Carolyn Schanen

Guidelines for reporting clinical features in cases with MECP2 mutations

An international group recommends that papers relating phenotypes to genotypes involving mutations in the X chromosome gene MECP2 should provide a minimum data set reporting the range of disturbances frequently encountered in Rett Syndrome. A simple scoring system is suggested which will facilitate comparison among the various clinical profiles. Features are described which should prompt screening for MECP2 mutations.

Phenotypic manifestations of MECP2 mutations in classical and atypical rett syndrome

Since the identification of mutations in MECP2 in girls and women with apparent Rett syndrome, numerous efforts have been made to develop phenotype‐genotype correlations. These studies have produced conflicting results in part related to use of different clinical severity scales, different diagnostic criteria, and different stratification by age and mutation group as well as the possible effects of unbalanced X‐chromosome inactivation. The present study applied a revised ordinal scoring system that allowed for correction for patient ages. We analyzed 85 patients with mutation in MECP2. Sixty‐five (76%) had one of eight common mutations. Patients with missense mutations had lower total severity scores and better language performance than those with nonsense mutations. No difference was noted between severity scores for mutations in the methyl‐binding domain (MBD) and the transcriptional repression domain (TRD). However, patients with missense mutations in TRD had the best overall scores and better preservation of head growth and language skills. Analysis of specific mutation groups demonstrated a striking difference for patients with the R306C mutation including better overall score, later regression, and better language with less motoric impairment. Indeed, these patients as a group accounted for the differences in overall scores between the missense and nonsense groups. Thus, the impact of specific mutations coupled with possible variation in X‐chromosome inactivation must be considered carefully in the derivation of phenotype‐genotype correlations. These results emphasize the limitations of such analyses in larger mutation groups, either by type or position.

Occurrence of Rett Syndrome in Boys

The neurologic disorder Rett syndrome was originally described exclusively in girls. We present two boys with clinical features of Rett syndrome. Other than head circumference deceleration, no longer considered mandatory, patient 1 meets all of the criteria. Using fluorescent in situ hybridization analysis, 97.6% of cells were found to be karyotypically normal (46,XY). No mutation was detected on screening of the coding region of the MECP2 gene. The second patient also has classic features of Rett syndrome. However, cytogenetic analysis of peripheral blood revealed a karyotype 47,XXY[23]/46,XY[7] confirming mosaicism for Klinefelters syndrome. A T158M missense mutation in the methylcytosine-binding domain of the MECP2 gene was identified. A diagnostic bias against the clinical identification of Rett syndrome in boys may exist. This presentation of the male phenotype could be more common than it would appear, although boys with MECP2 mutations might also manifest in other ways. Rett syndrome remains a clinical diagnosis that should not be dismissed in boys, and thorough evaluation including karyotype and mutation testing is warranted. (J Child Neurol 2001;16:333-338).

CADASIL Notch3 Mutant Proteins Localize to the Cell Surface and Bind Ligand

Cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a vascular dementia arising from abnormal arteriolar vascular smooth muscle cells. CADASIL results from mutations in Notch3 that alter the number of cysteine residues in the extracellular epidermal growth factor–like repeats, important for ligand binding. It is not known whether CADASIL mutations lead to loss or gain of Notch3 receptor function. To examine the functional consequences of CADASIL mutations, we engineered 4 CADASIL-like mutations into rat Notch3 and have shown that the presence of an unpaired cysteine does not impair cell-surface expression or ligand binding.

Functional analyses of MeCP2 mutations associated with Rett syndrome using transient expression systems

Rett syndrome, an X-linked neurodevelopmental disorder, is a major cause of mental retardation in females. Recent genetic analyses have revealed that mutations in the methyl-CpG-binding protein gene encoding MeCP2 are associated with Rett syndrome. In this study, we used transient expression systems to investigate the functional significance of mutations seen in patients with Rett syndrome. Missense mutations in the methyl-CpG-binding domain were analyzed by the transfection in mouse L929 cells and Drosophila SL2 cells. The L929 cells were utilized to investigate the effects of mutations on the affinity for heterochromatin, where methylated CpG dinucleotides are extremely enriched. The SL2 cells were utilized to analyze their effects on transcriptional repression activities. R106W and F155S mutations led to the substantial impairment of MeCP2 functions, showing the loss of accumulation of the mutated protein to mouse heterochromatin and the reduction of the transcriptional repressive activity in Drosophila SL2 cells. Intriguingly, the R133C mutant retained the functionality equivalent to MeCP2 in these analyses. On the other hand, the T158M mutation exhibited the intermediate level of the impairment of functions in both analyses. Thus, these functional assays are useful to evaluate the consequences of mutation in the methyl-CpG-binding domain of MeCP2 and provide an insight into the relationship between the genotype and the severity of Rett syndrome.

Rethinking the fate of males with mutations in the gene that causes Rett syndrome

Rett syndrome (RTT) is usually described as an X-linked dominant disorder that is lethal prenatally to males. While this hypothesis stemmed from the nearly exclusive recognition of the classical features in females and the scarcity of similarly affected or severely affected brothers of girls manifesting the classic phenotype, there are actually no pedigree data to support it. The identification of methyl-CpG-binding protein 2 gene (MECP2) as the causative gene for most cases of RTT allows us to examine the fate of males with a mutation in the gene. Although the number of cases is small, mutations in MECP2 that lead to the classical phenotype in females do not appear to result in prenatal lethality of affected hemizygous males. It is likely that sporadic cases are not ascertained because of the relative non-specific congenital onset encephalopathy. Males who have MECP2 mutations and Klinefelter syndrome or who are mosaic for the mutation are more likely to present with a RTT-like phenotype.

Functional characterisation of MeCP2 mutations found in male patients with X linked mental retardation

Rett syndrome1 is a neurodevelopmental disorder that primarily affects females and is characterised by a period of normal growth and development, followed by severe neurological dysfunction including dementia, autistic features, loss of purposeful use of the hands, jerky truncal ataxia, and seizures.2 Systematic genetic analyses have shown that mutations in the methyl-CpG binding protein gene MeCP2 3 are associated with Rett syndrome.4 Recently, MeCP2 mutations (A140V5,6 and E137G6) were also found in male patients with non-specific X linked mental retardation that is clinically distinct from Rett syndrome. To elucidate the functional significance of these mutations, we used transient expression assays to compare the effects of these mutations on MeCP2 function with those of Rett syndrome mutations.4 Wild type and mutant MeCP2/GFP fusion proteins expressed in mouse L929 cells were analysed to determine the effect of mutations on accumulation of MeCP2 in heterochromatin, where approximately half of the methyl-CpG dinucleotides occurring in the genome are located.7 In contrast to an R106W Rett syndrome mutant protein, which has no affinity to heterochromatin, both A140V and E137G mutants showed a clear focal heterochromatin staining pattern indistinguishable from the wild type protein. The effects of mutations on transcriptional repressive activities were also evaluated in Drosophila SL2 cells, which possess only marginal background activities of methyl-CpG binding proteins.8–10 Although R106W expression substantially reduced transcriptional repressive activity, the A140V and the E137G mutants retained the transcriptional repressive activity. In particular, the A140V mutant retained such repressive activity at a level comparable to the wild type protein. These results indicate that potential alterations in MeCP2 function resulting from the A140V and the E137G mutations are different from those associated with mutations observed in Rett syndrome and may explain why the manifestation of MeCP2 related mental disorder in …