MiR-122
Encyclopedia
miR-122 is a miRNA
that is conserved between vertebrate
species. miR-122 is not present in invertebrates, and no close paralogs of miR-122 have been detected. miR-122 expression is specific to the liver
, where it has been implicated as a regulator of fatty-acid metabolism
in mouse studies. Reduced miR-122 levels are associated with hepatocellular carcinoma
. miR-122 also plays an important positive role in the regulation of hepatitis C virus
replication.
of tissue-specific microRNAs in mouse, where its expression is restricted to the liver
. The liver-specific expression of miR-122 is conserved in zebrafish. miR-122 expression increases during embryogenesis
until it constitutes 72% of total miRNA in adult human liver, making it one of the most highly expressed miRNAs in any tissue. In humans, miR-122 is encoded at a single genomic locus
in chromosome
18. The primary miR-122 transcript (pri-miR-122) is a long non-coding RNA
. Transcription
is regulated by HNF4α. The miR-122 hairpin precursor consensus shown here is predicted based on base pairing and cross-species conservation. The mature sequence is excised from the 5' arm of the hairpin.
There is evidence that miR-122 is regulated by Rev-ErbA alpha
which is involved in circadian gene expression
, suggesting that miR-122 is a circadian metabolic regulator
. miR-122 regulates the expression of several mRNA molecules that are important in the circadian cycle, such as PPARβ/δ
. Mature miR-122 is subject to modification by the poly(A) polymerase
GLD-2, which adds a single adenosine to the miRNA 3' end. This results in an increase in miR-122 stability.
CAT-1
by binding to sites in the mRNA 3'UTR such that translation is repressed and the mRNA is targeted to P bodies. This repression can be relieved by the protein HuR
, which is released from the nucleus
under conditions of cell stress and binds to the CAT-1 3'UTR. The HuR interaction leads to release of the mRNA from the P bodies and resumption of active translation.
A number of other miR-122 targets, including CD320
, AldoA and BCKDK
, have been identified by microarray
analysis of changes in mRNA expression in the liver of mice treated with miR-122 inhibitors. The overall effect of miR-122 inhibition is to reduce the plasma cholesterol level, although the pathways involved in this regulation have not been fully elucidated. miR-122 also regulates systemic iron homeostasis
via the target mRNAs Hjv
and Hfe. miR-122 inhibition in mice or primates does not result in any detectable liver toxicity.
. Overexpression of miR-122 reduces tumorigenic properties in HCC cell lines, indicating that it functions as a tumor suppressor gene
, and increases the response of cells to the chemotherapeutic drugs sorafenib
and doxorubicin
. Several miR-122 target genes have been implicated in tumorigenesis, including ADAM10
, IGF1R, CCNG1
and ADAM17
.
of hepatitis C virus (HCV) is dependent on miR-122 expression. miR-122 regulates HCV by binding directly to two adjacent sites close to the 5' end of HCV RNA. Although these experiments were conducted using genotype
1a and 1b HCV RNA, the miR-122 binding sites are highly conserved across different genotypes, and miR-122 is also required for replication of infectious type 2a HCV. As miRNAs generally function to repress gene expression by binding to 3'UTR sites, this positive regulation of viral replication via a 5'UTR represents a novel function for miR-122. The mechanism of regulation is not yet clear. miR-122 stimulates translation of HCV RNA, but not to a sufficient extent to explain its effects on viral replication, indicating that a second stage of the viral replication cycle must also be regulated. HCV RNA synthesis is not affected by miR-122, suggesting that regulation of other processes such as RNA stability may occur. The extent to which the miRNA-induced silencing complex (miRISC) is involved in this regulation has not been fully deterined. The Argonaute
proteins (Ago1-4), which are essential for miRNA-directed repression, appear to be necessary for miR-122 to regulate HCV, although miR-122 overexpression may overcome this requirement. Another miRISC component, the DEAD-box RNA helicase DDX6, does not play a role in miR-122-facilitated HCV replication.
