Dysregulated microRNA expression in serum of non-vaccinated children with varicella

miRNAs: Targets to Investigate Herpesvirus Infection Associated with Neurological Disorders

Herpesvirus is associated with various neurological disorders and a specific diagnosis is associated with a better prognosis. MicroRNAs (miRNAs) are potential diagnostic and prognostic biomarkers of neurological diseases triggered by herpetic infection. In this review, we discuss miRNAs that have been associated with neurological disorders related to the action of herpesviruses. Human miRNAs and herpesvirus-encoded miRNAs were listed and discussed. This review article will be valuable in stimulating the search for new diagnostic and prognosis alternatives and understanding the role of these miRNAs in neurological diseases triggered by herpesviruses.

Shared miRNA landscapes of COVID-19 and neurodegeneration confirm neuroinflammation as an important overlapping feature

Introduction

Development and worsening of most common neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis, have been associated with COVID-19 However, the mechanisms associated with neurological symptoms in COVID-19 patients and neurodegenerative sequelae are not clear. The interplay between gene expression and metabolite production in CNS is driven by miRNAs. These small non-coding molecules are dysregulated in most common neurodegenerative diseases and COVID-19.

Methods

We have performed a thorough literature screening and database mining to search for shared miRNA landscapes of SARS-CoV-2 infection and neurodegeneration. Differentially expressed miRNAs in COVID-19 patients were searched using PubMed, while differentially expressed miRNAs in patients with five most common neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and multiple sclerosis) were searched using the Human microRNA Disease Database. Target genes of the overlapping miRNAs, identified with the miRTarBase, were used for the pathway enrichment analysis performed with Kyoto Encyclopedia of Genes and Genomes and Reactome.

Results

In total, 98 common miRNAs were found. Additionally, two of them (hsa-miR-34a and hsa-miR-132) were highlighted as promising biomarkers of neurodegeneration, as they are dysregulated in all five most common neurodegenerative diseases and COVID-19. Additionally, hsa-miR-155 was upregulated in four COVID-19 studies and found to be dysregulated in neurodegeneration processes as well. Screening for miRNA targets identified 746 unique genes with strong evidence for interaction. Target enrichment analysis highlighted most significant KEGG and Reactome pathways being involved in signaling, cancer, transcription and infection. However, the more specific identified pathways confirmed neuroinflammation as being the most important shared feature.

Discussion

Our pathway based approach has identified overlapping miRNAs in COVID-19 and neurodegenerative diseases that may have a valuable potential for neurodegeneration prediction in COVID-19 patients. Additionally, identified miRNAs can be further explored as potential drug targets or agents to modify signaling in shared pathways.

Graphical Abstract

Evaluation of Cellular miR-122 Expression in Association with the Presence of Varicella-Zoster Virus among Central Nervous System Tumors

Background: Brain tumors are all primary central nervous system (CNS) tumors with unclear etiologies and viral infections, especially human herpesviruses, which have emerged as a hot topic for comprehensive research. Objectives: The present study aimed at assessing the molecular epidemiology of varicella-zoster virus (VZV) and its association with microRNA 122 (miR-122) expression in CNS tumor samples. Methods: Fresh frozen tissue samples were collected from 60 CNS tumor patients and 45 healthy controls. A nested PCR assay was performed to detect the VZV-DNA. Subsequently, the expression level of miR-122 was evaluated in the CNS tumor tissue samples of patients and the brain tissue samples were obtained from healthy controls, using a real-time PCR assay. Results: Of 60 patients with CNS tumors, 29 were men and 31 were women. VZV-DNA was detected in 13.3% of the CNS tumor tissue specimens. There was no statistically significant association between the presence of VZV-DNA and different types of CNS tumors (P > 0.05). Furthermore, the expression level of miR-122 was significantly downregulated in the CNS tumor tissue samples obtained from the patients compared with those of the healthy controls (P < 0.05). Additionally, the expression level of miR-122 was significantly lower in the VZV-positive tumor samples as compared with those of the VZV-negative tumor samples and the healthy controls. Conclusions: Although VZV plays no direct role in the development of CNS tumors, the virus may affect the biology of CNS tumors by decreasing the expression levels of miR-122, which consequently leads to an increased risk of malignancy. However, the experimental data are not conclusive enough; so, further investigations are needed.

