The development of microarray-based technologies for comparative genomic hybridization (array-CGH) analysis has enabled the detection of submicroscopic microdeletions or microduplications, also referred to as copy number variations (CNVs). Recently, this approach has been widely used in neurologic and psychiatric disorders, including mental retardation (MR),^1-3 autism spectrum disorders (ASDs),^4-7 and schizophrenia.^8-11 Findings from these studies suggested that several genes involved in similar neurodevelopmental pathways may be associated with these conditions. However, so far only rare structural variants, sometimes present in a single case, have been identified. Therefore, it is difficult to decipher which of these variations are causative, which are risk factors, and which are only rare polymorphisms unrelated to any pathologic phenotype. De novo rearrangements are usually considered pathogenic, but this argument (which is acceptable for rare large rearrangements detectable by conventional cytogenetics) should be considered with caution for smaller CNVs, for which a high mutation rate is expected. Indeed, it has been estimated that a de novo segmental deletion occurs in 1 per 8 newborns and a de novo segmental duplication in 1 per 50 newborns, with most of these rearrangements being benign polymorphic variants.¹² Therefore, the disease association of CNVs has to be tested systematically by comparing the frequency of each candidate CNV in cases and in healthy control subjects. Given the low frequency of each CNV, this would require the study of huge series, achievable only in the context of forthcoming meta-analyses. Other problems arise because ascertainment of most of the published samples, initially recruited for linkage studies, is biased toward multiplex cases and because control samples, when present, are generally composed of subjects not screened for the studied pathologic conditions. The objectives of the present study were (1) to provide an estimate of the collective frequency of a set of recurrent or overlapping CNVs in 3 different groups of cases compared with controls and (2) to assess whether each CNV is present in more than 1 clinical category.

Following the recent discovery that the methyl-CpG binding protein 2 (MECP2) gene located on Xq28 is involved in Rett syndrome (RTT), a wild spectrum of phenotypes, including mental handicap, has been shown to be associated with mutations in MECP2. These findings, with the compelling genetic evidence suggesting the presence in Xq28 of additional genes besides RabGDI1 and FMR2 involved in non-specific X-linked mental retardation (MRX), prompted us to investigate MECP2 in MRX families. Two novel mutations, not found in RTT, were identified. The first mutation, an E137G, was identified in the MRX16 family, and the second, R167W, was identified in a new mental retardation (MR) family shown to be linked to Xq28. In view of these data, we screened MECP2 in a cohort of 185 patients found negative for the expansions across the FRAXA CGG repeat and reported the identification of mutations in four sporadic cases of MR. One of the mutations, A140V, which we found in two patients, has been described previously, whereas the two others, P399L and R453Q, are novel mutations. In addition to the results demonstrating the involvement of MECP2 in MRX, this study shows that the frequency of mutations in MECP2 in the mentally retarded population screened for the fragile X syndrome is comparable to the frequency of the CGG expansions in FMR1. Therefore, implementation of systematic screening of MECP2 in MR patients should result in significant progress in the field of molecular diagnosis and genetic counseling of mental handicap.