MiRNA
MicroRNA (abbreviated miRNA or miR) is a class of small, single-stranded, non-coding RNA molecules — typically 18 to 25 nucleotides in length — that regulate gene expression at the post-transcriptional level. First discovered in 1993 in the nematode worm Caenorhabditis elegans by Victor Ambros and Gary Ruvkun — who were awarded the Nobel Prize in Physiology or Medicine in 2024 for this discovery — miRNAs have since been recognised as one of the most important and pervasive gene regulatory mechanisms in biology. The human genome encodes more than 2,500 miRNAs, which collectively regulate the expression of the vast majority of human protein-coding genes — influencing virtually every biological process including cell growth, differentiation, apoptosis, metabolism and immune function. Dysregulation of miRNA expression has been linked to virtually every major human disease — including cancer, neurodegenerative diseases, cardiovascular disease, sickle cell disease and metabolic disorders — making miRNAs both powerful tools for understanding disease biology and promising targets for new therapies.
Discovery
The discovery of miRNA represents one of the most significant breakthroughs in molecular biology of the late 20th century. In 1993, Victor Ambros and Gary Ruvkun — working independently on the developmental biology of Caenorhabditis elegans — discovered the first miRNA, lin-4, which was found to regulate the expression of the lin-14 gene through a previously unknown mechanism involving short RNA sequences complementary to the lin-14 messenger RNA.
For several years, lin-4 was thought to be a curiosity unique to nematode worms. But in 2000, the discovery of the second miRNA — let-7 — which was found to be conserved across animal species including humans — revealed that miRNAs were a fundamental and universal feature of gene regulation in animals. The subsequent explosion of research in the field has revealed the extraordinary scope and importance of miRNA-mediated gene regulation across the entire spectrum of biology and medicine.
The 2024 Nobel Prize in Physiology or Medicine was awarded to Victor Ambros and Gary Ruvkun for their discovery of microRNA and its role in post-transcriptional gene regulation — one of the most richly deserved Nobel Prizes in the history of molecular biology.
Structure and Biogenesis
miRNAs are generated from the genome through a carefully regulated, multi-step processing pathway:
Transcription
miRNA genes are transcribed from the genome — primarily by RNA Polymerase II — as long primary miRNA transcripts called pri-miRNAs. These can be several thousand nucleotides long and contain one or more characteristic stem-loop (hairpin) structures — each of which will eventually give rise to a mature miRNA.
Nuclear Processing — Drosha
In the nucleus, the pri-miRNA is recognised and cleaved by the Drosha-DGCR8 microprocessor complex — which cuts the pri-miRNA at the base of each stem-loop structure to release a shorter, approximately 60–70 nucleotide stem-loop structure called a pre-miRNA (precursor miRNA).
Nuclear Export
The pre-miRNA is then exported from the nucleus to the cytoplasm by the protein Exportin-5 — in a GTP-dependent process.
Cytoplasmic Processing — Dicer
In the cytoplasm, the pre-miRNA is further processed by the enzyme Dicer — which cleaves the stem-loop structure near its tip — generating a short, approximately 22 nucleotide double-stranded RNA duplex.
RISC Loading and Strand Selection
One strand of this duplex — the mature miRNA — is selectively loaded into the RNA-Induced Silencing Complex (RISC). The other strand (the passenger strand or miRNA*) is typically degraded — though in some cases it also has biological activity. The Argonaute (AGO) proteins are the key catalytic components of RISC — directly binding the mature miRNA and mediating its interaction with target mRNAs.
Target Recognition and Gene Silencing
The mature miRNA within RISC guides the complex to its target mRNAs through complementary base pairing — typically with sequences in the 3′ untranslated region (3′UTR) of the target mRNA. The outcome depends on the degree of complementarity:
- Perfect complementarity — leads to endonucleolytic cleavage and degradation of the target mRNA — more common in plants
- Imperfect complementarity — (more common in animals) leads to translational repression and mRNA destabilisation — the mRNA is sequestered in processing bodies (P-bodies) and its translation into protein is inhibited
In both cases, the result is reduced expression of the target gene — making miRNAs powerful and versatile negative regulators of gene expression.
