This chapter covers the complete life cycle of microRNAs, from start to finish. Beginning with their location in the genome, how they are transcribed and some of the factors that switch microRNAs on and off, it moves next to the biochemical steps involved in the step-wise processing of the precursor RNA by the enzymes Drosha and Dicer, before the microRNA is eventually loaded into a pocket in the Argonaute protein ready to carry out gene silencing. For some steps, a deeper look is taken into the atomic structures of these biological nanomachines and how they pivot and adjust to join together proteins and RNA as they perform their functions. This includes the remarkable search strategy by which the gene silencing complex containing a microRNA probes for binding sites on mRNA targets. Finally comes the molecular decision-making behind how much protein is reduced and by what mechanism. The when and how a microRNA knows its work is done and what finally extinguishes its effects. Again, the chapter conveys the intense competition among scientists vying to answer the next question and the one after that, which was a potent accelerant for discovery.

Having covered the discovery of microRNAs, the expansion of their universe, the cataloguing of their presence in the kingdoms of life, how they control gene activity and why this is so important, and finally how they are applied in science and medicine, we come to the end. Here, there is an opportunity to ask, ‘What’s next?’ What are some of the most exciting directions in current microRNA research and what lies over the horizon? This final chapter explores some of the latest questions. What is the totality of the influence of microRNAs in the most complex systems in the body and what technologies will we use or need to answer these questions? Are there components of the microRNA pathway still to be found? Some of the most advanced applications of microRNAs are in the field of synthetic biology, where microRNAs can be useful in engineered cells and systems. After such a richness of discovery about microRNAs during development, research is now asking questions about what microRNAs do towards the ends of our lives. Finally, a speculation about whether microRNAs or molecules like them exist beyond the borders of Earth, wherever else life is found in the universe.

Epilepsy is one of the most common neurological disorders, affecting people of all ages. This chapter focusses on what has been learnt about the microRNA system in this important disease. Starting with an overview of epilepsy, it addresses what causes seizures to occur and some of the underlying mechanisms, including gene mutations and brain injuries. It explores how and which microRNAs drive complex gene changes that underpin but also oppose the enduring hyperexcitability of the epileptic brain. This includes by regulating amounts of neurotransmitter receptors, structural components of synapses, metabolic processes and inflammation. It also covers some of the earliest studies linking microRNAs to epilepsy as well as recent large-scale efforts to map every microRNA and its target in the epileptic brain. Finally, it highlights ways to model epilepsies and use of experimental tools such as antisense oligonucleotides to understand the contributions of individual microRNAs. Collectively, these studies reveal how microRNAs contribute to the molecular landscape that underlies this disease and offer the exciting possibility of targeting microRNAs to treat genetic and acquired epilepsies.

The process of how we get from gene to protein is one of the most intensely studied and best understood in biology. The reading of DNA, the generation of a messenger ribonucleic acid (mRNA) and the translation of that transcript into a protein through assembling chains of amino acids. But what we thought we knew about the gene pathway changed forever in 1993, when Gary Ruvkun and Victor Ambros discovered microRNAs. This chapter begins by explaining the basic biochemistry of genes and proteins before moving on to the seminal work of 30 years ago. The objective of those experiments was to understand which genes controlled the timing of animal development in a worm called Caenorhabditis elegans. That led to the realisation that a gene called lin−4, crucial for worms to transition from juvenile to adult stages, did not code for a protein; instead, its RNA acted by sticking to the mRNA of a protein-coding gene. Lin−4 was a gene silencer, working to lower the amounts of protein in cells. The finding of a new step on the journey from gene to protein would go on to transform our understanding of the biology of living organisms.

Seven years passed since the discovery of lin−4’s unique properties and then, around the turn of the millennium, the research floodgates opened. This chapter tracks the nascent field of microRNA research, the frenetic race to discover and catalogue new microRNAs and find the organisms in which they were made. MicroRNAs held a prominent position in evolution, their number and diversity expanding at key transitions to more complex life, including for our own species, Homo sapiens. MicroRNAs, it would become clear, are the genome’s solution to how to control the natural fluctuations, randomness and noise in gene expression. The chapter also covers the pivotal experiments that laid the ground rules for how microRNAs work and revealed their effects on gene expression. Along the way, a selection of the scientific toolkit gets special attention, including some of the models used to find microRNAs and the technologies that would prove that microRNAs, despite their small size and limited number in genomes, controlled the vast majority of gene activity in our cells.

