Genetic Prevention: Screening and Engineering

Q1, Classifying the technologies

Newborn PKU screen plus diet: detection, followed by treatment or management [1]. No DNA sequence is altered anywhere; the test identifies a biochemical pattern and the diet removes the substrate the broken pathway cannot handle, so the phenotype is prevented while the genotype and its heritability are unchanged [1].

PGT-M: selection [1]. Embryos are biopsied and tested, and one without the variant is chosen for transfer. No embryo's DNA is altered at any point, so it reduces the chance of transmitting a known variant by choosing between existing genotypes rather than by modification [1].

Casgevy: treatment, by somatic editing [1]. DNA is genuinely altered, but only in the patient's own blood stem cells, edited outside the body and returned. Blood stem cells do not become sperm or eggs, so the change cannot be inherited and the patient's children face the same risk as before [1].

Edited and implanted embryo: prevention of inheritance, by germline editing [1]. The change is made in a very early embryo, so it appears in every cell of the resulting person including their own gametes, which makes it heritable. This is the only genetic intervention that could remove an allele from a family line, and it is prohibited in Australia [1].

Q2, Prenatal test data

2.1 NIPT is a screening test: it analyses cell-free fetal DNA in maternal blood and reports a raised or lowered chance rather than a definitive result [1]. Because the underlying rate is only 1 in 500, even a highly sensitive test produces a substantial number of false positives relative to true positives, so a positive result identifies a pregnancy needing further investigation rather than an affected pregnancy [1]. Amniocentesis is a diagnostic test that examines fetal cells directly, so it gives a definitive answer on chromosome number and needs no confirmation [1].

2.2 NIPT can be done from about 10 weeks and carries no procedure-related miscarriage risk, so it can safely be offered to everyone [1]. Amniocentesis carries about a 0.5% procedure-related miscarriage risk and cannot be performed until 15 to 20 weeks, so exposing every pregnancy to it would cause more harm than the condition it detects in the great majority who are unaffected [1]. Screening first narrows the group to those with a raised chance, so the small procedural risk is only taken where the information gained justifies it, which is a trade-off between risk, certainty and timing [1].

2.3 The error is confusing detection with prevention [1]. Neither test alters any DNA, and trisomy 21 arises from a chromosomal error that has already occurred at the point of testing, so nothing about it can be prevented by testing [1]. What the tests do is provide information: NIPT identifies a raised chance and amniocentesis confirms it, after which non-directive counselling supports whatever decision the family reaches [1].

Q3, PKU cause-and-effect chain

Step 2. The enzyme phenylalanine hydroxylase is missing or non-functional [1].

Step 3. Phenylalanine from ordinary dietary protein cannot be converted to tyrosine [1].

Step 4. Phenylalanine accumulates in the blood and brain, where high concentrations are neurotoxic and damage developing neurons [1 to 2].

Step 6. The low-phenylalanine diet breaks the chain at Step 3 and 4 by removing the substrate, so phenylalanine never reaches the toxic range and the child develops normally [1]. The genotype is unaffected because diet does not change the DNA sequence: both faulty PAH alleles remain present and are still passed to the next generation. Only the phenotype has been prevented [1].

Maximum 6 marks.

Q4, Gene therapy case study

4.1 The vector must physically reach the cells that need the gene, so the route follows the target tissue [1]. In Luxturna the target is the retinal pigment epithelium, an enclosed and directly accessible tissue, so injecting under the retina delivers the vector straight to those cells and keeps the dose local [1]. In spinal muscular atrophy the target is motor neurons distributed throughout the spinal cord, which cannot be reached by a local injection, so Zolgensma is given intravenously to distribute the vector systemically [1].

4.2 No [1]. Zolgensma delivers a working SMN1 gene to the patient's motor neurons, which are body cells, so the edit is somatic and is not present in the parents' gametes at all [1]. The parents' carrier status is entirely unchanged by treating their child, so the recurrence risk in a future pregnancy is exactly what it was before, and reducing that risk would require carrier screening with counselling, or PGT-M within IVF, rather than gene therapy [1].

4.3 Any two distinct reasons, 2 marks each. Biological specificity: each therapy targets one gene in one accessible tissue, and the gene must be small enough to fit inside the viral vector, so the approach does not generalise to conditions with many contributing genes or with targets in inaccessible tissue [2]. Timing: Zolgensma works only if given before motor neurons are lost, so the therapeutic window can close before or shortly after diagnosis [2]. Cost and access: a single dose costs around 2.5 million dollars and is funded for a small number of infants through a national program, so it cannot be scaled to population level [2]. Vector immunity: the immune system can attack the vector, which limits repeat dosing [2]. It is somatic: it treats the patient without preventing inheritance, so it does not reduce the number of affected children born [2].

Q5, Comparison table

Cells changed. Somatic: body cells such as liver, retina or blood stem cells, which are not the cells that become sperm or eggs [1]. Germline: gametes or a very early embryo, so the change appears in every cell of the resulting person, including their own gametes [1].

Inherited by children? Somatic: no. The patient may be cured while their children face exactly the same risk they always did, so it is treatment rather than prevention of inheritance [1]. Germline: yes. The change is heritable and passes into the population permanently, which makes it the only genetic intervention that could remove an allele from a family line [1].

Legal status in Australia. Somatic: permitted, and several therapies are approved, including Casgevy, Luxturna and Zolgensma [1]. Germline: prohibited. The Prohibition of Human Cloning for Reproduction Act 2002 makes it an offence to implant a genetically modified embryo [1].

Main objections to germline editing. Consent is impossible from a person who does not yet exist; any off-target changes would themselves be inherited; and the effects cannot be recalled once the change is in the population. The 2018 case in China, in which twins were born after embryo editing, drew international condemnation and a prison sentence [2].