Tau seeding activity in skin biopsy differentiates tauopathies from synucleinopathies.

Most neurodegenerative diseases lack definitive diagnostic tests, and the identification of easily accessible and reliable biomarkers remains a critical unmet need. Since tau protein is highly expressed in skin of tauopathies patients, we aimed to exploit the ultrasensitive seeding activity assay (SAA) to assess tau seeding activity in skin of patients with tauopathies. In this multicentric, case-control study, patients with tauopathies and synucleinopathies were consecutively recruited and sex-matched to healthy controls (HC). Subjects underwent a double 3 mm skin biopsy in cervical area and ankle. Skin tau-SAA, using TauK18 and TauK19 as reaction substrates for 4R and 3R isoforms, seeding score, clinical scales, biochemical and morphological characterization of SAA end-products were evaluated. We analyzed 58 subjects: 24 tauopathies (18 progressive supranuclear palsy, PSP, and 6 corticobasal degeneration, CBD), 20 synucleinopathies (14 Parkinson's disease, PD, and 6 multiple system atrophy, MSA), and 14 HC. PSP and CBD showed higher tau seeding activity at both anatomical sites. A greater sensitivity of 4R-SAA than 3R-SAA was observed. 4R tau-SAA identified tauopathies with 71% sensitivity and 93% specificity. Accuracy was higher for PSP than CBD: PSP vs HC / PD (AUC 0.825), while CBD vs HC / PD (AUC 0.797), and PSP vs MSA (AU 0.778). SAA end-products showed differences in biochemical and morphological characterization according to the anatomical site. Skin tau-SAA identifies tauopathies with good accuracy and can be used to implement the in-vivo clinical diagnosis of patients with neurodegenerative diseases. Further characterization of peripheral tau seed in skin may elucidate the structure of tau deposits in brain.

Amyloid detection and typing yield of skin biopsy in systemic amyloidosis and polyneuropathy.

OBJECTIVE: Disease-modifying therapies are available for amyloidosis but are ineffective if end-organ damage is severe. As small fiber neuropathy is an early and common feature of amyloidosis, we assessed detection and typing yield of skin biopsy for amyloid in patients with confirmed systemic amyloidosis and neuropathic symptoms. METHODS: In this case-control study, patients with transthyretin and light chain amyloidosis (ATTRv, ATTRwt, and AL) were consecutively recruited. They were sex and age-matched to three control groups (1) non-neuropathic controls (NNC), (2) monoclonal gammopathy of undetermined significance (MGUS), and (3) other neuropathic disease controls (ONC). Patients underwent a double 3 mm skin biopsy in proximal and distal leg. Amyloid index and burden, protein typing by immuno-electron microscopy, intraepidermal nerve fiber density, electroneuromyography, and clinical characteristics were analyzed. RESULTS: We studied 15 subjects with confirmed systemic amyloidosis, 20 NNC, 18 MGUS, and 20 ONC. Amyloid was detected in 100% of patients with amyloidosis (87% in ankle and 73% in thigh). It was not detected in any of the control groups. A small fiber neuropathy was encountered in 100% of amyloidosis patients, in 80% of MGUS, and in 78% of ONC. Amyloid burden was higher in ATTRv, followed by AL and ATTRwt. The ultrastructural examination allowed the identification of the precursor protein by immunotyping in most of the cases. INTERPRETATION: Skin biopsy is a minimally invasive test with optimal sensitivity for amyloid. It allows amyloid typing by electron microscope to identify the precursor protein. The diagnostic work up of systemic amyloidosis should include a skin biopsy.

Tau protein quantification in skin biopsies differentiates tauopathies from alpha-synucleinopathies.