Changes in the MicroRNA Profile of the Giant Panda After Canine Distemper Vaccination and the Integrated Analysis of MicroRNA-Messenger RNA

Canine distemper (CD) is a significant threat to wild and captive giant panda populations. Captive giant pandas are inoculated with canine distemper virus (CDV) vaccination to prevent the infection with the CDV. As an important regulator, microRNA (miRNA) plays a crucial role in regulating gene expression, including in disease immunity. To understand the role of miRNA in immune response to CDV vaccination, we investigated the miRNA expression profile in five giant panda cubs after two inoculations, 21 days apart. A total of 187 conserved miRNAs and 96 novel miRNAs were identified. Among the 187 conserved miRNAs, 29 differentially expressed miRNAs were found postinoculation. The upregulation of miR-16, miR-182, miR-30b, and miR-101 indicated that the innate immune may be enhanced, whereas the upregulation of miR-142 and miR-19a are probably involved in the enhanced cellular immune response. However, the downregulated miR-155 and miR-181a might indicate the giant panda has weak ability to produce antibodies and memory B cells. Integrated analysis of miRNA-messenger RNA (mRNA) found 20 negatively regulated miRNA-mRNA pairs, where downregulated miR-204 might enhance giant panda cub innate immunity by increasing TLR6 expression, and downregulated miR-330 might activate macrophages and regulate the immune response by increasing TMEM106A expression. Our research provides key information for future development to enhance the immune response of giant pandas and potentially improve the survival of captive and wild giant panda populations threatened by CD.

MicroRNA analysis from acute to convalescence in Crimean Congo hemorrhagic fever

Crimean Congo hemorrhagic fever (CCHF) is one of the most important viral infections and is caused by Crimean Congo hemorrhagic fever orthonairovirus (CCHFV). Severity of CCHF can vary from a mild and nonspecific illness to a severe disease with fatal outcomes. MicroRNAs (miRNAs) have an increasing impact on the different pathways of viral infections. Within the transition process from acute phase to convalescence with 18 CCHF patients, we investigated the impacts on miRNA via microarray for the first time. We also compared miRNA gene expression in 16 severe and 15 mild cases. We identified Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) pathways associated with significant miRNAs utilizing DIANA TOOLS mirPath v.3. In this study, miR-15b-5p and miR-29a-3p were significantly downregulated in statistical terms; miR-4741, miR-937-5p, miR-6068, miR-7110-5p, miR-6126, and miR-7107-5p were upregulated in acute cases in comparison with convalescent patients (p ≤ .05). In total, 28 miRNAs (8 downregulated, 20 upregulated) were differentially expressed in severe CCHF patients as compared with mild cases (p ≤ .05). Whereas miR-6732-3p, miR-4436b-5p, miR-483-3p, and miR-6807-5p had the highest downregulation, miR-532-5p, miR-142-5p, miR-29c-3p, and let-7f-5p had the highest upregulation in severe patients in comparison with mild cases. Consequently, we determined that CCHF-induced miRNAs are associated with antiviral and proinflammatory pathways in acute and severe cases. In comparison with convalescence, these miRNAs in acute period may be therapeutic targets.

MicroRNA Biomarkers for Infectious Diseases: From Basic Research to Biosensing

In the pursuit of improved diagnostic tests for infectious diseases, several classes of molecules have been scrutinized as prospective biomarkers. Small (18–22 nucleotide), non-coding RNA transcripts called microRNAs (miRNAs) have emerged as promising candidates with extensive diagnostic potential, due to their role in numerous diseases, previously established methods for quantitation and their stability within biofluids. Despite efforts to identify, characterize and apply miRNA signatures as diagnostic markers in a range of non-infectious diseases, their application in infectious disease has advanced relatively slowly. Here, we outline the benefits that miRNA biomarkers offer to the diagnosis, management, and treatment of infectious diseases. Investigation of these novel biomarkers could advance the use of personalized medicine in infectious disease treatment, which raises important considerations for validating their use as diagnostic or prognostic markers. Finally, we discuss new and emerging miRNA detection platforms, with a focus on rapid, point-of-care testing, to evaluate the benefits and obstacles of miRNA biomarkers for infectious disease.

