represent a platform to deliver metabolic therapy.

The Mitochondrial Unfoldase-Peptidase Complex ClpXP Controls Bioenergetics Stress and Metastasis.

Mitochondria must buffer the risk of proteotoxic stress to preserve bioenergetics, but the role of these mechanisms in disease is poorly understood. Using a proteomics screen, we now show that the mitochondrial unfoldase-peptidase complex ClpXP associates with the oncoprotein survivin and the respiratory chain Complex II subunit succinate dehydrogenase B (SDHB) in mitochondria of tumor cells. Knockdown of ClpXP subunits ClpP or ClpX induces the accumulation of misfolded SDHB, impairing oxidative phosphorylation and ATP production while activating "stress" signals of 5' adenosine monophosphate-activated protein kinase (AMPK) phosphorylation and autophagy. Deregulated mitochondrial respiration induced by ClpXP targeting causes oxidative stress, which in turn reduces tumor cell proliferation, suppresses cell motility, and abolishes metastatic dissemination in vivo. ClpP is universally overexpressed in primary and metastatic human cancer, correlating with shortened patient survival. Therefore, tumors exploit ClpXP-directed proteostasis to maintain mitochondrial bioenergetics, buffer oxidative stress, and enable metastatic competence. This pathway may provide a "drugable" therapeutic target in cancer.

PI3K therapy reprograms mitochondrial trafficking to fuel tumor cell invasion.

Molecular therapies are hallmarks of "personalized" medicine, but how tumors adapt to these agents is not well-understood. Here we show that small-molecule inhibitors of phosphatidylinositol 3-kinase (PI3K) currently in the clinic induce global transcriptional reprogramming in tumors, with activation of growth factor receptors, (re)phosphorylation of Akt and mammalian target of rapamycin (mTOR), and increased tumor cell motility and invasion. This response involves redistribution of energetically active mitochondria to the cortical cytoskeleton, where they support membrane dynamics, turnover of focal adhesion complexes, and random cell motility. Blocking oxidative phosphorylation prevents adaptive mitochondrial trafficking, impairs membrane dynamics, and suppresses tumor cell invasion. Therefore, "spatiotemporal" mitochondrial respiration adaptively induced by PI3K therapy fuels tumor cell invasion, and may provide an important antimetastatic target.

Treatment with oncolytic vaccinia virus infects tumor-infiltrating regulatory and exhausted T cells.

BACKGROUND: Oncolytic viruses (OVs) are an attractive way to increase immune infiltration into an otherwise cold tumor. While OVs are engineered to selectively infect tumor cells, there is evidence that they can infect other non-malignant cells in the tumor. We sought to determine if oncolytic vaccinia virus (VV) can infect lymphocytes in the tumor and, if so, how this was linked to therapeutic efficacy. METHODS: To investigate infection of lymphocytes by VV, we used a GFP reporting VV in a murine head and neck squamous cell carcinoma tumor model. We also performed in vitro infection studies to determine the mechanism and consequences of VV lymphocyte infection by VV. RESULTS: Our findings show that VV carries the capacity to infect proportions of immune cells, most notably T cells, after intratumoral treatment. Notably, this infection is preferential to terminally differentiated T cells that tend to reside in hypoxia. Infection of T cells leads to both virus production by the T cells as well as the eventual death of these cells. Using a mouse model which overexpressed the antiapoptotic protein Bcl2 in all T cells, we found that reducing T cell death following VV infection in MEER tumors reduced the number of complete regressions and reduced survival time compared with littermate control mice. CONCLUSIONS: These findings suggest that OVs are capable of infecting more than just malignant cells after treatment, and that this infection may be an important part of the OV mechanism. We found that exhausted CD8+ T cells and regulatory CD4+ T cells were preferentially infected at early timepoints after treatment and subsequently died. When cell death in T cells was mitigated, mice responded poorly to VV treatment, suggesting that the deletion of these populations is critical to the therapeutic response to VV.

An oncolytic virus-delivered TGFß inhibitor overcomes the immunosuppressive tumor microenvironment.

