New technique adds precision and permanence to gene therapy

Mount Sinai School of Medicine researchers developed a technique for inserting genes into specific sites on the genome in liver cells. The genes are inserted into non-coding regions of the genome so there is no danger of interfering with the functioning of other genes. Once inserted, the gene remains a permanent part of the cell's genome. In a study published in this week's PNAS, the researchers used this technique to cure phenylketonuria (PKU) in mice.

"To date gene therapy has relied upon vectors that randomly insert genes into the cell's genome," explains Savio L. C. Woo, PhD, Professor and Chairman of Gene and Cell Medicine at Mount Sinai School of Medicine and corresponding author on the study. "The technique we developed identifies a specific sequence which only occurs in a few places in the mammalian genome. These sequences occur between genes so there is no danger of the insertion of the gene damaging existing genes in the cell.

"Because the genes are inserted permanently, a few applications would suffice to permanently correct a disease." Dr. Woo and his colleague Li Chen , PhD, a post-doctoral fellow at Mount Sinai) were able to cure PKU in mice with just three intravenous injections. The levels of phenylalanine in the treated mice dropped to normal range and remained stable thereafter. Their fur color also changed from gray to black, indicating that they were now producing normal levels of melanin, a pigmentation which is under-produced in mice and humans with PKU.

Drs. Woo and Chen used a gene from a bacteriophage that recognizes a specific DNA sequence. This sequence occurs only several times in the entire mouse genome and it is always found in the non-coding region between genes. Similar sequences are found in a few locations in the human genome that are also between existing genes.

"The current challenge is to identify a suitable means of introducing DNA into liver cells," said Dr. Woo. "Once that technology is developed, this new technique will provide a safe and efficient means of integrating the DNA into the cell's genome."

In addition to PKU, this technique could be used to cure other genetic diseases caused by missing liver enzymes including hemophilia and urea cycle enzyme deficiencies, as well as cholesterol clearing from the blood and others.

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