Neurologic aspects of MECP2 gene duplication in male patients

Duplications in Xq28 involving the methyl CpG binding protein 2 gene (MECP2) have been described in male patients with severe mental disability, delayed milestones, absence of language, hypotonia replaced by spasticity and retractions, and recurrent and often severe infections. In a study involving five patients in two families, multiplex ligation-dependent probe amplification was used to screen the Xq28 region that includes MECP2, focusing on the presence of gene duplications. Some manifestations of the disease observed in these patients may occur less regularly than the classical abnormalities. Epilepsy with frequent seizures of the myoclonic-astatic type was observed in these patients and was associated with a slowing of the background electroencephalographic activity, rather than the generalized spike-waves or polyspike-waves usually observed in this type of seizure. In addition, cerebral abnormalities were observed with magnetic resonance imaging that were inconstant and nonspecific but that could nonetheless assist in diagnosis of this genetic pathology.

A MECP2 mutation in a highly conserved aminoacid causing mental retardation in a male

MeCP2 is a protein that functions as a key factor in epigenetic transcriptional regulation. Mutations in MECP2 gene have been reported as being the major cause of Rett syndrome. These mutations may also cause a wide spectrum of neurological disorders in males. Here, we report the identification of the mutation p.P405L in a 19-year-old Brazilian male with mental retardation. This variant is localized in a highly conserved aminoacid from the carboxy terminal domain and may affect the protein function. Segregation analysis on the patient's mother revealed that this is a de novo mutation and it was not identified in the control sample. The programs SIFT, PolyPhen and A-GVGD considered that the p.P405L may be damaging. Despite the high frequency of non pathogenic variants that have been identified in this gene, our data lead us to consider the p.P405L a disease-causing mutation.

Low significance of MECP2 mutations as a cause of mental retardation in Brazilian males

MeCP2 is a protein that selectively binds to methylated cytosines through its methyl-CpG-binding domain (MBD) and connects DNA methylation to transcriptional repression. Mutations in MECP2 gene, located in Xq28, have been reported as being the major cause of Rett syndrome and are also associated with some cases of X-linked mental retardation in both males and females. In this study, we present the first screening in the MECP2 gene in a Brazilian cohort of 239 unrelated males with idiopathic mental retardation. Eight sequence variations were observed in 10 patients: one novel putative pathogenic variant, two never described variants of unknown pathogenic value and five non-pathogenic variations. We conclude that in mentally retarded Brazilian males, non-pathogenic variants in the MECP2 gene are more common than actual pathogenic mutations, and therefore alterations in this gene have a weak relationship with mental retardation in males.

Mutation screening of the MECP2 gene in a large cohort of 613 fragile-X negative patients with mental retardation

Mental retardation affects 2 to 3% of the population and is marked by significant etiological heterogeneity, including genetic and non genetic causes. FRAXA (FMR1) trinucleotide expansion is widely searched in routine screening, but found in only about 2% of the patients tested. Mutations of the MECP2 (methyl-CpG-binding protein) gene mainly cause Rett syndrome but were also shown to be involved in mental retardation. This study aimed to estimate the frequency of MECP2 gene mutations in a large group of mentally retarded patients without FRAXA expansion. Screening by heteroduplex analysis and SSCP followed by DNA sequencing of shifted bands were performed on 613 patients, including 442 males and 171 females. Eleven sequence variants were found, including nine polymorphisms. The two others may be pathogenetic. The first one, the double nucleotide substitution c.1162_1163delinsTA leading to a premature stop codon (p.Pro388X) was found in a female patient with random X-inactivation, presenting with borderline mental impairment without any features of Rett syndrome. The second one, the c.679C>G substitution, changing a glutamine to a glutamate in the transcriptional repression functional domain (p.Gln227Glu), was found in a female patient with a moderately biased X-chromosome inactivation profile and presenting with mild intellectual delay and minor psychotic features. The low mutation rate suggests that a large-scale routine screening for MECP2 in mentally retarded subjects is not cost-effective in clinical practice. Screening may be improved by a pre-selection based on clinical features that remain to be established.

Deleterious mutations in exon 1 of MECP2 in Rett syndrome

The MECP2 gene is responsible for 80-85% of typical cases of Rett syndrome with deleterious mutations affecting exons 3 and 4. Recently, an alternate transcript including exon 1 was discovered with a new protein isoform (MeCP2_e1) much more abundant in brain. We screened exon 1 of MECP2 for mutations and for large rearrangements in a panel of 212 typical cases of Rett syndrome and one family case with atypical Rett syndrome. We identified two deleterious mutations (c.48_55dup and c.62+2_62+3del) and four large rearrangements encompassing exon 1 of MECP2. We also identified the c.16_21dup alteration formerly reported as c.3_4insGCCGCC and give additional support to classify this sequence variation as polymorphic. In our large panel of typical Rett, mutations affecting exon 1 of MECP2 represent 1% of the deleterious alleles. This study confirms that mutations in exon 1 of MECP2 are a rare cause of Rett syndrome.