Rett syndrome: "We'll keep the genes on for you"

Recent advances in gene therapy for neurodevelopmental disorders with epilepsy

Neurodevelopmental disorders can be caused by mutations in neuronal genes fundamental to brain development. These disorders have severe symptoms ranging from intellectually disability, social and cognitive impairments, and a subset are strongly linked with epilepsy. In this review, we focus on those neurodevelopmental disorders that are frequently characterized by the presence of epilepsy (NDD + E). We loosely group the genes linked to NDD + E with different neuronal functions: transcriptional regulation, intrinsic excitability and synaptic transmission. All these genes have in common a pivotal role in defining the brain architecture and function during early development, and when their function is altered, symptoms can present in the first stages of human life. The relationship with epilepsy is complex. In some NDD + E, epilepsy is a comorbidity and in others seizures appear to be the main cause of the pathology, suggesting that either structural changes (NDD) or neuronal communication (E) can lead to these disorders. Furthermore, grouping the genes that cause NDD + E, we review the uses and limitations of current models of the different disorders, and how different gene therapy strategies are being developed to treat them. We highlight where gene replacement may not be a treatment option, and where innovative therapeutic tools, such as CRISPR‐based gene editing, and new avenues of delivery are required. In general this group of genetically defined disorders, supported increasing knowledge of the mechanisms leading to neurological dysfunction serve as an excellent collection for illustrating the translational potential of gene therapy, including newly emerging tools.

Clinical variability in Rett syndrome

The clinical variability of Rett syndrome, associated with mutations in the MECP2 gene, varies from classically symptomatic female patients to asymptomatic female patients, and male patients who have none of the diagnostic features considered pathognomonic of this disease. Multiple factors contribute to this variability. In our studies, mutations closer to the amino-terminus, prior to amino acid 255, led to severe clinical manifestations, such as inability to walk, severe dysphagia, and urinary organic acid abnormalities, compared with mutations toward the carboxyl-terminus. However, we found no correlation between severity and mutation type (missense versus nonsense). Despite the importance of mutation location to clinical severity, the widely varying severity within the same mutation suggests that in females, X-chromosome inactivation or other epigenetic phenomena also have roles in determining severity. We propose that stages 1 and 2 of the disease are a consequence of failed, time-linked, postnatal expression of MeCP2 in cerebellar neurons. This, in association with glutamate N-methyl-D-aspartate receptor-mediated neuroexcitotoxic injury to the differentiating neurons, results in the transient age-specific autistic-like behavior, motor, and cognitive dysfunction associated with these stages.

Rett syndrome: investigation of nine patients, including PET scan

BACKGROUND

We describe nine females with Rett Syndrome (RS), aged 14 to 26 years. All had had developmental delay before the end of their first year and had subsequently regressed to profound dementia with apraxia, ataxia, irregular respirations and often also seizures.

METHODS

The Revised Gesell developmental assessment and Alpern-Boll Developmental Profile were used in modified form. Volumetric measurements of basal ganglia using MRI were compared with the findings in nine age-matched volunteer females. Positron emission scans with [18F]-6-fluorodopa and [11C]-raclopride were performed under light anesthesia with intravenous Propofol, and the findings were compared with those in healthy control girls. Bidirectional sequencing of the coding regions of the MECP2 gene was investigated in blood samples for mutational analyses.

RESULTS

The RS females functioned at a mental age level ranging from about 4 to 15 months. The scores correlated with height, weight and head circumference. Magnetic resonance scans of basal ganglia showed a significant reduction in the size of the caudate heads and thalami in the Rett cases. Positron emission scans demonstrated that the mean uptake of fluorodopa in RS was reduced by 13.1% in caudate and by 12.5% in putamen as compared to the controls, while dopamine D2 receptor binding was increased significantly by 9.7% in caudate and 9.6% in putamen. Mutations in the coding regions of the MECP2 gene were present in all nine patients. No significant correlation between type and location of mutation and volumetric changes or isotope uptake was demonstrable.

CONCLUSIONS

Our findings suggest a mild presynaptic deficit of nigrostriatal activity in Rett syndrome.

Epilepsy in autism

There is an increased but variable risk of epilepsy in autism. Three main factors--age, cognitive level, and type of language disorder--account for variability in the reported prevalence of epilepsy. The prevalence is highest in studies that have included adolescents and young adults, individuals with moderate to severe mental retardation and those with motor deficits, and individuals with severe receptive language deficits. The association of autism with clinical or subclinical epilepsy might denote common genetic factors in some cases. Whether subclinical epilepsy has adverse effects on cognition, language, and behaviour is debated, as is the relation of autistic regression with an epileptiform electroencephalogram to Landau-Kleffner syndrome. There is no evidence-based treatment recommendation for individuals with autism, regression, and subclinical epilepsy. Double-blind studies with sufficient power to resolve this issue are urgently needed.

A Rett syndrome MECP2 mutation that causes mental retardation in men

OBJECTIVE

To characterize the clinical features of a new type of X-linked mental retardation associated with MECP2 mutation in the index family.

BACKGROUND

MECP2 mutations, originally described in a high percentage of patients with classic Rett syndrome, were considered lethal in men. The authors recently described a novel A140V MECP2 missense mutation in an Italian family with X-linked semidominant mental retardation.

METHODS

The neurologic features of six symptomatic relatives (two women and four men) carrying the mutation were compiled. Laboratory investigations included EEG, EMG, conduction velocity (CV) of peripheral nerves, brain MRI, and (1)H-MR spectroscopy.

RESULTS

Mental retardation and signs of neurologic impairment were present in all the affected members, but more pronounced in men. Neurologic features included slowly progressive spastic paraparesis/pyramidal signs (6/6), distal atrophy of the legs (6/6), ataxia (2/6), and postural tremor of the hands (3/6). Speech was preserved (6/6) but was dysarthric in the oldest brothers (2/6). Mild dysmorphic features were present in all cases.

CONCLUSION

The neurologic disorder associated with A140V MECP2 mutation is not necessarily lethal in men, but they are more severely affected than women of the same family.