Abnormal accumulation of microtubule-associated protein tau (τ) is a characteristic feature of atypical parkinsonisms with tauopathies, such as progressive supranuclear palsy and corticobasal degeneration. However, pathological τ has also been observed in α-synucleinopathies like Parkinson's disease and multiple system atrophy. Based on the involvement of the peripheral nervous system in several neurodegenerative diseases, we characterized and compared τ expression in skin biopsies of patients clinically diagnosed with Parkinson's disease, multiple system atrophy, progressive supranuclear palsy and corticobasal degeneration and in healthy control subjects. In all groups, τ protein was detected along both somatosensory and autonomic nerve fibres in the epidermis and dermis by immunofluorescence. We found by western blot the presence of mainly two different bands at 55 and 70 kDa, co-migrating with 0N4R/1N3R and 2N4R isoforms, respectively. At the RNA level, the main transcript variants were 2N and 4R, and both were more expressed in progressive supranuclear palsy/corticobasal degeneration by real-time PCR. Enzyme-linked immunosorbent assay demonstrated significantly higher levels of total τ protein in skin lysates of progressive supranuclear palsy/corticobasal degeneration compared to the other groups. Multivariate regression analysis and receiver operating characteristics curve analysis of τ amount at both sites showed a clinical association with tauopathies diagnosis and high diagnostic value for progressive supranuclear palsy/corticobasal degeneration versus Parkinson's disease (sensitivity 90%, specificity 69%) and progressive supranuclear palsy/corticobasal degeneration versus multiple system atrophy (sensitivity 90%, specificity 86%). τ protein increase correlated with cognitive impairment in progressive supranuclear palsy/corticobasal degeneration. This study is a comprehensive characterization of τ in the human cutaneous peripheral nervous system in physiological and pathological conditions. The differential expression of τ, both at transcript and protein levels, suggests that skin biopsy, an easily accessible and minimally invasive exam, can help in discriminating among different neurodegenerative diseases.

Alpha-synuclein oligomers and small nerve fiber pathology in skin are potential biomarkers of Parkinson's disease.

The proximity ligation assay (PLA) is a specific and sensitive technique for the detection of αSyn oligomers (αSyn-PLA), early and toxic species implicated in the pathogenesis of PD. We aimed to evaluate by skin biopsy the diagnostic and prognostic capacity of αSyn-PLA and small nerve fiber reduction in PD in a longitudinal study. αSyn-PLA was performed in the ankle and cervical skin biopsies of PD (n = 30), atypical parkinsonisms (AP, n = 23) including multiple system atrophy (MSA, n = 12) and tauopathies (AP-Tau, n = 11), and healthy controls (HC, n = 22). Skin biopsy was also analyzed for phosphorylated αSyn (P-αSyn) and 5G4 (αSyn-5G4), a conformation-specific antibody to aggregated αSyn. Intraepidermal nerve fiber density (IENFD) was assessed as a measure of small fiber neuropathy. αSyn-PLA signal was more expressed in PD and MSA compared to controls and AP-Tau. αSyn-PLA showed the highest diagnostic accuracy (PD vs. HC sensitivity 80%, specificity 77%; PD vs. AP-Tau sensitivity 80%, specificity 82%), however, P-αSyn and 5G4, possible markers of later phases, performed better when considering the ankle site alone. A small fiber neuropathy was detected in PD and MSA. A progression of denervation not of pathological αSyn was detected at follow-up and a lower IENFD at baseline was associated with a greater cognitive and motor decline in PD. A skin biopsy-derived compound marker, resulting from a linear discrimination analysis model of αSyn-PLA, P-αSyn, αSyn-5G4, and IENFD, stratified patients with accuracy (77.8%), including the discrimination between PD and MSA (84.6%). In conclusion, the choice of pathological αSyn marker and anatomical site influences the diagnostic performance of skin biopsy and can help in understanding the temporal dynamics of αSyn spreading in the peripheral nervous system during the disease. Skin denervation, not pathological αSyn is a potential progression marker for PD.

Tau affects P53 function and cell fate during the DNA damage response.

Cells are constantly exposed to DNA damaging insults. To protect the organism, cells developed a complex molecular response coordinated by P53, the master regulator of DNA repair, cell division and cell fate. DNA damage accumulation and abnormal cell fate decision may represent a pathomechanism shared by aging-associated disorders such as cancer and neurodegeneration. Here, we examined this hypothesis in the context of tauopathies, a neurodegenerative disorder group characterized by Tau protein deposition. For this, the response to an acute DNA damage was studied in neuroblastoma cells with depleted Tau, as a model of loss-of-function. Under these conditions, altered P53 stability and activity result in reduced cell death and increased cell senescence. This newly discovered function of Tau involves abnormal modification of P53 and its E3 ubiquitin ligase MDM2. Considering the medical need with vast social implications caused by neurodegeneration and cancer, our study may reform our approach to disease-modifying therapies.

Targeting Alpha Synuclein Aggregates in Cutaneous Peripheral Nerve Fibers by Free-floating Immunofluorescence Assay.

