Sickle Cell Disease and Thalassemia

TL;DR. Haemoglobin is the protein that carries oxygen, and roughly 250 million of them are packed into each red blood cell. Two inherited conditions break it in opposite ways. In sickle cell disease, a single letter change in the DNA swaps one amino acid, and the resulting haemoglobin sticks to itself when it releases oxygen, forming rigid fibres that deform the cell into a crescent. Those stiff cells jam in small vessels, causing episodes of excruciating pain and cumulative organ damage. In thalassemia, the body makes too little of one haemoglobin chain, so red cells are small, pale, and short-lived, and the marrow works itself into deformity trying to compensate. Both are common precisely because carriers are protected against malaria, one of the clearest examples of evolution trading one disease for another.

Key takeaways

• Sickle cell disease affects hundreds of thousands of newborns a year, most in sub-Saharan Africa, where without care the majority of affected children die before age 5.
• Sickle cell was the first "molecular disease" ever described: Linus Pauling showed in 1949 that the haemoglobin itself was abnormal, and Vernon Ingram showed in 1956 that a single amino acid was responsible.
• Carriers (sickle cell trait) are protected against severe malaria, which is why the variant reaches frequencies of 20 to 30 percent in parts of Africa.
• Three cheap interventions transformed childhood survival: newborn screening, daily penicillin from infancy, and pneumococcal vaccination. Where they are unavailable, children still die of the same infections.
• Hydroxyurea works by switching fetal haemoglobin back on, and it reduces pain crises, hospitalisation, and death.
• In 2023, the first CRISPR gene-editing therapy ever approved was for sickle cell disease and beta thalassemia. It works, and it costs around 2 to 3 million US dollars per patient.

What they are

In short: One condition makes a defective haemoglobin and the other makes too little of it, and carrying one copy is not the same as having the disease.

Haemoglobin is four protein chains, each cradling an iron-containing haem group that binds oxygen. Adult haemoglobin (HbA) is two alpha chains and two beta chains. Fetal haemoglobin (HbF) is two alpha and two gamma chains; it binds oxygen more tightly, which is how a fetus pulls oxygen across the placenta, and it is switched off in the months after birth.

Two ways to break this:

Sickle cell disease is a qualitative defect. A single base change in the beta-globin gene (GAG to GTG) replaces glutamic acid with valine at position 6. Glutamic acid is charged and water-loving; valine is greasy. The substitution creates a sticky patch on the molecule's surface, and when haemoglobin releases its oxygen and changes shape, that patch fits a pocket on a neighbouring molecule. Molecules chain together into long rigid polymers that distort the cell.

Thalassemia is a quantitative defect: not enough of one chain is made.

Don't be confused: sickle cell trait is not sickle cell disease. A person with one copy has normal life expectancy, does not have anaemia, and does not have pain crises. Conflating the two has caused real harm: people with trait have been denied jobs, insurance, and sporting opportunities. In the 1970s some US states passed mandatory screening laws that led to employment and insurance discrimination against carriers, an episode that is a standing lesson in genetic testing ethics. Trait does carry a small number of genuine considerations, including rare complications with extreme exertion and dehydration, and a risk of a rare kidney cancer, and it matters enormously for reproductive planning: two carriers have a 1 in 4 chance of an affected child with each pregnancy.

The history

In short: Sickle cell was the first disease ever defined at the molecular level, and the chain from one DNA letter to a deformed cell is still the cleanest in medicine.

1910: James Herrick, a Chicago physician, described "peculiar elongated and sickle-shaped red blood corpuscles" in a dental student from Grenada named Walter Clement Noel. The observation was actually made by Herrick's intern Ernest Irons; Noel lived until 32, dying of pneumonia, and the disease carried Herrick's name for decades.

1949: Linus Pauling and colleagues showed that haemoglobin from patients with sickle cell anaemia moved differently in an electric field from normal haemoglobin, proving the protein itself was abnormal. They called it a molecular disease, the first time any illness had been defined at that level.

