Like the MIR space station that orbited the earth for 15 years, miRs (microRNAs or miRNAs) orbit the circulatory system of mammals. miRNAs are small (20-23-nucleotide) RNA molecules, with primary regulatory functions in controlling expression levels of cellular messenger RNAs (mRNAs). The details of miRNA generation and targeting to cellular mRNAs are described in a recent comprehensive review. In brief, miRNAs act in conjunction with the RNA-induced silencing complex to target the 3′ untranslated region of cellular mRNAs, resulting in mRNA degradation. Alternatively, miRNAs can bind to mRNAs and inhibit translation. miRNAs also appear to have other functions in the context of infectious disease. For example, herpes viruses encode miRNAs that regulate pathogenesis and latent viral reservoirs. For hepatitis C virus (HCV), the cellular miRNA-122 has been shown to bind to HCV RNA to enhance RNA abundance,[3, 4] although miR-122-independent replication of HCV has been reported on. Thus, miRNAs may also serve as host cell factors to modulate virus replication. Interestingly, despite its ability to enhance HCV replication, miR-122 levels decrease with progression of HCV-induced liver disease.[6, 7]
Recent studies demonstrate that miRNAs are also associated with various diseases, including cancer. Because a single miRNA can regulate the expression of many cellular mRNAs, deregulated expression of only a few miRNAs has the capacity to significantly perturb cellular processes that lead to disease. As such, miRNAs have become attractive targets for novel approaches to control various disease processes. Additionally, because circulating miRNAs are also found in human serum and plasma, they are touted as candidate biomarkers for many diseases.
In this issue of Hepatology, Shrivastava et al. screened the plasma and serum of HCV-infected patients to identify miRNAs that correlated with different stages of histologically assessed liver disease severity and during the establishment of chronic HCV infection. miRNAs were characterized using a commercially available assay that measures expression of 84 miRNAs, which were subsequently validated by real-time reverse-transcriptase polymerase chain reaction.
In the first phase of the study, the team compared serum miRNA profiles among HCV-infected patients with fibrosis versus healthy volunteers. A total of 44 subjects with chronic HCV infection were studied, including 33 with early-stage fibrosis (F0-F2) and 11 with late-stage fibrosis (F3-F4). Twenty subjects with non-HCV fibrosis and 22 healthy subjects served as controls. In the second phase, plasma miRNA profiles of 10 healthy volunteers were compared to 29 patients with acute HCV infection, 18 who progressed to chronic HCV infection and 11 who spontaneously resolved the infection. Subjects were recruited from St. Louis University and Massachusetts General Hospital.
The investigators reported that serum miR-20a and miR-92a levels were significantly higher in HCV+ subjects with fibrosis, compared to healthy volunteers or non-HCV-associated liver disease. Moreover, the abundance of these two miRNAs was increased in patients with both acute and chronic infection, as compared to healthy volunteers. However, degree of enhancement of miR-20a and miR-92a in HCV infection was independent of viral load. In longitudinal samples, both miR-92a and miR-20a remained elevated and relatively stable during transition from acute to chronic infection, whereas miR-92a decreased as patients spontaneously resolved their acute infection. Receiver operating characteristic analyses suggested that these miRNAs discriminated infected from noninfected patients, HCV+ patients with or without fibrosis, acute versus noninfected, and chronic versus noninfected subjects. Finally, miR-20a and miR-92a were induced in cultured hepatoma cells after in vitro HCV infection.
Although miR-92a and many other miRNAs are implicated in liver disease in animal models and in humans,[1, 10] the article from Shrivastava et al. is the first report describing an association of miR-20a with HCV-associated fibrosis (Fig. 1). Other recent studies have shown that miRNAs associated with inflammation, such as miR-155, a positive regulator of tumor necrosis factor alpha production, is up-regulated in serum and circulating monocytes from patients with HCV infection, that miR-199 and miR-200 families in liver are associated with progression of fibrosis, that hepatic miR-21 correlates with viral load, fibrosis, and levels of serum liver transaminases, possibly through induction of transforming growth factor beta signaling, and that HCV infection is associated with decreased hepatic miR-29, which is associated with induction of extracellular matrix proteins by hepatic stellate cells.
Additional studies from different and larger patient cohorts are required to validate results on miR-20a and miR-92a. Future work should evaluate more broad-spectrum miRNA profiling on larger sample sizes. Clinical questions for future study include how well miR-20a and miR-92a discriminate HCV liver disease from liver disease by other factors, including hepatitis B virus, alcohol, and nonalcoholic fatty liver disease. Questions for basic researchers will be to determine how acute HCV infection leads to changes in serum miRNA levels. What are the cellular pathways that generate circulating miRNAs? Are cellular pathogen recognition receptors, such as retinoic acid inducible gene I and Toll-like receptor (TLR)3, known sensors of HCV infection in hepatocytes,[14, 15] or TLRs 2, 7, and 8, which appear to sense HCV RNA or proteins in immune cells, or C1q complement receptor, a sensor of HCV core protein, involved in miRNA induction? Furthermore, what cellular mRNAs are being regulated by these HCV-induced miRNAs? Which host cell types are being targeted, and importantly, how do host miRNA responses influence HCV infection and contribute to pathogenesis of HCV liver disease? Studies of this nature will undoubtedly keep miRs in scientific and clinical orbit for years to come as scientists continue to define the mechanisms for miRNA-associated liver disease as well as prognostic values of circulating miRNAs.