To date, for most neurodegenerative diseases only a post-mortem histopathological definitive diagnosis is available. For Parkinson's disease (PD), the diagnosis still relies only on clinical signs of motor involvement that appear later on in the disease course, when most of the dopaminergic neurons are already lost. Hence, there is a strong need for a biomarker that can identify patients at the beginning of disease or at the risk of developing it. Over the last few years, skin biopsy has proved to be an excellent research and diagnostic tool for peripheral nerve diseases such as small fiber neuropathy. Interestingly, a small fiber neuropathy and alpha synuclein (αSyn) neural deposits have been shown by skin biopsy in PD patients. Indeed, skin biopsy has the great advantage of being an easily accessible, minimally invasive and painless procedure that allows the analysis of peripheral nervous tissue prone to the pathology. Moreover, the possibility of repeating the skin biopsy in the course of the follow-up of the same patient allows studying the longitudinal correlation with the disease progression. We set up a standardized reliable protocol to investigate the presence of αSyn aggregates in skin nerve fibers of the PD patient. This protocol involves few short fixation steps, a cryotome sectioning and then a free-floating immunofluorescence double-staining with two specific antibodies: anti Protein Gene Product 9.5 (PGP9.5) to mark the cutaneous nerve fibers and anti 5G4 for detecting αSyn aggregates. It is a versatile, sensitive and easy to perform protocol that can also be applied for targeting other proteins of interest in skin nerves. The ability to mark αSyn aggregates is another step forward to the use of skin biopsy as a tool for establishing a pre-mortem histopathological diagnosis of PD.

T-Cell Leukemia/Lymphoma 1 (TCL1): An Oncogene Regulating Multiple Signaling Pathways.

Almost 30 years ago, Carlo Croce's group discovered the T-Cell Leukemia/Lymphoma 1A oncogene (TCL1A or TCL1). TCL1 protein is normally expressed in fetal tissues and early developmental stage lymphocytes. Its expression is deregulated in chronic lymphocytic leukemia (B-CLL) and most lymphomas. TCL1 plays a central role in lymphomagenesis as a co-activator of AKT kinases and other recently elucidated interacting protein partners. These include ATM, HSP70 and TP63, which were all confirmed as binding partners of TCL1 from co-immunoprecipitation experiments utilizing endogenously expressed proteins. The nature of these interactions highlighted the role of TCL1 in enhancing multiple signaling pathways, including PI3K and NF-κB. Based on its role in the aforementioned pathways and, despite the lack of a well-defined enzymatic activity, TCL1 is considered a potential therapeutic target for TCL1-positive hematological malignancies. This perspective will provide an overview of TCL1A and its interacting partners.

Publisher Correction: Compartmentalized activities of the pyruvate dehydrogenase complex sustain lipogenesis in prostate cancer.

In the HTML version of this article initially published, the name of author Diletta Di Mitri was miscoded in the XML such that Di was included as part of the given name instead of the family name. The error has been corrected in the HTML version of the article.

Compartmentalized activities of the pyruvate dehydrogenase complex sustain lipogenesis in prostate cancer.

The mechanisms by which mitochondrial metabolism supports cancer anabolism remain unclear. Here, we found that genetic and pharmacological inactivation of pyruvate dehydrogenase A1 (PDHA1), a subunit of the pyruvate dehydrogenase complex (PDC), inhibits prostate cancer development in mouse and human xenograft tumor models by affecting lipid biosynthesis. Mechanistically, we show that in prostate cancer, PDC localizes in both the mitochondria and the nucleus. Whereas nuclear PDC controls the expression of sterol regulatory element-binding transcription factor (SREBF)-target genes by mediating histone acetylation, mitochondrial PDC provides cytosolic citrate for lipid synthesis in a coordinated manner, thereby sustaining anabolism. Additionally, we found that PDHA1 and the PDC activator pyruvate dehydrogenase phosphatase 1 (PDP1) are frequently amplified and overexpressed at both the gene and protein levels in prostate tumors. Together, these findings demonstrate that both mitochondrial and nuclear PDC sustain prostate tumorigenesis by controlling lipid biosynthesis, thus suggesting this complex as a potential target for cancer therapy.

Split GFP technologies to structurally characterize and quantify functional biomolecular interactions of FTD-related proteins.