1954: Anthony Allison, working in East Africa, showed that the geographic distribution of the sickle gene matched the distribution of Plasmodium falciparum malaria and that carriers had lower parasite densities. This was the first demonstration of balanced polymorphism in humans.

1956: Vernon Ingram identified the exact change: a single amino acid substitution. The chain from DNA sequence to protein shape to cell shape to clinical disease was complete, and it remains the cleanest such chain in medicine.

1970s to 1990s: newborn screening, penicillin prophylaxis (the PROPS trial in 1986 showed an 84 percent reduction in pneumococcal infection and was stopped early), and the Multicenter Study of Hydroxyurea (1995), which showed roughly a halving of painful crises.

1998 to 2000s: the STOP trial showed that screening children with transcranial Doppler ultrasound and transfusing those with high cerebral blood flow velocities reduced stroke by about 90 percent.

2023: exagamglogene autotemcel (Casgevy) approved in the UK, US, and elsewhere: the first approved therapy using CRISPR gene editing for any disease, alongside a lentiviral gene therapy, lovotibeglogene autotemcel.

What actually goes wrong

In short: Deoxygenated haemoglobin polymerises into rigid fibres, and the speed of that reaction relative to a cell's transit time explains every treatment.

Sickle cell disease

Polymerisation. Deoxygenated HbS polymerises. The process has a delay time that depends very steeply on HbS concentration, and this detail explains almost everything therapeutic. Red cells normally pass through capillaries in about 1 to 2 seconds; if polymerisation takes longer than the transit time, the cell reoxygenates in the lungs and escapes unharmed. Anything that slows polymerisation (raising fetal haemoglobin, which does not participate in the polymer; keeping cells hydrated so HbS is less concentrated; avoiding acidosis and hypoxia, which favour the deoxygenated form) prevents sickling. Anything that lengthens transit time (inflammation, adhesion, slow flow) promotes it.

Two consequences follow, and both matter:

Vaso-occlusion. Sickled cells are rigid, and their membranes become damaged and sticky after repeated cycles. They adhere to the inflamed vessel lining and to white cells, obstructing small vessels. Downstream tissue is starved of oxygen: this is the pain crisis, and it is bone infarction, which is why it hurts as it does.

Haemolysis. Sickled cells survive 10 to 20 days instead of 120, so patients are chronically anaemic. Crucially, haemoglobin released into the plasma scavenges nitric oxide, the molecule that keeps blood vessels relaxed. Chronic nitric oxide depletion produces a vasculopathy that underlies pulmonary hypertension, priapism, leg ulcers, and stroke. This explains why sickle cell disease damages organs even between crises.

Thalassemia

In beta thalassemia, too few beta chains are made, so alpha chains accumulate unpaired. Unpaired alpha chains are unstable, precipitate inside developing red cells, and destroy them in the marrow (ineffective erythropoiesis). The body responds by expanding the marrow enormously, which in untreated severe disease causes the characteristic facial and skull bone changes, fragile bones, and an enlarged spleen and liver. Iron absorption also increases inappropriately, so patients accumulate iron even before transfusions add more.

What it does to the body

In short: Pain crises are bone infarction, and the spleen, brain, lungs, kidneys, hips, and eyes accumulate damage over a lifetime.

Sickle cell disease damages nearly every organ:

Thalassemia major, untreated: severe anaemia from a few months of age as fetal haemoglobin declines, failure to thrive, bone deformity, massive spleen, and death in childhood. Treated with transfusion but without iron chelation, patients died in their teens and twenties of iron overload, which damages the heart, liver, and endocrine glands. The addition of chelation changed the disease from fatal in adolescence to compatible with adult life.

Is it deadly?

In short: Over 95 percent of affected children reach adulthood where care exists, and most die before age five where it does not.

