When we feel our reality is escaping us, we search for the pulsation of blood through the capillaries in our fingers, or within the grooves of our wrists, or against the bones lining our chest.
Normal blood, that is.
Some people feel blood that should not be felt — blood trapped in the forks of their arteries that causes immeasurable pain (1). Those same people also cannot feel blood when it ought to be felt — blood the body recognizes as damaged and removes in haste, leaving little to circulate (2).
That’s not normal blood.
Sickle cell disease (SCD) is an autosomal recessive illness marked by the distortion of red blood cells (RBCs) as hemoglobin, the oxygen-carrying globular protein, devolves into a spindle shape through a process called polymerization (1). Consequently, these RBCs are shaped like the farming tool used to reap harvest, though the only benefit anyone with sickle cells is reaping is protection against severe malaria (2). These cells are inflexible and thus pose clotting and oxygen perfusion issues, possibly leading to strokes, pulmonary infarctions (lack of blood flow to lung tissue), and multi-organ failure (3). Currently, around 300,000 newborns suffer from the disease per year and are typically of sub-Saharan, Mediterranean, Middle Eastern, or Indian descent (4).
The irony is that it only takes one mutation in the beta-globin gene HBB to drive a disease of such international and physiological magnitude (2). The mutation is a point replacement of glutamic acid with valine (5). This lack of genetic complexity actually poses an advantage in therapeutic design.
One promising avenue for treating SCD is through base editing of hematopoietic stem and progenitor cells (HSPCs) which are precursors to blood cells (1). This novel method can correct single mutations without introducing unwanted effects as CRISPR-Cas9 gene editing does (6). Base editing avoids breaking double-stranded DNA by employing nickase Cas9, a mutated form of the original Cas9 enzyme that can recognize unusual DNA sequences and generate just a single-strand break (7). An enzyme called adenosine deaminase binds to nickase Cas9 and “nicks” the top strand (7). The DNA replication pathway gets triggered and ultimately a pair of nucleotides is converted (7).
Standard CRISPR-Cas9 gene editing, while revolutionary in correcting stretches of mutations, falls short for most diseases caused by point mutations (1,7). Cas9 can cause large deletions and activate the tumor suppression pathway, potentially inducing apoptosis (cell death) in these benign, engineered cells (8,9). Additionally, in administering gene therapies with external DNA, physicians run the risk of excess, toxic insertions; base editing relies on only ribonucleic acids and proteins, yielding safer results (1).
A team of researchers led by Dr. David Liu at the Broad Institute of Harvard and MIT employed adenine base editors (ABEs) to correct for the HBB mutation that causes SCD (1). ABEs cannot “undo” the amino-acid changing mutation. However, they can edit the middle nucleotide of the codon to yield a normal, non-pathogenic HBB variant called the Makassar allele (1). The group reported a conversion rate between 44% and 80% with an extraneous mutation rate of less than 2% to 5% (1). These results are truly promising since only 20% of HBB alleles need to be converted from pathogenic to Makassar to “rescue” the normal blood phenotype (1). Base editing yielded a lower percentage of sickle cells than current therapeutic procedures (1).
Base editing, though an excellent improvement from conventional methods of gene editing, still requires refinement. While off-target editing is rarer with ABEs, neighboring nucleotides can be inadvertently mutated in a process called “bystander editing” (1). Even if further extraneous mutations result, they will not exert the same toxic effect as double-strand breaks do post-CRISPR-Cas9 editing (1).
In the context of SCD, the method devised by Dr. Liu and his team can be translated into clinical practice as an autologous (from the same patient) stem cell transplant of ex vivo (outside the body) genetically engineered cells (1). While they discuss delivery in their Nature paper, the question remains about the accessibility of this treatment to SCD patients. Current gene therapies are projected to cost at least $1 million per patient and insurance companies remain sheepish at subsidizing the cost (10, 11). Especially in regions of the world with poor health infrastructure, such treatments exist as a pipe dream. The onus is on the healthcare sector to assure that a lack of affordability does not eclipse the immense value of base editing therapy.