Cytogenetics and Cochlear Expressed Sequence Tags (ESTs) for Identification of Genes Involved in Hearing and Deafness - Genetics of Auditory Disorders - page 110

suggested that WS1 may be homologous to the Splotch mouse that maps to
a region of homologous synteny on mouse chromosome 1. The Splotch
mouse has pigmentary anomalies similar to those seen in humans with WS1,
but, interestingly, does not have any hearing deficit. When the mouse Pax3
gene was cloned and shown to cause the Splotch phenotype (Epstein et al.
1991; Goulding et al. 1991), the human homolog, HuP2 or PAX3 , was
evaluated as a candidate gene in several WS1 families by heteroduplex
analysis (Mueller, Van Camp, and Lench, Chapter 4). Band shifts were
observed that showed complete concordance with the WS1 phenotype.
The PAX3 gene was sequenced in these families and found to contain
disease causing mutations (Tassabehji et al. 1992). To date, dozens of
mutations in PAX3 have been found that cause WS1. Identification of PAX3
as the WS1 gene was facilitated greatly by the cytogenetic finding of the
inv(2)(q35q37.3).
2.3.2 X-Linked Mixed Deafness
Loci for a number of disorders have been mapped to Xq21 by linkage analy-
sis and translocation studies, including retinal dystrophy choroideremia,
mental retardation, cleft lip and palate, and mixed deafness with stapes fixa-
tion and perilymphatic gusher (DFN3) (OMIM 1999). DFN3 is the most
common form of X-linked hearing impairment. Physical mapping of the Xq21
region using patients with these disorders and with cytogenetically visible
deletions of Xq21 allowed successive refinement of the locus for DFN3 (Bach
et al. 1992; Cremers et al. 1989), until ultimately it was reduced to 500 kb
(Fig. 5.9) (Huber et al. 1994). When the murine Pou3f4 gene was cloned and
mapped to the mouse X chromosome in a region of homologous synteny with
human Xq21 (Douville et al. 1994), POU3F4 became a positional candidate
gene for DFN3. Radiolabled mouse Pou3f4 probes hybridized to Southern
blots of genomic DNA from DFN3 males with cytogenetically visible dele-
tions, failed to detect any restriction fragments, suggesting the orthologous
POU3F4 gene was deleted in these individuals.The mouse sequence was used
to make primers to amplify and clone the human POU3F4 gene. SSCP analy-
sis showed frameshift mutations in POU3F4 in four DFN3 patients who did
not have cytogenetically visible deletions (de Kok et al. 1995), confirming that
loss of POU3F4 in the deletion patients was responsible for their hearing loss.
Additional DFN3 patients have been identified subsequently who have dele-
tions in Xq21 that do not encompass POU3F4 , but delete regions centromeric
to the gene. These patients may harbor deletions in unidentified POU3F4
regulatory sequences, or another gene whose product is similar to or interacts
with the POU3F4 protein (de Kok et al. 1996).
2.3.3 Branchio-Oto-Renal Syndrome
The autosomal dominant Branchio-Oto-Renal (BOR) syndrome is the
association of branchial arch anomalies, such as branchial cysts or fistulas,