Revise Genetic technology applied to medicine for Cambridge 9700 (19.2)

One of the earliest and most successful applications of genetic technology in medicine is the production of therapeutic human proteins in genetically modified organisms (GMOs). This process, known as recombinant protein production, allows for the large-scale, safe, and ethical manufacturing of vital medicines like insulin, human growth hormone, and clotting factors.

The general process involves isolating the human gene for the desired protein, inserting it into a vector (often a plasmid), and introducing this recombinant vector into a host organism, such as the bacterium Escherichia coli. These transgenic organisms are then grown in large industrial fermenters, where they express the human gene and produce the protein. The protein is then extracted and purified for medical use.

While prokaryotes like E. coli are widely used due to their rapid growth and simple nutritional needs, they are not always suitable. Many human proteins require complex folding and modifications after translation to become functional. These post-translational modifications, such as glycosylation or the formation of disulfide bridges, are carried out by organelles like the Golgi apparatus, which are absent in prokaryotic cells.

For this reason, eukaryotic hosts such as yeast (Saccharomyces cerevisiae) or cultured mammalian cells are often used to produce more complex recombinant proteins. These cells possess the necessary cellular machinery to correctly process the human protein, ensuring it is biologically active and identical to the one produced in the human body.

Genetic screening involves testing individuals or populations for genetic disorders or predispositions. It's crucial for early detection, informed decision-making, and disease prevention. Key applications include:

• Carrier screening: Identifying individuals who carry a recessive allele for a genetic disease (e.g., cystic fibrosis, sickle cell anaemia) without expressing symptoms themselves.
• Prenatal diagnosis: Testing a foetus for genetic abnormalities before birth, often via amniocentesis or chorionic villus sampling (CVS), followed by DNA analysis.
• Preimplantation genetic diagnosis (PGD): Used during in vitro fertilisation (IVF), where embryos are screened for specific genetic conditions before implantation into the uterus. This allows prospective parents to select embryos free from a particular disorder.

A key application is screening for cancer-predisposing genes like BRCA1 and BRCA2. Mutations in these tumour suppressor genes significantly increase the risk of breast and ovarian cancer. Identifying these mutations allows for increased surveillance and preventative measures. Another example is screening for cystic fibrosis, caused by mutations in the CFTR gene. This gene codes for a protein that transports chloride ions across cell membranes. A faulty CFTR protein leads to thick, sticky mucus, affecting multiple organs. Techniques often involve DNA probes, polymerase chain reaction (PCR), and DNA sequencing.

Gene therapy aims to treat genetic diseases by introducing new, functional genes into a patient's cells to replace or supplement faulty ones. This process can be categorised as:

• Ex vivo gene therapy: Cells are removed from the patient, modified genetically in the lab, and then returned to the patient. This offers more control but is invasive.
• In vivo gene therapy: The therapeutic genes are delivered directly into the patient's body. This is less invasive but harder to control the target cells.

The biggest challenge is efficient and safe delivery of the therapeutic gene. Common delivery systems, known as vectors, include:

• Viral vectors: Modified viruses (e.g., adenoviruses, retroviruses, adeno-associated viruses) are often used because they naturally infect cells and integrate their genetic material. They are engineered to be non-pathogenic and carry the desired gene.
  • Advantages: High efficiency of gene transfer, ability to target specific cell types.
  • Disadvantages: Potential immune response, risk of insertional mutagenesis (if integrating into host genome), limited carrying capacity.
• Liposomes: These are synthetic lipid vesicles that encapsulate the therapeutic DNA. They fuse with cell membranes, releasing their contents into the cell.
  • Advantages: Non-immunogenic, no risk of insertional mutagenesis, can carry larger DNA fragments.
  • Disadvantages: Lower transfection efficiency compared to viruses, short-term expression.

While gene therapy adds new genes, CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats and Cas9 nuclease) is a revolutionary gene editing tool that allows scientists to make precise, targeted changes to an organism's DNA.

The system works as follows:

1. A short guide RNA (gRNA) molecule is designed to match a specific target DNA sequence to be edited.
2. The gRNA forms a complex with the Cas9 enzyme, which acts like molecular 'scissors'.
3. The gRNA guides the Cas9 enzyme to the complementary target DNA sequence.
4. Cas9 then cuts both strands of the DNA at that precise location.
5. The cell's natural DNA repair mechanisms kick in. Scientists can then either:
   • Knock out a faulty gene by allowing imperfect repair.
   • Insert a new, correct DNA sequence by providing a template during repair.

CRISPR-Cas9 offers unprecedented precision and ease, opening doors for curing diseases like sickle cell anaemia and even potentially editing germline cells.

The power of genetic technology comes with significant ethical responsibilities. Cambridge examiners frequently test your ability to discuss these issues critically. Key considerations include:

• Privacy and data protection: Genetic information is highly personal. Who has access to it, and how is it used?
• Potential for eugenics and 'designer babies': Genetic screening and PGD raise concerns about selecting for 'desirable' traits (beyond disease prevention) and societal pressure to conform.
• Equity and access: Will these advanced therapies only be available to the wealthy, exacerbating health inequalities?
• Safety and unforeseen consequences: Long-term effects of gene therapy or editing are not fully understood. What are the risks of off-target edits or immune reactions?
• Germline vs. somatic cell therapy: Germline editing (changes passed to future generations) is far more controversial than somatic cell editing (changes affect only the individual treated).