Zeroing in on the fetal-to-adult hemoglobin switch and a new way to combat sickle cell disease

Normal red blood cell vs. sickle-shaped blood cell, which is found in sickle cell disease
Normal red blood cell vs. sickle-shaped blood cell.

It’s been known for more than 40 years that in rare individuals, lingering production of the fetal form of hemoglobin — the oxygen-transporting protein found in red blood cells — can reduce the severity of certain inherited blood disorders, most notably sickle cell disease and thalassemia. Typically, however, a protein called BCL11A switches off fetal hemoglobin production past infancy, but exactly how this happens has not been well understood until now.

In a new paper in Cell, researchers at Dana-Farber/Boston Children’s Cancer and Blood Disorders Center have revealed how BCL11A controls the switch in the body’s production of fetal hemoglobin to adult hemoglobin. It does so by binding to a DNA sequence — made up of the bases T-G-A-C-C-A — that lies just in front of the fetal hemoglobin genes.

Another approach to curing sickle cell disease is already being evaluated in a new clinical trial at Dana-Farber/Boston Children’s. The novel gene therapy restores fetal hemoglobin production by genetically suppressing BCL11A, which prevents it from blocking fetal hemoglobin production. Learn more.

“Genetically modifying this TGACCA segment could be another possible strategy to cure sickle cell disease by blocking BCL11A’s ability to bind to this DNA site and switch off fetal hemoglobin production,” says Stuart Orkin, MD, senior author on the study.

The BCL11A protein, once bound to this TGACCA segment, blocks fetal hemoglobin production. As fetal hemoglobin is shut off, adult hemoglobin production takes its place.

“Normally, this switch happens within the first six months after birth,” says Orkin, who is also the David G. Nathan Distinguished Professor of Pediatrics at Harvard Medical School (HMS) and an Investigator of the Howard Hughes Medical Institute. “Therefore, in individuals with a genetic mutation in the adult hemoglobin gene, such as is the case in sickle cell disease, it is starting at this time in a child’s life that production of the abnormal adult hemoglobin begins to cause adverse health consequences.”

Sickle cell disease is caused by a single change in the DNA of the adult hemoglobin gene. In contrast to normal, round-shaped hemoglobin, the abnormal adult hemoglobin (called HbS) assembles into rigid rod shapes that distort the normal disc-like shape of red cells. These deformed, sickle-shaped cells get stuck in small blood vessels, which eventually leads to anemia, pain crises and a risk of strokes.

“If we can prevent the mutant adult hemoglobin from causing these defective red blood cells, we can stop sickle cell disease in its tracks,” says Orkin. “To do this, increasing fetal hemoglobin in patients is a promising solution.”

Substituting fetal hemoglobin for the abnormal HbS is especially good at blocking formation of the rigid rods, giving red bloods cells a normal shape.

Finding a needle in a haystack

Orkin’s team was able to make the connection between BCL11A and the TGACCA DNA sequence through collaboration with Martha Bulyk, PhD, of HMS and Brigham and Women’s Hospital.

“Basically, Martha has developed a technique that allows you to determine which DNA sequence a protein will bind to if you don’t know it to begin with,” Orkin says.

After determining that BCL11A binds to a TGACCA sequence of DNA, Orkin and collaborators coupled this knowledge with a new technique for locating proteins inside chromosomes, the structures in cell nuclei that contain DNA. Called “CUT&RUN”, the technique allowed Orkin’s team to zero in on the specific TGACCA site that BCL11A uses to shut off the fetal globin genes.

Remarkably, this TGACCA site is the same place where other researchers have found genetic mutations in very rare individuals who continue to produce fetal hemoglobin at a high level into adulthood. These people are naturally protected from sickle cell disease and thalassemia.

“We’ve figured out what BCL11A binds to, we’ve linked it to naturally-occurring genetic mutations and we’ve shown that this must be the way BCL11A normally represses fetal hemoglobin production,” Orkin says.

And with that, a 40-year-long scientific case is finally closed. In turn, new doors toward treating adult hemoglobin disorders have been blown open.

This research was done in collaboration with Daniel E. Bauer, MD, a hematologist/oncologist at Boston Children’s, and Guo-Cheng Yuan, PhD, Associate Professor of Biostatistics and Computational Biology at Dana-Farber Cancer Institute. In addition to Orkin, Bulyk, Bauer and Yuan, additional authors on the study are Nan Liu, Victoria Hargreaves, Qian Zhu, Jesse Kurland, Jiyoung Hong, Woojin Kim, Falak Sher, Claudio Macias-Trevino, Julia Rogers, Ryo Kurita, Yukio Nakamura and Jian Xu.

The work was funded by the National Institutes of Health and the Howard Hughes Medical Institute.