First, let s go over an important concept in genetics - germ cell vs. somatic cell changes. BUT for a mutation to be passed in the germline it cannot severely compromise development. Many of the dominant mutations that create oncogenes are embryonic lethal and cannot be passed in the germline, but can create cancer as a somatic mutation. Since germ cells are not affected, the ethics of making changes that will be passed to progeny are not an issue.
Most people feel comfortable with changing the genes of the somatic cells of the human.
However, there are problems with gene therapy. In no case is any genetic change to the individual person going to affect their germline, so they will NOT pass any changes to progeny. These are the cells in which gene targeting with drug selection occurs. The stem cells from the fetal brains produced new neurons in the brain to partially compensate for the ones lost to the disease. This is how the idea of having banks of stem cells representing different lineages came about.