The existing HCV therapy of PEG-IFNα plus ribavirin
is poorly tolerated and frequently ineffective, so there is an urgent need for new drugs, and miR-122 inhibitors are an attractive possibility. The association between low miR-122 levels and hepatocellular carcinoma suggests that caution will be necessary when testing miR-122 inhibitors, and that long term treatment might be undesirable. However, miR-122 is a promising target as it can be very selectively and effectively inhibited with antisense oligonucleotides, and as it is a conserved host factor it is hoped that the virus would not be able to acquire resistance mutations to an anti-miR-122 therapeutic. Santaris Pharma have developed a locked nucleic acid
-based antisense oligonucleotide
that is delivered to the liver and effectively inhibits miR-122 following intravenous injection without inducing liver toxicity. This molecule reduced HCV viremia
in a small-scale trial in chimpanzees. The inhibitor is known as miravirsen and is in phase II clinical trials with the results expected in early 2012.
Miravirsen is currently undergoing Phase IIa trial with preliminary data presented on November 2011 at The Liver Meeting showing promising results from a group of chronic HCV genotype 1 infection patients. Patients were randomly assigned to placebo
groups or one of the three multiple ascending dose groups (3mg/kg, 5mg/kg, 7mg/kg of miravirsen). The naïve patients were given 5 weekly doses of miravirsen via subcutaneous injection
for four weeks, and 4 out of 9 patients receiving the highest dose (7mg/kg) exhibited significant decrease in Hepatitis C virus
RNA (p<0.001). The result was sustained even 10 weeks after the last miravirsen administration. Further trials testing for the safety and pharmokinetics of miravirsen will also be conducted in other countries such as Germany and Netherlands.
. This change is noted before increased amino-transferase
activity, making it a preferable indicator for liver disease. This study also suggested that miR-122 could be a “novel, predictive and reliable blood marker for viral-, alcohol- and chemical-induced liver injury.”
There is a great deal of research into the use of miR-122 as a biomarker for Hepatitis C
. While some studies dispute its efficacy for diagnosing Hep C, other research indicates that it may be useful in diagnosing specific forms of Hepatitis. For instance, decreased levels of miR-122 have been linked to a strain of Hepatitis C that is resistant to interferon
therapy. In this case, measuring miR-122 levels in the bloodstream of Hepatitis C positive patients would enable more accurate and personalized therapy for their disease.
Mirna
Mirna may refer to:geographical entities* Mirna , a river in Istria, Croatia* Mirna , a river in Slovenia, tributary of the river Sava* Mirna , a settlement in the municipality of Mirna in Southeastern Sloveniapeople...
that is conserved between vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
species. miR-122 is not present in invertebrates, and no close paralogs of miR-122 have been detected. miR-122 expression is specific to the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
, where it has been implicated as a regulator of fatty-acid metabolism
Fatty acid metabolism
Fatty acids are an important source of energy and adenosine triphosphate for many cellular organisms. Excess fatty acids, glucose, and other nutrients can be stored efficiently as fat. Triglycerides yield more than twice as much energy for the same mass as do carbohydrates or proteins. All cell...
in mouse studies. Reduced miR-122 levels are associated with hepatocellular carcinoma
Hepatocellular carcinoma
Hepatocellular carcinoma is the most common type of liver cancer. Most cases of HCC are secondary to either a viral hepatitide infection or cirrhosis .Compared to other cancers, HCC is quite a rare tumor in the United States...
. miR-122 also plays an important positive role in the regulation of hepatitis C virus
Hepatitis C virus
Hepatitis C virus is a small , enveloped, positive-sense single-stranded RNA virus of the family Flaviviridae...
replication.
Expression and regulation
miR-122 was originally identified by cloningCloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...
of tissue-specific microRNAs in mouse, where its expression is restricted to the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
. The liver-specific expression of miR-122 is conserved in zebrafish. miR-122 expression increases during embryogenesis
Embryogenesis
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...
until it constitutes 72% of total miRNA in adult human liver, making it one of the most highly expressed miRNAs in any tissue. In humans, miR-122 is encoded at a single genomic locus
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
in chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
18. The primary miR-122 transcript (pri-miR-122) is a long non-coding RNA
Non-coding RNA
A non-coding RNA is a functional RNA molecule that is not translated into a protein. Less-frequently used synonyms are non-protein-coding RNA , non-messenger RNA and functional RNA . The term small RNA is often used for short bacterial ncRNAs...
. Transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
is regulated by HNF4α. The miR-122 hairpin precursor consensus shown here is predicted based on base pairing and cross-species conservation. The mature sequence is excised from the 5' arm of the hairpin.
There is evidence that miR-122 is regulated by Rev-ErbA alpha
Rev-ErbA alpha
Rev-ErbA alpha also known as NR1D1 , is a protein that in humans is encoded by the NR1D1 gene.Rev-erbα is member of the Rev-ErbA family of nuclear receptors and is a transcriptional repressor...
which is involved in circadian gene expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
, suggesting that miR-122 is a circadian metabolic regulator
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
. miR-122 regulates the expression of several mRNA molecules that are important in the circadian cycle, such as PPARβ/δ
Peroxisome proliferator-activated receptor delta
Peroxisome proliferator-activated receptor beta or delta , also known as NR1C2 is a nuclear receptor that in humans is encoded by the PPARD gene....
. Mature miR-122 is subject to modification by the poly(A) polymerase
Polymerase
A polymerase is an enzyme whose central function is associated with polymers of nucleic acids such as RNA and DNA.The primary function of a polymerase is the polymerization of new DNA or RNA against an existing DNA or RNA template in the processes of replication and transcription...
GLD-2, which adds a single adenosine to the miRNA 3' end. This results in an increase in miR-122 stability.
miR-122 targets
miR-122 regulates the synthesis of the proteinProtein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
CAT-1
SLC7A1
High affinity cationic amino acid transporter 1 is a protein that in humans is encoded by the SLC7A1 gene.-Further reading:...
by binding to sites in the mRNA 3'UTR such that translation is repressed and the mRNA is targeted to P bodies. This repression can be relieved by the protein HuR
Hur
Hur may refer to:* Hur, a Korean family name, also spelled Heo.*HuR, the ubiquitous RNA binding protein also known as Drosophilla ELAVL1*HUR, Ukraine's military intelligence branch*Hurs , are a Muslim Sufi order in Sindh, Pakistan...
, which is released from the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
under conditions of cell stress and binds to the CAT-1 3'UTR. The HuR interaction leads to release of the mRNA from the P bodies and resumption of active translation.
A number of other miR-122 targets, including CD320
CD320
CD320 is a human gene....
, AldoA and BCKDK
BCKDK
[3-methyl-2-oxobutanoate dehydrogenase [lipoamide]] kinase, mitochondrial is an enzyme that in humans is encoded by the BCKDK gene.-Further reading:...
, have been identified by microarray
Microarray
A microarray is a multiplex lab-on-a-chip. It is a 2D array on a solid substrate that assays large amounts of biological material using high-throughput screening methods.Types of microarrays include:...
analysis of changes in mRNA expression in the liver of mice treated with miR-122 inhibitors. The overall effect of miR-122 inhibition is to reduce the plasma cholesterol level, although the pathways involved in this regulation have not been fully elucidated. miR-122 also regulates systemic iron homeostasis
Human iron metabolism
Human iron metabolism is the set of chemical reactions maintaining human homeostasis of iron. Iron is an essential element for most life on Earth, including human beings. The control of this necessary but potentially toxic substance is an important part of many aspects of human health and disease...
via the target mRNAs Hjv
Hemojuvelin
Hemojuvelin is a membrane-bound and soluble protein in mammals that is responsible for the iron overload condition known as juvenile hemochromatosis in humans, a severe form of hemochromatosis. In humans, the hemojuvelin protein is encoded by the HFE2 gene...
and Hfe. miR-122 inhibition in mice or primates does not result in any detectable liver toxicity.
Role in cancer
miR-122 levels are frequently reduced in hepatocellular carcinoma (HCC) compared to normal liver, and low miR-122 levels correlate with poor prognosisPrognosis
Prognosis is a medical term to describe the likely outcome of an illness.When applied to large statistical populations, prognostic estimates can be very accurate: for example the statement "45% of patients with severe septic shock will die within 28 days" can be made with some confidence, because...
