Alterations in ncRNA expression profiles may indicate a cell’s attempt to regain homeostasis. As such, they may be recorded before the cell is irreversibly damaged. Accordingly, relevant ncRNAs may be early predictors of different toxicological pathways making them potentially useful biomarkers for regulatory toxicology. The establishment of dose-response relationships is essential to building mechanistic understandings. Cells may counterbalance external insults with sufficient efficiency until a certain level or time of exposure. To understand mechanisms of toxicity, the point at which the system is irreversibly disrupted should be identified.
Extensive discussions addressed the question whether ncRNA expression changes were ‘cause or consequence’ of adverse effects and which implications this may have in making use of ncRNA technologies for RA. While ncRNA expression changes themselves may not be causative, their subsequent downstream effects can induce toxicity. Such changes can be thought of as causal. Those ncRNA expression changes that occur in response to a particular exposure and/or toxicity, but are not involved in the induction of adverse downstream effects, can be thought of as consequential. Also such changes may be useful in RA as biomarkers of exposure. Therefore, even without a full understanding of whether ncRNA expression changes are ‘cause or consequence’, ncRNAs may nevertheless be applied as biomarkers for RA.
From a toxicological point of view, concerns regarding the applicability of ncRNA profiling for RA are similar to the ones that have been voiced in respect to gene mapping. Data may indicate that, e.g. specific carcinogenic genes are enhanced or expressed. Upon too simplistic evaluation, this may be interpreted as giving rise to concern, even if no apical effect results from the genetic alteration. It is essential to understand the phenotypic consequences of a given ncRNA change, i.e. to perform a functional verification and validation of the ncRNA expression profile. At best, the physiological and pathological roles of all ncRNAs that rank high in expression profiles should be known. To date, such functionalities are investigated rather randomly by changing the expression of a specific ncRNA, e.g. by using knockout animals (or genetically modified cell lines) that lack a specific gene or ncRNA and searching for phenotypic (or cellular) alterations.
A way forward in identifying ncRNA fingerprints that may be applicable to determine, e.g. a substance’s carcinogenic potential, may be to comparatively assess ncRNA profiles from different animal species for substances with known carcinogenicity. Generally, the most relevant animal species and animal model for a potential human issue should be identified, just as the human health relevance of findings should be ensured. Research should also aim at investigating the biological implications of (different levels of) ncRNAs present in body fluids. Even though ncRNAs (just as other biomarkers in the blood) may be exceptionally stable when bound to proteins, they may nevertheless be removed from the blood very quickly, i.e. before sampling can be performed. It is unclear from which organs or tissues the miRNAs present in body fluids come from, or if they are truly specific to the process under investigation. Importantly, research reports should not only include all data that were collected, but they should also clearly describe how the data were collected and analysed. Further investigations should aim at identifying the technology that is best suited to determine biologically relevant ncRNA alterations (that are not merely technological artefacts or sample contaminations).