MicroRNA: a small molecule with a big biological impact.

One of the most significant achievements in biological science in the last decade is the discovery of RNA interference (RNAi), a process within living cells that regulates gene expression at post-transcriptional levels. Historically, this process was described by other more generic names, such as co-suppression and post transcriptional gene silencing. Only after the molecular mechanism underlying these apparently unrelated processes was fully understood did it become apparent that they all described the RNAi phenomenon. In 2006, Dr. Andrew Fire and Dr. Craig C. Mello were awarded the Nobel Prize in Physiology or Medicine for their work on RNAi interference. RNAi is an RNA-dependent gene silencing process that is controlled by the RNA-induced silencing complex (RISC) and is initiated by two types of small RNA molecules - microRNA (miRNA) and small interfering RNA (siRNA). However, the function of microRNA appears to be far beyond RNAi alone, including direct interaction with the gene promoter and epigenetic regulation of the DNA methylation and histone modification. By regulating gene expression, miRNAs are likely to be involved in diverse biological activities, such as tumorigenesis, immune response, insulin secretion, neurotransmitter synthesis, and circadian rhythm, to name a few. MicroRNAs are 21-23 nucleotide single stranded RNA molecules found in eukaryotic cells. The first miRNA, lin-4, was characterized in C. elegans in the early 1990s [1]. In the early years, the progress on microRNA research was slow and experienced substantial growing pains. The short length and uniqueness of each microRNA rendered many conventional hybridization based methods ineffective; very small RNAs are difficult to reliably amplify or label without introducing bias. In addition, hybridization-based methods for microRNA profiling relied on probes designed to detect known microRNAs or known microRNA species previously identified by sequencing or homology search. Recent evidence of target-directed editing of mature microRNA (trimming and tailing by 3'-to-5' exonuclase and terminal nucleotide transferase) [2] further highlighted the complexity of microRNA processing and regulation mechanisms. Moreover, the wide range of microRNA expression, from tens of thousands to just few molecules per cell, complicated the detection of microRNAs expressed at low copy numbers. Hence, many novel microRNAs may exist even in well-explored species. Nevertheless, recent advances in genomic technologies and data analysis / bioinformatics approaches have made a significant impact on microRNA research. For example, next generation deep sequencing platforms are ideal for detecting and quantifying both known and novel microRNA sequences with high sensitivity and for a relatively low cost [3]. The microRNA field has experienced a major explosion in recent years. The microRNA gene family is continuously growing with novel members discovered in association with rapid advances in genomic technologies, and reports on the functional characterizations of specific microRNA genes have been dominating the recent literature. We devote this new journal, MicroRNA, to the rapidly advancing field of microRNA research. We dedicate our new journal to the scientists who work tirelessly on this family of small molecules, and their immense contributions to the biological sciences. MicroRNA publishes letters, full-length research articles, review articles, drug and clinical trial studies and thematic issues on all aspects of microRNA research. The scope of the journal covers all experimental microRNA research and applied research in the fields of health and disease, including therapeutic, biomarker, and diagnostic applications of microRNA.