John R. Chevillet

Argonaute2 complexes carry a population of circulating microRNAs independent of vesicles in human plasma

MicroRNAs (miRNAs) circulate in the bloodstream in a highly stable, extracellular form and are being developed as blood-based biomarkers for cancer and other diseases. However, the mechanism underlying their remarkable stability in the RNase-rich environment of blood is not well understood. The current model in the literature posits that circulating miRNAs are protected by encapsulation in membrane-bound vesicles such as exosomes, but this has not been systematically studied. We used differential centrifugation and size-exclusion chromatography as orthogonal approaches to characterize circulating miRNA complexes in human plasma and serum. We found, surprisingly, that the majority of circulating miRNAs cofractionated with protein complexes rather than with vesicles. miRNAs were also sensitive to protease treatment of plasma, indicating that protein complexes protect circulating miRNAs from plasma RNases. Further characterization revealed that Argonaute2 (Ago2), the key effector protein of miRNA-mediated silencing, was present in human plasma and eluted with plasma miRNAs in size-exclusion chromatography. Furthermore, immunoprecipitation of Ago2 from plasma readily recovered non–vesicle-associated plasma miRNAs. The majority of miRNAs studied copurified with the Ago2 ribonucleoprotein complex, but a minority of specific miRNAs associated predominantly with vesicles. Our results reveal two populations of circulating miRNAs and suggest that circulating Ago2 complexes are a mechanism responsible for the stability of plasma miRNAs. Our study has important implications for the development of biomarker approaches based on capture and analysis of circulating miRNAs. In addition, identification of extracellular Ago2–miRNA complexes in plasma raises the possibility that cells release a functional miRNA-induced silencing complex into the circulation.

Absolute quantification by droplet digital PCR versus analog real-time PCR

Nanoliter-sized droplet technology paired with digital PCR (ddPCR) holds promise for highly precise, absolute nucleic acid quantification. Our comparison of microRNA quantification by ddPCR and real-time PCR revealed greater precision (coefficients of variation decreased 37–86%) and improved day-to-day reproducibility (by a factor of seven) of ddPCR but with comparable sensitivity. When we applied ddPCR to serum microRNA biomarker analysis, this translated to superior diagnostic performance for identifying individuals with cancer.

Quantitative and stoichiometric analysis of the microRNA content of exosomes

Significance Exosomes have been a subject of great interest in recent years, especially in the context of the microRNAs (miRNAs) that they contain. Exosome-mediated miRNA transfer between cells has been proposed to be a mechanism for intercellular signaling and exosome-associated miRNAs in biofluids have been suggested as potential minimally invasive biomarkers for multiple human disease states. Remarkably, we show here that most exosomes derived from standard preparations do not harbor many copies of miRNA molecules. These findings suggest a reevaluation of current models of the mechanism of exosome-mediated miRNA communication and indicate that stoichiometric analysis will be valuable for the study of other populations of extracellular vesicles and their associated RNAs as well. Exosomes have been proposed as vehicles for microRNA (miRNA) -based intercellular communication and a source of miRNA biomarkers in bodily fluids. Although exosome preparations contain miRNAs, a quantitative analysis of their abundance and stoichiometry is lacking. In the course of studying cancer-associated extracellular miRNAs in patient blood samples, we found that exosome fractions contained a small minority of the miRNA content of plasma. This low yield prompted us to perform a more quantitative assessment of the relationship between miRNAs and exosomes using a stoichiometric approach. We quantified both the number of exosomes and the number of miRNA molecules in replicate samples that were isolated from five diverse sources (i.e., plasma, seminal fluid, dendritic cells, mast cells, and ovarian cancer cells). Regardless of the source, on average, there was far less than one molecule of a given miRNA per exosome, even for the most abundant miRNAs in exosome preparations (mean ± SD across six exosome sources: 0.00825 ± 0.02 miRNA molecules/exosome). Thus, if miRNAs were distributed homogenously across the exosome population, on average, over 100 exosomes would need to be examined to observe one copy of a given abundant miRNA. This stoichiometry of miRNAs and exosomes suggests that most individual exosomes in standard preparations do not carry biologically significant numbers of miRNAs and are, therefore, individually unlikely to be functional as vehicles for miRNA-based communication. We propose revised models to reconcile the exosome-mediated, miRNA-based intercellular communication hypothesis with the observed stoichiometry of miRNAs associated with exosomes.

