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December 15, 2015

Stopping Premature Protein Synthesis Halts Genetic Diseases

  • Scientists at Cold Spring Harbor Laboratory report that they have inhibited a process cells have evolved to prevent imperfect proteins from being synthesized. Improperly formed proteins can cause a host of serious illnesses, from muscular dystrophy to cystic fibrosis. A question of enormous import in research, beyond the challenge of determining how malformed proteins contribute to specific disease processes, is figuring out ways to prevent or reduce the pathologies they cause.

    The team published its study (“Antisense oligonucleotide–directed inhibition of nonsense-mediated mRNA decay”) in Nature Biotechnology.

    Nonsense-mediated decay (NMD) provides cells with the ability to detect errors in RNA transcripts. Activation of a gene induces a cell to make an RNA copy of its code, edit unneeded segments out of that message, and splice together a final version of the message that provides ribosomes with a template to make one specific protein.

    Mutations in genes can foul up the works, right from the start. Although mutations come in many varieties, nonsense mutations involve the seemingly innocuous change of a single letter in the coded message—a change that causes the gene's message to prematurely read "stop." When things are working properly, the stop signal comes at the very end of the coded message. This tells the ribosome to end the protein manufacturing process at the appropriate place.

    When the stop signal comes too soon, due to a faulty letter in the code, the process ends prematurely and various outcomes are possible. Often, the error is noted by the cell and the NMD process is engaged, as a way of preventing bad outcomes. Via NMD, the faulty message is degraded and little or no protein is made. Other times, an abnormally short version of the protein is still manufactured. Such truncated proteins are sometimes harmless. Other times they are partially useful to the cell. Still other times a too-short protein can have toxic effects.

    A team led by Adrian Krainer, Ph.D., CSHL professor, has devised a method to prevent NMD from being engaged. The critical caveat is: his team has determined how to deactivate this cellular quality-control mechanism only when it is advantageous, i.e., only when it will result in the therapeutic restoration of protein manufacture, whether the resulting protein is full-length or at least partly functional.

    Part of the challenge is knowing in what situations it's going to be advantageous to inhibit NMD. The details are technical, regarding how far—how many coded RNA letters are involved—a premature stop signal occurs relative to a location on the message that serves as a loading site for an EJC (exon junction complex). EJCs are like chapter marks or tags deposited in the cell's nucleus as a gene's copied RNA message is edited, or spliced.

    The innovation tested successfully by Dr. Krainer and his team involves the use of an ASO (antisense oligonucleotide) that will block the site normally occupied by a particular EJC.

    "We only want to dislodge or replace the EJC at one spot in the gene's spliced RNA message," explains Dr. Krainer. "We design and test an ASO that will prevent a particular EJC from binding to the message; the others are unaffected."

    Why? Because when the cell detects an EJC within a certain distance from a "stop" signal in a gene message, that is when it calls the NMD machinery into action, to halt decoding of the message and resulting in its degradation. The trick is do this only in gene messages in which a premature stop signal, if ignored, will nevertheless result in the production of a full-length, or at least partly functional protein, one that will have a helpful impact in a disease process.

    "We are interfering with the NMD machinery's ability to target such mRNAs for destruction," continues Dr. Krainer. "This means the cell will make more protein. It may still be truncated, but that can be beneficial. In other cases, a truncated protein would not be functional, or could even be toxic."

    In the latter cases, either it would not make sense to intervene with an ASO; or, an ASO could be combined, according to Dr. Krainer, with so-called read-through drugs. These are experimental drugs that prevent premature stop signals from halting protein production by ribosomes, although NMD tends to limit their efficacy.

    The current work shows that combining a read-through drug with an appropriate EJC-blocking ASO results in cells manufacturing substantially more full-length protein.

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