Genetic Diseases: CF, PKU, Huntington's Disease, Type 1 Diabetes

Q1 — Sample Band 6 response (8 marks), annotated

CFTR normally functions as a chloride ion (Cl−) channel in the apical membrane of epithelial cells lining the airways. When it is open, Cl− moves from inside the cell into the airway lumen, and water follows by osmosis, keeping mucus hydrated and able to trap and clear pathogens. [1 — CFTR normal function, Cl− secretion, osmosis, mucus hydration]

The F508del mutation deletes the codon for phenylalanine at position 508. The resulting CFTR protein misfolds and is recognised by the cell’s protein quality-control machinery; it is ubiquitinated and degraded by the proteasome before it can be inserted into the cell membrane. No functional CFTR channel is present at the membrane → Cl− is not secreted → water does not follow by osmosis → mucus dehydrates and becomes thick and sticky → chronic bacterial colonisation (notably Pseudomonas aeruginosa) and progressive lung destruction. [1 — F508del mechanism: misfolding, degradation, absent from membrane, consequence cascade]

Trikafta targets specific steps in this pathway. The “correctors” elexacaftor and tezacaftor bind to the misfolded F508del CFTR protein and help it fold into the correct three-dimensional shape so it escapes proteasomal degradation and is trafficked to the cell membrane — directly addressing the folding and localisation failure. Once the corrected CFTR reaches the membrane, the “potentiator” ivacaftor binds to the channel and holds it open (increases channel gating), ensuring Cl− can flow. [1 — correctors fix folding/trafficking; potentiator restores gating; both steps explained]

The sweat chloride data provide a quantitative measure of CFTR restoration. Before Trikafta, mean sweat chloride was 86 mmol/L (well above the diagnostic threshold of 60 mmol/L). After treatment, it dropped to 45 mmol/L — into the borderline/normal range (30–59 mmol/L for intermediate, <30 normal). This indicates that CFTR function has been substantially restored, though not fully normalised (45 mmol/L is still above the healthy mean of ∼8–24 mmol/L seen in CC and Cc individuals). The >60% reduction in pulmonary exacerbations and +14% FEV₁ improvement confirm clinically meaningful benefit. [1 — quantitative evaluation using sweat Cl− data; notes partial rather than full normalisation]

Trikafta is not effective for approximately 10% of CF patients because its mechanism depends on having a misfolded but otherwise intact CFTR protein to “rescue.” Mutations that produce a premature stop codon (nonsense mutations) cause the ribosome to terminate translation early, producing a truncated, non-functional protein fragment or triggering nonsense-mediated mRNA decay — meaning little or no CFTR protein is produced at all. Correctors cannot help protein that does not exist; ivacaftor cannot potentiate a channel that is absent. Similarly, some rare mutations affect a completely different structural domain of CFTR, so the corrector drug binds in the wrong place or cannot stabilise the alternate fold. [1 — premature stop codons = no protein = correctors/potentiators have nothing to act on]

Trikafta is not a cure for CF. The CFTR gene mutation is still present in every cell; the drug must be taken daily for life to maintain its protein-rescuing effect. If Trikafta is stopped, the misfolded CFTR is degraded again and disease symptoms return. A cure would require correction of the CFTR DNA sequence itself — for example, using CRISPR-Cas9 base editing to repair the F508del deletion in airway stem cells, which remains under investigation but is not yet clinically available. Trikafta is therefore best described as a targeted, mechanism-based treatment that rescues protein function without correcting the genetic cause. [1 — not a cure; gene still mutated; daily drug required; distinguishes protein rescue from gene correction]

This case illustrates the power of the gene–protein–disease framework: by identifying the precise step at which F508del disrupts CFTR function (protein misfolding and premature degradation), researchers could design drugs that target exactly that step rather than simply managing symptoms downstream. [1 — synthesis: targeted therapy follows from understanding the pathway; integrates all components]

The remaining mark is available for a student who can also note the implication for the 10% — these patients need a fundamentally different therapeutic strategy (e.g. nonsense suppression with ataluren, RNA-based approaches, or gene therapy), because the problem is upstream: no protein is produced rather than a mis-produced protein. [1 — extension: alternative strategies needed for no-protein mutations; evaluative conclusion]

Marking criteria.

• 1 mark — Describes CFTR’s normal function accurately: Cl− channel in apical membrane → Cl− secretion → water follows by osmosis → mucus hydrated.
• 1 mark — Explains F508del mechanism accurately: deletion of codon 508 → misfolded protein → degraded before reaching membrane → no functional channel → thick dehydrated mucus.
• 1 mark — Correctly identifies how correctors (elexacaftor, tezacaftor) fix folding/trafficking and how ivacaftor (potentiator) restores gating — each drug addresses a specific step.
• 1 mark — Uses sweat chloride data quantitatively (86 → 45 mmol/L) to evaluate degree of CFTR restoration; notes partial but not complete normalisation.
• 1 mark — Explains why premature stop codons or rare mutations make Trikafta ineffective: no protein produced / wrong structural domain = nothing for corrector or potentiator to act on.
• 1 mark — Evaluates Trikafta as a protein rescue (not a cure): gene mutation still present, daily treatment required, symptoms return if stopped; contrasts with gene therapy/CRISPR as a genuine cure strategy.
• 1 mark — Synthesises the analysis into the broader lesson point: targeted therapy is only possible because the gene–protein–disease pathway was fully understood.
• 1 mark — Extension: identifies the implication for premature-stop-codon patients (different molecular strategy needed, e.g. nonsense suppression, RNA therapy, gene therapy) and evaluates what this means for CF as a heterogeneous genetic disease.

