DNA sequencing was not invented for another twenty years, but once fellow Cavendish biochemist Fred Sanger developed his eponymous Nobel-worthy sequencing method, it was natural to work out the sequence of the HbS gene mutation. One year after the mutation was genetically spelled out in 1977, Yuet Kan and Andrée Dozy reported the use of a polymorphic DNA marker adjacent to the beta-globin gene to perform prenatal genetic diagnosis for pregnant women with a family history of SCD.
Despite knowing the molecular basis of SCD for more than sixty years, a treatment has remained elusive, dreams of a cure a mirage. That may be about to change: a Bay Area biotech called Global Blood Therapeutics in 2019 had a drug called Voxelotor approved by the FDA; it binds to the mutant hemoglobin and increases its oxygen affinity, although further studies are needed to ensure the drug significantly reduces pain crises.
Several gene therapy biotechs have set their sights on treating SCD and beta-thalassemia. With a large proportion of beta-thalassemia patients lacking any beta-globin production, there are two main strategies for genetic therapy. The most straightforward would be to replace the defective gene by restoring copies of the healthy beta globin gene. On Boxing Day, 2017, a twenty-eight-year-old African American, Jennelle Stephenson, arrived at the NIH clinical center for the beginning of a major clinical trial. Asked to describe the pain she experiences on a scale of 1–10, she said it went beyond a 10, a sharp stabbing pain affecting her shoulders, back, elbows, arms, cheekbones—her entire body.19 Once she collapsed in a hospital emergency room, only to be accused by hospital staff of faking her distress to get narcotic drugs.
A team led by hematologist John Tisdale purified Stephenson’s stem cells and inserted the correct version of the beta globin gene. To ferry the gene into her cells, Tisdale and his collaborators at Bluebird Bio in Kendall Square chose a modified lentivirus vector. After chemotherapy to cripple her immune system, Stephenson received an infusion of her modified stem cells. (Bluebird’s first SCD patient, a French teenager, was treated with LentiGlobin three years earlier.)20
A few months later, Tisdale compared magnified images of Stephenson’s blood. Before, the sickled blood cells are plainly visible like a biology textbook photo. Tisdale meticulously scans the new slide but comes up empty. “Her blood looks normal,” he says. Stephenson is able to run, swim, and take judo classes, experiencing an endorphin high for the first time. NIH director Francis Collins, whose interest in the genetics of blood diseases traces back to meeting a sickle-cell patient as a young medical student in the 1970s, told 60 Minutes: “I’ve got to be careful, but from every angle that I know how to size this up, this looks like a cure.”21
Several other strategies are being tested to treat SCD patients, including tinkering with the regulatory switches of globin production. During pregnancy, fetuses produce a special form of hemoglobin, appropriately called fetal hemoglobin (HbF). This form has a higher affinity for oxygen than adult hemoglobin, all the better to pull oxygen from the maternal bloodstream. HbF is made up of four globin chains—a pair of alpha chains that also exist in adult hemoglobin, along with two gamma (γ-) chains. A few days after birth, production of γ-globin shuts down, replaced by beta globin. Reactivating the fetal form of hemoglobin in SCD and beta thalassemia patients offers a promising strategy. But where’s the switch?
In a 1948 study, Brooklyn pediatrician Janet Watson observed fewer sickle cells in two hundred “Negro newborn infants” than in older patients. “Fetal hemoglobin thus appears to lack the sickling properties of adult hemoglobin,” she concluded.22 In the 1950s, physician Richard Perrine was puzzled that some SCD patients at the Aramco oil company in Saudi Arabia had low levels of anemia, with only mild pain episodes.23 He too suspected an increase in levels of fetal hemoglobin compensated for the disease. Decades later, Collins began studying a benign genetic disorder called hereditary persistence of fetal hemoglobin (HPFH) in which, as the name suggests, fetal globin production curiously persists after birth into adulthood. Collins discovered a pair of mutations in HPFH patients located just in front, or upstream, of the start of the γ-globin gene. Collins had effectively located the site of a regulatory switch that tells the γ-globin gene to shut down. Disrupt that signal and γ-globin stays on, offering a lifeline to thalassemia and SCD patients.
It took more than twenty years to identify the genetic circuitry leading to the γ-globin switch. In 2008, researchers including Vijay Sankaran and Stuart Orkin at Boston Children’s Hospital conducted a genome-wide association study to identify gene variants associated with high levels of HbF. One of the biggest hits incriminated a gene for a zinc finger transcription factor called BCL11A, which governs this “fetal switch” by shutting down HbF by clamping onto the DNA in the regions identified by Collins’s sleuthing two decades earlier. Sankaran and Orkin nominated BCL11A as an attractive therapeutic target, suggesting that “directed down-regulation of BCL11A in patients would elevate HbF levels and ameliorate the severity of the major β-hemoglobin disorders.”24
How would one go about suppressing BCL11A? Disrupting the gene itself was the simplest route but that won’t work: BCL11A regulates many other genes including some that are expressed in the brain—patients with inherited mutations in BCL11A are on the autism spectrum. Orkin and Daniel Bauer came up with an alternative strategy. They identified a critical regulatory element that enhances BCL11A activity; if disrupted, BCL11A is switched off specifically in red blood cell precursors. By crippling the enhancer sequence, Orkin’s team would silence BCL11A selectively in the cells that give rise to mature red blood cells. In turn, this would remove the brake on HbF production while simultaneously dialing down sickle globin production.
In May 2018, twenty-one-year-old Emmanuel “Manny” Johnson Jr. became the first patient treated in a clinical trial conducted at Dana Farber Cancer Institute, led by Orkin’s colleagues David Williams and Erica Esrick. The Boston team isolated stem cells from Manny’s blood and, using a modified lentivirus, engineered the genes so that when the hematopoietic stem cell (HSC) becomes a red blood cell, the gene switch is automatically turned on. Williams’s team uses RNA interference, a Nobel Prize–winning technology. Following chemotherapy to allow the replacement cells to take hold, Manny’s modified cells were infused intravenously, setting up shop in his bone marrow, pumping out healthy red blood cells.
After seventeen years of monthly blood infusions—Manny suffered a stroke at the age of four—Manny is hoping for a cure not just for himself but also his younger brother Aiden, who has the same disease. “I’m doing this so that my brother might not need all the years of treatment I’ve had to go through,” he says. Six months later, Williams shows Manny before-and-after photographs of his magnified blood showing a frame of healthy round blood cells. “Oh wow, I’ve never seen this before, this is fantastic.”25 Manny hasn’t needed a blood transfusion in the nine months since the therapy. Williams hopes for similar results in his next patients, including twenty-six-year-old Brunel Etienne Jr., who started chemotherapy shortly after attending the Super Bowl. The tickets were a gift from New England Patriots star Devin McCourty, who has a relative with SCD.