Genetic differences in environmental toxicity and cancer susceptibility
among individuals in a human population often reflect polymorphisms
in the genes encoding drug-metabolizing enzymes (DMEs), drug
transporters, and receptors that control DME levels. This field
of study is called "ecogenetics", and a subset of this field-concerning
genetic variability in response to drugs-is termed "pharmacogenetics".
Although human-mouse differences might be 3- to perhaps 10-fold,
human interindividual differences can be as great as 20-fold
or more than 40-fold. It would be helpful, therefore, to study
toxicokinetics/pharmacokinetics of particular environmental
agents and drugs in mice containing these "high-" and "low-extreme"
human alleles. We hope to use transgenic "knock-in" technology
in order to insert human alleles in place of the orthologous
mouse gene. However, the knock-in of each gene has normally
been a separate event requiring the following: (a) construction
of the targeting vector, (b) transfection into embryonic stem
(ES) cells, (c) generation of a targeted mouse having germline
transmission of the construct, and (d) back-cross breeding of
the knock-in mouse (at least 6-8 times) to produce a suitable
genetically homogeneous background (i.e., to decrease "experimental
noise"). These experiments require 1
to 2 years to complete,
making this very powerful technology inefficient for routine
applications. If, on the other hand, the initial knock-in targeting
vector might include sequences that would allow the knocked-in
gene to be exchanged (quickly and repeatedly) for one new allele
after another, then testing distinctly different human polymorphic
alleles in transgenic mice could be accomplished in a few months
instead of several years. This "gene-swapping" technique will
soon be done by zygotic injection of a "human allele cassette"
into the sperm or fertilized ovum of the parental knock-in mouse
inbred strain or by the cloning of whole mice from cumulus ovaricus
cells or tail-snip fibroblasts containing the nucleus wherein
each new human allele has already been "swapped." In mouse cells
in culture using heterotypic lox sites, we and others have already
succeeded in gene swapping, by exchanging one gene, including
its regulatory regions, with a second gene (including its regulatory
regions). It is anticipated that mouse lines carrying numerous
human alleles will become commonplace early in the next millennium.