. Overexpression of miR-122 reduces tumorigenic properties in HCC cell lines, indicating that it functions as a tumor suppressor gene
Tumor suppressor gene
A tumor suppressor gene, or anti-oncogene, is a gene that protects a cell from one step on the path to cancer. When this gene is mutated to cause a loss or reduction in its function, the cell can progress to cancer, usually in combination with other genetic changes.-Two-hit hypothesis:Unlike...
, and increases the response of cells to the chemotherapeutic drugs sorafenib
Sorafenib
Sorafenib , is a drug approved for the treatment of primary kidney cancer and advanced primary liver cancer ....
and doxorubicin
Doxorubicin
Doxorubicin INN is a drug used in cancer chemotherapy. It is an anthracycline antibiotic, closely related to the natural product daunomycin, and like all anthracyclines, it works by intercalating DNA....
. Several miR-122 target genes have been implicated in tumorigenesis, including ADAM10
ADAM10
Disintegrin and metalloproteinase domain-containing protein 10, also known as ADAM10 or CDw156 or CD156c is a protein that in humans is encoded by the ADAM10 gene.- Function :...
, IGF1R, CCNG1
CCNG1
Cyclin-G1 is a protein that in humans is encoded by the CCNG1 gene.-Interactions:CCNG1 has been shown to interact with P16, Mdm2, PPP2R4 and P53.-Further reading:...
and ADAM17
ADAM17
ADAM metallopeptidase domain 17 , also called TACE , is a 70-kDa enzyme that belongs to the ADAM protein family of disintegrins and metalloproteases.- Chemical characteristics :...
.
Regulation of HCV
Recent studies have shown that replicationViral replication
Viral replication is the term used by virologists to describe the formation of biological viruses during the infection process in the target host cells. Viruses must first get into the cell before viral replication can occur. From the perspective of the virus, the purpose of viral replication is...
of hepatitis C virus (HCV) is dependent on miR-122 expression. miR-122 regulates HCV by binding directly to two adjacent sites close to the 5' end of HCV RNA. Although these experiments were conducted using genotype
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
1a and 1b HCV RNA, the miR-122 binding sites are highly conserved across different genotypes, and miR-122 is also required for replication of infectious type 2a HCV. As miRNAs generally function to repress gene expression by binding to 3'UTR sites, this positive regulation of viral replication via a 5'UTR represents a novel function for miR-122. The mechanism of regulation is not yet clear. miR-122 stimulates translation of HCV RNA, but not to a sufficient extent to explain its effects on viral replication, indicating that a second stage of the viral replication cycle must also be regulated. HCV RNA synthesis is not affected by miR-122, suggesting that regulation of other processes such as RNA stability may occur. The extent to which the miRNA-induced silencing complex (miRISC) is involved in this regulation has not been fully deterined. The Argonaute
Argonaute
Argonaute proteins are the catalytic components of the RNA-induced silencing complex , the protein complex responsible for the gene silencing phenomenon known as RNA interference . Argonaute proteins bind different classes of small non-coding RNAs, including microRNAs , small interfering RNAs and...
proteins (Ago1-4), which are essential for miRNA-directed repression, appear to be necessary for miR-122 to regulate HCV, although miR-122 overexpression may overcome this requirement. Another miRISC component, the DEAD-box RNA helicase DDX6, does not play a role in miR-122-facilitated HCV replication.
The existing HCV therapy of PEG-IFNα plus ribavirin
Ribavirin
Ribavirin is an anti-viral drug indicated for severe RSV infection , hepatitis C infection and other viral infections. Ribavirin is a prodrug, which when metabolised resembles purine RNA nucleotides...
is poorly tolerated and frequently ineffective, so there is an urgent need for new drugs, and miR-122 inhibitors are an attractive possibility. The association between low miR-122 levels and hepatocellular carcinoma suggests that caution will be necessary when testing miR-122 inhibitors, and that long term treatment might be undesirable. However, miR-122 is a promising target as it can be very selectively and effectively inhibited with antisense oligonucleotides, and as it is a conserved host factor it is hoped that the virus would not be able to acquire resistance mutations to an anti-miR-122 therapeutic. Santaris Pharma have developed a locked nucleic acid
Locked nucleic acid
A locked nucleic acid , often referred to as inaccessible RNA, is a modified RNA nucleotide. The ribose moiety of an LNA nucleotide is modified with an extra bridge connecting the 2' oxygen and 4' carbon. The bridge "locks" the ribose in the 3'-endo conformation, which is often found in the A-form...
