Therapies that edit a patient’s genes have moved from experimental to approved for specific conditions. That is a genuine milestone and the applicable list is narrow for reasons worth understanding.
What the first approvals treat
Blood disorders caused by a single well-characterised mutation. The treatment removes cells from the patient, edits them outside the body, and returns them.
Editing outside the body is far easier than editing inside it, because the edited cells can be checked before being returned. That is why blood disorders came first.
Why the list is short
Three conditions have to hold. The disease must be caused by a single identified gene. The relevant cells must be accessible. And the edit must be verifiable before it matters.
Most diseases fail at least one. Common conditions involve many genes each contributing slightly, and there is no single edit to make.
The delivery problem
Editing cells inside the body requires getting the editing machinery to the right tissue and nowhere else. That is the central unsolved challenge and most research effort now goes into it.
The liver is reachable. The brain, muscle and most solid tissues are considerably harder.
The cost problem
Current therapies cost enormous sums per patient – individually manufactured treatments for small populations. Health systems are struggling with how to pay, and proposals include paying over time contingent on the treatment continuing to work.
For a one-time cure of a lifelong condition, that arithmetic can work. It requires financial structures that do not exist yet.
The realistic near term
Expansion to more single-gene conditions, and to cancers where cells are edited outside the body. Not to common complex diseases, and not soon.