Effectiveness and failure rate of the varicella vaccine in an outbreak in Jiangsu, China: a 1:2 matched case-control study

Background: The varicella vaccine is not included in the national childhood immunization schedules in China, although one-dose varicella vaccine has been suggested for susceptible children aged 1-12 years in Jiangsu Province. However, varicella epidemics and outbreaks are frequently reported. We investigated a varicella outbreak in an elementary school to explore the risk factors for varicella transmission and vaccine failure.Methods: A 1:2 matched case-control study was carried out. Participant data were collected with standardized questionnaires. For each case, we enrolled two controls: a subject with high exposure in the same classroom as the case and a subject with low exposure in a different classroom. Data regarding vaccination status and medical and exposure histories were analyzed.Results: Fifty-one cases were reported during the outbreak; 26 cases (51%) were breakthrough varicella. Varicella vaccine immunization history (P < .001, OR = 0.19, 95% CI = 0.08-0.45) and the presence of siblings (P = .037, OR = 0.45, 95% CI = 0.21-0.95) were protective factors in preventing varicella infection. Contact with varicella patients increased the risk of varicella infection (P = .028, OR = 3.39, 95% CI = 1.14-10.09). Breakthrough varicella cases tended to present a milder rash (P = .049), fewer complications (P = .02), fewer rash sites (P = .02) and a shorter duration of active lesions (P = .001). One pneumonia case and one encephalitis case were reported in breakthrough cases. Age <15 months at the time of vaccination increased the risk of breakthrough varicella (P = .012). The adjusted vaccine effectiveness was 81%.Conclusions: One-dose varicella vaccine is effective at alleviating clinical manifestations. The moderate coverage provided by one dose cannot prevent varicella outbreaks, and vaccination after 15 months of age should be considered in the immunization schedule; a two-dose strategy is highly recommended.

Whole Transcriptome Analyses Reveal Differential mRNA and microRNA Expression Profiles in Primary Human Dermal Fibroblasts Infected with Clinical or Vaccine Strains of Varicella Zoster Virus

Licensed live attenuated vaccines have been developed to prevent varicella zoster virus (VZV) infection, which causes chickenpox and shingles. The genomic sequences of both clinical- and vaccine-derived VZV strains have been analyzed previously. To further characterize the molecular signatures and complexity of wildtype (clinical) versus attenuated (vaccine-derived) VZV-mediated host cellular responses, we performed high-throughput next generation sequencing to quantify and compare the expression patterns of mRNAs and microRNAs (miRNAs) in primary human dermal fibroblasts (HDFs) infected with wildtype (YC01 low passage) and attenuated (YC01 high passage, SuduVax, and VarilRix) VZV strains. 3D-multidimensional scaling of the differentially expressed genes demonstrated the distinct grouping of wildtype and attenuated strains. In particular, we observed that HDFs infected with attenuated strains had more differentially expressed genes (DEGs) involved in the retinoic-acid inducible gene–I-like receptor and interferon-mediated signaling pathways compared with wildtype strains. Additionally, miRNA expression patterns were profiled following the infection of HDFs with VZV. Small RNA sequencing identified that several miRNAs were upregulated, including miR-146a-5p, which has been associated with other herpesvirus infections, whereas let-7a-3p was downregulated in both wildtype and attenuated VZV-infected cells. This study identified genes and miRNAs that may be essential in VZV pathogenesis.

Differential miRNA expression in B cells is associated with inter-individual differences in humoral immune response to measles vaccination

Background

MicroRNAs are important mediators of post-transcriptional regulation of gene expression through RNA degradation and translational repression, and are emerging biomarkers of immune system activation/response after vaccination.

Methods

We performed Next Generation Sequencing (mRNA-Seq) of intracellular miRNAs in measles virus-stimulated B and CD4⁺ T cells from high and low antibody responders to measles vaccine. Negative binomial generalized estimating equation (GEE) models were used for miRNA assessment and the DIANA tool was used for gene/target prediction and pathway enrichment analysis.

Results

We identified a set of B cell-specific miRNAs (e.g., miR-151a-5p, miR-223, miR-29, miR-15a-5p, miR-199a-3p, miR-103a, and miR-15a/16 cluster) and biological processes/pathways, including regulation of adherens junction proteins, Fc-receptor signaling pathway, phosphatidylinositol-mediated signaling pathway, growth factor signaling pathway/pathways, transcriptional regulation, apoptosis and virus-related processes, significantly associated with neutralizing antibody titers after measles vaccination. No CD4⁺ T cell-specific miRNA expression differences between high and low antibody responders were found.