Protein multimerization in physiological and pathological conditions constitutes an intrinsic trait of proteins related to neurodegeneration. Recent evidence shows that TDP-43, a RNA-binding protein associated with frontotemporal dementia and amyotrophic lateral sclerosis, exists in a physiological and functional nuclear oligomeric form, whose destabilization may represent a prerequisite for misfolding, toxicity and subsequent pathological deposition. Here we show the parallel implementation of two split GFP technologies, the GFP bimolecular and trimolecular fluorescence complementation (biFC and triFC) in the context of TDP-43 self-assembly. These techniques coupled to a variety of assays based on orthogonal readouts allowed us to define the structural determinants of TDP-43 oligomerization in a qualitative and quantitative manner. We highlight the versatility of the GFP biFC and triFC technologies for studying the localization and mechanisms of protein multimerization in the context of neurodegeneration.

Epithelial growth factor receptor expression influences 5-ALA induced glioblastoma fluorescence.

The extent of 5-aminolevulinic acid (5-ALA) guided tumor resection has a determining impact in high-grade glioma and glioblastoma surgery. Yet the intensity of the 5-ALA induced fluorescence may vary within the tumor. We aimed to correlate 5-ALA induced fluorescence with the expression of epithelial growth factor receptor (EGFR) and its constitutively active version EGFRvIII in different glioblastoma (GBM) cell lines. To elucidate the role of EGFR in the metabolism of 5-ALA in GBM cell lines with variable EGFR expression status, we analyzed the activation of EGFR by its primary ligand EGF, and its downstream effect on Heme oxygenase-1 (HO-1), a key enzyme regulating the metabolism of Protoporphyrin IX (PpIX), the fluorescent metabolite of 5-ALA. Effects of direct pharmacological inhibition by Tin(IV)-Protoporphyrin (SnPP) or gene knockdown by small interfering RNA (siRNA) on HO-1 enzyme were analyzed in respect to 5-ALA induced fluorescence. Furthermore, inhibition of EGFR by Gefitinib was tested. A significant difference in 5-ALA induced fluorescence was obtained in U87MG (low EGFR expression) and LN229EGFR cells (EGFR overexpression) compared to BS153 (EGFR overexpression/EGFRvIII+). Treatment of U87MG and LN229EGFR cells with EGF significantly reduced cellular fluorescence, by promoting HO-1 transcription and expression in a concentration-dependent manner. This effect could be reversed by EGFR-specific siRNA treatment, which reduced protein expression of about 80% in U87MG. Remarkably, inhibition of HO-1 activity by SnPP or reduction of HO-1 protein levels by siHO-1 treatment restored fluorescence in all cell lines, independently of EGFR quantitative and qualitative expression. Gefitinib treatment was able to restore fluorescence after EGF stimulation in U87MG cells but not in BS153 cells, overexpressing EGFR/EGFRvIII. In GBM cell lines, 5-ALA induced fluorescence is variable and influenced by EGF-induced downstream activation of HO-1. HO-1 protein expression was identified as a negative regulator of 5-ALA induced fluorescence in GBM cells. We further propose that co-expression of EGFRvIII but not quantitative EGFR expression influence HO-1 activity and therefore cellular fluorescence.

Inhibition of Notch pathway arrests PTEN-deficient advanced prostate cancer by triggering p27-driven cellular senescence.

Activation of NOTCH signalling is associated with advanced prostate cancer and treatment resistance in prostate cancer patients. However, the mechanism that drives NOTCH activation in prostate cancer remains still elusive. Moreover, preclinical evidence of the therapeutic efficacy of NOTCH inhibitors in prostate cancer is lacking. Here, we provide evidence that PTEN loss in prostate tumours upregulates the expression of ADAM17, thereby activating NOTCH signalling. Using prostate conditional inactivation of both Pten and Notch1 along with preclinical trials carried out in Pten-null prostate conditional mouse models, we demonstrate that Pten-deficient prostate tumours are addicted to the NOTCH signalling. Importantly, we find that pharmacological inhibition of γ-secretase promotes growth arrest in both Pten-null and Pten/Trp53-null prostate tumours by triggering cellular senescence. Altogether, our findings describe a novel pro-tumorigenic network that links PTEN loss to ADAM17 and NOTCH signalling, thus providing the rational for the use of γ-secretase inhibitors in advanced prostate cancer patients.

A Fhit-mimetic peptide suppresses annexin A4-mediated chemoresistance to paclitaxel in lung cancer cells.

