The Questions of Developmental Biology

The second approach is candidate gene
mapping. This approach is similar to
positional cloning, but here, one starts
with a correlation between the genetic mapping of a
particular syndrome and the genetic mapping for a
particular gene. As an example, we will take
another condition that produces small eyes:
Waardenburg syndrome type 2. This autosomal dominant
condition is characterized by deafness, heterochromatic
(multicolored) irises, and a white forelock (Figure 4.25).
By analyzing the pedigrees of several families whose members had
this condition, Hughes and her colleagues 1994 showed that it is
caused by a mutation in a gene on the small arm of chromosome 3,
between bands 12.3 and 14.4. A strikingly similar condition is found in
mice, in which it is called microphthalmia. Mutations of the
microphthalmia gene (mi) cause a dominant syndrome involving deafness, a white patch of fur,
and eye abnormalities (Figure 4.25). Could Waardenburg syndrome type 2 be caused by
mutations in the human equivalent of the microphthalmia gene? The mouse mi gene was cloned,
and was found (by sequencing and in situ
hybridization) to encode a DNA-binding
protein that is expressed in the pigment
cells of the eyes, ears, and hair follicles of
embryonic mice (Hemesath et al. 1994;
Hodgkinson et al. 1994; Nakayama et al.
1998). DNA-binding proteins are important
because they often act to regulate the
transcription of other genes (as will be
detailed in the next two chapters).
The mouse mi gene was used to
make a probe to look for similar genes in
the human genome. Indeed, the probe
found a human homologue of the mouse
microphthalmia gene, and this homologous
sequence (MITF) mapped to the exact same
region as Waardenburg syndrome type 2
(Tachibana et al. 1994; Tassabehji et al.
1994). When Waardenburg syndrome
patients were studied, it was found that
they each had mutations of the MITF gene.