Shortly after microRNAs were discovered in humans they were found to be present in blood samples. This led to another branch of microRNA research with the potential to transform medicine, answering the question healthcare professionals ask every day. What’s wrong with my patient? This chapter introduces circulating microRNAs as biomarkers and their emergence as potential diagnostic tools. Core arguments in their favour as indicators of health and disease include tissue specificity, their known locations in the body enabling doctors to zero in on where a problem lies. It looks at what shelters microRNAs as they circulate in the bloodstream and the disruptive thinking that has interpreted such findings as evidence that extracellular microRNAs are conveyors of information between distant tissues in the body. It moves to efforts to probe ever-smaller volumes of biofluids to find the least-invasive source of microRNA biomarkers and the diseases for which microRNA-based diagnostic tests already exist or may emerge in the future. Finally, it looks at developments in RNA detection technology that might allow point-of-care testing and perhaps microRNA-based health monitoring at home.

MicroRNAs were discovered during experiments designed to learn how genes coordinate animal development. This chapter begins with the early studies that taught us the importance of microRNAs for mammalian development by studying what happened when key genes were deleted in mice. It ranges from studies that knocked out genes from the entire organism towards refined approaches that removed microRNAs at defined moments from specific tissues, including the heart and the visual system. A detailed review is taken of the genes that microRNAs regulate during brain development and their contribution to the diversity of cell types. These studies reveal the essential role for the microRNA system broadly, as well as how certain developmental events are more or less tolerant of disruption to the microRNA system. This chapter also reviews which microRNAs are the first to control gene activity after fertilisation and how environmental and parental experience can change microRNA activity. The chapter also includes explanations of the scientific toolkit needed to delete or deliver biogenesis components and microRNA genes, and how microRNAs have been used as tools in stem cell research.

The genome is the totality of information that directs the making and the maintenance of you and every other living organism. Scattered among the familiar genes that code for the proteins of life are other genes. This is a book about the genes we call microRNA. It is 30 years since their discovery. They are gene regulators, every bit as vital as their more famous gene cousins. MicroRNAs fine-tune how much protein is made in our cells, each one coordinating the activity of hundreds of genes and bringing precision to the ‘noise’ of gene expression. Without them, life is virtually impossible. This introduction provides a personal account of what fascinated the author about these genes enough to make him redirect his research to microRNAs. The journey from studying pharmacology in the UK, to the USA where his interest in the brain disease epilepsy began, and later to Dublin, to work at the Royal College of Surgeons in Ireland. It lays out the contents and style of the book, which is part history of science, describing what we know and the experiments that underpin our understanding, and part memoir of the author’s own research, and the applications of microRNAs in medicine.

The brain contains a greater diversity and abundance of microRNAs than any other organ in the body. MicroRNAs stay busy long after they’ve coordinated brain development, but doing what? In the brain, microRNAs serve two somewhat contradictory roles: enforcing the stable patterns of genes that define mature circuits while at the same time conferring the same cells with the flexibility to adapt to changing information. This chapter begins with the basic principles of brain function and some early discoveries on microRNAs in the brain. It explores how the microRNA system influences learning, memory and emotions. It also looks at the evidence that a rich and diverse pool of microRNAs contributed to evolved intelligence. It explains the molecular cues that signpost microRNAs to go to synapses, and how the amount of microRNA activity is linked to the incoming strength of signals. It then looks in depth at some specific microRNAs and their targets and how their competing actions adjust the strength of contacts between neurones. Finally, it looks at how genetic variation and erroneous amounts of certain microRNAs may contribute to risk of neuromuscular and psychiatric disease.

In 2010, only a decade since microRNAs were discovered in humans, the first patient was treated with a microRNA drug, miravirsen, for hepatitis C virus (HCV) infection. This chapter opens with the discovery that HCV contained binding sites for miR−122, an abundant liver-specific microRNA. It looks at the research showing how the virus hijacks miR−122 to replicate, and the groundbreaking drug development programme that took advantage of this to create the world’s first medicine to target a microRNA. It covers some of the microRNA-based therapies further back in the drug development pipeline, discussing the relative strengths but also the risks of this approach. It explores the method to target microRNAs, including recent developments to disrupt single microRNA–target interactions to create precision microRNA therapies, and the viruses being commandeered to deliver microRNA treatments into specific cell types in the body. Lastly, it looks at how new microRNAs are being identified and considers the future of microRNA-based treatments, focussing on prospects for neurological disorders and reflecting on how, by listening to patients, we can create better and safer medicines.