For about a century, studies in the realm of molecular biology elucidated the structure and relationships of DNA and its blueprint for protein synthesis. Within the past 50 years, numerous investigators have revealed the transcription process from which DNA yields messenger RNA (mRNA). mRNA can be considered the working copy of the blueprint. The construction site of this process is the ribosome. In plants and animals, ribosomes are free within the cytoplasm or attached on the endoplasmic reticulum, a tubular membranous network within the cell.
The 1998 RNA interference (RNAi) discovery by Fire and Mello led to their 2006 Nobel Prize in Physiology or Medicine (Fire et al. 1998). They noted that RNAi is a biological process that inhibits gene expression. This process involves the application of exogenous, small, and interfering double-stranded RNAs that bind to mRNA that have sequence homology. The double-stranded RNAs cleave the mRNA via an associated enzyme complex and effectively quell or silence the expression of the target gene or DNA. Thus the protein that was destined for construction is not produced (Fire 2015).
In agricultural applications,genes that produce proteins that are essential to parasite survival can be silenced through our understanding of specific RNAi. From an environmental perspective, this understanding can contribute to the reduced usage of toxic fumigants intended to combat nearly 2,500 nematodes that live in the soil and have a significant economic impact on world food crops. Other significant advantages that have been demonstrated in the laboratory include resistance to environmental stress, increased post-harvest stability, enhanced nutritional qualities, and lowered exposure to allergens and toxins, both innate and environmental (Saurabh et al. 2014; Sunilkumar et al. 2006; and Siritunga and Sayre 2003).
Since RNAi efficacy is based upon sequence homology between small RNAs and mRNAs, “off-target” genes with sufficient coincidentally similar sequence homology may be silenced as well as target genes, potentially leading to unwanted and adverse effects. In fact, experimental data in some plants suggest that gene silencing can occur even when there are mismatches between target genes and the small RNAs.
However, homology alone is not adequate to predict silencing; off-target mRNA may be protected in some instances by encapsidation (the enclosure of viral genetic material within the capsid), or there may be positional mismatches (Ramon et al. 2014).
These potential effects may be analogous to the uncertainty in phenotypic expression that is associated with the phenomenon of variable gene penetrance and gene expression as observed in plants and animals. Thus there is some theoretical uncertainty about prediction about the number and intensity of the pool of off-target effects. It is important to note that in at least two decades of RNAi research, there is little evidence for actual risk in growth, development, and reproduction of test organisms. Equally important is that there is even less evidence for human medical or environmental risks associated with RNAi.
The first successful large-scale use of RNAi technology to achieve disease control was reported in curbing the effects of disease in honeybees (Hunter et al. 2010). RNAi technology was used to prevent mortality and improve the health of bees affected by Israeli Acute Paralysis Virus. This was apparently the first successful demonstration of RNAi as a preventive treatment of Colony Collapse Disorder in honeybees. Honey production was as high as 300% greater than in those hives that went untreated.
Upon reflection of safety and our knowledge of RNAi, there are some 200 drugs, including human insulin, that are manufactured via genetically engineered techniques in which the temporal results of transcriptional change have been sustained, stable, and adaptive. Equivocal or worrisome animal studies have been reported and must be a motivation for caution and for additional investigation and compulsive monitoring of potential adverse health outcomes (Grimm et al. 2006).
Organizations such as Greenpeace emphasize the daunting complexity and theoretical likelihood of toxic effects associated with genetically modified RNAi constructs. The arguments are valid to an extent, but the logical extension of such reasoning would lead to the removal of scores of genetically engineered medicines and virtually all foods, cultured dairy products, and natural foods such as human breast milk. Human reproduction itself is a biological process fraught with possibilities for genetic disorders and even rare DNA quirks.
It is important to vigilantly guard against unintended risks, but that should not preclude the use of beneficial gene-based processes that may represent human adaptation at its best.
References cited in this column are available from the author.
Roger Clemens, DrPH, CFS,
Adjunct Professor, Univ. of Southern
California School of Pharmacy, Los Angeles, Calif.