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1992-08-28
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SCIENCE, Page 72The Generational Saga of The Vicious Gene
Researchers find a surprising type of heredity that can make
a defect more serious in a child than in the parent
For more than a century, scientists have built upon the basic
principles of heredity that Austrian monk Gregor Mendel gleaned
from his painstaking studies of garden peas. One of the most
strongly held beliefs has been that genes -- whether normal or
abnormal -- are passed from generation to generation
essentially unchanged. Now that assumption is being challenged.
Last week scientists announced that in people with a form of
muscular dystrophy, they had identified a segment of DNA that
can lengthen substantially with each succeeding generation. Most
disturbing, as the fragment lengthens, the illness becomes more
severe. "This is not your garden-variety genetic defect," says
Dr. Leon Charash, who chairs the medical advisory committee of
the Muscular Dystrophy Association.
The startling discovery, reported in Nature by an
international trio of research teams, marks only the third time
such a genetic phenomenon has been found. Last year researchers
revealed a similar process in two much rarer inherited diseases:
fragile X syndrome, a form of mental retardation; and spinal and
bulbar muscular atrophy, a wasting disease.
While forcing scientists to revise their thinking about
heredity, the findings are also raising ethical quandaries. "It
now appears we can identify people who may be asymptomatic but
whose risk for transmitting a devastating illness is very high
compared with the rest of the population," observes geneticist
David Housman of the Massachusetts Institute of Technology, a
member of one of the research teams. "Should they be informed?"
A man or woman with such a defect will have to consider the
brutal fact that not only is there a fifty-fifty chance that a
child will inherit the illness, but also that the disease may
be progressively worse in that child, the grandchildren and the
great-grandchildren.
Over the years, researchers have discovered that the DNA
that makes up the 46 chromosomes in the human cell is not as
stable as once thought. Mutations in DNA have long been known
to occur, but they usually involve relatively small changes in
genetic material. For example, between parent and child, there
may be a switch in the sequence of nucleotide bases that are the
building blocks of DNA. Sometimes an entire gene can jump to
another place on a chromosome. "But you don't usually see a big
increase in the absolute number of bases within a single gene,"
says Greg Lennon, a geneticist at Lawrence Livermore National
Laboratory in Livermore, Calif., and a member of one of the
teams that made last week's announcement. Moreover, mutations
tend to occur at a slow pace. "The rate is so low from one
generation to the next -- maybe 1 in 10,000 -- as to be
negligible," notes M.I.T.'s Housman.
In myotonic dystrophy, the most common form of muscular
dystrophy, the change can be far from negligible: a fragment of
DNA on chromosome 19 appears to repeat itself more frequently
with every generation. Just what triggers the repetition is a
mystery. Researchers surmise that a hitch occurs while DNA is
being copied in the cell, much as the same bar of music repeats
on a scratched record. The DNA repeat gets worse with each
generation, just as with each playing of a flawed record, the
music stutters for a longer period. "Presumably the replication
error occurs in the sperm or egg before conception," says
molecular geneticist Pieter de Jong, who headed the Livermore
team.
Scientists expect to find more stuttering genes. "Any time
a disease gets worse through generations, we're going to
suspect that this happens," notes Lennon. Researchers are also
intrigued by the possibility that gene growth occurs as cells
replicate in the body during a person's lifetime. That would
have implications for ailments such as cancer. The search begun
by Mendel for the secrets of heredity is far from complete.
-- By Anastasia Toufexis