Sickle cell disease, yes, and the gap between settings is stark:

• In sub-Saharan Africa, where most affected children are born and where newborn screening and prophylaxis are frequently unavailable, a large majority of children with sickle cell anaemia die before their fifth birthday, most from infection or severe anaemia.
• In high-income countries, over 95 percent of affected children survive to adulthood, and median life expectancy is roughly in the mid-fifties, compared with roughly 76 for the general population. That remaining two-decade gap is the current problem, and it is driven by cumulative organ damage in adults and by a documented drop-off in quality of care during the transition from paediatric to adult services.
• Roughly 300,000 to 500,000 affected babies are born each year, with the largest numbers in Nigeria, the Democratic Republic of the Congo, and India.

Thalassemia major is fatal in childhood without transfusion and, with transfusion plus chelation, allows survival into middle age and beyond.

Is it contagious?

No. These are inherited genetic conditions and cannot be transmitted by any form of contact.

They can only be passed from parents to children, and only in the pattern of autosomal recessive inheritance described in Chapter 15: two carriers have, with each pregnancy, a 1 in 4 chance of an affected child, a 1 in 2 chance of a carrier, and a 1 in 4 chance of a child with neither variant.

Who gets it, and why the genes are common

In short: The gene map traces the malaria map, because carrying one copy protects against severe malaria.

The distributions of sickle cell and thalassemia trace the historical distribution of malaria with remarkable fidelity.

Sickle cell trait protects against severe falciparum malaria, reducing the risk of severe disease and death by roughly 90 percent in some studies. Mechanisms include enhanced clearance of parasitised cells, impaired parasite growth, and reduced cytoadherence. So in a malarious environment, carriers survive better than either homozygote: better than people with two normal copies (who die of malaria) and better than people with two sickle copies (who die of sickle cell disease). That is balanced polymorphism, and it holds the variant at frequencies of 10 to 30 percent across the malaria belt.

Where: sub-Saharan Africa (highest), the Middle East, India (notably central India tribal populations), and, through the Atlantic slave trade, the Americas and the Caribbean. Thalassemias follow the Mediterranean basin, the Middle East, South and Southeast Asia, and southern China, with alpha thalassemia particularly common in Southeast Asia.

This is the clearest case in medicine of a "genetic disease of a population" that has nothing to do with any inherent property of that population and everything to do with where their ancestors lived and what was killing them. Chapter 62 develops the point.

Treatment, and how it works

In short: Three cheap childhood measures transformed survival, hydroxyurea works by switching fetal haemoglobin back on, and gene editing now cures it for two million dollars.

The childhood package that transformed survival

1. Newborn screening. Detects the disease before the first infection, which is the entire point.
2. Daily penicillin from around 2 months to at least age 5. Because the spleen is destroyed early, children are defenceless against pneumococcus. The PROPS trial found an 84 percent reduction in serious pneumococcal infection.
3. Pneumococcal, Hib, meningococcal, and other routine vaccination.
4. Parental education to recognise fever (an emergency requiring same-day assessment), splenic sequestration (a rapidly enlarging spleen and pallor), and priapism.
5. Folic acid to support high red cell turnover.

Where all four are delivered, childhood mortality falls by roughly an order of magnitude. None of it is expensive, and the gap between countries that deliver it and those that do not is the single largest determinant of survival in this chapter.

Disease-modifying treatment

Hydroxyurea is the workhorse. It increases production of fetal haemoglobin, which does not enter the sickle polymer and dilutes HbS, slowing polymerisation. It also reduces white cell and platelet counts (lowering the adhesion and inflammation that drive vaso-occlusion) and increases red cell volume and hydration. Trials show substantially fewer pain crises, fewer episodes of acute chest syndrome, less transfusion, and reduced mortality, and it works in African children as well as in high-income settings. It remains underused.

Transcranial Doppler screening and transfusion. Ultrasound measures blood flow velocity in cerebral arteries; high velocities predict stroke. Regular transfusion in those children reduced first stroke by around 90 percent in the STOP trial. This is one of the most effective preventive programmes in paediatrics.

Transfusion, either episodic for acute complications or chronic for stroke prevention. Chronic transfusion causes iron overload, requiring chelation, and can cause alloimmunisation, which is worsened by mismatch between predominantly African-ancestry patients and predominantly European-ancestry blood donors, an under-recognised argument for donor diversity.

