“Small interfering RNA (siRNA) holds therapeutic promise for silencing dominantly acting disease genes, particularly if mutant alleles can be targeted selectively”—at least, that is the hope of researchers at the University of Iowa School of Medicine. The results of their study, published in the June 10 Proceedings of the National Academy of Sciences, indicated that they may indeed be on the road to treating dominantly inherited disorders such as cancer and Huntington’s disease.

To explore the utility of siRNA in these disorders, Henry L. Paulson, MD, PhD, Associate Professor of Neurology, and colleagues used cellular models “to test whether we could target mutant alleles causing two classes of dominantly inherited, untreatable neurodegenerative diseases.” The researchers modeled polyglutamine degeneration in Machado-Joseph disease/spinocerebellar ataxia type 3 (MJD/SCA3) and frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). The cell cultures were transfected with siRNA, designed to abort the production of the mutant proteins while allowing the normal gene to continue its vital functions by targeting only the mutant messenger RNA for destruction by cellular mechanisms.

Dr. Paulson and colleagues designed siRNAs to target the transcript encoding ataxin-3, the disease protein in MJD/SCA3, in order to determine whether siRNA “could selectively silence a full-length polyglutamine disease protein.” Dr. Paulson related that “in transfected cells, this resulted in efficient but not allele- specific suppression of ataxin-3.”

To refine the specificity of siRNAs, the researchers targeted a single nucleotide polymorphism in the MJD1 gene that was “in linkage disequilibrium with the disease-causing expansion.” The modified siRNA “effectively suppressed mutant ataxin-3 expression … and also nearly eliminated the accumulation of aggregated mutant ataxin-3, a pathologic hallmark of disease.”

Additionally—and most important, Dr. Paulson noted—the siRNA worked without the spread of silencing signals, a significant problem in plant and worm experiments which, if present in mammalian cells, might have limited the applications of siRNA to nonessential genes. Fortunately, the siRNA employed by Dr. Paulson and colleagues in their study “inhibited only mutant allele expression.”

To test whether siRNA worked equally well to silence disease-causing mutations directly, the investigators then targeted missense Tau mutations responsible for FTDP-17. After focusing on the V337M mutation, they based their siRNA design on their approach to ataxin-3 and achieved “preferential inactivation of the mutant allele … without detectable loss of wild-type Tau,” Dr. Paulson reported.

Based on their findings, the researchers concluded that “allele-specific silencing should be possible for many dominant disease genes.” The researchers are now trying to find the best way to deliver interfering RNA in vivo. “That’s the next step in the process towards a therapy,” Dr. Paulson said. “Issues of in vivo delivery and efficacy remain to be resolved, of course. Notably, the long-term consequences of chronically triggering the interfering RNA pathway in vivo, as may be required to treat neurodegenerative conditions, are unknown.”

Still, Dr. Paulson is optimistic about the potential for this treatment technique. “This kind of technology says, ‘We don’t need to know [what causes the genetic mutation]. We know it’s a bad gene—let’s just eliminate the expression of that gene.’ I think there’s a lot of excitement in the Huntington’s disease community that this kind of technology might ultimately lead to a therapy.”

NR

—C. Justin Romano

Suggested Reading
Miller VM, Haibin X, Marrs GL, et al. Allele-specific silencing of dominant disease genes. Proc Natl Acad Sci U S A. 2003;100:7195-7200.

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