Chapter 116
Curing Rare Diseases — 5
This field is more than a decade ahead of genome editing for getting treatments out there. But the intrinsic limitations of gene therapy include its use restricted to recessive mutations, inability to deliver large genes, and the unregulated nature of the additional gene copy that is introduced (without native promoters and enhancers that regulate gene expression). The virus vector for the gene therapy delivery has the potential to integrate with DNA of the recipient as well as inducing inflammation. Nonetheless, many gene therapies have fulfilled critical unmet needs for people with rare diseases and have paved the way for a more precise and potentially curative approach.
ONE-SHOT GENOME EDITING
There has been a long string of success in experimental models, such as a single injection of base editors, delivered in vivo (injected into the animal) with an adeno-associated virus (AAV) vector for progeria, a devastating disease of accelerated aging, replacing a C-G base pair with a T-A. Likewise, in vivo base editors were used to restore hearing in a mouse model of a dominant hearing loss. CRISPR-Cas 9 injected to the brain with AAV in a model of familial Alzheimer’s disease reduced amyloid deposition and microglial activation, and improved cognitive performance. Prime editing of phenylketonuria, a metabolic disease of the liver, was also successful given in vivo, but there was requirement for high vector doses that induced immune response. Direct injection of base editors with AAV into the chest to treat hypertrophic cardiomyopathy, a genetic disorder that occurs in one of five hundred people, achieved 30 to 60 percent editing efficiency (proportion of heart cells fixed) and improvement in heart function and structure. Use of base editors, directly injected to the heart, has been shown to protect from lack of blood supply, which could be useful in people with prior heart disease and heart attacks.
While the above were all using mouse models, base editing moved on to primates to test correction of the PCSK9 mutation that is a frequent culprit for familial hypercholesterolemia in people. LDL cholesterol was reduced by 65 percent after a single shot of base editors was delivered in a lipid nanoparticle package into a vein, and this effect was durable for as long as the monkeys were assessed (beyond one year). That strategy has gone on to clinical trials, with results similar to the nonhuman primates.
That’s a good transition to the other clinical trials for genome editing, all of which were antedated by animal model work. They include CRISPR-Cas9 for treating hereditary amyloidosis, a life-threatening condition due to a misfolded protein called transthyretin (TTR) that accumulates in the heart and nervous system. The disruptive editing approach used a lipid nanoparticle delivery, the first genome editing given to people by vein, targeting the liver where the protein is made, and achieving marked (more than 90%), dose-dependent, and durable reduction of TTR in the blood.
Another life-threatening condition is hereditary angioedema, characterized by unpredictable and severe swelling attacks. Here again, a single infusion of the CRISPR-Cas 9 nanoparticle package targeting a kallikrein gene (KLKB1) achieved a potent, dose-dependent, and durable lowering of kallikrein levels and less angioedema attacks. Trials with base editing and Cas12a are showing good results for sickle cell disease, with plans to move to direct, in vivo injection rather than the cumbersome ex vivo approach currently approved.