Conclusion

Our study demonstrates that miRNA expression directly or indirectly influences humoral immunity to measles vaccination and suggests that B cell-specific miRNAs may serve as useful predictive biomarkers of vaccine humoral immune response.

Diagnosis and Assessment of Microbial Infections with Host and Microbial MicroRNA Profiles

MicroRNAs (miRNAs) encoded by viral genome or host have been found participating in host-microbe interactions. Differential expression profiles of miRNAs were shown linking to specific disease pathologies which indicated its potency as diagnostic/prognostic biomarkers of infectious disease. This was emphasized by the discovery of circulating miRNAs which were found to be remarkably stable in mammalian biofluids. Standardized methods of miRNA quantification including RNA isolation should be established before they will be ready for use in clinical practice.

Diagnostic and Prognostic Value of MicroRNA in Viral Diseases

Virology is probably the most rapidly developing field within clinical laboratory medicine. Adequate diagnostic methods exist for the diagnostics of most acute viral infections. However, emergence of pathogenic viruses or virus strains and new disease associations of known viruses require the establishment of new diagnostic methods, sometimes very rapidly. In the field of chronic or persistent viral diseases, particularly those involving potential of malignant or fatal development, there is a constant need for improved differential diagnostics, monitoring, prognosis and risk assessment. Increasing understanding of disease pathogenesis also enables better patient management and personalized medicine, where companion diagnostics can offer precise and specific tools for individual care. Very often the new tools are offered by molecular diagnostic techniques, and this includes the detection of microRNAs (miRNAs). miRNAs are small regulatory RNA molecules, which regulate the expression of their target genes. They are encoded both by viruses and their host, and both can target either viral or cellular gene expression. In this review the diagnostic possibilities offered by miRNA will be discussed. The focus will be on selected viral and human miRNAs in viral diseases, and examples of miRNAs of putative diagnostic potential will be presented.

Advances in Roles of miR-132 in the Nervous System

miR-132 is an endogenous small RNA and controls post-transcriptional regulation of gene expression via controlled degradation of mRNA or transcription inhibition. In the nervous system, miR-132 is significant for regulating neuronal differentiation, maturation and functioning, and widely participates in axon growth, neural migration, and plasticity. The miR-132 is affected by factors like mRNA expression, functional redundancy, and signaling cascades. It targets multiple downstream molecules to influence physiological and pathological neuronal activities. MiR-132 can influence the pathogenesis of many diseases, especially in the nervous system. The dysregulation of miR-132 results in the occurrence and exacerbation of neural developmental, degenerative diseases, like Alzheimer’s disease, Parkinson’s disease and epilepsy, neural infection and psychiatric disorders including disturbance of consciousness, cognition and memory, depression and schizophrenia. Regulation of miR-132 expression relieves symptoms, alleviates severity and finally effects a cure. This review aims to discuss the clinical potentials of miR-132 in the nervous system.

The Clinical Application of MicroRNAs in Infectious Disease

MicroRNAs (miRNAs) are short single-stranded non-coding RNA sequences that posttranscriptionally regulate up to 60% of protein encoding genes. Evidence is emerging that miRNAs are key mediators of the host response to infection, predominantly by regulating proteins involved in innate and adaptive immune pathways. miRNAs can govern the cellular tropism of some viruses, are implicated in the resistance of some individuals to infections like HIV, and are associated with impaired vaccine response in older people. Not surprisingly, pathogens have evolved ways to undermine the effects of miRNAs on immunity. Recognition of this has led to new experimental treatments, RG-101 and Miravirsen—hepatitis C treatments which target host miRNA. miRNAs are being investigated as novel infection biomarkers, and they are being used to design attenuated vaccines, e.g., against Dengue virus. This comprehensive review synthesizes current knowledge of miRNA in host response to infection with emphasis on potential clinical applications, along with an evaluation of the challenges still to be overcome.

MicroRNA mediated regulation of immunity against gram-negative bacteria

Evidence over the last couple decades has comprehensively established that short, highly conserved, non-coding RNA species called microRNA (miRNA) exhibit the ability to regulate expression and function of host genes at the messenger RNA (mRNA) level. MicroRNAs play key regulatory roles in immune cell development, differentiation, and protective function. Intrinsic host immune response to invading pathogens rely on intricate orchestrated events in the development of innate and adaptive arms of immunity. We discuss the involvement of miRNAs in regulating these processes against gram negative pathogens in this review.

