Two human observational studies have been carried out to date assessing the role of miRNAs as potential mediators of the impact of tobacco smoke on in-utero development. Maccani et al.  compared the expression level of candidate miRNAs in 25 placentas obtained at the time of delivery from women with and without a history of smoking during pregnancy. Out of the four selected targets, a significant downregulation of miR-16, miR-21, and miR-146a was observed due to maternal cigarette smoking. Furthermore, miR-146a was significantly downregulated following exposure to both nicotine and benzopyrene in TCL-1 (immortalized human trophoblast cell line), a cell line derived from third trimester extravillous cells, whereas no impact due to either agent was observed on the remaining miRNAs.
The impact on miRNA expression levels due to tobacco smoke exposure during the gestational period was also addressed in a study conducted by Herberth et al. [47▪]. As this study was particularly concerned with the effect of tobacco smoke on immune-related responses, two candidate miRNAs, miR-155 and miR-223, previously implicated in Treg cell formation and function, were selected for analysis. In this prospective study of mother–child pairs, increasing levels of miR-223 were observed in both maternal and cord blood with increasing levels of cotinine in maternal urine.
The effects of a maternal high-fat diet on the miRNA profile of the resulting offspring were assessed in a murine model by Zhang et al. . Microarray analysis of liver-derived miRNAs from female offspring identified 10 miRNAs that were upregulated and 23 miRNAs that were downregulated among offspring exposed to a high-fat diet. Although all targets were not successfully validated, the downregulation of miR-483* was consistently observed as the greatest fold-change among high-fat diet-exposed offspring. The authors determined that the genetic location of miR-483* lies within the intron of Igf2, a gene known to be critical in regulating fetal growth, indicating that the transcription of both is likely regulated by the same promoter. Paradoxically, among high fat-exposed mice, the downregulation of miR-483* was observed alongside an upregulation of Igf2.
Findings relaying the impact of in-utero exposures on miRNA expression levels thus far have raised several interesting questions that will need to be further addressed. Principal among these is determining whether the observed changes in miRNA expression levels reflect a causal pathway between exposure and outcome, identifying the downstream gene targets impacted by deregulated miRNAs, and delineating variability introduced because of methodological and biospecimen specifications.
To address this question, it will first have to be established whether the observed changes in miRNA expression levels due to environmental exposures are the result of a direct effect or a surrogate indication of a different mechanism. This issue was highlighted in the study by Herberth et al., in which the expression level of miR-223 in blood was found to vary among different blood cell-types, leading the authors to question whether the observed tobacco-related increase in miR-223 levels is indeed a direct response to the exposure or a bystander effect of smoke-induced changes in blood cell-type composition [47▪]. Although the utility as a marker of exposure is not diminished in either case, the former would more directly associate miRNAs to the causal pathway linking exposures to putative deregulated physiological processes.
Given a substantiated impact of environmental exposures on miRNA expression levels and related physiological processes, more efforts into how these changes in expression profiles translate into health outcomes will also be warranted. To date, studies relating changes in miRNAs to various health outcomes and studies relating exposures to changes in miRNAs are often conducted separately. Few studies have linked the observed changes in miRNAs due to environmental exposures to known health effects. One such example includes the finding by Herberth et al. [47▪] that higher levels of miRNA-223 in cord blood were also correlated with decreasing levels of Treg cells in newborns, which in turn was shown to be associated with a significantly higher risk of developing atopic dermatitis by the age of 3. Such findings pave the way to further understand the etiologic processes underlying the exposure–outcome relationship and offer possibilities for remediation and intervention.
In order to identify potential physiological processes that are affected by the exposure-incurred changes in miRNA profiles, studies often employ various existing web-based programs (e.g., miRBase, Targetscan, PicTar) to identify putative mRNA targets of the aberrantly expressed miRNAs. Using slightly varying algorithms, these programs scan a library of mRNA 3’UTR regions to identify potential binding sites for the miRNAs of interest. However, as binding of miRNAs to their targets requires only partial complementarity, prediction of targets typically yields up to 20% false positives . The substantial rate of false positives and the observed discrepancy in predicted targets across the various algorithms highlight the need to experimentally validate reported putative targets.
