MicroRNAs (miRNAs) are tiny but critical regulators of post-transcriptional gene expression that have been linked to various cellular processes and are associated with a number of disease pathologies. However, after more than 20 years since the discovery of the first animal miRNA, the exact number of sequences coding for these regulatory molecules continues to perplex scientists.

Now, researchers from Thomas Jefferson University in Philadelphia have released data that adds another 3,707 novel miRNA sequences to the human genome, in addition to the 1,900 sequences previously described. The results from this study were published recently in PNAS through an article entitled “Analysis of 13 cell types reveals evidence for the expression of numerous novel primate- and tissue-specific microRNAs”.

“By analyzing human deep-sequencing data, we discovered many new locations in the human genome that produce miRNAs. Our findings effectively triple the number of miRNA-generating loci that are now known,” explained Isidore Rigoutsos, Ph.D., director of the Computational Medicine Center at Thomas Jefferson University and senior author on the current study. “This new collection will help researchers gain insights into the multiple roles that miRNAs play in various tissues and diseases.”

Dr. Rigoutsos and his team gathered samples over a three-year period from an array of healthy and diseased individuals. The study included samples from 13 human tissues types, including brain, pancreas, prostate, breast, and blood. The combination of the samples that the Jefferson team collected and the publicly available sequence data, allowed them to analyze more than 1,300 total samples. The results from their analyses revealed 3,356 new sites within the human genome that generated the 3,707 previously undescribed miRNAs. 

Additionally, Dr. Rigoutsos’ team looked at the newly discovered miRNAs association with the protein Argonaute, which is a key part of the regulatory complex that allows miRNA to interact with its target sequence. The investigators found that almost 50% of the miRNAs were associated with Argonaute, which is indicative that these molecules are vital for gene expression.  

“We anticipate that many more of the newly discovered miRNAs will be found loaded on Argonaute as additional such data become available for the other tissues,” stated Eric Londin, Ph.D., assistant professor at Thomas Jefferson University and co-first author of the current study.

Since the researchers decided not to limit their bioinformatic searches to only conserved sequences, they were able to broadly gather interesting and potentially valuable data about the novel miRNAs they discovered. Specifically, they found that 57% of the sequences were specific to humans and that almost 95% of those sequences were found only in primates.  

“Advances in sequencing technology of the last several years made it easier to generate more data, from more tissues, and do so faster,” said Dr. Rigoutsos “Investigating the alluring possibility that miRNAs with important roles might exist only in humans was within reach. And this is what we set out to do.”

Dr. Rigoutsos and his team are optimistic that the primate and tissue specificity of the novel miRNAs will have a significant impact on the scientific community’s knowledge toward the etiology of disease.








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