Circulating microRNAs as Potential Biomarkers of Infectious Disease

microRNAs (miRNAs) are a class of small non-coding endogenous RNA molecules that regulate a wide range of biological processes by post-transcriptionally regulating gene expression. Thousands of these molecules have been discovered to date, and multiple miRNAs have been shown to coordinately fine-tune cellular processes key to organismal development, homeostasis, neurobiology, immunobiology, and control of infection. The fundamental regulatory role of miRNAs in a variety of biological processes suggests that differential expression of these transcripts may be exploited as a novel source of molecular biomarkers for many different disease pathologies or abnormalities. This has been emphasized by the recent discovery of remarkably stable miRNAs in mammalian biofluids, which may originate from intracellular processes elsewhere in the body. The potential of circulating miRNAs as biomarkers of disease has mainly been demonstrated for various types of cancer. More recently, however, attention has focused on the use of circulating miRNAs as diagnostic/prognostic biomarkers of infectious disease; for example, human tuberculosis caused by infection with Mycobacterium tuberculosis, sepsis caused by multiple infectious agents, and viral hepatitis. Here, we review these developments and discuss prospects and challenges for translating circulating miRNA into novel diagnostics for infectious disease.

Integrated analysis neurimmiRs of tilapia (Oreochromis niloticus) involved in immune response to Streptococcus agalactiae, a pathogen causing meningoencephalitis in teleosts

MicroRNAs (miRNAs) are a class of noncoding RNA molecules and play important roles in a wide spectrum of biological processes, including in immune response. Recent years have witnessed considerable amount of research interest in studies on miRNA-mediated modulation gene function during neuroinflammation. Here, we evaluated Streptococcus agalactiae infected tilapia (Oreochromis niloticus) brain for the expression profile of miRNAs, potential functions and their correlation with genes involved in inflammatory pathways. A total of 1981 miRNAs were identified, including in 486 miRNAs which have homologues in the currently available databases and 1945 novel miRNAs. The expression levels of 547 miRNAs were significantly altered at 6 h-48 h post-bacterial infection, and these miRNAs were therefore classified as differentially expressed tilapia miRNAs. Real-time PCR were implemented for 14 miRNAs co-expressed in five samples, and agreement was confirmed between the high-throughput sequencing and real-time PCR data. For the 486 differentially expressed miRNAs target 41,820 genes. GO and KEGG enrichment analysis revealed that some target genes of miRNAs were grouped mainly into the categories of apoptotic, signal pathwayand immune response. This is the first report of comprehensive identification of teleost miRNAs being differentially regulated in brain in normal conditions relating to bacterial infection.

Evaluation of microRNA Expression in Patients with Herpes Zoster

Reactivated varicella-zoster virus (VZV), which lies latent in the dorsal root ganglions and cranial nerves before its reactivation, is capable of causing herpes zoster (HZ), but the specific mechanism of virus reactivation and latency remains unknown. It was proposed that circulating microRNAs (miRNAs) in body fluids could potentially indicate infection. However, the connection between herpes zoster and circulating miRNAs has not been demonstrated. In this study, 41 HZ patients without superinfection were selected. The serum miRNA levels were analyzed by TaqMan low density array (TLDA) and confirmed individually by quantitative reverse transcription PCR (RT-qPCR) analysis. Thirty-five age-matched subjects without any infectious diseases or inflammation were selected as controls. The results showed that the serum miRNA expression profiles in 41 HZ patients were different from those of control subjects. Specifically, 18 miRNAs were up-regulated and 126 were down-regulated more than two-fold in HZ patients compared with controls. The subsequent confirmation of these results by qRT-PCR, as well as receiver operating characteristic (ROC) curve analysis, revealed that six kinds of miRNAs, including miR-190b, miR-571, miR-1276, miR-1303, miR-943, and miR-661, exhibited statistically significant enhanced expression levels (more than four-fold) in HZ patients, compared with those of healthy controls and herpes simplex virus (HSV) patients. Subsequently, it is proposed that these circulating miRNAs are capable of regulating numerous pathways and some may even participate in the inflammatory response or nervous system activity. This study has initially demonstrated that the serum miRNA expression profiles in HZ patients were different from those of uninfected individuals. Additionally, these findings also suggest that six of the altered miRNA could be potentially used as biomarkers to test for latent HZ infection.

