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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