Since an increasing number of miRNAs studies and clinical trial reports show miRNAs are used as biomarkers in numerous disorders and diseases (Bonneau et al. 2019; Hanna et al. 2019). The requirement of clinical and pre-clinical research of miRNAs in epilepsy is still needed to be forged. Similarly in PTE, lacunae remain in-vivo and in-vitro studies focusing to identify miRNAs. Hence, in this study using in-silico tools, we focused to provide an insight on miRNAs that can be involved in PTE. Here, we identified miR-27a, miR-502, miR-130b, miR-9, miR-625, miR-660, miR-138, miR-21, miR-30a and miR-1307 were reported to be dysregulated in clinical studies of both TBI and epilepsy.
One of the key finding of this review is identification of commonly upregulated and downregulated miRNAs in TBI as well as in epilepsy. Thereby, this suggest share mechanism of regulation of pathways in both TBI and epilepsy. Since PTE is progression of epilepsy after TBI, the predominant miRNAs hints a link of molecular mechanism and shared signaling pathways of epileptogenesis after TBI. Therefore, analysis by Gene Ontology and KEGG pathways, cancer pathways were found to be involved most individually with all miRNAs. Additionally, pathways such as cell cycle pathway, NF-κB, p53 and apoptosis were also found significantly affected by the targets of predicted miRNAs individually. For example, targets of miR-130b, miR-21 and miR-30 are involved in p53 pathway commonly. Since numerous studies already reported the involvement of p53 gene and apoptosis pathway in the pathogenesis of PTE and neurodegeneration, these pathways may be affected by the role of such predicted miRNAs in the regulation of epileptogenesis (Engel et al. 2007, 2010). Similarly, NF-κB pathway consists of gene targets of miR-502 and miR-9. As it is established that the inflammation pathway is one of the key pathways which is involved in the pathogenesis of PTE. Thus apparently, the targets of both miRNAs might be somehow involved in the regulation of pathogenesis of PTE. Additionally, the summarization of shared pathways regulated by miRNAs in PTE shows that centrally apoptosis and inflammation are major signaling which is affected by both TBI and epilepsy (Fig. 5). It is in concordance with previous studies in PTE that suggest that apoptosis-associated neuronal death occurs post-inflammation (Wang et al. 2021; Sun et al. 2021).
The classification of biomarkers is done in-between diagnostic biomarkers, prognostic biomarkers, predictive biomarkers and therapeutic biomarkers (Pitkänen et al. 2021). However, in current review, we are focusing here only on prognostic and therapeutic biomarkers for PTE. There is several biomarkers are under investigation for TBI and epilepsy. For example, GFAP, S100B, IL-6, HMBG1 and UCH-L1 are considered biomarkers in both TBI and epilepsy. Similarly, GFAP is considered as one of the biomarkers for PTE (Liliang et al. 2010; Okonkwo et al. 2013; Çevik et al. 2019; Pitkänen et al. 2021). Additionally, US FDA has recently approved GFAP and UCH-L1 as biomarkers for mild TBI (Pitkänen et al. 2021). Likewise, numerous studies on genetic biomarkers are also published (Pitkänen et al. 2021). However, both genetic and protein biomarkers fail to provide prior information on the probability of PTE after brain insult. Additionally, molecular biomarkers are highly stable in circulatory environment of human body (Sanz-Rubio et al. 2018). Besides, an earlier diagnosis can provide better chances for the treatment. Here, miRNAs may provide a crucial opportunity for earlier diagnosis of diseases including PTE. An increasing body of evidence suggests that numerous miRNAs are considered as biomarker candidates for post-traumatic epilepsy. For example, miR-106b, miR-130a, miR-301a, miR-30b, let-7d, miR-194, miR-4521, miR-134 can be biomarkers of epilepsy (Wang et al. 2015b, a; An et al. 2016) and maybe probable biomarker of PTE (Ma 2017). Evidently, It is necessary to uncover the field of biomarkers for PTE that can provide prior and predictive information about the response to therapy and ultimately improve the therapeutics overall against PTE.
Here in current review, miRNAs such as miR-21, miR-9 and miR-130b were found upregulated. Since, miR-21 shows up-regulation in the initial period of head injury 6hrs till the late period after 72hrs (Raoof et al. 2017; Pitkänen et al. 2021). It was also found upregulated in epilepsy as well. As, a prognostic biomarker consists of properties of showing quantitative changes in the body fluid or tissues before the onset of diseases. It provides information about the predisposed condition of a specific disorder that might turn into a disease. Hence, we predicted it may be a probable candidate for further study for the prognostic biomarker of PTE. Importantly, miR-21 is primarily targets apoptosis pathway. Thus miRNAs which shares the same signaling axis might be further studied for candidate prognostic biomarkers. For example, miR-9 and miR-130 can also be a molecule of interest for further analysis as a prognostic biomarker.
Since therapeutic markers are molecules that can be targeted by drugs for therapy of various diseases. Generally, proteins are considered as good therapeutic markers. For example, in case of cancer, several markers are being studied such as CDs, CLL123 BCL2, CHD-1, E-Catherine and FGF-18 (Wei et al. 2013; Xu et al. 2018; Liu 2019). Similarly, in epilepsy, HMGB1 has been contemplated as a potential therapeutic biomarker. As a result, it can differentiate patients with a higher risk of epilepsy (Au et al. 2012; Zhu et al. 2018; Kobylarek et al. 2019). In the current review, the attempt to understand the link of miRNAs as therapeutic biomarker for PTE. Since studies suggest that miR-27a, miR-21 and miR-30b are considered as therapeutic markers for epilepsy (Raoof et al. 2018; Venø et al. 2020). Our data also in concordance with that previous studies. Hence, it requires in-depth in-vitro and in-vivo studies for a concrete conclusion about the therapeutic marker of PTE. However, our review provides a narrowed down direction for miRNA which should be consider for further studies in PTE.
In summary, we focused on the miRNA candidates with commonalities in TBI and epilepsy to provide an insight into progression epileptogenesis and identified 10 miRNAs. Thus, we considered these 10 miRNAs linked with PTE. Also, apoptosis, inflammation, NFκB and cell cycle regulation pathways were found majorly in targets of miRNAs. Further we noted the shared regulatory pathway of 10 identified miRNA with putative linked with PTE. Though we have some hints about probable miRNAs associated with PTE, it is crucial for future miRNA research to understand a correlation of mechanism in PTE while addressing various factors such as age and severity of the injury.