Multiple Integrated Copies and High-Level Production of the Human Retrovirus XMRV (Xenotropic Murine Leukemia Virus-Related Virus) from 22Rv1 Prostate Carcinoma Cells

ABSTRACT The human retrovirus XMRV (xenotropic murine leukemia virus-related virus) is associated with prostate cancer, most frequently in humans with a defect in the antiviral defense protein RNase L, suggesting a role for XMRV in prostate carcinogenesis. However, XMRV has not been found in prostate carcinoma cells. Here we show that 22Rv1 prostate carcinoma cells produce high-titer virus that is nearly identical in properties and sequence to XMRV isolated by others and consist primarily of a single clone of cells with at least 10 integrated copies of XMRV, warranting further study of a possible role for XMRV integration in carcinogenesis.

Issues and Prospects of microRNA-Based Biomarkers in Blood and Other Body Fluids

Cell-free circulating microRNAs (miRNAs) in the blood are good diagnostic biomarker candidates for various physiopathological conditions, including cancer, neurodegeneration, diabetes and other diseases. Since their discovery in 2008 as blood biomarkers, the field has expanded rapidly with a number of important findings. Despite the initial optimistic views of their potential for clinical application, there are currently no circulating miRNA-based diagnostics in use. In this article, we review the status of circulating miRNAs, examine different analytical approaches, and address some of the challenges and opportunities.

Release of Cell-free MicroRNA Tumor Biomarkers into the Blood Circulation with Pulsed Focused Ultrasound: A Noninvasive, Anatomically Localized, Molecular Liquid Biopsy

Purpose To compare the abilities of three pulsed focused ultrasound regimes (that cause tissue liquefaction, permeabilization, or mild heating) to release tumor-derived microRNA into the circulation in vivo and to evaluate release dynamics. Materials and Methods All rat experiments were approved by the University of Washington Institutional Animal Care and Use Committee. Reverse-transcription quantitative polymerase chain reaction array profiling was used to identify candidate microRNA biomarkers in a rat solid tumor cell line. Rats subcutaneously grafted with these cells were randomly assigned among three pulsed focused ultrasound treatment groups: (a) local tissue liquefaction via boiling histotripsy, (b) tissue permeabilization via inertial cavitation, and (c) mild (<10°C) heating of tissue, as well as a sham-treated control group. Blood specimens were drawn immediately prior to treatment and serially over 24 hours afterward. Plasma microRNA was quantified with reverse-transcription quantitative polymerase chain reaction, and statistical significance was determined with one-way analysis of variance (Kruskal-Wallis and Friedman tests), followed by the Dunn multiple-comparisons test. Results After tissue liquefaction and cavitation treatments (but not mild heating), plasma quantities of candidate biomarkers increased significantly (P value range, <.0001 to .04) relative to sham-treated controls. A threefold to 32-fold increase occurred within 15 minutes after initiation of pulsed focused ultrasound tumor treatment, and these increases persisted for 3 hours. Histologic examination confirmed complete liquefaction of the targeted tumor area with boiling histotripsy, in addition to areas of petechial hemorrhage and tissue disruption by means of cavitation-based treatment. Conclusion Mechanical tumor tissue disruption with pulsed focused ultrasound-induced bubble activity significantly increases the plasma abundance of tumor-derived microRNA rapidly after treatment.

Tumor characterization by ultrasound-release of multiple protein and microRNA biomarkers, preclinical and clinical evidence

We have previously shown that low frequency ultrasound can release biomarkers from cells into the murine circulation enabling an amplification and localization of the released biomarker that could be used as a blood-based method to detect cancer earlier and monitor therapy. In this study, we further demonstrate that this technique could be used for characterization of tumors and/or identification of cellular masses of unknown origin due to the release of multiple protein and nucleic acid biomarkers in cells in culture, mice and patients. We sonicated colon (LS174T) and prostate (LNCaP) cancer cell lines in culture at a low frequency of 1 MHz and show that there were several-fold changes in multiple protein and microRNA (miRNA) abundance with treatment at various intensities and time. This release was dependent on the duration and intensity of the sonication for both cell lines. Significant increased release in biomarkers was also observed following tumor sonication in living mice bearing subcutaneous LS174T cell line xenografts (for proteins and nucleic acids) and in an experimental LS174T liver tumor model (for proteins only). Finally, we demonstrated this methodology of multiple biomarker release in patients undergoing ablation of uterine fibroids using MR guided high intensity focused ultrasound. Two protein biomarkers significantly increased in the plasma after the ultrasound treatment in 21 samples tested. This proof that ultrasound-amplification method works in soft tissue tumor models together with biomarker multiplexing, could allow for an effective non-invasive method for identification, characterization and localization of incidental lesions, cancer and other disease. Pre-treatment quantification of the biomarkers, allows for individualization of quantitative comparisons. This individualization of normal marker levels in this method allows for specificity of the biomarker-increase to each patient, tumor or organ being studied.

