Hope Not Hype - Third World Network

Appendix One: What is a GMO
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Once these conditions are met, then gene silencing results from “the cleavage or
translational repression of complementary single-stranded RNAs, such as messenger RNAs
or viral genomic/antigenomic RNAs. The short RNAs have also been implicated in guiding
chromatin modification”, DNA methylation, or histone modifications, the latter of which
are heritable by separate pathways (see also Chong and Whitelaw, 2004; Lippman and
Martienssen, 2004; quote from Meister and Tuschl, 2004, p. 343).
The classification of RNAi/PTGS as an epigenetic [heritable] phenomenon rests largely
upon its ability to provoke heritable changes in gene expression. Inheritance of silencing
could derive from either of two sources. The first is persistence of the signal. The second is
persistence of the silenced state (Bernstein et al., 2001, p. 1516).
When dsRNA is a signal that is maintained through sequence-specific degradation
and/or RNA-dependent RNA polymerase (RdRP) amplification (Mello and Conte Jr., 2004),
the trait is dependent on propagation of the signal (e.g., dsRNA). One way for this to
occur is through “stable incorporation of transgene arrays into the genome, the presence
of endogenous repetitive elements such as transposons, or the enforced expression of
hairpin RNAs. Such cases require no additional mechanisms to explain heritable silencing
because the trigger is expressed from an endogenous and heritable genetic element”
(Bernstein et al., 2001, p. 1516). By “heritable genetic element” these authors mean DNA,
a case already recognized by ERMA. “The latter case is more provocative and requires
consideration of mechanisms that propagate either the signal or the silenced state
independently of the silencing trigger” (Bernstein et al., 2001, p. 1516). This is the case I
argue should also be recognized by ERMA as a GMO under the existing wording of the
Act.
Numerous types of instigating events lead to gene silencing, including: in vitro
synthesized dsRNA, a novel mRNA precursor (as could arise from a transgene, a new
open reading frame resulting from a DNA insertion, or a new intron), or a product of
transcription expressed in a cell type or time of development where it would not normally
occur, as could happen if a transgene activated a region of heterochromatic DNA (Denli
and Hannon, 2003; Grewal and Elgin, 2007). New dsRNAs could arise when endogenous
RNA editing pathways act on a foreign RNA substrate (Heinemann and Bungard, 2005;
Yang et al., 2006). Finally, novel dsRNAs may be created in one organism and transferred
through food to, and amplified in, another organism (Baum et al., 2007; Mao et al., 2007).
The initiating event can require nothing more than common promoters or other types
of DNA-level regulatory elements on two different genes or any “aberrant RNA” (Al-
Kaff et al., 2000; Bhullar et al., 2003; Heinemann, 2007; Herr et al., 2006). Thus, a new
form of dsRNA can arise when two different transgenes run by common promoters combine
through breeding in the open environment, or when a virus, such as the CaMV, that carries
the original promoter for a transgene, for example the 35S promoter 4 , infects a transgenic
plant.
4
Currently the most commonly used promoter in GM crops.