While checkpoint blockade immunotherapies have widespread success, they rely on a responsive immune infiltrate; as such, treatments enhancing immune infiltration and preventing immunosuppression are of critical need. We previously generated αPD-1 resistant variants of the murine HNSCC model MEER. While entirely αPD-1 resistant, these tumors regress after single dose of oncolytic vaccinia virus (VV). We then generated a VV-resistant MEER line to dissect the immunologic features of sensitive and resistant tumors. While treatment of both tumor types induced immune infiltration and IFNγ, we found a defining feature of resistance was elevation of immunosuppressive cytokines like TGFß, which blunted IFNγ signaling, especially in regulatory T cells. We engineered VV to express a genetically encoded TGFßRII inhibitor. Inhibitor-expressing VV produced regressions in resistant tumor models and showed impressive synergy with checkpoint blockade. Importantly, tumor-specific, viral delivery of TGFß inhibition had no toxicities associated with systemic TGFß/TGFßR inhibition. Our data suggest that aside from stimulating immune infiltration, oncolytic viruses are attractive means to deliver agents to limit immunosuppression in cancer.

Obesity Is Associated with Altered Tumor Metabolism in Metastatic Melanoma.

PURPOSE: Overweight/obese (OW/OB) patients with metastatic melanoma unexpectedly have improved outcomes with immune checkpoint inhibitors (ICI) and BRAF-targeted therapies. The mechanism(s) underlying this association remain unclear, thus we assessed the integrated molecular, metabolic, and immune profile of tumors, as well as gut microbiome features, for associations with patient body mass index (BMI). EXPERIMENTAL DESIGN: Associations between BMI [normal (NL < 25) or OW/OB (BMI ≥ 25)] and tumor or microbiome characteristics were examined in specimens from 782 patients with metastatic melanoma across 7 cohorts. DNA associations were evaluated in The Cancer Genome Atlas cohort. RNA sequencing from 4 cohorts (n = 357) was batch corrected and gene set enrichment analysis (GSEA) by BMI category was performed. Metabolic profiling was conducted in a subset of patients (x = 36) by LC/MS, and in flow-sorted melanoma tumor cells (x = 37) and patient-derived melanoma cell lines (x = 17) using the Seahorse XF assay. Gut microbiome features were examined in an independent cohort (n = 371). RESULTS: DNA mutations and copy number variations were not associated with BMI. GSEA demonstrated that tumors from OW/OB patients were metabolically quiescent, with downregulation of oxidative phosphorylation and multiple other metabolic pathways. Direct metabolite analysis and functional metabolic profiling confirmed decreased central carbon metabolism in OW/OB metastatic melanoma tumors and patient-derived cell lines. The overall structure, diversity, and taxonomy of the fecal microbiome did not differ by BMI. CONCLUSIONS: These findings suggest that the host metabolic phenotype influences melanoma metabolism and provide insight into the improved outcomes observed in OW/OB patients with metastatic melanoma treated with ICIs and targeted therapies. See related commentary by Smalley, p. 5.

RB and p53 cooperate to prevent liver tumorigenesis in response to tissue damage.

BACKGROUND & AIMS: The tumor suppressors retinoblastoma (RB) and p53 are important regulators of the cell cycle. Although human cancer cells inactivate RB and p53 by many mechanisms, the cooperative roles of these proteins in tumorigenesis are complex and tissue specific. We analyzed the cooperation of RB and p53 in liver development and pathogenesis of hepatocellular carcinoma. METHODS: Spontaneous and carcinogen-induced (diethylnitrosamine) tumorigenesis were studied in mice with liver-specific deletions of Rb and/or p53 (Rbf/f;albcre+, p53f/f;albcre+ and Rbf/f; p53f/f;albcre+ mice). Genotype, histologic, immunohistochemical, microarray, quantitative polymerase chain reaction, immunoblot, and comparative genomic hybridization analyses were performed using normal and tumor samples. Comparative microarray analyses were performed against publicly available human microarray data sets. RESULTS: Deletion of RB and p53 from livers of mice deregulated the transcriptional programs associated with human disease. These changes were not sufficient for spontaneous tumorigenesis; potent quiescence mechanisms compensated for loss of these tumor suppressors. In response to hepatocarcinogen-induced damage, distinct and cooperative roles of RB and p53 were revealed; their loss affected cell cycle control, checkpoint response, and genome stability. In damaged tissue, combined loss of RB and p53 resulted in early lesion formation, aggressive tumor progression, and gene expression signatures and histologic characteristics of advanced human hepatocellular carcinoma. CONCLUSIONS: The effects RB and p53 loss are determined by the tissue environment; cell stresses that promote aggressive disease reveal the functions of these tumor suppressors.

Association of RB/p16-pathway perturbations with DCIS recurrence: dependence on tumor versus tissue microenvironment.

