Genetic modification, sometimes referred to as gene therapy, changes the genetic content – the DNA sequence – of a cell, many cells, or a whole organism. Genetic modification is possible in bacteria, plants, and animals. There are two "types" of human genetic modification: somatic and germline.
Somatic gene therapy involves introducing a gene or gene segment into specific tissues or organs in a patient to treat or cure an existing condition. Gene therapy has been successful only once in clinical trials, in treating X-chromosome linked severe combined immunodeficiency syndrome (X-SCID). However, some of the children who underwent this gene therapy developed leukemia as a result of the therapy. Somatic gene therapy alters the genetic make-up of some of the cells or tissues of the person treated, but cannot be passed to future generations because the altered gene does not exist in the person's egg or sperm.
Human germline genetic modification (HGGM) involves introducing into a person’s germline cells – the eggs or sperm – a gene or gene segment. Genetic modification of germline cells is meant to permanently alter the genetic make-up of future generations. No attempts of HGGM have been reported.
In HGGM, the transferred gene is meant to replace and correct or overcome deleterious effects of an existing non functioning or malfunctioning gene. Alternatively, one could add a new gene or genes to introduce a new and previously non-existent gene function. In addition to adding or replacing whole genes, HGGM could alter gene segments to enhance or abolish the function of an existing gene.
The techniques that might be used for HGGM draw from successful germline genetic modification studies in laboratory animals, human stem cell research, and human somatic gene therapy techniques where non-heritable genetic changes are used in attempt to cure or treat disease. All potential approaches to HGGM remain theoretical.
One potential use of human genetic modification might be genetic enhancement. Genetic enhancement aims to modify non-health related human traits to make a person "better than well" by optimizing attributes, possibly to improve performance levels. Animal experiments have shown that somatic transfer involved in growth can increase muscle mass in aged mice. These mice potentially are the first stage in developing future human treatments for degenerative diseases like muscular dystrophy.