HBB

In one line: A single A→T substitution in the HBB gene turns one amino acid of beta-globin from glutamic acid into valine — and that lone change is enough to bend a round red blood cell into a sickle. It took 75 years to go from understanding that to curing it.

---

The gene

HBB sits near the tip of the short arm of chromosome 11 (band 11p15.4) and codes for beta-globin, one of the four protein chains of adult hemoglobin (two alpha, two beta — Hb A = α₂β₂), the molecule that ferries oxygen around your body. Beta-globin is just 146 amino acids long. Change one of them, and the oxygen-carrier you depend on can turn against you.

The hunt (1949 → 1957)

In 1949, Linus Pauling, Harvey Itano, S. J. Singer and Ibert Wells ran hemoglobin from healthy people and from sickle-cell patients through an electrophoresis cell and watched the two migrate differently: the sickle hemoglobin carried a slightly different electric charge. The defect, they argued, wasn't in the cell or the tissue — it was in a single kind of molecule. They named the category that paper created: the first "molecular disease." Crucially, they had not yet found what was different about the molecule.

That came from Vernon Ingram. In 1956–57, using protein "fingerprinting" (digesting the chain into peptides and separating them), he located the difference to a single spot: the sixth amino acid of the beta chain, normally glutamic acid, was replaced by valine. One substitution, in one protein — the first time a human disease was pinned to a change that precise. (Modern HGVS nomenclature writes the variant as HBB c.20A>T, p.Glu7Val, counting the initiator methionine; the classic literature — and this video — use the mature-protein numbering, position 6.)

The mechanism

The gene is read three letters at a time. The healthy codon GAG specifies glutamic acid; flip the middle base — A→T — and it becomes GTG, which specifies valine. Glutamic acid is hydrophilic; valine is hydrophobic. That swapped residue creates a sticky patch on the surface of the hemoglobin molecule, and when oxygen is low, the mutant molecules (HbS) polymerize into stiff fibers. Those fibers distort the normally round, flexible red cell into a rigid sickle, which lodges in small blood vessels — the cause of the vaso-occlusive crises, pain, and cumulative organ damage of sickle cell disease.

The 75-year gap, and the cure (2023)

For most of the twentieth century we understood sickle cell almost perfectly and could do very little about it — it remains one of the most common inherited diseases in the world. Then, on December 8, 2023, the FDA approved Casgevy (exagamglogene autotemcel), the first approved medicine built on CRISPR gene-editing.

The elegant twist: Casgevy does not repair the HBB mutation at all. Instead it uses CRISPR-Cas9 to disable the erythroid-specific enhancer of the BCL11A gene in a patient's own blood stem cells. BCL11A is the switch that, after infancy, shuts off fetal hemoglobin (HbF, α₂γ₂). Knock the switch out, and the patient's cells start making fetal hemoglobin again — and because HbF has no beta chains, it cannot sickle. The edited cells are infused back, and in trials the recurrent pain crises largely disappeared. One molecule understood in 1949; one infusion, 75 years later.

The open thread isn't the biology anymore — it's access. A one-time list price around $2.2M, plus the hospital course it requires, puts this cure out of reach for most of the people who carry the gene. That gap is the next story.

Sources

Full claim-by-claim evidence is in references.md. Primary/authoritative anchors:

• Pauling L, Itano HA, Singer SJ, Wells IC. "Sickle Cell Anemia, a Molecular Disease." Science 110:543 (1949). https://www.science.org/doi/10.1126/science.110.2865.543
• Ingram VM. (1956, 1957) single amino-acid substitution in sickle hemoglobin — see https://www.pnas.org/doi/10.1073/pnas.0406677101
• HBB gene (location 11p15.4, structure): NCBI Gene/GTR 3043 — https://www.ncbi.nlm.nih.gov/gtr/genes/3043/ ; MedlinePlus — https://medlineplus.gov/genetics/gene/hbb/
• HbS variant HBB c.20A>T (p.Glu7Val), rs334: ClinVar — https://www.ncbi.nlm.nih.gov/clinvar/RCV000016573.11/
• Casgevy mechanism (BCL11A enhancer → HbF) + Dec 8 2023 approval: PMC10913280 — https://pmc.ncbi.nlm.nih.gov/articles/PMC10913280/ ; Drugs.com — https://www.drugs.com/history/casgevy.html

Accuracy note: the discovery beats are deliberately kept distinct — Pauling (1949) showed sickle cell is a molecular disease; Ingram (1956–57) identified the amino-acid substitution. And Casgevy edits BCL11A to reactivate fetal hemoglobin; it does not correct HBB.