The prevalence of ductal carcinoma in situ (DCIS) diagnoses has significantly increased as a result of active radiographic screening. Surgical resection and hormone and radiation therapies are effective treatments, but not all DCIS will progress to invasive breast cancer. Therefore, markers are needed to define tumors at low risk of recurrence and progression that can be treated by surgery alone rather than by adjuvant therapies. Initial analyses indicate that retinoblastoma (RB)-pathway perturbations occur at high frequency in DCIS and mirror the molecular alterations observed in invasive breast cancer. Particularly, the elevated expression of p16ink4a in DCIS was associated with loss of RB function and estrogen receptor-negative biology. Furthermore, high expression of p16ink4a in conjunction with Ki-67 was associated with increased risk of DCIS recurrence and progression to invasive disease in multivariate analyses. These data are consistent with a functional role for RB in modulating the invasive behavior of mammary epithelial cells. The tissue microenvironment is particularly relevant to the behavior of DCIS, and, surprisingly, elevated expression of p16ink4a in nonproliferative stroma was observed in a substantial fraction of cases. In this tissue compartment, p16ink4a expression was strongly associated with disease recurrence, independent of standard histopathologic features. Together, the data herein describe dual aspects of RB-pathway biology that are associated with disease recurrence through the epithelial or stromal compartment of DCIS.

Fully murine CD105-targeted CAR-T cells provide an immunocompetent model for CAR-T cell biology.

The modeling of chimeric antigen receptor (CAR) T cell therapies has been mostly focused on immunodeficient models. However, there are many advantages in studying CAR-T cell biology in an immunocompetent setting. We generated a fully murine CAR targeting CD105 (endoglin), a component of the TGFß receptor expressed on the surface of certain solid tumors and acute leukemias. CD105-targeted CAR-T cells can be grown from various murine backgrounds, tracked in vivo by congenic marks, and be activated by CD105 in isolation or expressed by tumor cells. CD105-targeted CAR-T cells were toxic at higher doses but proved safe in lower doses and modestly effective in treating wild-type B16 melanoma-bearing mice. CAR-T cells infiltrating the tumor expressed high levels of exhaustion markers and exhibited metabolic insufficiencies. We also generated a human CD105 CAR, which was efficacious in treating human melanoma and acute myeloid leukemia in vivo. Our work details a new murine model of CAR-T cell therapy that can be used from immunologists to further our understanding of CAR-T cell biology. We also set the foundation for further exploration of CD105 as a possible human CAR-T cell target.

Proliferative suppression by CDK4/6 inhibition: complex function of the retinoblastoma pathway in liver tissue and hepatoma cells.

BACKGROUND & AIMS: Hepatocellular carcinoma is the third leading cause of cancer mortality worldwide; current chemotherapeutic interventions for this disease are largely ineffective. The retinoblastoma tumor suppressor (RB) is functionally inactivated at relatively high frequency in hepatocellular carcinoma and hepatoma cell lines. Here, we analyzed the ability of CDK4/6 inhibition to inhibit hepatocyte proliferation and the effect of RB status on this process. METHODS: Hepatoma cell lines and xenograft models harboring RB knockdown and mice harboring liver-specific Rb deletion were used to define the role of RB function in response to CDK4/6 inhibition. RESULTS: Our study shows that CDK4/6-dependent cell cycle progression in hepatoma cells was readily arrested by inhibition of CDK4/6 by PD-0332991 or p16ink4a irrespective of RB status. Interestingly, upon CDK4/6 inhibition, p107 protein stability was dramatically increased as a function of RB loss. This engagement of compensatory mechanisms was critical for cell cycle inhibition in the absence of RB, because both the E1A oncoprotein and overexpression of E2F proteins were capable of overcoming the effect of CDK4/6 inhibition. These findings were recapitulated in xenograft models. Furthermore, to determine how these findings relate to hepatocyte proliferation in vivo, mice were exposed to carbon tetrachloride to induce liver regeneration followed by treatment with PD-0332991. This treatment significantly inhibited hepatocyte proliferation. Strikingly, this facet of PD-0332991 function was retained even in RB-deficient livers. CONCLUSIONS: These data show that CDK4/6 inhibition is a potent mediator of cytostasis and that RB loss can be readily compensated for in the context of both hepatoma cell lines and liver tissue.

Antitumor T-cell Reconditioning: Improving Metabolic Fitness for Optimal Cancer Immunotherapy.

