The potential of RNA interference is exciting, offering the possibility of shutting down expression of specific proteins—perhaps in specific cells—for both investigative and therapeutic purposes (see ARF live discussion on this subject). But potential is one thing; delivering on it, quite another. Doing so will likely require many incremental successes, such as the one reported in the November 23 online Nature Medicine by Ralph DiLeone and colleagues at the University of Texas Southwestern Medical Center in Dallas.

DiLeone and colleagues provide in-vivo evidence that one can use RNAi to knock down the expression of a brain enzyme, creating a possible Parkinson’s disease model in the process. Working in mice, the researchers used an adeno-associated virus (AAV) vector to deliver short hairpin (sh) RNA matching a sequence of the RNA for tyrosine hydroxylase, an enzyme critical to dopamine synthesis. The vector was injected unilaterally into the substantia nigra (SN), with a control vector injected into the contralateral SN. Beginning 12 days later, the researchers noted significant reductions in tyrosine hydroxylase in the SN injected with interfering RNA. This reduction was detectable even 50 days after the injections. Accompanying the tyrosine hydroxylase reductions, the researchers found several behavioral deficits, including motor performance (rotarod) deficit and a reduced locomotor response to an amphetamine challenge.

As the authors point out, technologies such as microarrays are going to be dropping countless candidate genes and proteins into the laps of scientists in the near future. Working out the in-vivo roles of these molecules will be very slow if transgenic mice are the only models available. “With AAV vectors, genetic disease models could be rapidly created in many species, including rodents and primates,” write the authors.—Hakon Heimer.

Reference:
Hommel JD, Sears RM, Georgescu D, Simmons DL, DiLeone RJ. Local gene knockdown in the brain using viral-mediated RNA interference. Nat Med. 2003 Dec;9(12):1539-44. Epub 2003 Nov 23. HREF="/pap/annotation.asp?powID=33386">Abstract

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  1. Tools to selectively knock down specific mRNA species in cells and tissues have developed at a spectacular pace over the past few years (reviewed by Scherer and Rossi, 2003). A major advance was the development of short interfering RNA technology (siRNA), where short stretches of antisense or hairpins, generally less than 20 nucleotides, have been used in invertebrate and mammalian systems to remarkable specificity. This report raises the bar a little further by grafting a hairpin antisense RNA into a viral vector, adeno-associated virus, which can be used in the brain of rodents in vivo. As the authors rightly state, this may be a way to generate disease models in species where knockouts are technically difficult, such as non-human primates, or to test regional specificity in gene function.

    As an example of the latter application, Hommel et al. show that knockdown of tyrosine hydroxylase (TH) in the substantia nigra is sufficient to induce motor deficits, which they suggest might be useful for modeling Parkinson’s disease. In contrast, knockdown in another midbrain dopaminergic region, the ventral tegmental area (VTA), decreases amphetamine-induced hyperactivity. What are not shown, probably for reasons of space in the journal, are the important reciprocal experiments; does VTA knockdown cause any rotorod deficits, or does SN knockdown induce hyperactivity after amphetamine challenge? It’s not quite clear that this is a substantially better PD model than current strategies, as the dopaminergic neurons are still alive, which is not the case in the human disease. Such discussion aside, the experiment described in this paper would have been extremely difficult to achieve using current knockout technology; homozygous TH knockout is lethal and promoters that drive expression in the VTA or SN selectively are poorly characterized, so selective knockout would be difficult. One can now imagine any number of ways to use these approaches; finding genes that normally maintain dopaminergic cell survival, modeling recessive genetic lesions in different species, and refining our understanding of the different properties of neurons that express similar transmitter phenotypes, to name a few.

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  1. . Local gene knockdown in the brain using viral-mediated RNA interference. Nat Med. 2003 Dec;9(12):1539-44. PubMed.

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  1. . Local gene knockdown in the brain using viral-mediated RNA interference. Nat Med. 2003 Dec;9(12):1539-44. PubMed.