August 11, 2006
CONTACT: Karen Kreeger
karen.kreeger@uphs.upenn.edu
Penn Researchers Discover “Remote
Control” for Expression
of Human Growth Hormone Gene
Mistakes in Expression Implied in Growth Disorders
(Philadelphia, PA) - Researchers at the University of Pennsylvania
School of Medicine recently discovered a novel mechanism that
works over an extensive genomic distance and controls the expression of
human growth hormone (hGH) in the pituitary gland. This
mechanism involves a newly discovered set of “non-coding RNAs”
expressed in the vicinity of the hGH gene.
By examining the relationship between these non-coding RNAs and the hGH
gene, researchers hope to understand how these remote regions impact hGH
gene expression and dysfunction. Such insight may aid researchers in the
development of therapeutics for growth hormone defects and lead to a greater
understanding of the causes of other genetic disorders.
The human genome is comprised of both non-coding DNA and coding regions,
or genes. While researchers once believed that only genes were transcribed
into messenger RNA (mRNA), investigators have recently discovered that
non-coding DNA is copied into mRNA as well. Unlike coding mRNAs, which
are translated into functional proteins and peptides, the function of
most non-coding RNAs is unclear. Although non-coding RNAs fail to produce
functional proteins, researchers believe that in some cases these RNAs
may control gene expression.
Using a genetically modified mouse model, Nancy E. Cooke, MD,
Stephen A. Liebhaber, MD, Professors of Genetics and
Medicine, and colleagues, demonstrated a critical role of two non-coding
regions on the activation of the hGH gene. They described
their recent findings in the August issue of Molecular Cell.
Synthesized by the pituitary gland, human growth hormone activates growth
and cell reproduction. In addition to serving as a major contributor to
height growth during childhood, hGH plays a role in strengthening
bones and increasing muscle mass throughout life. While mutations to the
hGH gene often lead to abnormal growth in children and
adults, these mutations have provided researchers with key clues regarding
the genomic areas that appear to control expression of the hGH
gene.
Previous work in the laboratories of Cooke and Liebhaber found that the
hGH gene is controlled by a non-coding DNA region, or
locus control region. Remarkably, this region is located more than 14,000
base pairs away from the hGH gene. At the genomic level,
a 14,000 base-pair separation is equal to the size of 10 growth hormone
genes lined end to end. “The effects of the locus control region
on human growth hormone expression is as if you turn a key in the lock
of a house at one end of your street, and find that this action opens
the lock and door of a house a block away,” notes Liebhaber.
By carefully analyzing the 14,000 base pairs separating the hGH
gene and its locus control region, co-authors Yugong Ho, PhD,
an Instructor of Genetics at Penn and a Cooke/Liebhaber lab member, and
Felice Elefant, PhD, Assistant Professor at Drexel University and former
member of the Cooke/Liebhaber lab, found that the locus control region
was copied into RNA, and discovered a gene called CD79b
within this region. Remarkably this CD79b gene was also
copied into RNA in the pituitary. While the CD79b gene
normally codes for a protein in blood lymphocytes, researchers discovered
that CD79b appears to play a very different role in the pituitary
gland. Here, CD79b was actively transcribed into mRNA, but this
mRNA failed to translate into a functional protein. Instead, the non-coding
RNA was suspected to play a role in hGH gene regulation.
In order to determine whether the CD79b RNA in the pituitary
gland served a function, Ho inserted a segment of human DNA that included
hGH, the hGH locus control region, and CD79b
into a group of mice. As a result, the transgenic mice expressed high
levels of human growth hormone in the pituitary as well as mouse growth
hormone. To test whether the transcription of the locus control region
and CD79b played a significant role in hGH expression
in transgenic mice, Ho then inserted a special piece of DNA into the locus
control region. This DNA insertion specifically blocked the copying of
the CD79b gene into RNA in the pituitary. This blockade led to
the five-fold repression of hGH gene expression. These findings
confirm that the CD79b non-coding DNA actively contributes to
hGH expression. The relationship between CD79b, the
hGH locus control region, and the hGH gene may aid researchers
in the development of treatments for patients suffering from hGH
deficiency.
“Our data predict that a subset of children with short stature and
low growth hormone may be suffering from a mutation in the hGH
locus control region, which blocks full levels of hGH gene activity,”
explains Ho. “We are now actively screening the appropriate clinical
populations for such genetic defects.”
In the future, Cooke, Liebhaber, and Ho will continue to search for how
transcription contributes to long-range activation of hGH gene
expression through the development of new transgenic mouse models and
the biochemical analysis of the hGH locus.
“By understanding how non-coding DNA functions at the human growth
hormone locus, researchers may be able to identify similar situations
at other genetic loci,” says Liebhaber.
“With every step forward in understanding how genes are expressed,
we increase our awareness of how naturally occurring and acquired mutations
interfere with this process,” adds Cooke. “Our research sets
the groundwork for advances in diagnosis and eventual treatment of genetic
diseases.”
These studies were funded by the National Institutes of Health.
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This release is available online at
http://www.uphs.upenn.edu/news/News_Releases/aug06/hghgene.htm