With the rapid rise of immunotherapy for cancer treatment, attention has focused on gaining a better understanding of T-cell biology in the tumor microenvironment. Elucidating the factors underlying changes in their function will allow for the development of new therapeutic strategies that could expand the patient population benefiting from immunotherapy, as well as circumvent therapy resistance. Cancers go beyond avoiding immune recognition and inducing T-cell dysfunction through coinhibitory molecules. Recent work has demonstrated that the tumor microenvironment elicits metabolic changes in T cells that dampen their ability to respond and that manipulating these metabolic changes can strengthen an antitumor immune response. Here we review the metabolic status of various types of T cells, the energetic state of the tumor microenvironment, and proposed modalities for improvement of immunotherapy through metabolic remodeling. Clin Cancer Res; 24(11); 2473-81. ©2018 AACR.

4-1BB costimulation induces T cell mitochondrial function and biogenesis enabling cancer immunotherapeutic responses.

Despite remarkable responses to cancer immunotherapy in a subset of patients, many patients remain resistant to these therapies. The tumor microenvironment can impose metabolic restrictions on T cell function, creating a resistance mechanism to immunotherapy. We have previously shown tumor-infiltrating T cells succumb to progressive loss of metabolic sufficiency, characterized by repression of mitochondrial activity that cannot be rescued by PD-1 blockade. 4-1BB, a costimulatory molecule highly expressed on exhausted T cells, has been shown to influence metabolic function. We hypothesized that 4-1BB signaling might provide metabolic support to tumor-infiltrating T cells. 4-1BB costimulation of CD8+ T cells results in enhanced mitochondrial capacity (suggestive of fusion) and engages PGC1α-mediated pathways via activation of p38-MAPK. 4-1BB treatment of mice improves metabolic sufficiency in endogenous and adoptive therapeutic CD8+ T cells. 4-1BB stimulation combined with PD-1 blockade results in robust antitumor immunity. Sequenced studies revealed the metabolic support afforded by 4-1BB agonism need not be continuous and that a short course of anti-4-1BB pretreatment was sufficient to provide a synergistic response. Our studies highlight metabolic reprogramming as the dominant effect of 4-1BB therapy and suggest that combinatorial strategies using 4-1BB agonism may help overcome the immunosuppressive metabolic landscape of the tumor microenvironment.

Syntaphilin controls a mitochondrial rheostat for proliferation-motility decisions in cancer.

Tumors adapt to an unfavorable microenvironment by controlling the balance between cell proliferation and cell motility, but the regulators of this process are largely unknown. Here, we show that an alternatively spliced isoform of syntaphilin (SNPH), a cytoskeletal regulator of mitochondrial movements in neurons, is directed to mitochondria of tumor cells. Mitochondrial SNPH buffers oxidative stress and maintains complex II-dependent bioenergetics, sustaining local tumor growth while restricting mitochondrial redistribution to the cortical cytoskeleton and tumor cell motility. Conversely, introduction of stress stimuli to the microenvironment, including hypoxia, acutely lowered SNPH levels, resulting in bioenergetics defects and increased superoxide production. In turn, this suppressed tumor cell proliferation but increased tumor cell invasion via greater mitochondrial trafficking to the cortical cytoskeleton. Loss of SNPH or expression of an SNPH mutant lacking the mitochondrial localization sequence resulted in increased metastatic dissemination in xenograft or syngeneic tumor models in vivo. Accordingly, tumor cells that acquired the ability to metastasize in vivo constitutively downregulated SNPH and exhibited higher oxidative stress, reduced cell proliferation, and increased cell motility. Therefore, SNPH is a stress-regulated mitochondrial switch of the cell proliferation-motility balance in cancer, and its pathway may represent a therapeutic target.

Survivin promotes oxidative phosphorylation, subcellular mitochondrial repositioning, and tumor cell invasion.

Survivin promotes cell division and suppresses apoptosis in many human cancers, and increased abundance correlates with metastasis and poor prognosis. We showed that a pool of survivin that localized to the mitochondria of certain tumor cell lines enhanced the stability of oxidative phosphorylation complex II, which promoted cellular respiration. Survivin also supported the subcellular trafficking of mitochondria to the cortical cytoskeleton of tumor cells, which was associated with increased membrane ruffling, increased focal adhesion complex turnover, and increased tumor cell migration and invasion in cultured cells, and enhanced metastatic dissemination in vivo. Therefore, we found that mitochondrial respiration enhanced by survivin contributes to cancer metabolism, and relocalized mitochondria may provide a "regional" energy source to fuel tumor cell invasion and metastasis.

