The Single-Gene Diseases

TL;DR. Most disease is polygenic and probabilistic. A small group is neither: one broken gene, one broken protein, and a predictable consequence. Cystic fibrosis breaks a chloride channel, so secretions everywhere become thick. Duchenne muscular dystrophy breaks the shock absorber that protects muscle fibres, so muscle tears itself apart with use. Huntington's disease adds too many repeats to one gene, and the resulting protein kills specific neurons on a schedule set by the repeat count. Haemophilia removes one clotting factor from a cascade of twelve. These conditions are individually rare and collectively common, and they are where medicine is currently changing fastest, because when you know exactly which molecule is broken, you can design something that fixes it. Two of them now have treatments that were science fiction fifteen years ago.

Key takeaways

• Rare diseases affect around 300 million people worldwide; roughly 72 percent are genetic, and about 70 percent of those begin in childhood.
• Cystic fibrosis is the clearest success story in this book. Modulator drugs that repair the folding and gating of the defective protein have moved median predicted survival from under 5 years in the 1960s to well beyond 50 for children born today.
• Huntington's disease is the cleanest genetic disease and the cruellest: autosomal dominant, essentially fully penetrant, with the age of onset predicted by the number of DNA repeats, and testable decades before symptoms.
• Gene therapy has arrived and it is extraordinarily expensive. Approved one-time treatments for spinal muscular atrophy, haemophilia, and other conditions carry list prices from roughly 1 to 4 million US dollars.
• Newborn screening is the highest-value intervention in this field: a heel-prick blood spot that finds treatable conditions before irreversible damage.
• Getting a diagnosis at all remains the biggest problem for most families, with a typical diagnostic odyssey of several years and multiple wrong diagnoses.

Cystic fibrosis

In short: One broken chloride channel thickens every secretion in the body, and drugs that repair the protein have moved survival from childhood to beyond fifty.

What it is. An autosomal recessive disease caused by mutations in CFTR, a gene encoding a channel that moves chloride ions across cell membranes. Water follows chloride, so the channel determines how hydrated secretions are.

What goes wrong. Without functioning CFTR, the fluid layer on epithelial surfaces is thin and salty and the mucus above it is thick and sticky. The consequences follow that one fact:

There are over 2,000 CFTR variants, grouped by what they do to the protein: not made at all, misfolded and destroyed before reaching the membrane (F508del, the commonest, present in about 70 percent of alleles in European-descended populations), reaching the membrane but failing to open properly, or opening too briefly.

Who gets it. About 1 in 2,500 to 3,500 births in populations of European descent, with a carrier frequency around 1 in 25. Far less common in African and Asian populations, which has produced a serious diagnostic bias: CF in a non-white child is frequently diagnosed later, because it is not considered.

Treatment, and the revolution. Conventional care is a daily grind: airway clearance physiotherapy, inhaled mucolytics (dornase alfa, which digests the DNA released by dead neutrophils that makes the mucus so viscous), inhaled hypertonic saline, inhaled and oral antibiotics, pancreatic enzyme replacement with every meal, high-calorie nutrition, fat-soluble vitamins, and, for advanced disease, lung transplantation.

Then came the CFTR modulators, drugs designed against the specific molecular defect:

• Potentiators (ivacaftor) hold the channel open in variants that reach the membrane but gate poorly. Approved in 2012 for a variant affecting about 4 percent of patients, with striking effects.
• Correctors (lumacaftor, tezacaftor, elexacaftor) help the misfolded F508del protein fold correctly and reach the cell surface.
• Triple therapy (elexacaftor/tezacaftor/ivacaftor) treats the great majority of patients and produced improvements in lung function, sweat chloride, weight, and exacerbations larger than anything previously seen in CF.

The result: median predicted survival for children born with CF today in countries with access is now beyond 50 years, compared with early childhood in the 1960s. The remaining problems are patients with variants no modulator addresses (roughly 10 percent, disproportionately those of non-European ancestry, whose variants were studied less), and price, which has kept these drugs out of many health systems.

