From: tilley314@attbi.com
Date: Mon May 27 2002 - 15:35:54 MDT
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Study: Errors May Explain Clone Woes
Sun May 26, 2:21 PM ET
By ANDREW BRIDGES, AP Science Writer
LOS ANGELES (AP) - Researchers working with clones of a
Holstein cow say genetic programming errors may explain
why so many cloned animals of all types die, either as
fetuses or newborns.
In cloning, the DNA of an adult animal is inserted into
a donor egg emptied of its own DNA. For that cell to
develop, genes that may have been turned off in the
adult animal that was being cloned must be turned on
again to guide the egg to form a new, genetically
identical individual.
In females, the embryo receives two X chromosomes, each
containing several hundred genes. In natural
reproduction, the genes on the two X's are active in
female embryos; one of the X's is later inactivated to
match the male complement of one X and one Y chromosome.
However, female clones receive an active X and an
already inactive X; the latter, and all its genes, must
be reprogrammed and then, later in development,
inactivated again.
Scientists at the University of Connecticut studying how
the normal patterns of X chromosome inactivation are
erased and then re-established during cloning found
abnormalities in nine of 10 genes they examined on the X
chromosome.
The scientists found the genes had been incompletely
reprogrammed in five dead cow clones and one aborted
fetus. Looking at four live clones, as well as control
animals conceived naturally, the scientists found the
same genes were normal.
"Our study demonstrates that in clones, even though they
can develop to full term, many abnormalities in gene
expression exist, which may be partially responsible for
the developmental abnormalities frequently observed,
including death," said Xiangzhong "Jerry" Yang, lead
author of the study. Results appear online Sunday in the
journal Nature Genetics.
Dr. Robert Lanza, medical director of Advanced Cell
Technology and an expert in the cloning of cows, called
the study "solid" and said that it helps explain the
high death rate in cloning. More than 80 percent of
clones die during pregnancy or shortly after birth.
"This work gives us a handle on what the problems might
be so we can screen for them at the various steps,"
Lanza said.
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