Newer agents: L-glutamine (modest effect), crizanlizumab (an antibody blocking P-selectin, a molecule mediating cell adhesion, whose confirmatory trial did not replicate the earlier benefit), and voxelotor (which increases haemoglobin's oxygen affinity so it stays in the non-polymerising form; it raised haemoglobin levels but was withdrawn in 2024 after post-marketing data raised safety concerns). These illustrate how hard it is to improve on hydroxyurea.

Stem cell transplantation from a matched sibling donor is curative, with cure rates above 90 percent in children, and is limited by donor availability and by the risks of graft-versus-host disease and conditioning chemotherapy.

Gene therapy. Two approaches, both requiring collection of the patient's own stem cells, modification in a laboratory, and reinfusion after chemotherapy that destroys the existing marrow:

• Exagamglogene autotemcel (exa-cel, Casgevy) uses CRISPR-Cas9 to disable BCL11A, the switch that turns off fetal haemoglobin after birth. Silencing it in the patient's own stem cells reactivates fetal haemoglobin production, reproducing pharmacologically what hydroxyurea attempts. In trials, the large majority of patients became free of vaso-occlusive crises.
• Lovotibeglogene autotemcel (lovo-cel) uses a lentiviral vector to add a modified, anti-sickling beta-globin gene.
• Betibeglogene autotemcel (beti-cel) does the equivalent for beta thalassemia, freeing most recipients from transfusion.

These are genuine cures for many patients and they carry three large problems: the conditioning chemotherapy causes infertility and carries a risk of later blood cancers; the process requires months in a specialist centre; and the price, roughly 2 to 3 million US dollars, is out of reach for essentially every health system in the countries where the disease is most common. The technology arrived first for the disease whose patients can least afford it.

Thalassemia

Regular transfusion to suppress ineffective erythropoiesis, plus iron chelation (deferoxamine by infusion, or oral deferasirox and deferiprone) to remove the accumulated iron. Splenectomy in some. Luspatercept improves red cell maturation and reduces transfusion need. Stem cell transplant and gene therapy as above.

Managing a pain crisis

Hydration, oxygen if hypoxic, treatment of any precipitant such as infection, and prompt, adequate analgesia, typically including opioids for severe crises, given within 30 to 60 minutes of arrival according to guidelines.

That last point requires stating plainly. Patients with sickle cell disease report systematically worse pain management than patients with other severe pain conditions, with longer waits, lower doses, and frequent suspicion of drug-seeking. Studies in several countries have documented longer time to first analgesia in sickle cell crisis than in other acute pain presentations, and the patient population in Western countries is predominantly of African descent. This is a well-documented instance of racial bias affecting clinical care, and it is one reason individual pain protocols, held by the patient and honoured on arrival, are recommended.

What treatment costs

• Hydroxyurea: marrow suppression requiring blood count monitoring, and theoretical concerns about fertility and later malignancy that long-term follow-up has not substantiated. It is under-prescribed partly because of those concerns.
• Chronic transfusion: iron overload, alloimmunisation, and infection risk where blood screening is imperfect.
• Iron chelation: deferoxamine requires overnight subcutaneous infusions, which is a major adherence burden; oral agents cause gastrointestinal effects, kidney and liver effects, and, for deferiprone, agranulocytosis requiring monitoring.
• Transplant and gene therapy: conditioning chemotherapy causes infertility (fertility preservation should be discussed first), infection risk during marrow recovery, and a small risk of secondary malignancy.
• Opioids for chronic sickle pain: dependence and hyperalgesia are real considerations, and fear of them is used to justify under-treatment far more often than it is justified.

What the person can do

In short: Stay hydrated, take hydroxyurea, treat fever as an emergency, and carry a written pain plan because delays in analgesia are documented and unequal.

