1. Playing God? – An
Introduction to Genetic
Modification
ETHAN SUNG – DOCTOR’S SOCIETY – 3/11/17
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2. Editing Techniques : History of CRISPR-Cas9
First discovered by Yoshizumi Ishino, who accidentally cloned part of
CRISPR when researching the iap gene
Found unusual repeated sequences of nucleotides interspaced throughout
DNA, the purpose was not yet known
In 2006, scientists hypothesised that CRISPR and associated Cas genes
created bacterial immunity
2008, discovered that regions of CRISPR corresponded to nucleotide
sequences of invaders (spacers)
Cas9 protein observed in Streptococcus, uses CRISPR-RNA to guide to
target DNA and trans-activating RNA to cut, fused together to create a
locating/cutting tool
2015, testing in tripronuclear zygotes showed CRISPR could cut the HBB
gene when reprogrammed, but caused mutations due to off-target cuts
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3. Editing Techniques :
Mechanics of CRISPR-Cas9
Guide-RNA – composed of CRISPR RNA and trans-activated
CRISPR RNA to produce a hairpin loop active site
PAM – small DNA sequence following the target, allowing Cas9
to bind at NGG nucleotides
Non-Homologous End Joining – re-joins cut using Ku protein
and DNA Ligase for deletions, insertions facilitated by DNA with
suitable overhangs
Homologous Directed Repair – allows DNA sequence to be
incorporated via displacement as long as both sequences have
long homologous elements
CRISPRi – uses a ‘dead’ form of Cas9 unable to cut DNA,
repressing transcription through methylation
Dead CRISPR-Cas9 associated with activation-induced cytidine
deaminase, to cleave nitrogenous components of cytosine to
uracil (C:G to U:A)
In RNA, the TadA enzyme can convert adenine to inosine, which
is functionally similar to guanine
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4. Human Editing : Contentions
Germline editing could eradicate inherited
genetic diseases such as β-thalassaemia,
preventing a child being born and suffering with
the condition
Somatic editing could be used to treat severe
conditions like immunodeficiencies, without
having to worry about transmission to offspring
Preferential phenotypes are often dependent on
multiple genes, have mechanics that are not yet
known, and too difficult to engineer
Cons
CRISPR-Cas9 may introduce unexpected off-
target mutations, causing more issues than
initially hypothesised
Designer babies could be potentially created
by inserting preferential genes (i.e.
intelligence, beauty etc.), pushing eugenics
Religious arguments contest the idea of
‘playing God’ by interfering with nature
Could bring legal disputes, gene modifying
techniques are often patented, leading to
arguments about ownership
Germline-edited offspring have not
consented to the modifications made to their
DNA
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Pros
5. Medical Applications : Organ Transplantation
Shortage for human organ donation means that other mammals are often
looked at for alternatives (e.g. Baby Fae with baboon heart for hypoplastic
heart syndrome)
Rejection caused by foreign antibodies stimulating immune system to
produce xenoreactive natural antibodies or T-cells
Porcine-derived organs have a low disease transmission, are anatomically
comparable and easily sourced
Xenozoonosis – transmission of animal infections to humans is a risk as
implantation bypasses physical barrier
Porcine endogenous retroviruses shown to infect human cell lines in-vitro,
showing a risk of possible reactivation, deactivated all 62 using CRISPR
Chimeras produced using gene-knockout in embryos to prevent porcine
pancreas development, replacing them with stem cells, but could
undesirable differentiate elsewhere
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6. Medical Applications : Recombinant Organisms
Recombinant organisms have their DNA modified by inserting genetic
information from another organism
Scientists used human β-pancreas cells to extract the DNA sequence for
insulin using polymerase, inserting it into a bacterial plasmid for mass
production
Plasmids re-adopted by bacteria through transformation, using calcium
ions and temperature changes to alter membrane permeability, success
detected using antibiotics
Better than using an animal vector (porcine/bovine), as insulin is already
humanised, preventing infection or rejection
Pharming process use genes for pharmaceuticals into host plants
(tobacco or potato), cultivating them in bioreactors, secreting products
into growth medium
Strains of rice modified to contain β-carotene genes (psy and crtl), a
precursor of vitamin A to combat deficiency, but faced controversy
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7. Medical Applications : iPS Cells
Induced stem cells have pluripotency restored from most adult
cells, using four ‘Yamanaka’ transcription factors
Use of viral vectors to deliver transcription factors may express
tumour-creating genes, have been successfully removed post-
procedure
Has less ethical and legal implications as embryos are not used, due
to sanctity of life arguments
Can be used to model genetic diseases, such as using spinal
muscular atrophy iPS cells, showing that less motor neurone
differentiation occurred, bypassing limitations in sourcing cells
2017, iPS cells used to combat age-related macular degeneration
(progressive blindness), by forming retinal epithelial cells
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