Mar. 10, 2003 CHAPEL HILL -- New research at the University of North Carolina at Chapel Hill offers an important contribution to a new wave of thinking in genetics: the idea that not all human disease states are due to alterations in DNA sequence.
A growing body of research on these "epigenetic" changes are leading geneticists to rethink the conventional view that all human disease is fundamentally tied to DNA sequence variation (changes in the actual sequence of the DNA nucleic acid code of A's, C's, G's and T's within any given gene).
The DNA within human cells contains the information for roughly 35,000 different proteins that carry out the body's functions. But not all of these genes are active all of the time. Like switches, epigenetic modifications to proteins surrounding the DNA regulate a given gene's activity, such that only those that are required in a particular cell are active (switched on). These changes constitute a "memory" of gene activity that can be passed on each time a cell divides.
If these epigenetic modifications do not occur properly, the result can cause some genes to become switched on or off incorrectly, thereby having profound biological consequences. Incorrect epigenetic modifications have been implicated in many human disorders including several types of cancer, birth defects and mental retardation.
"These expression changes are heritable and are not related to sequence changes in the gene that is directly affected", said Dr. Terry Magnuson, Kenan professor of genetics and director of the Carolina Center for Genome Sciences. "Sequencing the human genome will not necessarily lead one to discover why these genes are expressed abnormally."
For example, there is a recent awareness among scientists of a new type of health threat posed by environmental chemicals that can disrupt endocrine signals during critical periods of development through epigenetic alterations. Additionally, researchers studying Rett syndrome (a rare neurological disease) have found that the mutated gene exerts its effect by influencing epigenetic modifications elsewhere in the genome.
A new study, published online in Nature Genetics on Monday (March 10) makes a case for epigenetics by identifying a gene that may be critical for proper epigenetic changes. The new study shows for the first time that the gene called "Eed" is required for the proper epigenetic regulation of a subset of genes that normally show parent of origin expression, known as genome imprinting. Genome imprinting is a phenomenon in which only one copy of specific genes are active, or switched on. Which copy is active depends upon whether they are inherited from the mother or the father.
When Eed is mutated or its function impaired, loss of imprinting can occur - both the maternal and paternal copy become active.
The new work builds on previous research by Magnuson's group. In that work, Eed was shown to maintain the molecular brakes on the paternal X chromosome, keeping many of its genes inactive, thus preserving female embryo survival. The study team created female mouse embryos that lacked a copy of Eed. As a result, the paternal X chromosome was shut down only temporarily and then came back on with subsequent problems in placental formation.
"Basically, Eed forms a complex of proteins and alters those chromosomal proteins that affect the configuration of the chromosome so as to allow or not allow expression. If this gene, Eed, isn't functioning properly, the imprint is lost resulting in incorrect activity of specific genes," Magnuson said.
"We learned from the Human Genome Project that in a complex organism like humans, the 35,000 genes must act in concert with one another in many different combinations at many different times," he added. "So understanding how genes are regulated in terms of their expression, how they are turned on and off, and if they are off how they are maintained in that 'off' state, becomes critical. This study opens up new possibilities for looking into mechanisms responsible for epigenetic alterations in human disorders."
Along with Magnuson, study co-authors are Jesse Mager and Nathan Montgomery, graduate students in the Curriculum in Genetics and Molecular Biology; and Dr. Fernando Pardo-Manuel de Villena, assistant professor of genetics and member of the UNC Lineberger Comprehensive Cancer Center.
The research was funded by the National Institute of Child Health and Human Development, part of the National Institutes of Health.
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