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Genetically Modified Stem Cells may Have Therapeutic Application to X-SCID

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A study demonstrating therapeutic utilization of human stem cells, including potential application to X-linked Severe Combined Immunodeficiency (XSCID or “Bubble-Boy” disease) was presented at the 10th Annual Meeting of the American Society of Gene Therapy (ASGT) in Seattle.

X-linked Severe Combined Immunodeficiency (XSCID) is a rare genetic disease caused by defects in an essential immune hormone receptor called the common gamma chain (?c), found on the surface of lymphocytes and other immune blood cells.

Patients with XCID have immune cells that lack this receptor and both fail to develop properly, as well as do not function normally to protect patients against infections.

A team of scientists led by Dr. Luigi Naldini from the San Raffaele Telethon Institute for Gene Therapy, Milan and Sangamo BioSciences Inc., Richmond, CA, are developing a novel approach to genetically modify embryonic and tissue stem cells for therapeutic use. This approach can potentially be applied to many diseases, including X-SCID.

Using technology developed by Sangamo, the scientists showed gene correction or precise addition of a gene in human stem cells at unprecedented levels.

“This is a significant advance for the potential therapeutic use of stem cells,” says Dr. Angelo Lombardo, lead author of the study. “Stem cells are the body’s natural resource for regeneration and repair and the ability to efficiently insert a therapeutic gene in a predetermined location or to correct a mutated gene in a patient’s stem cells may enable us to provide a long term solution for many genetic diseases. The powerful combination of Sangamo’s ZFN™ technology and our highly efficient viral delivery platform was the key factor for obtaining unprecedented gene modification efficiency in these therapeutically important cells”.

Using this approach, an inherited mutation in a gene can be directly corrected or modified in a cell or a new copy of a gene can be added into the genome at a specific and predetermined site. This is a significant advance over traditional approaches that rely on the random insertion of a new copy of a gene to cells to restore the missing function of an inherited mutant gene.

In fact, despite the successful outcome of gene therapy reported in many SCID patients, random insertion of genes into the cellular genome may in some circumstances lead to serious adverse effects due to altered expression of the therapeutic gene or its neighboring genes at the insertion site.

In contrast, Dr. Naldini’s study demonstrates that ZFN-mediated gene editing can be used to correct mutations in the IL-2R gamma gene, the defective gene in X-linked SCID, and to add a therapeutic gene to a pre-determined ‘safe-harbor’ site in the genome of both human hematopoietic progenitors and human ES cells with high efficiency. This ‘safe-harbor site’ was selected by the investigators for its capacity to allow efficient expression of the therapeutic gene and to tolerate an insertion event without adverse effects.

ZFN-mediated gene editing overcomes some major limitations of standard gene therapy approaches and potentially provides a unique means to treat disease with HSCs and ES cells in a safe and effective manner.

The scientists used designer proteins, named zinc finger DNA-binding protein nucleases (ZFN), that were developed at Sangamo BioSciences. ZFN can be engineered to specifically cut DNA at a chosen site.

Dr. Naldini’s team exploited the high infectious capacity of lentiviral vectors derived from HIV to express ZFN and provide the template DNA for gene correction in target cells. When they introduced ZFN into target cells with an appropriately designed donor DNA molecule that encodes the correct gene sequence, the DNA break was repaired in a natural process by the cell’s DNA-repair machinery leading to correction of the mutation or the precise insertion of a novel gene into the target site.

Using the new delivery system, high rates of gene editing were obtained in a panel of human cells, including hematopoietic and embryonic stem cells.