RETT SYNDROME IS TRACED TO A SURPRISING GENETIC CULPRIT

Less than 20 years after Rett syndrome became a familiar diagnosis to pediatric neurologists in the United States, researchers have identified the responsible gene. Two new studies demonstrate that Rett syndrome—a severe neurodevelopmental disorder characterized by apraxia, mental retardation, and other neurologic symptoms—occurs when a mutation disrupts the function of MECP2, a gene that participates in "silencing" numerous other genes at key points in development. The discovery has immediate implications for the diagnosis of developmentally impaired infants, as well as possible future implications for the development of effective treatments.

Rett syndrome was first described in the German literature in 1966 by Dr. Andreas Rett, but received little attention in the United States until a 1983 paper by Dr. Bengt Hagberg and colleagues was published in the Annals of Neurology. The syndrome occurs almost exclusively in females, affecting approximately one of every 10,000 to 15,000 girls born in the world. Infants with Rett syndrome develop normally for the first six to 18 months, then motor skills and speech typically regress; walking is often delayed or accomplished with great difficulty. Perhaps the most striking feature, however, is the near-constant hand-wringing that is typical of the disorder. About 80% of children with Rett syndrome have seizures; also common are breathing irregularities, scoliosis, retarded growth, and EEG abnormalities.

Because documented cases of Rett syndrome in boys are extremely rare—and almost always occur in conjunction with Klinefelter's syndrome (two X chromosomes)—researchers have long suspected that Rett syndrome is caused by a dominant mutation on the X chromosome. Presumably, the defective gene is fatal to boys with only one X chromosome, but girls (and XXY boys) are able to survive because they possess a second, normal copy of the gene. In addition, because familial Rett syndrome accounts for less than 1% of known cases, de novo mutations have been suspected.

The recent research, published in the October issue of Nature Genetics, confirmed that the mutation is located in the Xq28 region of the X chromosome, and that the disease is typically caused by sporadic mutations. More unexpected, however, was the identity of the defective gene; MECP2 is expressed by nearly all cell types, making it an unlikely culprit in a disease with predominantly neurologic symptoms. "Our first candidates were neurotransmitters and receptors—genes that have known function in neuronal differentiation or synaptogenesis," noted study author Huda Y. Zoghbi, MD, Professor of Pediatrics, Neurology, and Molecular Genetics at Baylor College of Medicine, and an Investigator with the Howard Hughes Medical Institute. "We spent quite a bit of energy on such genes, and of course none of them panned out. So we were quite surprised to find that the mutation is in a gene that is ubiquitously expressed."

Of the 21 patients with sporadic Rett syndrome described in the Nature Genetics study, five (24%) had point mutations that disrupted one of the binding sites in the protein encoded by MECP2, Dr. Zoghbi and colleagues reported. "Since [completing the paper] we've looked at a lot more patients, and [at a greater proportion] of the gene, and our detection rate is much higher now," Dr. Zoghbi said in an interview with Neurology Reviews. "Now we're finding MECP2 mutations in about 70% of the patients. I anticipate that as we look at more of the gene, and refine the technology, the percentage will be even higher. That's why I think that this will probably be the predominant, if not only, gene for Rett syndrome."

Although the identification of the relevant gene means that prenatal or postnatal testing can now determine which infants are fated to develop Rett syndrome, study coauthor Uta Francke, MD, noted that routine screening makes little sense because there is currently no effective treatment. Once a treatment is discovered, researchers hope, the six- to 18-month postnatal window may allow interventions to be initiated before permanent neuronal damage occurs.

There are, nonetheless, immediate clinical applications for MECP2 testing, particularly for diagnosis, added Dr. Francke, who is Professor of Genetics and Pediatrics at Stanford University School of Medicine. If an infant girl begins developing normally but then regresses, an exact diagnosis is often not possible before the age of 3 or 4 years, "because [Rett syndrome] is a clinical diagnosis and you need to see certain manifestations for patients to meet the criteria. It's an absolutely agonizing time for parents, because you need to do many tests to rule out other conditions; it's an extensive workup to rule out everything else it could be. And some of these [other disorders] might be treatable, so you don't want to miss them. Now we have this gene, and we can look for mutations in any girl whose development doesn't seem to be quite right."

Genetic testing might also be useful in family planning for women who have already given birth to one daughter with Rett syndrome. Because the cause of the disorder has been unknown, until now these women were often "reluctant to try another pregnancy, just hanging in there in a state of uncertainty," Dr. Francke said. Now testing can determine which women with Rett infants are carriers of the disease and which are not; if a woman who carries a MECP2 mutation does decide to have a child, said Dr. Francke, "we can do prenatal testing in the first trimester, and then she doesn't have to wait until nine months or longer to find out what's going on. We can say, 'This is what you've got; this is what is going to happen to your daughter.' "

Although the initial batch of mutations reported in the Nature Genetics paper each occurred at a different site in the MECP2 gene, subsequent screening has identified several "hot spots" prone to mutation. Dr. Zoghbi and her colleagues are conducting "a blinded study to look at the spectrum of the mutations" in Rett syndrome and whether "the heterogeneity of the clinical features can be accounted for by the heterogeneity of the mutations."

However, individual differences in symptom severity may be due to a less predictable factor—X chromosome inactivation. At the point in development when a female embryo is composed of only about 400 cells, each cell turns off one of its X chromosomes, apparently at random. If most of the cells slated to become neuronal precursors in an embryo with an MECP2 mutation happen to inactivate their intact copy of MECP2, leaving them with only the mutated version, the infant may wind up severely disabled, Dr. Francke suggests. If the mutant MECP2 genes are preferentially turned off, however, the girl may have relatively few developmental difficulties. She will remain a carrier of the defective gene, however, and her offspring may develop the disorder if they inherit the mutation. MECP2 mutations in such families, including an affected 46,XY male with congenital encephalopathy, have been discussed in a paper by Wan et al, in the American Journal of Human Genetics.

Because relatively little is known about MECP2's role in development, including which genes it silences, it remains unclear precisely how a mutant form leads to the neurologic symptoms of Rett syndrome. However, Dr. Francke and colleagues are looking for clues by screening expression levels of about 20,000 genes in Rett patients and controls. Genes that are abnormally expressed in Rett patients, Dr. Francke notes, are perhaps not being properly shut off by MECP2 and thus may play a role in the pathogenesis of the disorder. When the Human Genome Project reaches completion early in the next century, she adds, the other 80,000 genes in the genome will be screened as well.

—Peter Doskoch
Senior Editor

Suggested Reading
Amir RE, Van Den Veyver IB, Wan M, et al. Rett syndrome is caused by mutations in X-linked MECP2, encoding methyl-CpG-binding protein 2. Nature Genet. 1999;23:185-188.
Wan M, Lee SSJ, Zhang X, et al. Rett syndrome and beyond: recurrent spontaneous and familial MECP2 mutations at CpG hotspots. Am J Hum Genet. [in press].