Scientists at the University of Pennsylvania School of Veterinary Medicine have successfully used a retrovirus to modify genes in spermatogonial stem cells in a mouse, the first instance, in any species, of a transgenic animal created by inserting a gene into male germ-line stem cells.
The inserted gene subsequently appeared in approximately 4.5% of offspring of mice transplanted with the altered stem cells, and was transmitted to at least three succeeding generations.
The findings should enable the creation of transgenic individuals in a wide range of species, permitting scientists to develop research models for study of numerous human diseases.
Led by Ralph Brinster, Penn's Department of Animal Biology, the scientists succeeded in inserting a foreign gene, the common reporter gene lacZ, whose product is the enzyme ß-galactosidase into 2 to 20% of mouse spermatogonial stem cells in laboratory experiments, a tenfold improvement over previous attempts.
"These results indicate that there is no intrinsic barrier to the genetic engineering of spermatogonial stem cells using retroviruses, and that once inserted, the foreign genes will continue to be transmitted and expressed from one generation to the next," said Brinster, the Richard King Mellon Professor of Reproductive Physiology at Penn.
All male mammals harbour many spermatogonial stem cells, key repositories of genetic material whose daughter cells give rise after puberty to sperm. In the human male, approximately 1,000 sperm cells, each carrying a different combination of genetic material, are generated in this manner with each heartbeat. Whereas the female germ cell, the egg, stops dividing before birth, male germ-line stem cells continue to divide throughout life.
Of the various types of stem cells, only two, spermatogonial and hemapoeitic stem cells, can be positively identified using functional assays. This makes them valuable models for other types of stem cells, such as those that give rise to skin, the lining of the intestines, brain, muscle and liver.
Spermatogonial stem cells are of additional interest for transgenics applications because they are the only cells, including all other stem cells, which undergo self-renewal throughout an animal's lifetime and contribute genes to subsequent generations. Previous attempts to genetically alter this unique type of stem cell, either through retroviruses or other methods, have met with little success.