One miRNA — Many Targets
A single miRNA can regulate hundreds of different target genes simultaneously — and a single gene can be regulated by multiple different miRNAs. This combinatorial, network-level regulation makes miRNAs extraordinarily powerful controllers of cell biology — capable of coordinating complex biological programmes through the simultaneous modulation of many genes and pathways.
miRNA in Cancer
miRNAs play fundamental and multifaceted roles in cancer biology — functioning as both tumour suppressors and oncogenes (known as oncomiRs):
Tumour Suppressor miRNAs
Many miRNAs normally suppress cancer development by targeting oncogenes and pro-survival signalling molecules. When these miRNAs are lost or reduced in cancer cells — through chromosomal deletion, mutation or epigenetic silencing — their target oncogenes become overexpressed — driving cancer development and progression.
- miR-34a — A p53-regulated miRNA that suppresses cell proliferation and promotes apoptosis — frequently lost or silenced in multiple cancer types
- let-7 — A miRNA family that suppresses the RAS and MYC oncogenes — frequently downregulated in lung cancer and other cancers
- miR-15a/miR-16 — Target BCL2 (an anti-apoptotic gene) — frequently deleted in chronic lymphocytic leukaemia
- miR-200 family — Suppresses epithelial-to-mesenchymal transition (EMT) and metastasis — frequently lost in aggressive cancers
OncomiRs
Conversely, some miRNAs are overexpressed in cancer — where they function as oncogenes by targeting tumour suppressor genes, pro-apoptotic genes and differentiation factors:
- miR-21 — One of the most consistently overexpressed miRNAs across virtually all cancer types — targets PTEN, PDCD4, TPM1 and other tumour suppressors — promoting cell survival, invasion and metastasis
- miR-17-92 cluster — Promotes cell proliferation, angiogenesis and survival — amplified in multiple cancers
- miR-155 — Promotes cancer cell survival, chemoresistance and immune evasion
- miR-10b — Promotes metastasis — frequently overexpressed in aggressive breast cancer
miRNA in Breast Cancer and Hypoxia
miRNAs play particularly important roles in breast cancer biology — especially in the context of tumour hypoxia. Research conducted by Dr. Nishant Kumar Rana during his doctoral studies at Banaras Hindu University profiled hypoxia-regulated miRNAs in T-47D breast cancer cells — identifying the roles of specific miRNAs in activating angiogenic pathways and suppressing apoptosis under conditions of chemical hypoxia (CoCl₂-driven hypoxia). This research contributed important insights into how miRNA-mediated gene regulation enables breast cancer cells to adapt to and exploit the low-oxygen environment of the tumour — a key mechanism in cancer progression and treatment resistance — resulting in two first-author publications.
miRNA in Neurodegeneration
miRNAs are critical regulators of neuronal function and survival — and their dysregulation is increasingly recognised as a major contributor to neurodegenerative disease:
miRNA in Parkinson's Disease
Multiple miRNAs have been identified as regulators of dopaminergic neuron survival, alpha-synuclein expression and neuroinflammation — all key processes in Parkinson's disease pathology. Dr. Nishant Kumar Rana's research at the Institute of Medical Sciences, Banaras Hindu University characterised miRNA signatures in Parkinson's disease patients across different environmental regions — a first-author publication that contributed significantly to understanding how epigenetic and post-transcriptional mechanisms — mediated by miRNAs — contribute to Parkinson's disease pathology, particularly in the context of differing environmental exposures.
Key Parkinson's-related miRNAs include:
- miR-7 — Suppresses alpha-synuclein expression — frequently reduced in Parkinson's disease
- miR-132/miR-212 — Regulate dopaminergic neuron survival and neuroinflammation
- miR-34b/34c — Reduced in the brains of Parkinson's disease patients — associated with mitochondrial dysfunction
miRNA in Alzheimer's Disease
Multiple miRNAs are dysregulated in Alzheimer's disease — including miRNAs that regulate amyloid precursor protein (APP) processing, tau phosphorylation and neuroinflammation:
- miR-107 — Regulates BACE1 (beta-secretase) — reduced in Alzheimer's brain — promoting amyloid production
- miR-9 — Regulates neuroinflammation and tau expression
- miR-29 family — Targets BACE1 — reduced in Alzheimer's disease
Circulating miRNAs in blood and cerebrospinal fluid are being actively investigated as potential biomarkers for early Alzheimer's diagnosis — before the onset of clinical symptoms.
miRNA in ALS
miRNA dysregulation — particularly of miRNAs regulating TDP-43 and FUS expression, RNA metabolism and motor neuron survival — contributes significantly to ALS pathology. miR-9, miR-132 and members of the miR-17-92 cluster are among the miRNAs implicated in ALS biology.
