3 Recent Advances 49fish, determined that the myostatin gene is highly
conserved among vertebrates, which suggests that
its function may be conserved as well. The re-
searchers also demonstrated that the increase in
muscle mass (double-muscling) phenotype observed
in some breeds of cattle, such as Belgian Blue and
Piedmontese, is due to mutations in the myostatin
gene. In myostatin-null mice that had the myo-
statin gene knocked out by gene targeting, individ-
ual muscles weighed on average two to three times
more than those in wild-type mice, and body weight
was 30% higher. This difference was not due to an
increase in fat amount, but to an increase in the
cross-sectional area of the muscle fibers (hypertro-
phy) and an increase in the number of muscle fibers
(hyperplasia) (McPherron and Lee 1997).
It is believed that myostatin prodomain may bind
noncovalently with the mature myostatin, resulting in
inhibition of the biological activity of myostatin
(Thies et al. 2001). Based on the general molecular
model of TGF proteins, Yang et al. (2001) hypothe-
sized that overexpression of the prodomain segment
would interfere with mature myostatin, resulting in
the promotion of muscle development. The myosta-
tin prodomain DNA was cloned into a pMEX-
NMCS2 vector, which contained a rat myosin light
chain 1 (MLC1) promoter, a SV40 poly adenylation
sequence, and a MLC enhancer. This construct was
then inserted into the mouse genome using the
pronuclear microinjection technique. In comparison
with wild-type mice, overexpression of the myo-
statin prodomain in the transgenic mice resulted in a
17–30% increase in body weight; a 22–44% in-
crease in total carcass weight at 9 weeks of age (Fig.
3.10); and a significant decrease in epididymal fat
pad weight, which is an indicator of body fat mass
(Yang et al. 2001). No undesirable phenotypes or
health or reproductive problems were observed in
the transgenic animals. These results indicate that
this same approach can be used to develop farm ani-
mals with enhanced growth performance, increased
muscle mass, and decreased fat content, which would
equate to a healthier meat product for consumers
(Yang et al. 2001).
REDUCEDFATCONTENT INTRANSGENIC
SWINE
In the human diet, ingested exogenous fats serve as
the raw material for the synthesis of fat, cholesterol,
and many phospholipids. Since fat energy content is
two times greater than the energy obtained from car-
bohydrates and proteins, most of the energy that is
stored in the body is in the form of fat. Fats are a
group of chemical compounds that contain fatty
acids. The most common fatty acids found in animal
fats are palmitic acid, stearic acid, and oleic acid.
The human body is able to synthesize these fats, but
there is one more class of fatty acids called the
essential fatty acids (linolenic acid, linoleic acid,
and arachidonic acid), which the body cannot pro-
duce; they therefore must be obtained from diet
(Campbell and Reece 2002). There are two main
types of naturally occurring fatty acids: saturated
and unsaturated. Saturated fatty acids (SFA) are
mainly animal fats. They are called saturated
because all the carbon chains are completely filled
with hydrogen and there are no double bonds
formed between the carbon atoms. Saturated fatty
acids are believed to be “bad” fats since they raise
both high-density lipoprotein (HDL) and low-densi-
ty lipoprotein (LDL) cholesterol (Keys et al. 1965,
NRC 1988). Unsaturated fats are found mainly in
products derived from plant sources and are divided
into two categories: monounsaturated fatty acids
(MUFA), which have one double bond; and polyun-
saturated fatty acids (PUFA), which have two or
more double bonds in the carbon chain. It has been
observed that the increased consumption of these
“good” fats actually reduces LDL levels and en-
hances HDL levels (Grundy 1986, NRC 1988). It isFigure 3.10.Comparison between wild-type and trans-
genic mice overexpressing myostatin prodomain.
(Courtesy of Dr. J. Yang, Univ. of Hawaii.)