26 November 2002. Dinucleotide repeats are scattered throughout the human genome, but do they actually do anything? A report in yesterday's Nature Structural Biology reveals that in some cases at least, such repeats may be novel enhancers of RNA splicing.
In this process, blocks of coding (exon) and noncoding (intron) nucleotides are removed from the nascent RNA to reveal mature messenger RNA. It is one way of increasing the protein complement, or proteome, within a given cell. Splicing of a single RNA can, in fact, yield hundreds, if not thousands, of mRNAs from a single gene (see Graveley and related news item).
Now, researchers from Albrecht Bindereif's laboratory at the Justus-Liebig-University in Giessen, Germany, with the aid of the Proteomics Analysis Facility at Harvard University, show that a polymorphic CA dinucleotide repeat located in intron 13 of the gene for endothelial nitric oxide synthase (eNOS) enhances RNA splicing. The finding is significant not only for understanding the regulation of splicing activity, but also because other researchers have found a correlation between the number of eNOS CA repeats and risk for coronary heart disease (see Stangl et al.). eNOS plays a role in vascular homeostasis.
CA repeats in eNOS fall between exons 13 and 14. First author Jingyi Hui et al. engineered eNOS genes to show that doubling the number of repeats, from 19 to 38, dramatically enhanced splicing of the flanking exons in a variety of cell lines. The authors were also able to affinity purify two proteins that bind to transcripts containing the repeats. One of them, YB-1, is a known splicing factor, while the other was identified as the ribonucleoprotein hnRNP L. When the authors depleted the latter from cells, CA-induced splicing diminished by half.
Given that CA repeats occur throughout the genome, the question now is, how many other splicing sites are similarly regulated and could such regulation be involved in other disease processes?
The involvement of eNOS in this story is fascinating in light of recent data suggesting that expression of the three forms of NOS, inducible (iNOS), neuronal (nNOS) and endothelial, are altered in Alzheimer's disease (AD).
Reporting in October's Brain Research, first author Hans-Joachim Lüth and colleagues from Thomas Arendt's lab at the University of Leipzig, Germany, showed that in postmortem AD brain both iNOS and eNOS expression was elevated in astrocytes. iNOS appeared in cells surrounding amyloid plaques, while eNOS-expressing astrocytes were found to be associated with either plaques or blood vessels. Lüth et al. had previously demonstrated that in AD brains, but not control tissue, nNOS is expressed in pyramidal neurons of the isocortex.-Tom Fagan.
Hui J, Stangl K, Lane WS, Bindereif A. HnRNP L stimulates splicing of the eNOS gene by binding to variable-length CA repeats. Nature Struct. Biol. 2002 November 25.Abstract
Luth H-J, Munch G, Arendt T. Aberrant expression of NOS isoforms in Alzheimer's disease is structurally related to nitrotyrosine formation. Brain Research 2002 October 953:135-143. Abstract.
Graveley BR. Alternative splicing: increasing diversity in the proteomic world. Trends Genet. 2001 Feb ; 17(2):100-7. Abstract
See related news item.
Stangl et al. High CA repeat numbers in intron 13 of the endothelial nitric oxide synthase gene and increased risk of coronary artery disease. Pharmacogenetics. 2000 Mar ; 10(2):133-40. Abstract
Luth et al. Aberrant expression of NOS isoforms in Alzheimer's disease is structurally related to nitrotyrosine formation. Brain Res. 2002 Oct 25 ; 953(1-2):135. Abstract