Analysis of TaqMan Array Cards Data by an Assumption-Free Improvement of the maxRatio Algorithm Is More Accurate than the Cycle-Threshold Method

Quantitative PCR diagnostic platforms are moving towards increased sample throughput, with instruments capable of carrying out thousands of reactions at once already in use. The need for a computational tool to reliably assist in the validation of the results is therefore compelling. In the present study, 328 residual clinical samples provided by the Public Health England at Addenbrooke's Hospital (Cambridge, UK) were processed by TaqMan Array Card assay, generating 15 744 reactions from 54 targets. The amplification data were analysed by the conventional cycle-threshold (CT) method and an improvement of the maxRatio (MR) algorithm developed to filter out the reactions with irregular amplification profiles. The reactions were also independently validated by three raters and a consensus was generated from their classification. The inter-rater agreement by Fleiss' kappa was 0.885; the agreement between either CT or MR with the raters gave Fleiss' kappa 0.884 and 0.902, respectively. Based on the consensus classification, the CT and MR methods achieved an assay accuracy of 0.979 and 0.987, respectively. These results suggested that the assumption-free MR algorithm was more reliable than the CT method, with clear advantages for the diagnostic settings.

The role of microRNAs in enteroviral infections

The genus Enterovirus, a member of the Picornavirus family, are RNA viruses that can cause poliomyelitis, hand-food-mouth disease, viral meningitis or meningoencephalitis, viral myocarditis and so on. MicroRNAs are a class of highly conserved, small noncoding RNAs recognized as important regulators of gene expression. Recent studies found that MicroRNAs play a significant role in the infection of Enterovirus, such as enterovirus 71, coxsackievirus B3 and other Enterovirus. Enteroviral infection can alter the expression of cellular MicroRNAs, and cellular MicroRNAs can modulate viral pathogenesis and replication by regulating the expression level of viral or host's genes. Herein, this review summarizes the role of MicroRNAs in enteroviral infection.

The oncomiR miR-197 is a novel prognostic indicator for non-small cell lung cancer patients

Background:

MicroRNA expression signatures can promote personalised care for non-small cell lung cancer (NSCLC) patients. Our aim was to evaluate the previously unexplored prognostic potential of miR-197, a key oncogenic molecule for NSCLC.

Methods:

Total RNA isolation (n=124 NSCLC and n=21 tumour-adjacent normal tissues), was performed using the QIAsymphony SP workstation. The quantity and quality of RNA were assessed by spectrophotometric analysis and an Agilent 2100 bioanalyzer. Polyadenylation and reverse transcription were subsequently carried out. MiR-197 expression levels were measured by qPCR, after quality control (inter-assay CV=7.8%). Internal validation procedures were followed by assigning training and test sets and robust biostatistical analyses were performed, including bootstrap resampling.

Results:

MiR-197 is associated with larger tumours (P=0.042) and the squamous cell carcinoma histotype (P=0.032). Interestingly, after adjusting for important prognostic indicators, miR-197 expression was identified as a novel independent predictor of unfavourable prognosis for NSCLC patients (HR=1.97, 95% CI=1.10–3.38, P=0.013). We also demonstrate that miR-197 retains its prognostic performance in both early-stage I (P=0.045) and more advanced-stage individuals (P=0.036).

Conclusions:

The cost-effective expression analysis of miR-197 could constitute a novel molecular tool for NSCLC management.

miRNAs as non-invasive biomarkers for lung cancer diagnosis

Lung cancer is a leading cause of cancer death and late diagnosis is one of the most important reasons for the high mortality rate. Circulating microRNAs (miRNAs) represent stable and reproducible markers for numerous solid tumors, including lung cancer, and have been hypothesized as non-invasive diagnostic markers. Serum, plasma or whole peripheral blood can be used as starting material, and several methodological approaches have been proposed to evaluate miRNA expression. The present review provides an in depth summary of current knowledge on circulating miRNAs in different types of biological samples used as diagnostic markers of lung cancer. We also evaluate the diagnostic accuracy of each miRNA or group of miRNAs in relation to the different housekeeping miRNAs used. Finally, the limitations and potential of miRNA analysis are discussed.