Biology Reference
In-Depth Information
oocytes just after fertilization. The oocytes are then implanted into a foster
mother whose uterus has been prepared for pregnancy by treatment with
hormones. The transgene may incorporate anywhere in the genome, and
several copies (sometimes as many as 200 copies) are usually found at a
single location. In general, between 10% and 30% of the progeny are found
to have the injected gene in their germline DNA, and can therefore pass it
on to their offspring, thus allowing the development of a colony of trans-
genic mice.
The construction of a knockout mouse is a much more controlled and
precise experiment than generating a transgenic mouse. The first step is to
replace the normal copy of the gene with a copy containing the mutation
of interest. This procedure is carried out in embryonic stem (ES) cells
derived from mouse blastocysts (an early stage of embryonic development)
and grown in tissue culture. The mutant gene may insert anywhere in the
genome, but only the cells in which the normal copy is replaced by the
mutant copy are selected for the next step. The selected ES cells are
then injected into a recipient blastocyst, which is implanted into a foster
mother. The resulting offspring will be chimeric, meaning that some of their
cells are derived from the ES cell line, and some are from the recipient
blastocyst. Mating experiments may then be set up in order to deter-
mine if the ES cells have contributed to the germline and to generate
a colony of knockout mice. Knockout mice provide excellent animal
models for studying the effects of gene mutations associated with human
disorders.
Because of the high degree of similarity (orthology) of genes in humans
and mice, studies of mouse mutants have made many valuable contribu-
tions to human disease gene identification (Meisler 1996), and hearing
impairment is no exception (Brown and Steel 1994). Major advantages of
using the mouse for finding disease genes are the ability to set up specific
matings, and the relative ease of obtaining large numbers of informative
progeny to localize the genes by linkage analysis. A relevant example is
human
USH1B
and the mouse deafness mutant
shaker-1
(
sh1
), which were
hypothesized to be caused by mutations in orthologous genes because they
had been mapped by linkage to a conserved region on human chromosome
11q13 and mouse chromosome 7. This hypothesis was proven to be correct
when Gibson et al. (1995) showed that the
sh1
gene encoded myosin VIIa,
and Weil et al. (1995) found mutations in the human
MYO7A
gene in Usher
type IB patients soon thereafter. Note that symbols for human genes are
uppercase, while those for mouse genes are lowercase. Thus, the
USH1B
gene is
MYO7A
, and the
sh1
gene is
myo7a
. Steel, Erven, and Kiernan
(Chapter 8) cover the application of mouse models to studies of human
hearing impairment.
Mapping and sequencing of genes in non-mammalian species is also
proving to be useful for studies of human diseases. In particular, the
