Biologists Develop Efficient Genetic Modification Of Human Embryonic Stem Cells

One of the non-ethical problems with embryonic stem cell research has been the lack of efficient ways to study genetics using these cells.  Now, using bacterial artificial chromosomes  (BACs) to swap in defective copies of genes, researchers have successfully transferred a defective copy of the gene p53, which suppresses cancer, into a human embryonic stem cell line.

"This will help open up the whole human embryonic stem cell field. Otherwise, there are really few efficient ways you can study genetics through them," said Yang Xu, professor of biology at the University of California, San Diego who directed the research.

BACs are synthesized circles of human DNA, which bacteria will replicate just like their own native chromosomes. Commercially available BACs can be modified within bacterial cells to insert altered copies of specific genes. Once the modified BACs are introduced into human cells, they will sometimes pair up with a matching segment of a human chromosome and swap segments of DNA, a process called homologous recombination.

The advantage of using BACS to alter the genetic code in human cells comes from the long flanking sequences on either side of the modified gene, which increases the chance that the BAC with line up with native DNA in position for a swap. Other genetic approaches have been limited by shorter segments of DNA.

Using BACs, the team was able to substitute modified genes in 20 percent of treated cells. Standard methods of genetic modification typically achieve modification in fewer than one percent of cells.

After the group successfully transferred a defective copy of the gene p53 into a human embryonic stem cell line, they repeated the process in a second round, developing a line of cells in which both copies of the genes were disrupted.

The researchers also report success with a different gene, ATM, which when mutated in humans causes Ataxia-telangiectasia, a disease characterized by a host of systemic defects including increased cancer risk, degeneration of specific types of brain cells and degraded telomeres, the protective caps at the end of each chromosome.

Genetically engineered mice with two bad copies of the ATM gene share some of these traits with human patients, but not all. Neurons don't degenerate in ATM mice, for example, and the telomeres are long. "If you want to study accelerated shortening of telomeres," said Xu, "you can't do it in the mouse. You can only do it in human cells."

Adapted from the UCSD announcement.