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Linking genetic findings to clinical symptoms in Rett syndrome

In 2014, the Ontario Rett Syndrome Association (O.R.S.A.) Board of Directors had unanimously approved the funding of a $50,000 Research Grant to a joint grant application from Dr. Juan Ausio (Biochemistry and Microbiology, University of Victoria, BC) and Dr. John Vincent (Neurogenetics, University of Toronto). This was the first grant from The Hope Fund and was largest grant released at the time.

We are pleased to announce that with our support, their work, From Function to Phenotype: Impaired DNA Binding and Clustering Correlates with Clinical Severity in Males with Missense Mutations in MECP2 has been published in the Nature Journal “Scientific Reports”. A summary that explains the findings and context can be found below and to read the full article, please follow this link, www.nature.com/articles/srep38590/

Mutations in the MECP2 gene cause Rett syndrome (RTT). MeCP2 binds to chromocentric DNA through its methyl CpG-binding domain (MBD) to regulate gene expression.While it’s well established that single DNA changes can lead to Rett syndrome — one of the most common genetic forms of intellectual disability – a new CAMH study has for the first time linked DNA mutations directly to how severe the syndrome is among those affected.

The results could pave the way for improved diagnosis and potentially new treatments for Rett syndrome, which is marked by problems with communication, language, learning and physical coordination.

The study was led by Dr. John Vincent, Senior Scientist in CAMH’s Campbell Family Mental Health Research Institute, and PhD student and first author Taimoor Sheikh. It was published in the journal Scientific Reports in December.

The DNA in question is part of the MECP2 gene, which is made up of thousands of pairs of DNA – one part from our mother, one from our father. This DNA can be thought of as “letters” that create words to generate genetic instructions. In many cases of Rett syndrome, any one of about 160 known changes to these letters on the MECP2 gene can garble the words and then the instructions.

There’s a reason these single DNA changes have such serious effects: the MECP2 gene carries instructions for a fundamental protein in the brain. The MECP2 protein binds to DNA at various places, and then acts as a “switch” to turn nearby genes on or off.

The research team found that if the change, or mutation, occurred in a part of the protein where it disrupted the switch, the syndrome was more serious. Changes that were further from the parts of MECP2 protein that bind to DNA, instead affected how the protein interacted with other proteins. This led to milder symptoms.

“The discovery raises the possibility of finding compounds to treat Rett syndrome, if we could create a ‘patch’ to make the protein function the way that it’s supposed to,” says Dr. Vincent, who heads the Molecular Neuropsychiatry & Development (MiND) lab at CAMH.

In addition, the nature of each change – which “letters” were swapped out – also affected how serious the syndrome was, the researchers note. They used a series of tests to study MECP2 proteins, with 12 different mutations including many most commonly reported in girls with Rett syndrome.

Clinical implications

With further development, the findings could help in cases where Rett syndrome is suspected in a patient. Symptoms typically appear in girls before the age of two. Genetic screening would

yield information on the type of mutation, to predict the type and severity of illness, says Dr. Vincent. As there is currently no treatment for Rett syndrome, only symptom management, screening could point to where more supports are needed.

A study in boys

Another novel aspect of the study was the fact that it also focused on boys with MECP2 mutations. In boys, Rett syndrome is extremely rare. This approach provided researchers with a clearer picture of the effect of protein changes in terms of symptoms and severity.

The MECP2 gene is located on X chromosomes. As girls have two copies of X, those with Rett syndrome have a second unaffected MECP2 gene on the other X, which may compensate for the garbled instructions. This is seen by the fact that clinical symptoms of Rett syndrome vary greatly in girls.

As boys carry X and Y chromosomes, any mutation on the X has an even more serious effect. It’s believed that most such cases do not survive pregnancy or early infancy.

By studying 11 rare cases of boys identified with MECP2 mutations, the researchers were able focus on DNA changes and their molecular and cellular effects in relation to clinical severity, without any compensating effects of the second MECP2 gene on the other X chromosome.

Ultimately, they hope to find compounds to help treat Rett syndrome using an approach called “peptide panning.” It involves testing many different peptides, which are small pieces of proteins, to see which ones bind to the mutant MECP2 protein to help recover its normal functioning.

“By further developing the molecular and cellular tests used in our study, we hope to be able to predict the likely clinical outcomes for newly discovered MECP2 mutations, as well as screen for compounds that can make MECP2 protein with specific mutations revert back to its normal function,” says Dr. Vincent.

From Function to Phenotype: Impaired DNA Binding and Clustering Correlates with Clinical Severity in Males with Missense Mutations in MECP2.

Sheikh TI, Ausió J, Faghfoury H, Silver J, Lane JB, Eubanks JH, MacLeod P, Percy AK, Vincent JB.Published in Scientific Reports, 8th Dec 2016; funded through an ORSA grant to John B. Vincent and Juan Ausió.