This study consists of a large series of individuals with typical RTT, formal clinical assessment, and complete MECP2 mutation analysis. This allows comparisons of the severity between participants with common specific mutations. In general, specific MECP2 mutations confer different clinical severity in typical RTT. Furthermore, specific mutations (R133C, R294X, C-terminal truncations) are less severe than other mutations (R168X, large deletions). These differences seem to be largely independent of XCI status. The difference in severity appears to result primarily from variation in three clinical features: ambulation, hand use, and language.
One particular mutation, R306C, has an unusual dissociation of the amount of preserved function. A large percentage of those with R306C are able to walk but very few are able to use words. In contrast, a previous study found that individuals with R306C have a milder phenotype31
and specifically had better language skills. A significant difference between this study and the previous work is the inclusion of atypical RTT in that study and the exclusion of such atypical individuals in this study. The improved language skills in the R306C group in the prior study may be due primarily to inclusion of atypical RTT. This argues that the R306C mutation can either severely affect language thus causing typical RTT, or does not dramatically affect language and allows the expression of milder atypical RTT or non-RTT neurodevelopmental disorders. The clinical variability of this mutation suggests that it plays a major role in the function of the MeCP2 protein.
This study presents two features of clinical relevance. First, it provides the basis for clinical counseling. Although the study lacked the power to discern the severity of any specific common mutation, it does distinguish five mutations that represent the extremes of the severity spectrum. Furthermore, within these five mutations, predictions for the possible clinical outcomes can be made. For example, this series shows that R168X confers an increased risk that the affected individual will not be able to walk, will not use words and will not have any retained hand use, whereas individuals with C-terminal truncations are much more likely to use words and walk alone. Such knowledge provides the framework for clinical counseling and assists in tailoring therapies towards the problems that result more frequently from particular MECP2 mutations.
The second clinically relevant finding concerns the design of clinical trials for Rett syndrome. Because clear differences in clinical severity exist between mutations in Rett syndrome, any intervention trial must take this into account in study design. Without such planning, false negative and false positive results might occur due to a skewed distribution of mutations amongst the treatment groups. The simplest way to account for this is to design trials that compare individuals pre- and post-treatment.
A major point of this work is the need for a larger cohort to analyze for these genotype effects. Although this study has the largest clinical population of typical RTT published to date, the smallest mutation group (R106W) only represents 3.7% of the total population, necessitating a very large sample size to acquire an adequate number of individuals for this group. Because a number of genotype-phenotype comparisons have been performed in the past, it is possible to perform a meta-analysis on the published data to look for specific mutation effects. A challenge with such an analysis is the variation in the clinical rating systems used across studies. We propose that use of the simplified, compressed system presented here would allow an easy method to perform both a meta-analysis of previously published genotype-phenotype comparisons in RTT and the collation of existing international data sets for de novo analysis.
Beyond clinical relevance, this work highlights a MECP2
allelic series which indicates that particular regions of the MeCP2 protein have unique genetic and protein interactions that determine dissociable functions of MeCP2. For example, the common missense mutations (R133C, R306C, T158M, and R106W) are dissimilar with respect to the overall clinical severity conferred by these mutations. Although R106W appears to disrupt interaction with methylated cytosines, the other mutations do not37, 38
. This suggests that these mutations alter distinct functional and possible physical interactions. An alternative explanation is that these mutations may have different effects on the mRNA or protein stability, which could be tested experimentally. An interesting comparison is between R133C and R306C, both of which are relatively mild but have differential effects on language. Understanding the interactions disrupted by these specific mutations will help elucidate the molecular mechanisms involved in the acquisition and maintenance of important neurodevelopmental skills and will identify key proteins important for control of ambulation, language and hand use.