We break down a few interesting results recently published.
Multiple mutations on MeCP2
In most cases of Rett syndrome, one mutation occurs on (a single copy of) the MeCP2 gene. But, rarely, multiple mutations occur on the same copy. Researchers from the University of Alabama at Birmingham characterized the largest group of individuals to date, 15 of them, who have more than one mutation. (The girls were participating in the Rett Syndrome Natural History Study, which aims to gather detailed historical and physical examination data on a large cohort of females with Rett — of which there are now more than 800.)
In contrast to two previous case studies suggesting that girls with more than one MeCP2 tend to have more severe disease, the new study found that participants with multiple mutations assessed using two quantitative measures — the Clinical Severity Scale and the motor behavioral analysis — showed no difference from those individuals with single, similar mutations. The findings appear in the American Journal of Medical Genetics Part A.
Other factors, including the individual mutations themselves or how much of the mutated gene is inactivated as part of normal development, can worsen symptoms, the authors suggest. Future drug therapies that account for a person’s genetics should consider this small group of individuals, the authors add.
Rett Syndrome and epilepsy
Seizures affect 50% to 90% of individuals with Rett syndrome. According to a recent review in Pediatric Neurology by Alison Dolce and her colleagues at Johns Hopkins Hospital in Baltimore, Maryland:
- It’s unclear whether earlier onset of seizures signals a poorer overall prognosis.
- EEG, or electroencephalography, an indirect measure of brain activity, is an important diagnostic tool in children with Rett syndrome because it allows clinicians to identify true seizures — as opposed to symptoms, ranging from hyperventilation or motor abnormalities, that are often mistaken for seizures. From 2 to 4 years of age and beyond, all girls with Rett syndrome develop abnormal EEGs, the authors write.
- There are few studies assessing anticonvulsant therapies in Rett, and many of them are short-term and involve only a small number of participants. “At our centers, we have circumstantial evidence of good responses to levetiracetam, lamotrigine, and topiramate,” the authors write. Although the clinicians also use valproate, the side effects can be problematic. In general, many children with Rett require more than one medicine.
- It’s still difficult to predict which patients will tend to develop seizures early and also whose seizures will be easier to treat.
- According to the authors, physicians should begin to consider the ketogenic diet and vagal nerve stimulation earlier in the clinical course of Rett, because these treatments “might be helpful.”
More on MeCP2 mechanisms
MeCP2 protein has several defined parts that researchers are studying. The methyl-CpG-binding domain or MBD, for example, binds to DNA in studies conducted in vitro and is thought of as crucial for the protein’s function of turning genes on or off, or altering the way DNA is stored in a cell. Given that the interactions between MeCP2 and DNA are difficult to mimic in a dish, a team of Canadian scientists wanted to see what would happen to the interaction in a mouse whose front end of the protein was deleted. The Mecp2tm1.1Jae model, commonly used in studies of Rett syndrome, is missing its first 116 amino acids—which include the 48 amino acids that make up the MBD. (What’s more, MeCP2 mutations in people with Rett occur in this region, such as R106W, R1333C, P152R, F155S and T158M.)
In the study, mutant mice still had MeCP2 in their tissues but only about half as much as healthy mice did. Although the mutated MeCP2 still bound to DNA and chromatin, the interactions were weaker and less specific for methylated DNA. In addition, although the ‘nuclear localization signal’ region of the mutated protein was still intact, protein was less able to move from the cytoplasm to the nucleus, where it can influence the production of other proteins. The paper is published in the May issue of Nucleic Acids Research.