Gene Therapy

Lentiviral and retroviral gene therapies permanently insert DNA semi-randomly into the genome. If they land near a cancer gene, they can turn it on — this caused leukemia in 6 of 20 patients in early X-SCID trials. CAR-T therapies face the same risk. FDA requires 15 years of post-treatment monitoring.

Nearly all approved gene therapies treat single-gene diseases. But the diseases that kill the most people — heart disease, diabetes, Alzheimer's, most cancers — involve dozens to hundreds of genes. We don't know how to safely edit multiple genes simultaneously, and multiplexed editing exponentially increases off-target risks.

Base editors (~5.2 kb) and prime editors (~6.3 kb) exceed AAV's 4.7 kb packaging limit. Split-intein dual-AAV approaches work but lose 40-70% of activity. This means the safest, most precise editing tools can't use the most established delivery vehicle.

Precise gene correction via homology-directed repair (HDR) only works in dividing cells. But the cells you most want to fix — brain neurons, heart muscle, skeletal muscle — are non-dividing, so they default to error-prone repair that creates random mutations instead of precise corrections. Prime editing helps but achieves only 1-10% efficiency in vivo.

Both AAV and LNP vectors naturally end up mostly in the liver after IV injection. This is great for liver diseases but means lungs, kidneys, pancreas, and immune cells receive very little therapy. Cystic fibrosis lung gene therapy has failed for 30 years because of this delivery problem.

CRISPR sometimes cuts at wrong locations in the genome that look similar to the target. Beyond point mutations, these unintended cuts can cause large deletions (megabases), inversions, and translocations — potentially activating cancer genes. This is the biggest regulatory bottleneck for expanding CRISPR therapies.

30-70% of the population has pre-existing antibodies against AAV from natural childhood exposure, making them ineligible for gene therapy. Worse, patients who receive one dose develop strong immunity preventing a second dose — even if the first wears off years later.

The most commonly used gene therapy delivery vehicle (AAV) can only carry ~4,700 base pairs of DNA. But many serious diseases involve much larger genes: dystrophin for Duchenne muscular dystrophy is 14 kb, CFTR for cystic fibrosis needs ~6 kb with regulatory elements, ABCA4 for Stargardt disease is 6.8 kb.