Droplet Digital PCR for Absolute Quantification of Extracellular MicroRNAs in Plasma and Serum: Quantification of the Cancer Biomarker hsa-miR-141

Droplet-based digital PCR provides high-precision, absolute quantification of nucleic acid target sequences with wide-ranging applications for both research and clinical diagnostic applications. Droplet-based digital PCR enables absolute quantification by counting nucleic acid molecules encapsulated in discrete, volumetrically defined water-in-oil droplet partitions. The current available systems overcome the previous lack of scalable and practical technologies for digital PCR implementation. Extracellular microRNAs in biofluids (plasma, serum, urine, cerebrospinal fluid, etc.) are promising noninvasive biomarkers in multiple diseases and different clinical settings (e.g., diagnosis, early diagnosis, prediction of recurrence, and prognosis). Here we describe a protocol that enables highly precise and reproducible absolute quantification of extracellular microRNAs using droplet digital PCR.

High intensity focused ultrasound-induced bubbles stimulate the release of nucleic acid cancer biomarkers

Recently, several nucleic acid cancer biomarkers, e.g., microRNA and mutant DNA, have been identified and shown promise for improving cancer diagnostics. However, the abundance of these biomarker classes in the circulation is low, impeding reliable detection and adoption into clinical practice. Here, the ability of HIFU-induced bubbles to stimulate release of cancer-associated microRNAs by tissue fractionation or permeabilization was investigated in a heterotopic syngeneic rat prostate cancer model. A 1.5 MHz HIFU transducer was used to either mechanically fractionate subcutaneous tumor with boiling histotripsy (BH) (~20 kW/cm2, 10 ms pulses, and duty factor 0.01) or to permeabilize tumor tissue with inertial cavitation activity (p- = 16 MPa, 1 ms pulses, duty factor 0.001). Blood was collected immediately prior to and serially up to 24-hours after treatments. Plasma concentrations of microRNAs were measured by quantitative RT-PCR. Both exposures resulted in a rapid (within 15 min), short (≤3 h) and dramatic (over ten-fold) increase in relative plasma concentrations of tumor-associated microRNAs, Histologic examination of excised tumor confirmed complete fractionation of targeted tumor by BH and localized areas of intraparenchymal hemorrhage and tissue disruption by cavitation-based treatment. These data suggest a clinically useful application of HIFU-induced bubbles for non-invasive molecular biopsy. [Grant support: NIH 1K01EB015745, R01CA154451, R01DK085714.]

Abstract 1118: Circulating microRNA biomarkers for cancer exist in multiple biophysical states.

MicroRNAs (miRNAs) are small regulatory RNA molecules that function in a wide range of normal and pathologic cellular processes. Cell-free miRNAs circulate in blood (i.e. plasma, serum), are stable and can be diagnostic for cancer. Circulating microRNAs in cancer have been reported to exist in multiple biophysical states, including most famously as cargo of 50-100 nm extracellular vesicles known as exosomes. In prior studies of healthy individuals, we have found that a majority of plasma miRNAs are not vesicle-associated and are substantially present as part of a ribonucleoprotein complex containing Argonaute-2 (Ago2). In order to determine the state of miRNAs in the circulation of cancer patients, we quantitatively profiled the distribution of >300 miRNAs in progressive physical and biochemical fractions of plasma from individuals with metastatic prostate cancer and matched, healthy controls. Stepwise differential centrifugation was performed at 12,000g (to collect cell debris, etc.) and 120,000g (to collect exosomes), followed by Ago2-immunoprecipitation. Exosome fractions generally contained little miRNA, both in cases and in controls. Prostate cancer miRNA biomarkers (miR-141, miR-375) were predominantly soluble, partially associated with Ago2 (15-35% of total) and only marginally present in the exosome fraction ( Citation Format: John R. Chevillet, Emily N. Gallichotte, Heather H. Cheng, Lucia N. Vojtech, Jason D. Arroyo, Hilary Briggs, Robert L. Vessella, Celestia S. Higano, Evan Y. Yu, Era L. Pogosova-Agadjanyan, Derek L. Stirewalt, Florian Hladik, Muneesh Tewari. Circulating microRNA biomarkers for cancer exist in multiple biophysical states. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1118. doi:10.1158/1538-7445.AM2013-1118