Huntington's disease

In short: The number of DNA repeats predicts the age of onset, which makes this the cleanest genetic disease in medicine and the cruellest to be tested for.

What it is. An autosomal dominant neurodegenerative disease caused by an expanded CAG repeat in the HTT gene. CAG codes for glutamine, so the protein carries an abnormally long polyglutamine tract, which makes it misfold and aggregate and gives it toxic new functions.

Anticipation: the repeat tends to expand further when transmitted, particularly through the father, so successive generations can have earlier onset. Repeat length inversely correlates with age of onset: more repeats, earlier disease.

What it does. Onset typically between 30 and 50, with three domains of decline:

• Movement: chorea, involuntary, flowing, dance-like movements, later giving way to rigidity and difficulty with voluntary movement, swallowing, and speech.
• Cognition: progressive loss of executive function, planning, and flexibility, ending in dementia.
• Psychiatric: depression, irritability, apathy, and obsessive behaviour, often appearing years before the movement disorder. Suicide risk is substantially elevated, particularly around the time of diagnosis and early functional decline.

Neurons of the striatum die first, particularly the indirect-pathway neurons that suppress unwanted movement, which is exactly why the earliest motor sign is movement that cannot be suppressed, the mirror image of Parkinson's disease (Chapter 38).

Progression takes 15 to 20 years from onset to death, usually from pneumonia or complications of immobility.

Treatment. Nothing yet slows it. Chorea can be suppressed with tetrabenazine, deutetrabenazine, or antipsychotics; depression, irritability, and psychosis are treated; speech, swallowing, and physiotherapy support function; and genetic counselling supports the family.

The testing question. Because the gene was identified in 1993 and the test is definitive, an at-risk 25-year-old can learn with certainty whether they will develop an untreatable fatal disease in their forties. Only a minority choose to be tested, commonly estimated at 10 to 25 percent, and formal protocols require counselling before and after. Reasons to test include reproductive planning and life decisions; reasons not to include the absence of any treatment and the psychological weight of certainty. Both choices are legitimate, and the field's guidelines are explicit that the decision belongs to the person.

What's next. Huntington's is the ideal target for gene silencing, because the cause is one known toxic gene. Antisense oligonucleotides and RNA interference agents that lower huntingtin production have entered trials, with the largest antisense trial halted in 2021 for lack of benefit and signs of harm, and refined approaches (allele-selective silencing, and a gene therapy delivered directly into the brain reporting encouraging early results in 2025) continuing. This remains the field where a genuine disease-modifying treatment is most plausible in the near term.

Duchenne and Becker muscular dystrophy

In short: Without dystrophin, muscle tears itself apart with ordinary use, and steroids plus ventilation have moved survival from the teens into the thirties.

What it is. X-linked recessive, caused by mutations in the DMD gene encoding dystrophin, the largest gene in the human genome. Dystrophin links the muscle fibre's internal skeleton to the membrane and surrounding matrix, acting as a shock absorber during contraction.

Duchenne results from mutations that abolish dystrophin entirely: muscle fibres tear with normal use, degenerate, and are progressively replaced by fat and fibrous tissue. Becker results from mutations producing a shortened but partly functional protein, and is milder and later.

What it does. Boys with Duchenne appear normal at birth, walk late, and by around 3 to 5 years show difficulty running, climbing stairs, and rising from the floor. The Gowers manoeuvre, walking the hands up the thighs to stand, is characteristic. Calves look enlarged (pseudohypertrophy) because muscle is replaced by fat. Without treatment, wheelchair dependence follows around age 10 to 12, then scoliosis, then respiratory muscle failure, then cardiomyopathy. About a third have some degree of learning difficulty, since dystrophin is also expressed in the brain.

Incidence is roughly 1 in 3,500 to 5,000 male births. About a third of cases arise from new mutations, so there is often no family history.