• Stay hydrated, since dehydration concentrates HbS and promotes polymerisation.
• Avoid extremes: cold exposure, very high altitude, unpressurised flight, and extreme exertion without acclimatisation are recognised triggers.
• Take hydroxyurea if prescribed, and know that it is disease-modifying rather than symptomatic.
• Treat fever as an emergency, especially in children, because of functional asplenia. Same-day assessment, not next-day.
• Attend annual screening: transcranial Doppler in childhood, retinal screening, kidney function, and blood pressure.
• Know the emergency symptoms: chest pain and breathlessness (acute chest syndrome), sudden weakness or speech difficulty (stroke), a rapidly enlarging tender abdomen with pallor (splenic sequestration), and priapism lasting more than 2 to 4 hours. All are same-hour emergencies.
• Have a written individual care plan covering usual analgesia, doses that work, and known complications, to shortcut the delays described above.
• Get carrier testing before pregnancy if you or your partner have relevant ancestry. Many countries offer antenatal screening, and knowing carrier status changes reproductive options and removes an unnecessary shock later.
• In thalassemia trait, be aware you may be repeatedly and wrongly treated for iron deficiency: the anaemia looks similar on a basic blood count, and iron supplementation will not help and can cause overload.

Living with it

Sickle cell disease is a case study in how disease burden and research funding diverge. It affects millions of people, predominantly of African ancestry, and has historically received a small fraction of the research and philanthropic funding of conditions such as cystic fibrosis, which affects far fewer people. Analyses comparing funding per affected patient between the two have found differences of an order of magnitude.

The daily reality for adults includes chronic pain, unpredictable crises that disrupt work and education, repeated emergency department visits where they are frequently disbelieved, and a transition from paediatric care (usually excellent and specialised) to adult care (often fragmented), during which mortality measurably increases.

Thalassemia's burden is different: not pain but the relentless schedule of transfusion every few weeks and chelation daily, for life, and the endocrine complications of iron that appear in early adulthood.

What's next

• In vivo gene editing, delivering the editing machinery directly into the body rather than removing, editing, and returning stem cells. This would eliminate the conditioning chemotherapy and the specialist infrastructure, and it is the only plausible route to affordable global access.
• Cheaper, simpler curative therapy, which is the central equity problem: the disease is concentrated in low-income countries and the cure costs millions.
• Hydroxyurea scale-up in Africa, where trials such as REACH have shown it is safe and effective in children in malaria-endemic settings, and where coverage remains low.
• Universal newborn screening in high-burden countries, which is cheap, proven, and still absent in most of sub-Saharan Africa.
• Better fetal haemoglobin inducers, taken as tablets, aiming to reproduce the gene therapy effect pharmacologically.

Sources and notes

Herrick's case: Archives of Internal Medicine, 1910. Pauling et al., "Sickle Cell Anemia, a Molecular Disease," Science, 1949. Ingram, Nature, 1956 and 1957. Allison, BMJ, 1954, on malaria protection. PROPS penicillin prophylaxis trial: Gaston et al., NEJM, 1986 (84 percent reduction). MSH hydroxyurea trial: Charache et al., NEJM, 1995. STOP transcranial Doppler trial: Adams et al., NEJM, 1998. Stroke incidence by age 20: Ohene-Frempong et al., Blood, 1998. Life expectancy estimates: US registry and modelling studies, commonly cited as mid-fifties versus roughly 76 for the general population. Under-five mortality in sub-Saharan Africa: Grosse et al., American Journal of Preventive Medicine, 2011, and WHO estimates. Exa-cel: Frangoul et al., NEJM, 2021 and 2024; approvals from late 2023. Voxelotor withdrawal: 2024 regulatory action. Analgesia disparities in sickle cell crisis: multiple emergency medicine studies, including Haywood et al. and Lazio et al. Research funding comparisons: Farooq et al., JAMA Network Open, 2020.

Open questions. Why crisis frequency varies so much between patients with identical genotypes is not fully explained. Whether gene therapy's benefits are lifelong is not yet known. How to deliver curative therapy at a price the affected countries can pay is unresolved.

Next: the rest of the diseases written into single genes, and the therapies that are starting to correct them. 👉