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EPA Releases Advisory Panel Recommendations on Agricultural RNAi Risk Assessment

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NEW YORK (GenomeWeb) – The US Environmental Protection Agency last week formally released the recommendations of a scientific advisory panel assembled earlier this year to explore the possible risks associated with plants modified to express gene-silencing dsRNAs, particularly for insecticidal applications.

Generally, the panel agreed that there appears to be little danger associated with such RNAi molecules for humans and other mammals, but expressed concern over uncertainties about their potential effects on the environment and non-target insects.

Viewing the EPA's existing risk-assessment framework for agricultural biomolecules as inadequate for dsRNA, the panel called for additional research in the area and proposed a new model for evaluating the ecological impact of these products.

In an email to Gene Silencing News, an EPA spokesperson said that the agency is now conducting a review of the advisory panel's report, which it expects to complete "in several months." After that review the EPA will have a "better sense of next steps," according to the spokesperson.

Though more developed as a therapeutic modality, RNAi is also under active development for agricultural applications by a number of groups, most notably Monsanto, which has already introduced an RNAi-modified line of soybeans called Vistive Gold.

But the company is also working on various crop plant strains that express pest-killing RNAi molecules, known as plant-incorporated protectants (PIPs). Its most developed is Smart Stax Pro, a type of corn that expresses two widely used Bt proteins along with dsRNA that silence a gene essential to the Western corn rootworm.

Should Smart Stax Pro reach the market in the next few years as planned, it would be the first such RNAi PIP to be commercialized in the US, setting the stage for similar products from Monsanto and others. These include non-PIP RNAi products such as pesticidal sprays.

In anticipation of this, the EPA has been considering whether its current approach for evaluating the risks of PIPs is sufficient when it comes to the gene-silencing technology. In January, it held a meeting of agency scientists and outside researchers to get their input on the matter.

Last week, the EPA released the recommendations of the panel, which found little chance that RNAi PIPs would directly impact humans.

Agreeing with the EPA that the primary route of exposure for the RNAi molecules would be through oral ingestion, the panel noted dietary RNA is extensively degraded in the mammalian digestive system and that "there is no convincing evidence that ingested dsRNA is absorbed from the mammalian gut in a form that causes physiologically relevant adverse effects."

Still, the panel suggested that the EPA take steps to build upon these data by collecting information on dsRNA PIP abundance and tissue distribution to evaluate factors that may affect the absorption of the molecules; conduct experiments on mammalian blood and exposed tissues to ensure that physiologically active siRNAs are not processed from PIP dsRNAs; and investigate the stability of different forms of dsRNA to address the possibility of dermal or inhalation routes of exposure.

The panel took greater issue with the possible ecological risks of RNAi PIPs and found the EPA's current biomolecule risk-assessment approach lacking when it comes to the gene-silencing technology.

"Uncertainties in [RNAi's] potential modes of action in non-target species, potential for chronic and sub-lethal effects, and potential unintended consequences in the various life stages of non-target organisms are sufficient justification to question whether the current … framework for ecological effects testing is applicable to dsRNA PIPs," as well as dsRNA products applied exogenously, it wrote.

Importantly, the panel noted that the different modes of action of RNAi in different animals mean that no one set of test species can adequately represent non-target species for all pesticidal products using RNAi technology. "The classic approach of developing and assembling effects data for a standard set of test species will likely not work well for this technology," it added.

Additional data are required to better assess the environmental fate of dsRNA and assess the risks such molecules pose, the panel stated, but the questions of how to gather these data and which test species to study cannot be answered with a clearer understanding of exposure to dsRNA PIPs and exogenously applied dsRNA products.

The panel therefore proposed an exposure-based conceptual model that would help in the identification of non-target organisms that are at risk for exposure to RNAi PIP and non-PIP products, while narrowing the spectrum of such organisms that would need to be tested.

The proposed model includes characterizing the RNAi agent, including its target gene(s) and dosage levels; identifying non-target species likely to face exposure and their susceptibility to RNAi; and conducting in vitro feeding assays on non-target organisms to measure dsRNA effects on mortality, immune responses, and synergies with other PIPs and pesticides.

The model then calls for cellular and molecular studies that examine the effects of differential gene regulation in non-target organisms; field studies to examine the impact of RNAi products on biological networks and food webs under real-world conditions; and the development of strategies to mitigate off-target effects, including ones based on redesigning dsRNAs with wide spectrums of activity and resistance to RNAi by target species.

Finally, the panel called for the continued collection of additional data on both the kinetics of dsRNA degradation in soil and in plants; the effects of physical barriers to dsRNA degradation and uptake in non-target organisms; the potential for chronic and sub-lethal effects of dsRNA on non-target organisms, as well as unintended consequences at different stages of the life cycles of such animals; and the potential for resistance among target pests.

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