Ultraconserved elements are associated with homeostatic control of splicing regulators by alternative splicing and nonsense-mediated decay

  1. Julie Z. Ni1,
  2. Leslie Grate1,
  3. John Paul Donohue1,
  4. Christine Preston2,
  5. Naomi Nobida2,
  6. Georgeann O’Brien2,
  7. Lily Shiue1,
  8. Tyson A. Clark3,
  9. John E. Blume3, and
  10. Manuel Ares, Jr.1,2,4
  1. 1 Center for Molecular Biology of RNA and Department of Molecular, Cell, and Developmental Biology, University of California at Santa Cruz, Santa Cruz, California 95064, USA;
  2. 2 Hughes Undergraduate Research Laboratory, University of California at Santa Cruz, Santa Cruz, California 95064, USA;
  3. 3 Affymetrix, Inc., Santa Clara, California 95051, USA

Abstract

Many alternative splicing events create RNAs with premature stop codons, suggesting that alternative splicing coupled with nonsense-mediated decay (AS-NMD) may regulate gene expression post-transcriptionally. We tested this idea in mice by blocking NMD and measuring changes in isoform representation using splicing-sensitive microarrays. We found a striking class of highly conserved stop codon-containing exons whose inclusion renders the transcript sensitive to NMD. A genomic search for additional examples identified >50 such exons in genes with a variety of functions. These exons are unusually frequent in genes that encode splicing activators and are unexpectedly enriched in the so-called “ultraconserved” elements in the mammalian lineage. Further analysis show that NMD of mRNAs for splicing activators such as SR proteins is triggered by splicing activation events, whereas NMD of the mRNAs for negatively acting hnRNP proteins is triggered by splicing repression, a polarity consistent with widespread homeostatic control of splicing regulator gene expression. We suggest that the extreme genomic conservation surrounding these regulatory splicing events within splicing factor genes demonstrates the evolutionary importance of maintaining tightly tuned homeostasis of RNA-binding protein levels in the vertebrate cell.

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