-based antisense oligonucleotide
Oligonucleotide
An oligonucleotide is a short nucleic acid polymer, typically with fifty or fewer bases. Although they can be formed by bond cleavage of longer segments, they are now more commonly synthesized, in a sequence-specific manner, from individual nucleoside phosphoramidites...
that is delivered to the liver and effectively inhibits miR-122 following intravenous injection without inducing liver toxicity. This molecule reduced HCV viremia
Viremia
Viremia is a medical condition where viruses enter the bloodstream and hence have access to the rest of the body. It is similar to bacteremia, a condition where bacteria enter the bloodstream.- Primary versus Secondary :...
in a small-scale trial in chimpanzees. The inhibitor is known as miravirsen and is in phase II clinical trials with the results expected in early 2012.
Miravirsen is currently undergoing Phase IIa trial with preliminary data presented on November 2011 at The Liver Meeting showing promising results from a group of chronic HCV genotype 1 infection patients. Patients were randomly assigned to placebo
Placebo
A placebo is a simulated or otherwise medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient...
groups or one of the three multiple ascending dose groups (3mg/kg, 5mg/kg, 7mg/kg of miravirsen). The naïve patients were given 5 weekly doses of miravirsen via subcutaneous injection
Subcutaneous injection
A subcutaneous injection is administered as a bolus into the subcutis, the layer of skin directly below the dermis and epidermis, collectively referred to as the...
for four weeks, and 4 out of 9 patients receiving the highest dose (7mg/kg) exhibited significant decrease in Hepatitis C virus
Hepatitis C virus
Hepatitis C virus is a small , enveloped, positive-sense single-stranded RNA virus of the family Flaviviridae...
RNA (p<0.001). The result was sustained even 10 weeks after the last miravirsen administration. Further trials testing for the safety and pharmokinetics of miravirsen will also be conducted in other countries such as Germany and Netherlands.
Use as a Biomarker
miR-122 has recently been explored as a potential biomarker for various hepatic conditions. A change in levels of miR-122 in the blood has been confirmed as an indicator for liver diseaseLiver disease
Liver disease is a broad term describing any single number of diseases affecting the liver.-Diseases:* Hepatitis, inflammation of the liver, caused mainly by various viruses but also by some poisons , autoimmunity or hereditary conditions...
. This change is noted before increased amino-transferase
Transferase
In biochemistry, a transferase is an enzyme that catalyzes the transfer of a functional group from one molecule to another . For example, an enzyme that catalyzed this reaction would be a transferase:In this example, A would be the donor, and B would be the acceptor...
activity, making it a preferable indicator for liver disease. This study also suggested that miR-122 could be a “novel, predictive and reliable blood marker for viral-, alcohol- and chemical-induced liver injury.”
There is a great deal of research into the use of miR-122 as a biomarker for Hepatitis C
Hepatitis C
Hepatitis C is an infectious disease primarily affecting the liver, caused by the hepatitis C virus . The infection is often asymptomatic, but chronic infection can lead to scarring of the liver and ultimately to cirrhosis, which is generally apparent after many years...
. While some studies dispute its efficacy for diagnosing Hep C, other research indicates that it may be useful in diagnosing specific forms of Hepatitis. For instance, decreased levels of miR-122 have been linked to a strain of Hepatitis C that is resistant to interferon
Interferon
Interferons are proteins made and released by host cells in response to the presence of pathogens—such as viruses, bacteria, or parasites—or tumor cells. They allow communication between cells to trigger the protective defenses of the immune system that eradicate pathogens or tumors.IFNs belong to...
therapy. In this case, measuring miR-122 levels in the bloodstream of Hepatitis C positive patients would enable more accurate and personalized therapy for their disease.