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Unintended Effects of Genetic Manipulation
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Transgenic expression of sheep growth hormone in sheep increased the incidence of reproductive problems and premature death.

Manipulated Organism: Sheep (Ovis aries).

Inserted Transgenes: Sheep growth hormone (GH) gene.

Background: "Injections of growth hormone are used to increase the efficiency of milk production in dairy cattle in the United States, and to improve production efficiency of swine in Australia. Accordingly, enhanced growth hormone expression is an obvious target for genetic manipulation in domestic animals" (Adams et al., 2002, p. 2325).

Goal of This Study: Compare the growth and overall health of sheep containing the GH transgene with their non-GM siblings, sired from rams with only one copy of the GH transgene.

Intended Effect: The GM sheep gained weight more quickly than the non-GM cohort.

Results of This Study:
  • The GM sheep had a consistently lower wool yield due to an increased content of suint, which is salt deposited in the fleece from sweating. The authors point out that increased sweating is characteristic of humans with acromegaly, a disorder characterized by elevated levels of growth hormone.

  • The GM sheep had higher average fecal worm counts at all ages sampled (9, 13, and 16 months), after which the sheep were put on strong anti-worm medicine.

  • Undescended testicles were discovered in 4 of the 21 male GM lambs but only 1 of the 55 male non-GM lambs.

  • The feet of all the GM sheep were swollen, and this problem worsened with age. As a result of joint softening, the toes splayed in such a way that the hooves did not come in normal contact with the ground. As a result of the decreased wear on the hooves, the GM sheep had to have their hooves trimmed twice as often.

  • Of the 35 non-GM sheep alive in Oct. 2001, none died before the end of the study in March 2003. By contrast, 9 of the 26 GM sheep died from a variety of causes: 3 died from cardiac arrest during shearing or herding, 2 died just before lambing, 2 were found dead and the cause was not determined, and 2 were euthanized because of skin cancer or severe lameness.

Source: Adams, N. R., and J. R. Briegel (2005). "Multiple Effects of an Additional Growth Hormone Gene in Adult Sheep," Journal of Animal Science vol. 83, pp. 1868-74.

Adams, N. R., J. R. Briegel, K. A. Ward (2002). "The Impact of a Transgene for Ovine Growth Hormone on the Performance of Two Breeds of Sheep," Journal of Animal Science vol. 80, pp. 2325-33.

Author Affiliations: CSIRO Livestock Industries, Australia.

Funding: Not mentioned.

Product Status: Not on the market as of 2009.

Copyright 2009 The Nature Institute.

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