Human Transgenesis - Definitions  Technical Possibilities and Moral Challenges
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Human Transgenesis - Definitions Technical Possibilities and Moral Challenges


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opportunity for an HGT event.
Like the Polydnavirus, viruses with genomes that are also able to integrate into the
genomes of higher vertebrates, including humans, are known. The most notorious
viruses that are capable of integrating into the genomes of vertebrates are the
retroviruses. Members of the Retroviridae virus family present RNA genomes that
are used as templates to generate proviral DNA. This is accomplished by a virus-
coded enzyme, reverse transcriptase. Once the proviral DNA is generated, it is able to
integrate into the host genome, persist indefinitely within the cell nucleus, and
Human Transgenesis: Technical Possibilities and Moral Challenges
generate new virus particles. Such a virus strategy creates a striking opportunity for
an HGT event to take place. Indications of ancient retroviral activity in the human
genome are given by the existence of genetic elements termed human endogenous
retroviruses (HERVs). These elements are remnants of retroviral infections that
occurred more than 25 Ma ago in mammalian cells at the birth of the primate lineage.
During these ancient infections, proviral DNA became permanently integrated into
the genome of germline cells that were passed from generation to generation as a part
of the host genome (Belshaw et al. 2004).
2.3 Transgenesis in Humans
In comparison to what is known from experimenting with the genetic manipulation of
other organisms, there is very little scientific indication of what would be the impact
of biotechnological transgenesis in humans, either individually or as a species.
Because this hypothesis cannot currently be openly and comprehensively tested,
one could look to the past and ask the question: has transgenesis ever occurred in
man? The answer would be: yes, and very successfully.
When the human genome was finally sequenced, it was verified that HERVs, also
termed LTR retrotransposons (Cordaux and Batzer 2009), comprise nearly 8 % of
human DNA. Combined with non-LTR retrotransposons and regular DNA trans-
posons, these mobile elements account for an astonishing 50 % of the human genome
(Venter et al. 2001). As a general concept, retrotransposons are genetic elements that
can jump from one locus to another within the genome or among different genomes.
Unlike regular transposons, retrotransposons are able to spread through a \u201ccopy-and-
paste\u201d mechanism by which the original TE copy is not excised from its original
locus. Instead, it works as a template to generate an RNA copy of itself. Then, a
retrotransposon-encoded reverse transcriptase is used to generate, from the RNA, a
new DNA copy that can integrate into another locus or into another genome
(Faulkner 2011). Non-LTR transposons are the most abundant of all TEs in the
human genome and occupy about one third of the total genomic size. We still lack
evidence to suggest how these TEs first appeared in the genome of man; some types
of retrotransposons are very old with evidence of ongoing activity, dating back to
approximately 160 Ma (Lander et al. 2001). At any rate, it is likely that at least some
of these elements were horizontally transferred to the genomes of human ancestors.
This transfer has already been demonstrated to have happened in many other mam-
malian species, such as in bats of the vespertilionid lineage, murine rodents, lemurs,
squirrel monkeys and opossums (Schaack et al. 2010).
For many years, the amount of apparently non-functional DNA sequences within
the human genome that correspond to TEs has puzzled scientists. Current studies
have indicated that these movable elements are far from being non-functional.
Retrotransposon activity yields specific genetic diversity that underlies phenotypic
variation mostly by generating insertional mutagenesis and structural changes in the
DNA. According to Schaack et al. (2010), when exchanged among different species
via HGT, transposons can trigger a series of events that actively shape the genomic
architecture of the organism and give rise to biological innovation. Indeed, these TE-
induced variations account for much of the inter-individual genomic diversity that is
observed among humans (Konkel and Batzer 2010). Such intense mutagenic activity
F.G. da Fonseca et al.
may eventually present deleterious results, including those that have been demon-
strated by a number of genetically transmitted diseases that are linked to retrotrans-
poson activity (Faulkner 2011). Still, the genetic variations that are provided by TEs
have been considered to be essential mechanisms of genomic evolution. A good
example of such mechanism is the inactivation of a gene, which encodes for a specific
hydroxylase enzyme, after the evolutionary divergence between the Homo (man) and
Pan (chimpanzee) lineages. This particular gene inactivation, caused by the simple
insertion of a retrotransposon, is believed to have influenced the expansion of the
human brain and the appearance of phenotypic and cognitive traits that are unique to
the human species (Chou et al. 2002).
We still lack clear scientific evidence of a recent HGT event between the human
genome and that of any other species. However, as reported by Schaack et al. (2010),
this lack could be due to the technical fact that only approximately 1,000 full
eukaryotic genomes from an estimated 1.5 million species have been sequenced.
Because the discovery of HGT events is based on comparisons between the different
available genomes, such restricted sampling may be impairing our ability to find
evidence of recent lateral gene transfers to humans. However, this phenomenon has
been unequivocally demonstrated in many other mammals, including new world
primates, which are closely related to the hominid lineage. If one considers the
evolution of the human species not as a pinpoint event in time, but rather as a
cumulative chain of events that involved many different but related ancestors, then
the influence of HGT contributing to the human genome becomes absolutely unde-
niable. Transgenesis in humans has already occurred.
2.4 The Feasibility of Transgenic Treatments in Humans: Gene Therapy
We have established that \u201cnatural\u201d transgenesis is not new to humans. But how close
are we to harnessing deliberately the therapeutic potential of transgenesis? Although
there are no failure-proof or adverse effect-free techniques for carrying out trans-
genesis in humans, methods for the stable introduction of exogenous DNA into the
genome of humans have been known for many years.
The most feasible method utilises retroviral vectors that carry exogenous genes. As
mentioned before, these viruses are able to integrate their own proviral DNA into the
genome of the host cell, and thus they are also capable of inserting any other piece of
foreign DNA. This principle was experimentally demonstrated in 1981 for the first
time when Wei et al. (1981) inserted a herpes virus gene into the genome of a mouse
using a retroviral vector. Since then, hundreds of different experimental studies
evaluating efficient therapeutic gene delivery into mammalian cells by retroviral
vectors have been conducted, and more than 250 human gene therapy clinical trials
employing retroviral vectors have been initiated in the last decade (Cockrell and Kafri
2007). Expectations were high when a group of independent researchers announced
in the mid-2000s the successful correction of three hereditary hematopoietic disorders
in humans with the use of simple retroviral-based vectors (Cavazzana-Calvo et al.
2000; Aiuti et al. 2002; Ott et al. 2006). However, the euphoria turned into disap-
pointment when three patients from one of the clinical trials developed leukaemia as a
result of mutagenesis that was caused by the uncontrolled insertion of the retroviral
vectors into the patients' DNA (Check 2005). Safety concerns and the inability of
Human Transgenesis: Technical Possibilities and Moral Challenges
simple retroviruses to insert their proviral DNA into the genome of cells that are not
prone to divide blocked all further