| | SO WHAT IS ELASTIN?
Some Questions and Answers
*Howard M. Lenhoff, Executive Vice President
Williams Syndrome Foundation, and
Professor of Developmental and Cell Biology
University of California, Irvine, CA 92697-2300
A gene for elastin is missing: Dr. Colleen Morris of the
University of
Nevada and her coworkers at the University of Utah
discovered that all
individuals with Williams syndrome tested lacked a small
piece of one of
their two chromosomes called
Chromosome # 7
[NOTE: Our chromosomes come
in pairs, one from each parent]. In individuals without WS
both of their
chromosome # 7s were intact. One of the genes in the
missing piece of
chromosome #7 was identified as the gene for making
elastin. In the
following, I try to answer some of the questions that you
may have
regarding genes, elastin, and the consequences of this
discovery for
your children.
***
How do most genes work? Most genes are responsible for the
synthesis of
a specific protein. Thus, for each kind of protein in our
body, we have
a specific gene responsible for its synthesis.
***
What is a protein? Our bodies can make about 100,000
different kinds of
proteins, each one having a special role to play. All
proteins in our
bodies are made up of small molecules, called amino acids,
linked
together in coiled chains. There are two general classes of
proteins.
Most are functional proteins which make the chemical
reactions in our
bodies go very quickly. One example is ptyalin, the protein
in our
saliva that digests starch into sugar. The other proteins,
the
structural proteins, help organize the structure of our
tissues and
organs, and give them strength and flexibility.
Some of these structural proteins are long and fibrous. The
most common
fibrous protein is collagen. This tough protein is found in
our tendons,
ligaments, and in the connective tissue of our skin, blood
vessels,
lungs, and other tissues and organs. The rope-like collagen
fibers gives
those tissues and organs the rigidity they require to
function.
***
What is elastin and where is it found? Elastin is our
body's structural
protein that gives elasticity to our tissues and organs.
Elastin is
found predominantly in the walls of our arteries, in our
lungs,
intestines, and skin, as well as in other elastic tissues.
It functions
in connective tissue in partnership with collagen. Whereas
collagen
provides rigidity, elastin is the protein which allows the
connective
tissues in our blood vessels and heart tissues, for
example, to stretch
and then recoil to their original positions.
Imagine elastin within the body's connective tissue to act
like a bunch
of rubber bands that are tied together at a number of
places. When the
elastic bands are pulled, they will stretch, and when there
is no longer
a pull, they will return to their original relaxed state.
You can't pull
the elastin chain too far because the companion stiff
collagen fibers in
the connective tissue limit the stretching of the elastin
fibers in the
tissue.
***
Why is elastin a particularly unusual protein? Elastin is
considered by
scientists to be a very tough and relatively stable protein
because it
has many internal linkages. Those linkages make elastin
resistant to the
normal breakdown characteristic of most proteins.
***
Since elastin is relatively stable, do we need to make
elastin
throughout our lives? No! Normally the body stops making
elastin once
the body reaches maturity soon after puberty. A geneticist
would say the
same thing by stating that "the gene for elastin is turned
off just
after puberty." In other words, once the body has made its
elastin, it
will not make that protein any more.
***
What is the consequence of not being able to make any more
elastin after
we mature? In two words, aging begins.
***
So what is the significance of all this to our child with
Williams
syndrome? On the one hand it helps us understand why our WS
children
have some complications with organs requiring elasticity,
such as
arteries and the intestines. On the other hand, at this
time we do not
know many of the consequences of the absence of the gene.
For example,
although we know that people without WS possess two doses
of the gene
for elastin and that people with WS have only one dose, we
do not know
if the gene from those with WS may work overtime to
compensate.
Furthermore we do not know how the absence of the one
elastin gene
affects the structure of the arteries, lung, and other
tissues and
organs of the body. That is precisely why further research
is needed.
What is the difference between the elastin made by
individuals with WS
and individuals who have inherited supravalvular arterial
stenosis
(SVAS)? Unfortunately, we can not say for certain until
further research
is done. The following statements, however, reflect the
current thinking
of research scientists: Those with WS produce elastin that
has a normal
structure, but probably not enough of it. On the other
hand, individuals
who have inherited SVAS, have a mutation of the gene for
elastin in one
of their chromosomes; therefore for every molecule of
normal elastin
they produce, they also produce one of abnormal elastin
thereby giving
their "hybrid elastin" an abnormal structure which leads to
a defective
elastin protein.
***
Will the discovery of the missing elastin gene, and the
anticipated
future discovery of the properties of elastin in
individuals with WS
explain all of the symptoms of WS? Probably not. Remember,
a piece of
Chromosome # 7 is missing, and the size of the missing
piece (called a
microdeletion) may vary among individuals. We still do not
know the
identity of other genes which may also be missing. Perhaps
when we
identify the other missing genes, we may be in a better
position to
understand more of the symptoms of WS and gain insight into
how to
counteract some of the negative aspects of the syndrome.
***
I'm confused; although we know that an elastin gene is
missing, are we
right back at square one? WRONG! Dr. Morris and her
colleagues have
shown that one chromosome, chromosome # 7, is involved, and
that one
specific location on that chromosome, the site of the
elastin gene, is
important. Now that scientists have the elastin gene as a
landmark, they
can start looking on either side of the elastin gene for
other genes
that may affect WS. The needle in the haystack of 100,000
genes has been
found! Now we must find out what genes are on both sides of
that needle.
*Dr. Lenhoff began teaching biochemistry at the University
of California
in 1969. He published his first research on collagens in
1957 while he
directed the Biochemistry Laboratory of the Armed Forces
Institute of
Pathology at the Walter Reed Medical Center.
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