**15. IsomiRs and their impact on miRNAs in dengue fever vector**  *Ae. aegypti*

microRNAs may exist in various lengths and sequence variations, which are known as isomiRs [101]. Earlier, those were considered as sequencing errors; however, some studies predicted to be physiologically relevant and posttranscriptionally modified miRNA variants. IsomiRs may express affinities for different targets than their canonical miRNA counterparts. Furthermore, nucleotide heterogeneity could be found at both ends of miRNA sequence in the form of nucleotide substitutions despite the variations are more frequent at 3′ end. The molecular biology, genome structure, and epigenetics of isomiRs are still poorly understood, and the mechanisms of their biogenesis are also considered very complex and even cell-type specific. Some variations in miRNA sequences are supposed to be the product of template variations, which might be brought by the exonuclease activity of Drosha and Dicer [101].

It is a well-documented fact that miRNAs may play a vital role to modulate the capability of vectors to propagate the infection for widely damaging arboviruses (e.g., dengue virus) [101]. It has been demonstrated that miRNA could be modified in mosquitoes upon the induction of DENV infection. As genetic variations in isomiR prevalence have significant impact on gene regulation, a clear understanding of the role of isomiR profile of mosquitoes in DENV and other arboviruses infection propagation is urgently needed. Etebari et al. in one study found this altered posttranscriptional modification role of miRNAs in *Ae. aegypti* mosquitoes after DENV infection in comparison with uninfected mosquitoes. For this purpose, they utilized already published RNA-seq data from 2-, 4-, and 9-day DENV-infected and uninfected mosquitoes. The findings showed significant variations in miRNA prevalence in response to dengue virus infection although the effects were not ubiquitous, and no remarkable alteration in overall pattern of isomiR expression was noted upon DENV infection. They calculated the exact/all read count ratio as an index for isomiR production rate for all known *Ae. aegypti* miRNAs. DENV-infected mosquitoes increased the isomiR production of 3 miRNAs (miR-2c, miR-210, and miR-34) with notable impact on two miRNAs (miR-276 and miR-10) with less read count by DENV infection. The data also demonstrated that 3 isomiRs of miR-34-5p were also significantly altered by dengue virus infection. Collectively, those alterations might have net benefit to determine the mosquito's role upon DENV infection propagation; however, potential biological significances of these modifications are still unclear except to infer that some evolutionary function of miRNAs. Similarly, it is also ambiguous that why some specific isomiRs potentially modify more in response to DENV infection in *Ae. aegypti*. Furthermore, it was also noted that one particular miRNA with significant increase in one specific isomiR variants upon DENV infection also contained a common variant in at least one other isomiR as well. The authors speculated that establishment and persistence of DENV infection might cause significant changes in activity levels of some enzymes involved in the production of isomiRs, which ultimately modify the isomiR production frequencies of some specific miRNAs. The isomiR production power against different *Ae. aegypti* miRNAs was also demonstrated differently as observed by the read count. The "true" miRNAs were considered as those which had an exact match to the canonical sequence (already reported and available in miRbase), while the "false" ones were classified as those which significantly differed to that reported in miRbase. The authors explicated that in *Ae. aegypti*, most abundant miRNA sequences matched to true miRNAs were just 45%, while 55% miRNAs produced by *Ae. aegypti* miRNAs were false miRNAs, and interestingly, this overall trend was not altered by DENV infection. Although the findings are striking and indicate that little variations in miRNA sequences may significantly impact target affinities in DENV-host interaction as well as in infection propagation, still extensive studies are required in the field to validate such hypothesis [101].

RNA and microRNA as anti-mRNA-based treatment strategies with strong evidences are evolutionary, but unfortunately still not a successful and reliable treatment line in viral infections with certain issues of drug delivery and long-term therapeutic effects. The involvement of lincRNAs and variations in miRNA epigenetic profile in disease-specific arboviruses (e.g., dengue virus) during infection also indicate complex genome interactions between virus and hosts. RNA association as a novel therapeutic approach to treat infection by gene activation, to find out some specific modulators of gene regulation in viral replication as well as in disease progression is still in an explanatory phase specifically for disease-specific or insect-specific flaviviruses. In conjunction, the molecular approaches to find out disease progression pathways in disease-specific arboviruses are still investigative and antiviral treatment approaches in the form of siRNAs, microRNAs as well as RNA association still need to be explored fully before their practical implementations in diagnostics and as some antiviral therapeutics.

RNA Association, RNA Interference, and microRNA Pathways in Dengue Fever Virus-Host…

http://dx.doi.org/10.5772/intechopen.80334

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**Conflict of interest**

**Author details**

Imran Shahid1,2\*

**References**

The author declare that there is no conflict of interest.

\*Address all correspondence to: iyshahid@uqu.edu.sa

(CEMB), University of the Punjab, Lahore, Pakistan

University, Makkah, Saudi Arabia

Research. 2009;**81**:6-15

1 Department of Pharmacology and Toxicology, College of Pharmacy, Umm-Al-Qura

2 Applied and Functional Genomics Laboratory, Center of Excellence in Molecular Biology

[1] Gubler DJ. Dengue/dengue hemorrhagic fever: History and current status. Novartis

[2] Tatmen AJ, Hay SI, Rogers DJ. Global traffic and disease vector dispersal. Proceedings of the National Academy of Sciences of the United States of America. 2006;**103**:6242-6247 [3] Clyde K, Kyle JL, Harris E. Recent advances in deciphering viral and host determinants of dengue virus replication and pathogenesis. Journal of Virology. 2006;**80**:11418-11431 [4] Lindenbach BD, Pragai BM, Montserret R, Beren RKF, Pyle AM, Penin F, et al. The C terminus of hepatitis C virus NS4A encodes an electrostatic switch that regulates NS5A hyperphospholylation and viral replication. Journal of Virology. 2007;**81**:8905-8918 [5] Sampath A, Padmanabhan R. Molecular targets for flavivirus drug discovery. Antiviral

Foundation Symposium. 2006;**277**:3-16. discussion 16-22, 71-13, 251-253

#### **16. Conclusions**

An over-growing number of reports and continuous publication of journals and books illustrate an intricate interplay between virus-host interactions in dengue fever virus infection. Similarly, an increase in morbidity and mortality rate with DENV infection and still unavailability of standard care to treat the infection have diverted the DENV-research to explore new paradigms in treatment and to map/identify cellular/molecular pathways to better understanding of disease progression in infected vectors, either mosquitoes or humans. Small interference RNA and microRNA as anti-mRNA-based treatment strategies with strong evidences are evolutionary, but unfortunately still not a successful and reliable treatment line in viral infections with certain issues of drug delivery and long-term therapeutic effects. The involvement of lincRNAs and variations in miRNA epigenetic profile in disease-specific arboviruses (e.g., dengue virus) during infection also indicate complex genome interactions between virus and hosts. RNA association as a novel therapeutic approach to treat infection by gene activation, to find out some specific modulators of gene regulation in viral replication as well as in disease progression is still in an explanatory phase specifically for disease-specific or insect-specific flaviviruses. In conjunction, the molecular approaches to find out disease progression pathways in disease-specific arboviruses are still investigative and antiviral treatment approaches in the form of siRNAs, microRNAs as well as RNA association still need to be explored fully before their practical implementations in diagnostics and as some antiviral therapeutics.
