Article: Does Environment Influence Genes? Researcher Gives Hard Thoughts On Soft Inheritance
- From: "Robert Karl Stonjek" <rstonjek@xxxxxxxxxxxxxx>
- Date: Thu, 10 Aug 2006 15:48:09 -0400 (EDT)
Does Environment Influence Genes? Researcher Gives Hard Thoughts On Soft
Inheritance
Organisms, including humans, all inherit DNA from generation to generation,
what biologists call hard inheritance, because the nucleotide sequence of
DNA is constant and only changes by rare random mutation as it is passed
down the generations.
But there also is evidence, especially in plants, that non-genetic factors
modifying the DNA can also be inherited. The modifications of the genetic
material take the form of small chemical additions to one of the DNA bases
and the alternative packaging of the DNA. These so-called epigenetic
modifications are known to be important for turning genes on and off during
the course of an organism's life, but their importance in controlling
inheritance has been debated. Many biologists are skeptical of any form of
soft inheritance, where the genetic material is not constant, believing that
it is only genetic information - DNA -- that can be passed onto generations.
Now Eric Richards, Ph.D., professor of biology at Washington University in
St. Louis, writing in the May issue of Nature Reviews Genetics, analyzes
recent and past research in epigenetics and the history of evolution and
proposes that epigenetics should be considered a form of soft inheritance,
citing examples in both the plant and mammalian kingdoms.
In doing so, he evokes the pre-Darwinian evolutionist Jean-Baptiste Lamarck
(1744-1829), a name that evolutionary biologists thought long ago left the
stage, and Soviet agronomist T.D. Lysenko. Lamarck, and more recent
neo-Lamarckian researchers, believed that the environment plays a key role
in a species acquiring inherited characteristics that drive variation and
evolution. Lamarck, for instance, believed that shore birds acquired their
long legs by constantly stretching their legs to lift themselves out of the
water and that generations later that kind of environment gave rise to birds
with long legs. Neo-Lamarckian views of evolutionary change stress the
importance of the environment in altering inheritance.
"When most biologists hear the name Lamarck or the term soft inheritance,
the reaction is, 'Oh my God, here we go again'," Richards says. "But from a
molecular biology point of view there is a mechanism to do soft inheritance,
and epigenetic inheritance can be construed as a form of soft inheritance.
That's all I'm saying. The really heretical thing to say is that the
environment could be pushing the epigenetic information in a direction that
is beneficial. This is the more extreme variation of soft inheritance that
raises the hackles."
Packing DNA
Epigenetic mechanisms leave DNA sequence unaltered but can affect DNA by
preventing the expression of genes. Richards cites a study that shows
certain epigenetic alleles can be inherited that affect tumor suppressor
genes. His own work in plants has often shown epigenetic information can be
inherited. The Richards lab specializes in epigenetics, a biological field
that deals with information stored "above and beyond the gene," referring to
the Greek meaning of the term. A classic epigenetic mechanism is a process
known as DNA methylation, a chemical modification of cytosine, one of the
four chemical subunits of DNA. Without proper DNA methylation, higher
organisms from plants to humans have a host of developmental problems, from
dwarfing in plants to certain death in mice.
The next level of gene regulation studied in epigenetics is DNA packaging.
DNA is wrapped around proteins similar to the way that thread is wrapped
around a spool. Loosely wrapped DNA is more readily accessible and therefore
more easily expressed than tightly wrapped DNA, allowing another mechanism
for regulation of gene expression. The location of DNA within the nucleus
also influences gene expression.
"Epigenetics as soft inheritance in mammals puts us on a slippery slope that
many people don't want to visit," Richards says.
'Different strokes' for rat folks
Still, recent studies in mice and rats have fueled the controversy. Richards
cited "a whole new world called nutritional epigenomics," where researchers
are trying to influence epigenetic information by of all things diet. In a
study with mice hybrids, researchers provided pregnant moms with varying
levels of folate and B vitamins, to affect DNA methylation.
"The idea was : If you pump these pregnant moms up with these dietary
supplements, you might be able to skew the DNA methylation patterns, and
thus skew the way the mice come out at the end of the day, and it works,'"
Richards says. "In this particular instance that says what you're getting
fed in the womb influences your phenotype - physical and physiological
attributes. "
Another study showed that early grooming and nurturing of rat pups by rat
moms affects methylation of a glucocorticoid receptor gene in the
hippocampus in the brain. If the pups get lots of nurturing the
glucocorticoid gene gets turned on and expressed early at a critical period,
providing pups the beneficial outcome to handle stress later in life. Not
enough nurturing and grooming, and the gene never gets turned on. Richards
says that whole mechanism appears to be the result of changes in DNA
methylation associated with changes in DNA packaging.
"These studies do not demonstrate inheritance between generations, but they
do show that the early nutritional environment in the mice and early
behavioral environment in the rat studies can change the DNA packaging on
the genome, and that that is 'remembered' in the cell divisions that make
the rest of the organism, " Richards says. "But this is not from one
generation to another. No one has shown that yet.
"To get to the issue of the more extreme variations of soft inheritance, it
has to be determined whether the environment can induce an epigenetic change
in an organism that can be inherited in subsequent generations. Certainly,
nobody has shown that an epigenetically induced beneficial or adaptive
change has been inherited. Mechanistically, there is no reason to discount
epigenetic inheritance. The biochemical nuts and bolts are there to support
it. The big questions to resolve are how many epigenetic changes are induced
by the environment, what types of phenotypes result from these changes, and
how many of these epigenetic changes are inherited."
Source: Washington University in St. Louis
http://www.sciencedaily.com/releases/2006/08/060807154715.htm
Posted by
Robert Karl Stonjek
.
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