Treatment. Corticosteroids delay loss of ambulation by roughly 2 to 3 years and are standard despite substantial side effects. Non-invasive ventilation at night, cardiac drugs (ACE inhibitors and beta blockers started before symptoms), scoliosis surgery, and physiotherapy have together moved median survival from the late teens into the thirties.

Genetic approaches are advancing unevenly. Exon-skipping antisense drugs make the cell skip over the faulty exon, restoring the reading frame and producing a shortened Becker-like dystrophin; several are approved for specific mutations on the basis of increased dystrophin production, with clinical benefit still debated. A micro-dystrophin gene therapy was approved in 2023, delivering a shortened but functional gene by viral vector; the full gene is far too large to package. Editing approaches are in development.

Haemophilia

In short: One missing clotting factor, a history entangled with European royalty and with the contaminated blood disaster, and now an antibody that does the missing factor's job.

What it is. X-linked recessive deficiency of a clotting factor: factor VIII in haemophilia A (about 1 in 5,000 male births) and factor IX in haemophilia B (about 1 in 30,000). Clotting is a cascade in which each factor activates the next; remove one and the cascade stalls, so a clot forms slowly and poorly.

What it does. Bleeding into joints (haemarthrosis) is the characteristic problem: repeated bleeds destroy cartilage and produce a crippling arthropathy by early adulthood in untreated patients. Also muscle bleeds, prolonged bleeding after injury or surgery, and, most dangerously, intracranial haemorrhage.

Its history is entangled with two others. It is the disease of European royalty, carried by Queen Victoria and transmitted through her daughters into the Spanish, German, and Russian royal houses, most consequentially to the Tsarevich Alexei, whose illness drew Rasputin into the Russian court. And in the 1980s, factor concentrates pooled from thousands of plasma donors transmitted HIV and hepatitis C to a large proportion of people with haemophilia worldwide, a contaminated-blood disaster that killed thousands and has been the subject of public inquiries in several countries.

Treatment. Modern care is prophylactic factor replacement, given intravenously several times a week, or extended-half-life products requiring less frequent dosing. Emicizumab is an ingenious workaround: a bispecific antibody that grabs factor IXa with one arm and factor X with the other, physically holding them together the way factor VIII normally does. It is given subcutaneously every one to four weeks and works even in patients who have developed inhibitory antibodies against factor VIII, which had been the hardest problem in haemophilia care.

Gene therapy for both haemophilia A and B has been approved, delivering a working factor gene to liver cells with a viral vector. Single infusions have produced years of factor production and freedom from routine treatment, with durability still being established. Prices are among the highest of any medicine, and one product was withdrawn commercially despite working, because too few patients were treated to sustain it, which is a warning about the economics of one-time cures for rare diseases.

Down syndrome and the chromosomal conditions

In short: An extra chromosome rather than a broken gene, and life expectancy more than doubled once society changed what it did with the people who had it.

Down syndrome is not a single-gene disease but a chromosomal one: three copies of chromosome 21 instead of two (trisomy 21), usually from failure of the chromosome pair to separate during egg formation. Incidence rises with maternal age, from roughly 1 in 1,500 at age 20 to about 1 in 100 at 40, though most affected babies are born to younger mothers because younger women have more babies.

The extra chromosome means an extra dose of about 200 genes, producing characteristic facial features, low muscle tone, intellectual disability of variable degree, and a specific medical profile: congenital heart defects in roughly half, higher rates of hearing and vision problems, thyroid disease, coeliac disease, atlantoaxial instability, leukaemia in childhood, and early-onset Alzheimer's disease, the last because the APP gene sits on chromosome 21 (Chapter 37).

Life expectancy has risen from around 25 in 1980 to about 60 today, driven mostly by cardiac surgery and by the end of institutionalisation. That change is a reminder that outcomes attributed to a chromosome were substantially determined by what society did with the people who had it.

