miRNAs are short sections of RNA (typically between 20 and 25 nucleotides). Their expression is altered in a variety of diseases including sepsis, cancer and diabetes. This makes miRNAs an interesting category of biomarker with clinical relevance, where a highly sensitive and accurate assay with fast time-to-result would be useful.
miRNAs are typically present in human blood at very low concentrations, in the high femtomolar to low nanomolar range. This is a challenge for technologists wishing to develop new miRNA-based diagnostic tools, as accurate sensing is needed without introducing bias.
Using nanopores to detect and quantify miRNAs
Oxford Nanopore uses the properties of nanopores to sense miRNAs directly. The technology measures the current passing through a nanopore and detects specific alterations in that current when events interrupt the current flow through the nanopore. In the case of miRNAs, the current is interrupted when the aperture of the nanopore is blocked by a probe bound to a target miRNA.
Oxford Nanopore has developed complementary probes to miRNAs of interest. When the probe binds to the target miRNA, the single stranded tail enters the barrel of the nanopore, but the duplex (double stranded) portion temporarily stalls the movement of the bound probe–miRNA complex though the pore. A characteristic current disruption occurs in the partially obstructed pore that is easily detectable by Oxford Nanopore's high-speed electronics.
Certain modifications to the probe bring the range of this assay into the physiological range, allowing the technique to sense miRNAs in clinically relevant samples. Barcodes can be added to the probe tails, which can be read during the duplex-stalling event, allowing multiplexed miRNA sensing using complex sample types such as serum or blood.