Such properties would explain why only girls develop RTT. A female fetus could survive the effects of the otherwise lethal mutation because it has two X chromosomes -- one that harbors a normal copy of the gene and the other a defective copy. The normal gene can compensate partially for the defective gene, allowing the fetus to survive. In a male fetus, however, there would be no opportunity to inherit a compensating, functional copy of the gene since the male fetus has but one X chromosome. As a result, development is so out of kilter that the male fetus dies either before or shortly after birth.
Zoghbi, aided by Francke and members of her laboratory, narrowed the search for the RTT gene by analyzing shared and unshared DNA sequences in a small number of Rett families. By 1998, other scientists helped to further narrow the search to about 200 candidate genes. Postdoctoral fellows Ruthie Amir of Zoghbi's laboratory and Mimi Wan of Francke's laboratory, assisted by technician Charles Tran, split up the work of analyzing those 200 genes.
A quarter of the way through a set of 50 candidate genes, Igna Van den Veyver, a member of Zoghbi's lab, suggested to Amir that MECP2 would be an excellent candidate gene. Sure enough, Amir found mutations in MECP2 that were shared by about 30 percent of their RTT patients.
Zoghbi said that it is not yet clear exactly how MECP2 defects cause RTT. One hypothesis, she said, is that the defective MeCP2 protein allows genes to remain active that should have been silenced at points along the precisely timed process of nervous system development.
"The exciting part of this discovery is not just what it may mean for RTT
patients," she explained. "Now we know about a whole genetic pathway involved in
neural development, and that human disease can result from its destruction. Many
other disorders could be
'"/>
Contact: Jim Keeley
keeleyj@hhmi.org
301-215-8858
Howard Hughes Medical Institute
1-Oct-1999