Prenatal screening by cell-free fetal DNA in maternal blood is now widespread and highly accurate, and has reduced the number of babies born with Down syndrome substantially in several countries. This has prompted a serious ethical debate, in which disability rights advocates argue that screening programmes communicate a judgement about which lives are worth living, and that prospective parents are frequently given outdated and negative information about outcomes. The medical facts and the ethical questions are separable, and both belong in any honest account.

Other chromosomal conditions: Turner syndrome (a single X in females: short stature, ovarian failure, cardiac and renal anomalies), Klinefelter syndrome (XXY in males: tall stature, reduced testosterone, infertility, often diagnosed late or never), 22q11.2 deletion syndrome (heart defects, immune deficiency, palate abnormalities, and high rates of psychiatric illness), and fragile X syndrome (a repeat expansion on the X chromosome, the commonest inherited cause of intellectual disability and a leading single-gene cause of autism).

Others worth knowing

In short: Six conditions where a specific intervention changed everything, from a diet in PKU to a two-million-dollar infusion in spinal muscular atrophy.

Is any of this contagious?

No. Nothing in this chapter can be transmitted between people. These conditions pass only from parents to children, in the patterns set out in Chapter 15, and a substantial fraction arise as brand new mutations in a child with no family history at all.

How they are found

In short: Newborn screening is the highest-value intervention, and sequencing has shortened the diagnostic odyssey that used to take families years.

Newborn screening is the single highest-value intervention. A few drops of blood from a heel prick in the first days of life, tested for a panel of conditions that are treatable and where early treatment prevents irreversible damage. Panels vary by country and typically include phenylketonuria, congenital hypothyroidism, cystic fibrosis, sickle cell disease, and a set of metabolic disorders; spinal muscular atrophy has been added in many countries precisely because presymptomatic treatment works so much better.

Carrier screening before or during pregnancy, either targeted by ancestry or as expanded panels covering hundreds of conditions.

Prenatal testing: cell-free fetal DNA screening from maternal blood, with diagnostic confirmation by chorionic villus sampling or amniocentesis. Preimplantation genetic testing allows embryos created by IVF to be tested before transfer.

Diagnostic sequencing for a child with unexplained symptoms. Exome or genome sequencing now resolves roughly 30 to 50 percent of previously undiagnosed cases, and rapid genome sequencing in critically ill newborns can return an actionable answer within days. Before this, families spent an average of several years and many specialist referrals reaching a diagnosis, a process families call the diagnostic odyssey, and a name for the condition, even without a treatment, changes prognosis discussions, recurrence risk, and access to support.

Treatment: the four strategies

1. Replace the missing product. Factor VIII in haemophilia, enzyme replacement in Gaucher and Fabry disease, thyroxine in congenital hypothyroidism.
2. Remove or restrict what accumulates. The phenylalanine-restricted diet in PKU, venesection in haemochromatosis, copper chelation in Wilson's disease.
3. Fix the protein. CFTR modulators, which repair folding and gating of the defective channel. This works only where a protein is made and is fixable, and it is the model everyone wants to copy.
4. Fix or bypass the gene. Antisense oligonucleotides that change how a message is spliced (nusinersen, exon-skipping in Duchenne), gene addition by viral vector (haemophilia, SMA, Duchenne), and gene editing (CRISPR for sickle cell and beta thalassemia, Chapter 48).

What it costs

In short: The technical problem is being solved faster than the economic one, and one working gene therapy has already been withdrawn for commercial reasons.

The technical problem is being solved faster than the economic one.

Approved one-time gene therapies carry list prices from roughly 1 million to over 4 million US dollars. The manufacturers' argument is that a single curative treatment replaces a lifetime of expensive care, which for haemophilia is arithmetically defensible. The problems are that health systems budget annually rather than over a lifetime, that durability is unproven for most of these products, that the patient populations are too small to spread development costs, and that essentially none of this reaches the low- and middle-income countries where many affected children live.

