What is "isomir"

n. (context genetics English) A variant form of miRNA

isomiR is a term created by Morin et al. to refer to those sequences that have variations with respect to the reference MiRNA sequence. miRBase is considered to be the gold-standard miRNA database—it stores miRNA sequences detected by thousand of experiments. In this database each miRNA is associated with a miRNA precursor and with one or two mature miRNA (-5p and -3p). Similarly, VIRmiRNA is the only available resource for miRNA in the field of virus. It not only contains viral miRNAs but also harbors their targets. It also provides extensive list of anti-viral microRNAs encoded by human to provide them immune defense against viral infections. Currently this resource harbors 9133 records; out of which there are 1308 experimentally validated viral miRNAs from 44 viruses In the past it had always been said that the same miRNA precursor generates the same miRNA sequences. However, the advent of deep sequencing has now allowed researchers to detect a huge variability in miRNA biogenesis, meaning that from the same miRNA precursor many different sequences can be generated potentially have different targets. It has been found that isomiR expression profiles can also exhibit race, population, and gender dependencies.

There are four main variation types:

• 5' trimming—the 5' dicing site is upstream or downstream from the reference miRNA sequence
• 3' trimming—the 3' dicing site is upstream or downstream from the reference miRNA sequence
• 3' nucleotide addition—nucleotides added to the 3' end of the reference miRNA
• nucleotide substitution—nucleotides changes from the miRNA precursor. It is thought that may be similar process than post-transcriptional modifications.