Genetic diseases have ultimate causes that undermine normal phenotypes associated with homeostatic functions and structures.
The goal of treatment, as we know it, is to restore homeostasis, currently and usually by modalities that modify proximate events. These euphenic strategies attempt to modify phenotype; they do not alter genotype.
The modalities include restriction of (toxic) substrates, replacement of (deficient) products, activation or stabilization of mutant enzymes with pharmacologic doses of coenzymes or other agents, replacement of normal protein (by several approaches, including tissue transplantation); surgical repair; and combinations of these approaches.
The effectiveness of these modalities has been measured in a series of studies over the past two decades. The response to treatment was only partial at best in the majority of genetic diseases but is steadily improving as new knowledge of pathogenesis is combined with new strategies and tactics for treatment. The prospects for combinatorial drug design to fit therapy to phenotype are encouraging.
There are no unequivocal examples yet of persistent success with somatic gene therapy for a human genetic disease.