We recently reported that Fhit is in a molecular complex with annexin A4 (ANXA4); following to their binding, Fhit delocalizes ANXA4 from plasma membrane to cytosol in paclitaxel-resistant lung cancer cells, thus restoring their chemosensitivity to the drug. Here, we demonstrate that Fhit physically interacts with A4 through its N-terminus; molecular dynamics simulations were performed on a 3D Fhit model to rationalize its mechanism of action. This approach allowed for the identification of the QHLIKPS heptapeptide (position 7 to 13 of the wild-type Fhit protein) as the smallest Fhit sequence still able to preserve its ability to bind ANXA4. Interestingly, Fhit peptide also recapitulates the property of the native protein in inhibiting Annexin A4 translocation from cytosol to plasma membrane in A549 and Calu-2 lung cancer cells treated with paclitaxel. Finally, the combination of Tat-Fhit peptide and paclitaxel synergistically increases the apoptotic rate of cultured lung cancer cells and blocks in vivo tumor formation.Our findings address to the identification of chemically simplified Fhit derivatives that mimic Fhit tumor suppressor functions; intriguingly, this approach might lead to the generation of novel anticancer drugs to be used in combination with conventional therapies in Fhit-negative tumors to prevent or delay chemoresistance.

Activation of the Lin28/let-7 Axis by Loss of ESE3/EHF Promotes a Tumorigenic and Stem-like Phenotype in Prostate Cancer.

Although cancer stem-like cells (CSC) are thought to be the most tumorigenic, metastatic, and therapy-resistant cell subpopulation within human tumors, current therapies target bulk tumor cells while tending to spare CSC. In seeking to understand mechanisms needed to acquire and maintain a CSC phenotype in prostate cancer, we investigated connections between the ETS transcription factor ESE3/EHF, the Lin28/let-7 microRNA axis, and the CSC subpopulation in this malignancy. In normal cells, we found that ESE3/EHF bound and repressed promoters for the Lin28A and Lin28B genes while activating transcription and maturation of the let-7 microRNAs. In cancer cells, reduced expression of ESE3/EHF upregulated Lin28A and Lin28B and downregulated the let-7 microRNAs. Notably, we found that deregulation of the Lin28/let-7 axis with reduced production of let-7 microRNAs was critical for cell transformation and expansion of prostate CSC. Moreover, targeting Lin28A/Lin28B in cell lines and tumor xenografts mimicked the effects of ESE3/EHF and restrained tumor-initiating and self-renewal properties of prostate CSC both in vitro and in vivo These results establish that tight control by ESE3/EHF over the Lin28/let-7 axis is a critical barrier to malignant transformation, and they also suggest new strategies to antagonize CSC in human prostate cancer for therapeutic purposes. Cancer Res; 76(12); 3629-43. ©2016 AACR.

A chemogenomic screening identifies CK2 as a target for pro-senescence therapy in PTEN-deficient tumours.

Enhancement of cellular senescence in tumours triggers a stable cell growth arrest and activation of an antitumour immune response that can be exploited for cancer therapy. Currently, there are only a limited number of targeted therapies that act by increasing senescence in cancers, but the majority of them are not selective and also target healthy cells. Here we developed a chemogenomic screening to identify compounds that enhance senescence in PTEN-deficient cells without affecting normal cells. By using this approach, we identified casein kinase 2 (CK2) as a pro-senescent target. Mechanistically, we show that Pten loss increases CK2 levels by activating STAT3. CK2 upregulation in Pten null tumours affects the stability of Pml, an essential regulator of senescence. However, CK2 inhibition stabilizes Pml levels enhancing senescence in Pten null tumours. Taken together, our screening strategy has identified a novel STAT3-CK2-PML network that can be targeted for pro-senescence therapy for cancer.

Interaction of CDCP1 with HER2 enhances HER2-driven tumorigenesis and promotes trastuzumab resistance in breast cancer.

Understanding the molecular pathways that contribute to the aggressive behavior of HER2-positive breast cancers may aid in the development of novel therapeutic interventions. Here, we show that CDCP1 and HER2 are frequently co-overexpressed in metastatic breast tumors and associated with poor patient prognosis. HER2 and CDCP1 co-overexpression leads to increased transformation ability, cell migration, and tumor formation in vivo, and enhanced HER2 activation and downstream signaling in different breast cancer cell lines. Mechanistically, we demonstrate that CDCP1 binds to HER2 through its intracellular domain, thereby increasing HER2 interaction with the non-receptor tyrosine kinase c-SRC (SRC), leading to trastuzumab resistance. Taken together, our findings establish that CDCP1 is a modulator of HER2 signaling and a biomarker for the stratification of breast cancer patients with poor prognosis. Our results also provide a rationale for therapeutic targeting of CDCP1 in HER2-positive breast cancer patients.

