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Summary
of Testimony
Recently,
cloning of humans by nuclear transplantation has been proposed. In this testimony I will focus on the
scientific concerns about human cloning that have resulted from the experience
with animal cloning.
1.
To
date, five mammalian species (sheep, mice, goats, cows and pigs) have been
cloned, however, survival of the nuclear clones has been uniformly low. The
great majority of all clones (of all five species) die either at various stages
of embryonic development, at birth, or soon after birth. Most newborn clones are overweight and have
an increased and dysfunctional placenta.
Those that survive the immediate perinatal period may die within days or
weeks after birth with defects such as kidney or brain abnormalities, or with a
defective immune system. Even
apparently healthy adult clones may have subtle defects that cannot be
recognized in the animal.
2.
The
most likely cause of abnormal clone development is faulty reprogramming of the
genome. This may lead to abnormal gene
expression of any of the 30,000 genes residing in the animal.
3.
Faulty
reprogramming does not lead to chromosomal or genetic alterations of the
genome, so methods that are used in routine prenatal screening to detect
chromosomal or genetic abnormalities in a fetus cannot detect these
reprogramming errors. There are no
methods available now or in the foreseeable future to assess whether the genome
of a cloned embryo has been correctly reprogrammed.
4.
The
experience with animal cloning allows us to predict with a high degree of
confidence that few cloned humans will survive to birth and of those, the
majority will be abnormal.
The arguments given in this outline have been summarized in more detail
in an article by Jaenisch and Wilmut that will be published in Science magazine on March 30,
2001. A copy of the article will be available at
the Committee meeting.
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