Q2 — Sample Band 6 source critique (6 marks)

Overall judgement: The statement contains one broadly correct element, three significant biological errors, and one important over-simplification. [1 — overall judgement]

Claim 1: “HD must be inherited from an affected parent”. This is mostly correct for Huntington’s disease but requires nuance. HD is autosomal dominant with 100% penetrance (for 40+ repeats), so an affected offspring almost always has an affected parent — but the repeat can also expand from a parental allele in the “intermediate” range (27–35 repeats, which does not cause disease in the parent) to a disease-causing length in the offspring. So new cases can arise without an obviously affected parent. The claim about Mendelian inheritance is correct for HD but CF is autosomal recessive (not dominant), meaning affected individuals are typically born to two unaffected carrier parents — this is not Mendelian dominance. Both do follow Mendelian inheritance but with different patterns. [1 — correct element identified and nuanced]

Claim 2: “CF and HD work the same way at the molecular level — a protein is missing.” This is incorrect. CF is a loss-of-function disease — the CFTR protein is absent or non-functional, and it is the absence of ion-channel activity that causes disease. HD is a gain-of-function disease — the mutant huntingtin protein (with its abnormally long polyglutamine tract) is present and actively toxic to neurons; having one normal HTT allele does not protect you from the toxic protein produced by the other allele. This mechanistic difference directly explains their different inheritance patterns: CF is recessive (one normal allele is enough), HD is dominant (the toxic protein acts regardless). Reformulation: “CF and HD differ fundamentally at the molecular level. CF is a loss-of-function disease (absent Cl− channel), while HD is a gain-of-function disease (a toxic mutant protein accumulates in neurons). This difference in mechanism explains why CF is autosomal recessive and HD is autosomal dominant.” [1 — correctly identifies CF=loss-of-function vs HD=gain-of-function; links to inheritance]

Claim 3: “Type 1 diabetes is not truly a genetic disease — it is really an infectious disease.” This is incorrect. Type 1 diabetes is classified as a genetic disease because the predisposition arises from inherited HLA gene variants; the disease is non-infectious and cannot be transmitted from person to person. The viral trigger (e.g. enteroviral infection) is an environmental trigger that initiates the autoimmune response in genetically susceptible individuals — it is not the direct cause of the disease and the virus does not cause diabetes in most people who encounter it. The ∼50% identical twin concordance proves the role of both genetic predisposition (if purely environmental, concordance should be ~baseline; if purely genetic, ~100%) and environmental factors. A viral trigger interacting with genetic susceptibility produces a genetic disease with an environmental component — gene–environment interaction, not an infectious disease. Reformulation: “Type 1 diabetes is a genetic disease with an environmental trigger. Inherited HLA gene variants create predisposition; environmental factors such as viral infection initiate the autoimmune destruction of beta cells in genetically susceptible individuals. It is non-infectious and cannot be transmitted between people.” [1 — correctly identifies T1D as genetic (non-infectious), explains gene–environment interaction, uses twin concordance evidence]

Claim 4: “Genetic counselling tells you exactly whether children will inherit the condition.” This is an over-simplification. For conditions with 100% penetrance and autosomal dominant inheritance (HD with 40+ repeats), genetic counselling can confirm whether an individual carries the allele and will (with very high probability) develop the disease. But for autosomal recessive conditions (CF, PKU), genetic counselling provides probabilities based on parental carrier status (e.g. 25% risk for two carriers) — not a definitive answer for each individual child. For polygenic conditions such as Type 1 diabetes, genetic counselling provides relative risk estimates, not certainties. Penetrance also varies (especially for HD at 36–39 repeats). Reformulation: “Genetic counselling provides probability-based risk estimates for a couple, based on parental genotypes and inheritance patterns. It can confirm or rule out the presence of disease-causing alleles in an individual, but except in specific high-penetrance dominant conditions, it provides risk percentages rather than certainties about whether each child will be affected.” [1 — correctly identifies probabilistic rather than deterministic nature of most genetic counselling]

Summary: The statement is substantially misleading. The only defensible core claim is that Huntington’s disease tends to be inherited from an affected parent (though with important exceptions). The conflation of loss-of-function with gain-of-function mechanisms, the misclassification of Type 1 diabetes as infectious, and the over-confidence attributed to genetic counselling all require correction. [included in marking above; 1 overall mark for structured, evidence-based overall evaluation]

Marking criteria.

• 1 mark — States an overall evaluative judgement (one correct element, three errors/over-simplifications).
• 1 mark — Correctly identifies the defensible element (HD is usually inherited from an affected parent; both diseases follow Mendelian patterns) while noting the nuance (intermediate repeats, different inheritance modes).
• 1 mark — Correctly refutes “same molecular mechanism”: identifies CF as loss-of-function (absent CFTR channel) and HD as gain-of-function (toxic polyQ protein); explains why this difference explains the difference in inheritance pattern (recessive vs dominant).
• 1 mark — Correctly refutes “Type 1 = infectious”: defines genetic disease (non-infectious, inherited, present from conception); explains gene–environment interaction; cites twin concordance as evidence; reformulates accurately.
• 1 mark — Correctly identifies “exactly” as an over-simplification: explains that genetic counselling gives probabilities (for recessive) or conditional certainties (for dominant with 100% penetrance) rather than universal exact predictions; notes polygenic conditions like T1D involve risk estimates only.
• 1 mark — Provides biologically accurate reformulations for each incorrect claim (or synthesises all corrections into a coherent overall reformulated statement) using precise lesson terminology.