Several high-throughput methodologies are currently in use to determine miRNA expression levels. Utilization of microarrays relies on nucleic-acid hybridization of labeled miRNA-derived cDNAs to complementary oligonucleotide probes immobilized on the array, with fluorescence intensity indicating the level of expression. This methodology is currently the most widely applied means of high-throughput determination of miRNA expression levels. However, innate properties of miRNAs limit the sensitivity and specificity achieved with this method. For example, miRNAs of low abundance likely fall below the limit of detection on the array. There is also considerable sequence homology among miRNAs, which probes on the array may not be sensitive enough to distinguish. Next generation sequencing is a methodology that is able to address several of these limitations. Additionally, unlike array-based methods, profiles generated using sequencing are not limited to previously identified miRNAs. However, the multiple steps involved in setting up the sequencing reaction offer various opportunities to introduce bias . Furthermore, although the price is continuously dropping, the cost of sequencing at present is still higher than array-based methods.
As with any biomarker-related study, the biospecimen source of miRNAs greatly influences the interpretation of meaningful results. Placental tissue, umbilical cord blood, and maternal sera are common sources for biomarkers reflecting the in-utero experience in human observational studies. All three serve as easily obtainable, noninvasive sources of miRNAs. However, there are factors driving distinctions in the expression profile generated from these biospecimens that need to be considered. Both placenta and blood are composite tissues consisting of heterogeneous cell types. Therefore, minimizing variability in cell-type composition across samples needs to be accounted for prior to analysis to reduce the likelihood of introducing bias into the study. Cord blood does provide access to specific cell lineages that make this particular biospecimen an attractive source for studies focusing on the impact of environmental exposures on the differentiation potential of stem cell populations, such as neuronal precursors, and on the immune response of cytokines.
While the expression profile generated from placental tissue and cord blood reflects the exposure experience toward the end of pregnancy, maternal plasma offers a means to monitor dynamic changes in expression level throughout pregnancy, enabling the focus on specific gestational periods. However, the proportion of placental miRNA in circulation likely fails to account for the comprehensive expression profile of the placenta. Therefore, maternal plasma is best suited to develop screening markers, whereas cord blood and placental levels can also be analyzed to further etiologic understanding.
The implementation of miRNAs as indicators of environmental exposures relevant to children's health is still a nascent field, and the promise of their utility is just beginning to be realized. While mRNA transcript levels, the ultimate targets of miRNAs, can also be utilized for this purpose, one of the major attractions over their labile transcript counterparts is the inherent stability and robustness of miRNAs in bodily fluids under various conditions [58,59]. Furthermore, technological advances have made high-throughput assessment affordable, providing a means of feasible and sensitive detection.
The relevance of this marker in these types of studies is also established by the fact that miRNAs are known to be involved in processes with heightened activity during early development. Hence, beyond reflecting extent of exposure to an environmental agent, an observed deregulation of miRNA levels can also point to the etiologic mechanism, ultimately linking a given exposure to an outcome.
Already various exposure and outcome-related signatures have been identified. However, the lack of comparability in experimental conditions prevents deriving meaningful conclusions from the findings reported thus far. Even in studies focusing on the same exposure of interest, variability exists in the form of dosage, study population, biospecimen analyzed, detection methods utilized, and definitions for fold cut-offs. Hence, while technological advances yet to come will further facilitate the ability to assay and analyze expression profiles under various conditions, resulting in novel contributions to the literature, future studies should also focus on replicating existing expression profiles. Only by establishing reproducible expression profiles and validating putative gene targets can the ultimate goal of building a cohesive narrative tying environmental exposures to children's health outcomes be realized.
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