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Replacement and alteration of defective genes is a milestone in gene therapy

FIVE years after the first human beingreceived gene therapy to correct aninherited disease, the results are in. TheWashington Post (October 21, 1995)reports that two seriously ill childrenwho were given immune-system genesthat they lacked from birth, are "healthyand thriving" now. Gene therapy is atechnique in which doctors give patientshealthy genes to replace the defectiveones inherited from their parents or toenhance the action of the genes they already have.

Researchers led by R Michael Blaeseof the us-based National Centre forHuman Genome Research (NCHGR)selected two young girls to debut thetechnique. They were born with anextremely rare genetic disorder calledadenosine deaminase (ADA) deficiency.The disease occurs when a child inheritsmutated versions of a gene responsiblefor making a crucial detoxifyingenzyme. Without the enzyme, toxinsbuild up in the blood, killing immunesystem cells and leaving patientsdefenseless against infections.

In the experiment, Blaese and hiscolleagues iried to add normal versionsof the genes to the patients' cells.Beginning in 1991, four-year-oldAshanthi De Silva was administeredabout one billion of her own whiteblood cells, after they had been removedfrom her body and genetically altered.Soon after, the investigators treatedanother eight-year-old patient in thesame way.

In a recently published paper, theresearchers report that half ofAshanthi's cells carry the added ADAgene, and her cells are making 25 percent of the normal amQunt of theenzyme. It is held that people who makejust 10 per cent of norM.&I ADA levelsshow no symptoms of immune dysfunction. "I couldn't have wished for a niceroutcome", says Blaese, satisfied with theresult of his experiment.

The result of the experiment significantly shows the potential of gene therapy. "I think people will no longer beable to say that gene therapy has neverbenefitted a patient", remarks F SCollins, director of the Nc#iGR. The success of the experiment is a milestoneindeed, the first in a field that has so farfailed to live up to expectations.

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