Enhancing chemotherapy efficacy in Pten-deficient prostate tumors by activating the senescence-associated antitumor immunity.

Prosenescence therapy has recently emerged as a novel therapeutic approach for treating cancer. However, this concept is challenged by conflicting evidence showing that the senescence-associated secretory phenotype (SASP) of senescent tumor cells can have pro- as well as antitumorigenic effects. Herein, we report that, in Pten-null senescent tumors, activation of the Jak2/Stat3 pathway establishes an immunosuppressive tumor microenvironment that contributes to tumor growth and chemoresistance. Activation of the Jak2/Stat3 pathway in Pten-null tumors is sustained by the downregulation of the protein tyrosine phosphatase PTPN11/SHP2, providing evidence for the existence of a novel PTEN/SHP2 axis. Importantly, treatment with docetaxel in combination with a JAK2 inhibitor reprograms the SASP and improves the efficacy of docetaxel-induced senescence by triggering a strong antitumor immune response in Pten-null tumors. Altogether, these data demonstrate that immune surveillance of senescent tumor cells can be suppressed in specific genetic backgrounds but also evoked by pharmacological treatments.

Tumour-infiltrating Gr-1+ myeloid cells antagonize senescence in cancer.

Aberrant activation of oncogenes or loss of tumour suppressor genes opposes malignant transformation by triggering a stable arrest in cell growth, which is termed cellular senescence. This process is finely tuned by both cell-autonomous and non-cell-autonomous mechanisms that regulate the entry of tumour cells to senescence. Whether tumour-infiltrating immune cells can oppose senescence is unknown. Here we show that at the onset of senescence, PTEN null prostate tumours in mice are massively infiltrated by a population of CD11b(+)Gr-1(+) myeloid cells that protect a fraction of proliferating tumour cells from senescence, thus sustaining tumour growth. Mechanistically, we found that Gr-1(+) cells antagonize senescence in a paracrine manner by interfering with the senescence-associated secretory phenotype of the tumour through the secretion of interleukin-1 receptor antagonist (IL-1RA). Strikingly, Pten-loss-induced cellular senescence was enhanced in vivo when Il1ra knockout myeloid cells were adoptively transferred to PTEN null mice. Therapeutically, docetaxel-induced senescence and efficacy were higher in PTEN null tumours when the percentage of tumour-infiltrating CD11b(+)Gr-1(+) myeloid cells was reduced using an antagonist of CXC chemokine receptor 2 (CXCR2). Taken together, our findings identify a novel non-cell-autonomous network, established by innate immunity, that controls senescence evasion and chemoresistance. Targeting this network provides novel opportunities for cancer therapy.

RNAi-mediated silencing of Myc transcription inhibits stem-like cell maintenance and tumorigenicity in prostate cancer.

Several studies link disease progression, recurrence, and treatment failures to the cancer stem-like cell (CSC) subpopulation within the heterogeneous tumor cell population. Myc is a transcription factor having a central function in stem cell biology and in human cancers. Hence, Myc represents an attractive target to develop CSC-specific therapies. Recent findings suggest that Myc transcription can be silenced using an RNA interference (RNAi)-based strategy that targets noncoding promoter-associated RNA (paRNA) overlapping the transcription start site. In this study, we investigated the effects of silencing Myc transcription on prostate CSC in cell culture and xenograft models of human prostate cancer. Treatment with an effective promoter-targeting siRNA reduced the fraction of CSCs, leading to reduced self-renewal, tumor-initiating, and metastatic capability. Combined analysis of stem-like cells and senescence markers indicated that Myc silencing triggered a phenotypic shift and senescence in the CSC subpopulation. Notably, systemic delivery of the promoter-targeting siRNA in the xenograft model produced a striking suppression in the development of prostate tumors. Our results support a pivotal role for Myc in CSC maintenance and show that Myc targeting via RNAi-based transcriptional silencing can trigger CSC senescence and loss of their tumor-initiating capability. More generally, our findings demonstrate the efficacy of RNAi-based transcriptional strategies and the potential to target regulatory noncoding paRNAs for therapeutic applications.