CDK4/6 inhibition antagonizes the cytotoxic response to anthracycline therapy.

Triple-negative breast cancer (TNBC) is an aggressive disease that lacks established markers to direct therapeutic intervention. Thus, these tumors are routinely treated with cytotoxic chemotherapies (e.g., anthracyclines), which can cause severe side effects that impact quality of life. Recent studies indicate that the retinoblastoma tumor suppressor (RB) pathway is an important determinant in TNBC disease progression and therapeutic outcome. Furthermore, new therapeutic agents have been developed that specifically target the RB pathway, potentially positioning RB as a novel molecular marker for directing treatment. The current study evaluates the efficacy of pharmacological CDK4/6 inhibition in combination with the widely used genotoxic agent doxorubicin in the treatment of TNBC. Results demonstrate that in RB-proficient TNBC models, pharmacological CDK4/6 inhibition yields a cooperative cytostatic effect with doxorubicin but ultimately protects RB-proficient cells from doxorubicin-mediated cytotoxicity. In contrast, CDK4/6 inhibition does not alter the therapeutic response of RB-deficient TNBC cells to doxorubicin-mediated cytotoxicity, indicating that the effects of doxorubicin are indeed dependent on RB-mediated cell cycle control. Finally, the ability of CDK4/6 inhibition to protect TNBC cells from doxorubicin-mediated cytotoxicity resulted in recurrent populations of cells specifically in RB-proficient cell models, indicating that CDK4/6 inhibition can preserve cell viability in the presence of genotoxic agents. Combined, these studies suggest that while targeting the RB pathway represents a novel means of treatment in aggressive diseases such as TNBC, there should be a certain degree of caution when considering combination regimens of CDK4/6 inhibitors with genotoxic compounds that rely heavily on cell proliferation for their cytotoxic effects.

Oxidative stress in cancer associated fibroblasts drives tumor-stroma co-evolution: A new paradigm for understanding tumor metabolism, the field effect and genomic instability in cancer cells.

Loss of stromal fibroblast caveolin-1 (Cav-1) is a powerful single independent predictor of poor prognosis in human breast cancer patients, and is associated with early tumor recurrence, lymph node metastasis and tamoxifen-resistance. We developed a novel co-culture system to understand the mechanism(s) by which a loss of stromal fibroblast Cav-1 induces a "lethal tumor micro-environment." Here, we propose a new paradigm to explain the powerful prognostic value of stromal Cav-1. In this model, cancer cells induce oxidative stress in cancer-associated fibroblasts, which then acts as a "metabolic" and "mutagenic" motor to drive tumor-stroma co-evolution, DNA damage and aneuploidy in cancer cells. More specifically, we show that an acute loss of Cav-1 expression leads to mitochondrial dysfunction, oxidative stress and aerobic glycolysis in cancer associated fibroblasts. Also, we propose that defective mitochondria are removed from cancer-associated fibroblasts by autophagy/mitophagy that is induced by oxidative stress. As a consequence, cancer associated fibroblasts provide nutrients (such as lactate) to stimulate mitochondrial biogenesis and oxidative metabolism in adjacent cancer cells (the "Reverse Warburg Effect"). We provide evidence that oxidative stress in cancer-associated fibroblasts is sufficient to induce genomic instability in adjacent cancer cells, via a bystander effect, potentially increasing their aggressive behavior. Finally, we directly demonstrate that nitric oxide (NO) over-production, secondary to Cav-1 loss, is the root cause for mitochondrial dysfunction in cancer associated fibroblasts. In support of this notion, treatment with anti-oxidants (such as N-acetyl-cysteine, metformin and quercetin) or NO inhibitors (L-NAME) was sufficient to reverse many of the cancer-associated fibroblast phenotypes that we describe. Thus, cancer cells use "oxidative stress" in adjacent fibroblasts (i) as an "engine" to fuel their own survival via the stromal production of nutrients and (ii) to drive their own mutagenic evolution towards a more aggressive phenotype, by promoting genomic instability. We also present evidence that the "field effect" in cancer biology could also be related to the stromal production of ROS and NO species. eNOS-expressing fibroblasts have the ability to downregulate Cav-1 and induce mitochondrial dysfunction in adjacent fibroblasts that do not express eNOS. As such, the effects of stromal oxidative stress can be laterally propagated, amplified and are effectively "contagious"--spread from cell-to-cell like a virus--creating an "oncogenic/mutagenic" field promoting widespread DNA damage.