Ever since gene therapy was introduced almost ten years ago, there's been much apprehension about genes from the disabled viruses used in the therapies inserting their own genetic material into a recipient's reproductive cells. The fear is that viral DNA could reach sperm or egg cells and potentially the genome of subsequent generations. Deciding on an acceptable level of this potential insertion has remained a contentious issue in setting policies for gene-therapy trials. The Federal Drug Administration (FDA) has proposed that a desirable limit would be no more than 1 insertion in about every 6,000 sperm cells, or individuals.
To help put this level into context, Haig Kazazian, MD, professor of genetics at the University of Pennsylvania School of Medicine estimated the natural incidence of gene insertions. He found that naturally occurring insertions in sperm cells were 100 times more common than the suggested FDA limit. His report is published in the June issue of Nature Genetics.
"There's a lot of concern about insertions of viral or other DNA from exogenous somatic gene therapy into gametes that could be passed on to the next generation," says Kazazian. "The FDA has been concerned about allowing studies where there was a chance of this." Kazazian's estimates -- along with other considerations -- were sufficiently convincing that the FDA no longer absolutely requires investigators to analyze gametic insertion before approving gene-therapy studies.
The major cause of naturally occurring genomic insertions is retrotransposons, so-called jumping genes. These are sequences of genetic material that are capable of duplicating and randomly reinserting themselves into the genome. Protein-coding sequences of retrotransposons are quite similar to some bacterial sequences, so are thought to be very old and therefore also important from an evolutionary standpoint.
Based on data found in the scientific literature and the Human Gene Mutation Database, Kazazian estimated that roughly 1 in 600 mutations arise from retrotransposon-mediated insertions and that there are on average 75 mutations derived from sperm cells in an individual. This means that 1 individual in every 8 (75 times 1/600) will carry a new retrotransposon insertion.
"Even if we overestimate this number by a factor of ten and say that one naturally occurring insertion is expected in every 50 to 100 individuals, this is still far greater than the one insertion in 6,000 sperm cells that has been suggested by the FDA," notes Kazazian. In addition, researchers consider most insertions harmless because critical sequences of DNA are thought to make up less than 5 percent of the human genome.
Materials provided by University Of Pennsylvania Medical Center. Note: Content may be edited for style and length.
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