Chapter 119
Curing Rare Diseases — 8
Ideally, the field will move to the in vivo, one-shot, in the body route, since ex vivo is laborious, expensive, and typically necessitates a considerable time delay to intervention. It seems riskier in principle. For now, however, genome editing in the body is greatly constrained by its reach, to the blood, which also accesses the liver, or by direct injection into the eye or ear. But there’s a blueprint for improved delivery that will ultimately help us reach the goal of broad access throughout the body.
In vivo access to most parts of the body will rely on better nanoparticles and virus-like particles. But even with what we have now, there has been successful restoration of a critical missing enzyme to patients with a rare disease using an mRNA that codes for it packaged within a nanoparticle, with no editors. However, unlike what can be accomplished with editing, it requires repetitive injections.
The importance of nanoparticles can’t be emphasized enough, aptly put by Megan Molteni, for a liver gene target:
Precious CRISPR instructions were billions of microscopic balls of fat, each enrobing an mRNA’s rigid backbone like a sticky, squishy sock. These lipid nanoparticles, or LNPs, work like sebaceous sherpas, ferrying their genetic cargo past peril after peril. To get where it needed to go, the CRISPR-encoded mRNA would have to withstand being buffeted about in the bloodstream where immune cells roved, then fan out into the forking network of vessels winding through the liver, slip through a pore in those vessels’ walls, fuse with the surface of a hepatocyte, get sucked across its protective membrane into an endosome, and somehow escape before the endosome became mRNA-meltingly acidic. Only then could CRISPR even have a chance to do its job.
Every one of the four key components of nanoparticles (fig. 8.3) is being worked on for improved stability and reach. We’ve already seen the enhanced potential for gold nanoparticles to get editing done in blood stem cells. Synthetic nanoparticles, such as adding a fifth lipid or an attached (conjugate) antibody, have now been enabled, getting editors into the brain, spleen, and muscle with ongoing work to get them to the heart, lungs, and bone marrow.
Figure 8.3. The anatomy of nanoparticles, with multiple layers of lipid, packaging the mRNA. Adapted from Aditya Raguram et al., “Therapeutic in vivo delivery of gene editing agents,” Cell 185, no. 15 (July 2022): 2806–27, https://doi.org/10.1016/j.cell.2022.03.045.
The other main direction is engineering the delivery vehicle, the broader term of enveloped delivery vehicles (EDVs), or more specifically, virus-like particles (VLP), to hold much more cargo and enhance its release at the target. For virus-like particles, the virus is gutted out, making it noninfectious and providing increased room for mRNA or proteins. There’s been progress on multiple fronts, such as in vivo liver delivery with base editors or retina delivery of prime editors with 65- to 170-fold increase efficiency. An advantage of VLPs with high efficiency is their short exposure time, which reduces the potential for off-target editing. Besides using human proteins as a capsule structure around the editor and guide, there’s even been use of bacteriophage to make better artificial viral vectors with enhanced cargo capacity. With all these tactics for improving efficiency and reach of delivery throughout the body, we’ll surely see progress in the future. What’s next?