Micro RNAs [miRNA] were discovered in worms about 14 years ago, but they have been found in a wide range of organisms (including humans) in the last few years. A subset of our genes encodes miRNAs which are transcribed in the usual fashion creating primary transcripts (pri-miRNA). These long single stranded pri-miRNAs fold back on themselves to create hairpin stem-loop structures.
The hairpins in the pri-miRNA are cleaved into short 70 nucleotide pre-miRNAs through drosha processing. These Pre-miRNAs are then exported from the nucleus. In the cytoplasm, dicer processing takes place where the RNA-induced silencing complex [RISC] binds to the hairpin RNAs and further cuts the RNA to make the final miRNA.
The miRNA is partially complementary to a messenger RNA [mRNA] that has been transcribed. This allows the miRNA/RISC complex to bind to the mRNA and inhibit its translation (production of proteins) either by directly blocking the translation machinery, or by stimulating the degradation of the mRNA. In essence, miRNAs work the same way as transcription factors in that they can control protein expression.
Interestingly, miRNAs have been observed to be overexpressed in some cancers. Increasing miRNAs should decrease protein expression, so you can imagine that these miRNAs might inhibit the expression of tumor suppressors although it is difficult to determine what genes miRNAs target.
How can biotechnology companies make money from miRNAs?
Reagents - The easiest way for scientists to understand the function of a protein is to eliminate it from the cell and see what happens. The synthetic equivalent of miRNAs is small interfering RNAs [siRNAs] which can be used to inhibit translation and thus expression of a protein. Since miRNAs occur naturally in the cell, they may be favored by researchers if for no other reason than that they have been validated (i.e. they have been shown to lower protein levels). Additionally, since understanding the impact of miRNAs on the cell is important, kits that measure the concentration of miRNAs may be marketable.
Diagnostics - Currently protein levels are measured as an indication of disease in many diagnostic tests. Since miRNAs control the protein levels, the amount of miRNA in cells could also be used as a test for diseases like cancer where the levels of miRNAs have been shown to be altered.
Therapeutics - miRNAs could be used to decrease a protein’s level in a diseased cell by overexpressing the miRNA that controls it. Conversely the level of the protein could also be up-regulated by inhibiting the miRNA in the cells with molecules that are capable of binding to (and inhibiting) the endogenous miRNAs.
Are miRNAs going to cure every disease? Probably not, but I’m looking forward to digging deeper into Rosetta Genomics' plan for monetizing their intellectual property.