Two consequences are already visible: at least one approved gene therapy has been withdrawn from the market for commercial rather than clinical reasons, and health systems are experimenting with outcome-based payment models in which the manufacturer is paid only if the treatment keeps working.

There is also a structural gap: for the several thousand rare diseases with fewer than a handful of known patients, no commercial development model exists at all, which is what orphan drug legislation, academic gene therapy centres, and n-of-1 antisense programmes are trying to address.

What the person and family can do

In short: Push for a genetic diagnosis, get counselling, find the patient organisation, and insist on specialist centre care, which measurably changes outcomes.

• Ask for a genetic diagnosis when a child has unexplained developmental delay, muscle weakness, recurrent unexplained illness, or a pattern that does not fit. Sequencing is now fast and comparatively cheap, and the odyssey is shorter than it used to be if someone starts it.
• Get genetic counselling, before testing and after. It is not an administrative step; it is the part that determines whether the information is useful.
• Take up newborn screening and, if a condition is found, engage with the specialist centre early, because presymptomatic treatment is dramatically better for several of these conditions.
• Consider carrier screening before pregnancy if there is relevant family history or ancestry.
• Find the patient organisation. For rare disease, these groups frequently hold better practical knowledge than any individual clinician, run registries that make research possible, and connect families to specialist centres.
• Ask about clinical trials. For many of these conditions, trials are where the effective treatments are.
• Insist on multidisciplinary specialist care. Outcomes in cystic fibrosis, Duchenne, and haemophilia differ measurably between specialist centres and general care.

What's next

• In vivo editing, correcting genes inside the body rather than in a laboratory dish, which would remove the chemotherapy conditioning and the specialist infrastructure that make current gene therapy so restricted.
• Base and prime editing, which change single DNA letters without cutting both strands, in early human trials.
• Personalised antisense drugs designed for a single patient's mutation, following the precedent of milasen, developed in under a year for one child with a unique variant, which has created an entirely new regulatory category.
• Newborn genomic screening, currently in pilot programmes sequencing healthy newborns for hundreds of treatable conditions. The technical case is strong and the ethical questions (consent for a person who cannot give it, findings of uncertain significance, and what to do about untreatable conditions) are unsettled.
• Affordability, which is now the binding constraint on the entire field.

Sources and notes

Rare disease population estimates: Nguengang Wakap et al., European Journal of Human Genetics, 2020 (approximately 300 million people worldwide; about 72 percent genetic in origin). CF median predicted survival: Cystic Fibrosis Foundation and UK CF Registry annual reports. Elexacaftor triple therapy: Middleton et al., NEJM, 2019. Huntington gene identification: Huntington's Disease Collaborative Research Group, Cell, 1993. Predictive testing uptake: multiple international cohorts, commonly 10 to 25 percent of at-risk individuals. Duchenne incidence and natural history: standard neuromuscular references; corticosteroid effect on ambulation from long-term cohort data. Emicizumab: HAVEN trial programme, NEJM, 2017 to 2018. Contaminated blood products: national inquiry reports, including the UK Infected Blood Inquiry, 2024. Down syndrome life expectancy: Presson et al., Journal of Pediatrics, 2013, and national registry data. Tay-Sachs carrier screening impact: Kaback et al., JAMA, 1993. SMA treatments: Finkel et al., NEJM, 2017 (nusinersen) and Mendell et al., NEJM, 2017 (gene therapy). Milasen: Kim et al., NEJM, 2019. Diagnostic yield of exome and genome sequencing: multiple meta-analyses, typically 30 to 50 percent depending on phenotype and prior testing.

Open questions. The durability of gene therapy effects is unknown for most products. Whether exon-skipping drugs for Duchenne produce clinically meaningful benefit is genuinely contested. Whether population newborn genomic screening should be implemented, and with what consent model, is unresolved.

Next: the diseases that arrive simply because the body has been used for a long time. 👉