Emerging Role of microRNAs as Biomarkers

Ciaran Faherty, Ph.D.

The study and application of microRNAs is a burgeoning area of research. MicroRNAs are an endogenous class of non-coding ~22 nucleotide RNAs that impose transcriptional regulation within the cell.¹ Base pairing of the microRNA with the 3’UTR of mammalian protein-coding genes exerts posttranscriptional regulation thus exerting control over a large number of mRNA transcripts in an evolutionarily conserved fashion.^2-5

Fundamental Role in Biology
The finding that microRNAs play a central role in many developmental processes seemed to suggest their control was mediated through a network of eloquently regulated mechanisms.^4,6 Indeed, microRNAs exert their effects on target mRNAs in a temporal and contextual manner thus conferring stringent control over a plethora of systems not only during development but within the adult cell population.^7,8 Therefore, microRNAs have emerged as a unique class of gene regulators superimposing control over a somewhat ‘noisy’ transcript level within the cell.⁹ Their intimate role in controlling gene expression has made them attractive biomarkers in the pathophysiology and etiology of many disease states.¹⁰

Figure 1. MicroRNA Expression Profiling to Biomarker Discovery A mutli-pronged approach to understand the role of microRNAs. Workflow for (A) extraction and expression profiling and (B) combining phenotypic screening and transcript evaluation to decipher the true role of microRNAs.

MicroRNAs as Biomarkers
The use of microRNAs as biomarkers is now widely accepted in the peer-reviewed literature, as well as by government and regulatory agencies.^11,12 Recently extraction of microRNAs from post-mortem prefrontal cortex allowed researchers to study their role in the etiology of Schizophrenia.¹³ Moreover, while identifying differences in the expression levels of several microRNAs from human brain the group also noted changes in microRNA levels in the brains of rats administered Haloperidol. Thus, microRNAs are being used not only to differentiate between disease and normal tissue but may provide useful insight to therapeutic response.

Impact of Regulation
MicroRNAs have been shown to affect cell proliferation and cell death, processes that are fundamental components of tumorogenesis.^14,15 One study demonstrated that microRNA expression profiles could be used to classify different tumors that hitherto have been difficult to routinely denote even with established pathological critieria.¹⁶ Importantly, this study demonstrated that while single indices have conventionally been used as biomarkers, microRNAs either singularly or as a “signature” can serve to decipher critical information about the disease or therapeutic intervention. In addition, Lu and colleagues highlighted the robustness of microRNA levels to tease-out critical information as opposed to non-robust mRNA levels in deciphering the same information, i.e. origin of tissue. For a full review of the role of microRNAs in tumor development and progression see Calin et al. (2006).¹⁷

      
Figure 2. MicroRNA Expression Profiling Array. Quantitative two-color microRNA expression array based on miRBase 9.0

MicroRNA profiles as Indicators of Pathophysiology
Owing to their tissue-specific nature, one study used expression profiling of skeletal muscle-associated microRNA levels to understand their role in muscular dystrophy.¹⁸ The expression of miR-206 was significantly altered in a number of muscle-rich tissues in the dystrophin-deficient (mdx) mouse model suggesting a putative role in the pathophysiology of muscular dystrophy.¹⁸

A study by van Rooij and colleagues demonstrated that a ‘signature’ of microRNAs was indicative of stress-response that led to cardiac hypertrophy and ultimately heart failure.¹⁹ Different types of stress-injury to the murine heart led to both up- and down-regulation of microRNAs. These microRNA expression levels were similarly altered in patients suffering from heart failure compared to controls. Furthermore, the overexpression of miR-195, in vitro and in vivo resulted in cardiomyopathy thus providing a causal link for the development of cardiac hypertrophy and heart failure. It was also shown that reducing the endogenous concentration of miR-195 with selective microRNA inhibitors might be advantageous in understanding the exact mechanism of cardiomyopathy following this type of stress-injury.^{19, 20}

Summary
MicroRNAs have gained a valuable foothold in building an understanding of disease progression and response to therapeutic intervention. In addition, these neo-biomarkers are poised to create a bridge between pre-clinical observations and clinical efficacy and safety.

References

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3. Lim et al. Microarray analysis shows that some microRNAs downregulate larger numbers of target mRNAs. Nature, 2005. 433: p.769-773.
4. Stark et al. Animal MicroRNAs Confer Robustness to Gene Expression and Have a Significant Impact on 3’UTR Evolution. Cell, 2005. 123: p.1133-1146.
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12. Methods of Determining the Prognosis of Hepatocellular Carcinoma-http://ott.od.nih.gov/db/abstxt.asp?refno=1500
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