Expression Patterns
Expression patterns of many miRNAs are timing and organ specific, especially those involved in the regulation of development. miRNA libraries constructed from cloning and sequencing short RNAs derived from cultures of mouse embryonic stem cells have shown miR-92 to be expressed. These totipotent cells represent the earliest stages of mammalian development (they are derived from the inner cells of the blastocyst ). However, mir-92 was also present in libraries constructed from four day old fully differentiated cells, as well as libraries constructed from adult tissues. An miRNA that remains constant in its expression through these stages is proposed to have a role in regulating general aspects of cell physiology. Thus it was becoming evident in 2003 shortly after its discovery, that the mir-92 miRNA and associated family members are providing functional roles to the cell cycle and to cell signalling, and to general cell physiology.
As there is a relatively small number of miRNAs encoded in the human genome compared with the number of protein coding genes, miRNAs are becoming the biomarkers of choice to characterise the state of a tissue or cell sample. Also there are no mRNA markers that show consistent differential expression between tumours and normal tissues but profiling all of the miRNAs has proved much more informative with respect to cancer diagnosis and its developmental origins. The extent of differential expression of miRNAs is even high enough that hierarchical clustering has enabled petitioning of samples within a single developmental lineage reflecting mechanisms of transformation and providing miRNA finger prints that encode the development of typical human cancers. As mir-92 is expressed in the majority of cells at all times, its expression profiles will form a reliable part of diagnosis and detection of disease. miR-92a is also present in the blood plasma of humans along with 91 other miRNAs.
The level of miR-92a in the circulation has been shown to reflect the onset of certain types of leukaemia (see experimental evidence section).
Read more about this topic: Mir-92 Micro RNA Precursor Family
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