Cloned Pigs Modified for Use in Human Transplants 
      Bijal P. Trivedi
      National Geographic Today
      January 3, 2002  
      Two competing teams have cloned pigs that have been genetically modified
      to produce organs more suitable for transplantation into

      Pig organs are well suited for transplantation; they are
      approximately the same size as human organs and have similar plumbing,
      which makes reconnecting blood vessels much easier. Also, the size of
      pig litters tends to be large and pigs reproduce quickly, raising the
      prospect of a large supply of "spare" organs.

      A problem with using pig organs, however, is that they are coated with 
sugar molecules that trigger acute rejection in people. Human antibodies attach 
themselves to these sugar molecules and quickly destroy the newly transplanted 
pig organ.

      To circumvent the rejection, scientists are working to produce pigs that 
lack the sugar-producing gene. 

      In a significant step toward that goal, a team of scientists led by 
Randall Prather of the University of Missouri in Columbia created four cloned 
piglets in which one copy of the sugar-producing gene was "knocked out" (an 
organism receives two copies of a gene, one from the mother and one from the 
father). The piglets were born in September and October. A description of the 
work was published online by the journal Science. 

      Earlier this week, PPL Therapeutics PLC of Scotland, the company that 
helped clone Dolly the sheep, announced the birth of five cloned piglets that 
also lack a copy of the sugar-producing gene. The piglets were born December 25 
and were named Noel, Angel, Star, Joy, and Mary.

      By selectively breeding the experimental pigs, both teams of scientists 
hope to produce pigs lacking both copies of the gene. It's expected that the 
organs of these modified pigs could be transplanted into people without the 
problem of tissue rejection.

      The new results are a significant advance over many other attempts at 
genetic modification in animals because in both of the studies, the scientists 
were able to modify-in this case, "knock out"-a gene at a specific location. 
Although genes from other organisms have been inserted into the genomes of 
sheep, cattle, and pigs, scientists have had little control over where on a 
chromosome the new gene is incorporated. 

      "This is the first time a specific genetic modification has been made in 
the pig," said Prather. 

      Prather's team at the University of Missouri and his colleagues at 
Immerge BioTherapeutics Inc. in Charlestown, Massachusetts, genetically altered 
fetal pig cells, which were used to create embryo clones. Of 3,000 genetically 
modified pig embryos that were implanted into 28 surrogate sows, only seven 
piglets were born, three of which died later. 

      The cloning of Dolly almost five years ago raised expectations of 
creating identical, genetically modified organs for transplantation into 
humans. The cloning of genetically modified piglets brings scientists closer to 
their goal of "xenotransplantation"-the transfer of cells and organs from one 
species into another.

      A concern that has dampened the prospects of xenotransplantation is the 
possibility of spreading viruses from one species to another. Porcine 
endogenous retrovirus (PERV), for example, is part of a pig's natural genetic 
makeup and does not cause any disease in the animal. There is no guarantee, 
however, that PERV would be harmless in humans. 

      To minimize the risk of spreading PERV, Prather's team used a line of 
pig-miniature swine-that was developed specifically for the purpose of 
transplantation. A major advantage of these pigs is that they are unable to 
spread PERV. 

      Prather's team expects to produce a miniature swine that lacks both 
copies of the sugar-producing gene within the next 18 months.

      National Geographic Today, 7 p.m. ET/PT in the United States, is a daily 
news magazine available only on the National Geographic Channel. Click here to 
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      © 1996-2008 National Geographic Society. All rights reserved.  


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