June 16, 1999
Inherited Deafness Studies May Affect Genetic Counseling
Researchers have confirmed that one type of genetic mutation causes
inherited profound deafness, while another mutation thought to cause
deafness does not. These results, say the investigators, emphasize the
value of basing genetic counseling on data derived from detailed
genetic studies.
Writing in the June 16, 1999 issue of the Journal of the American
Medical Association, the team, which included HHMI investigator Val Sheffield of the
University of Iowa College of Medicine, reported that about three
percent of their sample population carried a mutant form of the gene
GJB2. The lead author of the study was Glenn Green at the
University of Iowa College of Medicine.
“We don't want people doing genetic screening and basing counseling on any genetic mutation they might find until it's been more carefully characterized.”
Val C. Sheffield
Genetic tests on 52 young people with moderate to profound
congenital deafness showed that 42 percent had mutations in the
GJB2 gene. This finding confirms earlier studies by Sheffield's
team and others which suggest that mutations in GJB2 contribute
to inherited deafness. The Iowa team found that the majority of those
people with a defective GJB2 gene had a specific mutation, which
researchers call 35delG.
Studies of 560 randomly selected healthy infants showed that about
2.5 percent of these children are "carriers" of the 35delG mutation.
These infants were not deaf because they carried only one copy of the
defective gene. (Two mutant copies of such a "recessive" gene are
required to cause genetic disease.) Overall, however, the researchers
found that about 3 percent of the infants had some type of mutation in
the GJB2 gene.
But the incidence of a second type of mutation in GJB2 (which
researchers call M34T) was found to be very low in the group with
profound deafness. An earlier study by other investigators suggested
that the M34T mutation is a "dominant mutation," which means that
inheriting only one copy of the mutant gene would be enough to cause
deafness.
"Initially, based on a very small sample, it had been suggested that
the M34T variant was a dominant gene for deafness," Sheffield
explained. "But our subsequent screening studies found that it was not
uncommon for normal individuals to carry this variant, and we do not
believe that it causes deafness. Based on this latest study, we don't
find this mutation in deaf people at the level you would predict based
on how prevalent it is in non-deaf people."
Sheffield says that it is important for researchers to do these
types of carrier-rate studies "so that good genetic counseling can be
given based on solid scientific information. This finding means that a
test for 35delG, which thanks to advances in molecular genetics can be
done in a couple of hours, is a good place to start in determining the
cause of congenital deafness."
Sheffield also advises that genetic counselors and physicians use
caution when advising parents on the possibility that a child's illness
or disability may have genetic roots. "The take-home message from the
findings on both 35delG and M34T is that mutations really need to be
proven deleterious before giving genetic counseling that is based on
their presence. We don't want people doing genetic screening and basing
counseling on any genetic mutation they might find until it's been more
carefully characterized."
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