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Human body is made up of proteins that are encoded by the DNA.
Our existence is completely dependent on the function of these proteins. The food we take is digested and made available to the body by the action of enzymes, indeed proteins. They perform various functions like catalyzing metabolic reactions, responding to stimuli, and transporting molecules from one location to another.
Both, over expression and under expression of proteins is harmful and hence, has to be controlled.
The class of molecules which control gene expression are called gene regulatory molecules.
Caenorhabditis elegans is a free-living (not parasitic), transparent nematode (roundworm), about 1 mm in length, that lives in temperate soil environments.
Victor Ambros, Rosalind Lee and Rhonda Feinbaum.
Length of mRNA: 1500-200nt in eukaryotes
Drosha: Drosha is a Class 2 ribonuclease III enzyme
DGCR8: The DGCR8 microprocessor complex subunit (DiGeorge syndrome chromosomal [or critical] region 8) is a protein that in humans is encoded by the DGCR8 gene. DGCR8 is localized to the cell nucleus and is required for microRNA (miRNA) processing. It binds to Drosha, an RNase III enzyme, to form the Microprocessor complex that cleaves a primary transcript known as pri-miRNA to a characteristic stem-loop structure known as a pre-miRNA.
Dicer: Dicer, also known as endoribonuclease Dicer or helicase with RNase motif, is an enzyme that in humans is encoded by the DICER1 gene. Being part of the RNase III family, Dicer cleaves double-stranded RNA (dsRNA) and pre-microRNA (pre-miRNA) into short double-stranded RNA fragments called small interfering RNA and microRNA respectively. Dicer facilitates the activation of the RNA-induced silencing complex (RISC), which is essential for RNA interference. RISC has a catalytic component argonaute, which is an endonuclease capable of degrading messenger RNA (mRNA).
RISC contains two enzymes: Slicer/TRNC6 and Argonaute. Argonaute has two domains: PIWI domain consisting of RNase H activity, PAZ domain required for the attachment with one strand of RNA.
The strand that binds with PAZ domain is called the Guide strand and the other is called the Passenger strand.
The passenger is degraded using the RNase H activity and the guide strand i.e., the miRNA is released into the cytoplasm.
• Introduction of microRNA
• Discovery of microRNA
• Micro RNA biogenesis
• Mechanism of action of microRNA
• miRNAs are non-coding RNAs found only in
• Single stranded and small in size (˷22 nt long)
• Comprise one of the more classes of gene regulatory
• Play important regulatory roles in animals and plants
by targeting mRNAs for cleavage or translational
Discovery of miRNA
• The first miRNA was discovered in 1993 during a study of the lin-4
gene, which was known to control the timing of C. elegans larval
development by repressing the lin-14 gene.
• When they isolated the lin-4 gene, they found that instead of
producing an mRNA encoding a protein, it produced short
noncoding RNAs, one of which was a ~22-nucleotide RNA that
contained sequences partially complementary to multiple
sequences in the 3' UTR of the lin-14 mRNA.
• This complementarity was proposed to inhibit the translation of the
lin-14 mRNA into the LIN-14 protein
• At the time, the lin-4 small RNA was thought to be a nematode
• Only in 2000 was a second small RNA characterized: let-7 RNA,
which represses lin-41 to promote a later developmental transition
in C. elegans.
• The let-7 RNA was soon found to be conserved in many species,
leading to the suggestion that let-7 RNA and additional "small
temporal RNAs" might regulate the timing of development in
diverse animals, including humans.
• A year later, the lin-4 and let-7 RNAs were found to be part of a very
large class of small RNAs present in C. elegans, Drosophila and
• The many newly discovered RNAs of this class resembled the lin-4
and let-7 RNAs, except their expression patterns were usually
inconsistent with a role in regulating the timing of development,
which suggested that most might function in other types of
regulatory pathways. At this point, researchers started using the
term “microRNA” to refer to this class of small regulatory RNAs.
MicroRNA expression in human cancer
• oncogenes or oncosuppressor genes
o Presence of miR‐15a and miR‐16‐1 instead of tumor
supressor gene in deleted chromosome in CLL
o Increased expression of the microRNA
Tumor supressor gene is not all expressed because of
inhibition by miRNA.
o cell migration and metastasis
• Regulator of fatty-acid metabolism
• MiR-122 - hepatocellular carcinoma
• Regulation of hepatitis C virus replication
• Hematopoiesis & Immune system
• Cardiovascular diseases
• Lymphocyte malignancies
• First microRNA as an oncomir
• MiR-21- failing murine and human heart
• chemically modified and cholesterol-conjugated miRNA inhibitors
(antagomirs) was shown to inhibit interstitial fibrosis and improve cardiac
• miR-92a in acute myeloid leukaemia and acute lymphoblastic leukaemia
• miR-92 hepatocellular Cancer
• MiR-92a in neoangiogenesis
• Lipid metabolism
o Cholesterol and HDL generation
o Fatty acid degradation
o MiR-33 & miR-122 in metabolic disorders
and cardiovascular diseas
• miR-451- hematocrit(erythroid differentiation defect) -
Challenges for Micro RNA therapy
• Numerous molecular targets for miRNA
• Degradation by nucleases and phosphodiesterases
• Optimal chemistry and delivery systems have to be
• E. van Rooij, A.L. Purcell, A.A. Levin, Developing
microRNA therapeutics, Circulation research, 110 (2012)
• E. van Rooij, W.S. Marshall, E.N. Olson, Toward
MicroRNA–Based Therapeutics for Heart Disease The
Sense in Antisense, Circulation research, 103 (2008) 919-
• M.S. Ebert, J.R. Neilson, P.A. Sharp, MicroRNA sponges:
competitive inhibitors of small RNAs in mammalian cells,
Nature methods, 4 (2007) 721-726.