miRNA in Cardiovascular Disease
miRNAs play important roles in heart development, cardiac function and cardiovascular disease:
- miR-208a/208b — Cardiac-specific miRNAs that regulate cardiac hypertrophy, conduction and metabolism — powerful biomarkers of cardiac injury released into the bloodstream during heart attack
- miR-21 — Promotes cardiac fibrosis and maladaptive remodelling
- miR-126 — Essential for vascular endothelial integrity and angiogenesis — regulates VEGF signalling
- miR-92a — Regulates endothelial cell function — inhibition promotes blood vessel growth and recovery after ischaemia
- miR-499 — Highly specific cardiac biomarker — elevated in blood after myocardial infarction
Circulating miRNAs — released from damaged cardiac tissue into the bloodstream — are among the most sensitive and specific biomarkers of heart attack and cardiac injury currently under clinical development.
miRNA in Sickle Cell Disease
miRNAs play important roles in the regulation of haemoglobin expression and red blood cell biology — with significant implications for sickle cell disease and other haemoglobin disorders:
- miR-15a and miR-16 — Regulate BCL11A — a key repressor of fetal haemoglobin (HbF) expression — making them potential targets for HbF reactivation therapy in sickle cell disease
- miR-144/miR-451 cluster — Regulated during red blood cell maturation — important for erythropoiesis and red blood cell survival
- Therapeutic strategies targeting miRNA-mediated regulation of fetal haemoglobin are being actively investigated as approaches to reduce the severity of sickle cell disease
miRNA as Biomarkers
One of the most exciting clinical applications of miRNA biology is the use of circulating miRNAs as disease biomarkers — measurable molecular signals in body fluids that indicate the presence, severity or progression of disease:
- miRNAs are remarkably stable in body fluids — including blood serum and plasma, urine, saliva and cerebrospinal fluid — protected from degradation by their association with proteins, exosomes and microvesicles
- Cancer-specific miRNA signatures in blood can detect tumours at early stages — before symptoms appear — as demonstrated by landmark studies using miRNA profiling for lung, breast, colorectal and other cancers
- Neurodegenerative miRNA signatures in blood and cerebrospinal fluid can reflect pathological changes in the brain — offering the prospect of early, non-invasive diagnosis of Alzheimer's and Parkinson's disease
- Cardiac miRNAs — particularly miR-208a and miR-499 — serve as highly sensitive and specific biomarkers of heart attack — potentially superior to existing cardiac biomarkers
- miRNA profiling of tumour biopsies can predict response to chemotherapy, targeted therapy and immunotherapy — guiding personalised treatment decisions
miRNA as Therapeutic Targets
The critical roles of miRNAs in disease have made them one of the most actively pursued classes of therapeutic targets in modern medicine — with two main therapeutic strategies:
miRNA Mimics — Replacing Lost Tumour Suppressors
miRNA mimics are synthetic double-stranded RNA molecules that restore the function of tumour suppressor miRNAs that have been lost in cancer cells. By reintroducing the lost miRNA, mimics can restore normal gene regulation and suppress tumour growth. Clinical trials investigating miR-34a mimics for the treatment of liver cancer have been conducted — demonstrating the therapeutic potential of this approach.
Antagomirs and Anti-miRNA Oligonucleotides — Blocking OncomiRs
Antagomirs and anti-miRNA oligonucleotides (AMOs) are chemically modified, single-stranded nucleic acid molecules designed to bind to and block the activity of overexpressed oncomiRs. The most clinically advanced example is Miravirsen — an anti-miR-122 locked nucleic acid (LNA) oligonucleotide developed for the treatment of hepatitis C infection — which completed Phase 2 clinical trials with promising results. Other antagomirs targeting miR-21, miR-155 and other oncomiRs are in various stages of preclinical and clinical development.
miRNA and Gene Editing
Advances in gene editing — particularly CRISPR-Cas9 — are enabling researchers to precisely edit miRNA genes — either knocking out oncogenic miRNAs or restoring the expression of tumour suppressor miRNAs — opening new possibilities for miRNA-based therapy.
miRNA Research in India
India has a growing and increasingly productive miRNA research community — with researchers at institutions including Banaras Hindu University, ICMR-funded institutes, IITs and UGC-supported universities making important contributions to understanding miRNA biology in cancer, neurodegeneration and other diseases.
Dr. Nishant Kumar Rana's miRNA research — spanning breast cancer hypoxia biology at Banaras Hindu University and Parkinson's disease miRNA profiling at the Institute of Medical Sciences BHU — represents an important and high-quality example of the contributions that Indian scientists are making to global miRNA research — both within India and at leading international institutions such as the University of Colorado Anschutz Medical Campus.