Asymmetric distribution of miRNA's in the rat brain

MicroRNA's are non-coding strands of ~20 nucleotides that regulate mRNA activity by partially base pairing to certain complementary strands and inhibiting translation. Mutations in miRNA's have been linked to schizophrenia by Feng et al, who found that 8 out of 193 patients with schizophrenia had a mutation in a miRNA on their X chromosome as opposed to only 1 out of 191 control patients. Now Olsen et al have shown via miRNA isolation in rat brains that the distribution of micro RNA's is variable throughout the brain and is clustered according to biological activity. The researchers conducted unsupervised hierarchical clustering on their normalized expression data to compute the similarity of miRNA expression data in various regions of the brain. Longer branch lengths correspond to more variability in branch length. Here are the results of their cluster dendogram:

"Cb" refers to cerebellum, "hip" to hippocampus, "am" to amygdala, "hyp" to hypothalamus, and "sn" to substantia nigra.

This may be an evolutionary accident but more likely the miRNA's regulate cerebellar or forebrain-specific activities. The authors suggest particularly that the miRNA's may have a role in neural development, having some sort of interaction with insulin-like growth factor 1. But if miRNA's are important in the function of the active adult human brain this seems like the kind of thing whose activity would be difficult to simulate in a computer.

References

Feng J, et al. 2009 Evidence for X-chromosomal schizophrenia associated with microRNA alterations. PLoS One Online. PubMed.