Biology Reference
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associated with one form of nonsyndromic sensorineural hearing impair-
ment/deafness,
DFNB3
(Wang et al. 1998a).
7. Positional Candidate Gene Cloning
If a gene for a disorder is mapped to a specific chromosomal region, a
known gene mapping to that interval is a positional candidate by virtue of
its location. One of the first examples of the use of this approach was in
Waardenburg syndrome type 1 (WS1). It was recognized that the region of
human homology for the mouse mutant known as
Splotch
(
Sp
) (which is
deaf and has areas of depigmentation similar to that seen in persons with
Waardenburg syndrome) was 2q35-q37.3. A structural rearrangement of
this region of chromosome 2 had previously been reported in an individual
with WS1 and subsequent linkage analysis confirmed this location (Foy et
al. 1990). The
Pax3
gene in the mouse and the human homologue
PAX3
mapped to the same intervals as the
Sp
and WS1 loci suggesting positional
candidate genes for both disorders. This was confirmed by the identifica-
tion of mutations in the
Pax3
gene in the
Sp
mutant and in the
PAX3
gene
in WS1 patients (Tassabehji et al. 1992; Baldwin et al. 1992).
8. Mutation Detection
One of the essential prerequisites for confirmation that a gene causes an
inherited disorder or disease is the identification of DNA sequence differ-
ences that are present in affected individuals, but not in unaffected indi-
viduals from the general population. In other words, differences that are
normal sequence variation between individuals need to be distinguished
from deleterious mutations that result in inherited disorders.
8.1 Prioritizing Candidate Genes for Mutation
Detection/Screening
Determining which candidate genes to screen for mutations may not be
obvious. A first step is to examine those that have been characterized. The
function of the proteins encoded by these genes may suggest that some are
more likely candidates than others. For example,
MYO15
was considered a
probable candidate gene for
DFNB3
because mutations in another uncon-
ventional myosin,
MYO7A
had been associated with inherited hearing
impairment (Wang et al. 1998a). If function is unknown, evidence of expres-
sion in the inner ear helps in prioritizing candidates for mutation screening
(Giersch and Morton, Chapter 3). For example, a transcript
(COCH)
from
a fetal cochlear cDNA library (Robertson et al. 1994) was localized to the
region of the long arm of chromosome 14 to which the
DFNA9
locus had
