Classical Rett syndrome (RTT) is a neurodevelopmental disorder where most of cases carry MECP2 mutations. Atypical RTT variants involve mutations in CDKL5 and FOXG1. However, a subset of RTT patients remains that do not carry any mutation in the described genes. Whole exome sequencing was carried out in a cohort of 21 female probands with clinical features overlapping with those of RTT, but without mutations in the customarily studied genes. Candidates were functionally validated by assessing the appearance of a neurological phenotype in Caenorhabditis elegans upon disruption of the corresponding ortholog gene. We detected pathogenic variants that accounted for the RTT-like phenotype in 14 (66.6 %) patients. Five patients were carriers of mutations in genes already known to be associated with other syndromic neurodevelopmental disorders. We determined that the other patients harbored mutations in genes that have not previously been linked to RTT or other neurodevelopmental syndromes, such as the ankyrin repeat containing protein ANKRD31 or the neuronal acetylcholine receptor subunit alpha-5 (CHRNA5). Furthermore, worm assays demonstrated that mutations in the studied candidate genes caused locomotion defects. Our findings indicate that mutations in a variety of genes contribute to the development of RTT-like phenotypes.Electronic supplementary materialThe online version of this article (doi:10.1007/s00439-016-1721-3) contains supplementary material, which is available to authorized users.

MeCP2 is an abundant protein that binds to methylated cytosine residues in DNA and regulates transcription. Mutations in MECP2 cause Rett syndrome, a severe neurological disorder that affects approximately 1:10 000 females. Mice lacking MeCP2 have been generated and constitute important models of Rett syndrome. However, it is yet unclear whether certain physiological events are sensitive to a decrease, rather than a complete lack of MeCP2. Here we report that a Mecp2 floxed allele (Mecp2(lox)) that was generated to allow conditional mutagenesis behaves as a hypomorph and the corresponding mutant mice exhibit phenotypical alterations including body weight gain, motor abnormalities and altered social behavior. Our data reinforce the view that the central nervous system is extremely sensitive to MeCP2 expression levels and suggest that the 3'-UTR of Mecp2 might contain important elements that contribute to the regulation of its stability or processing.

Rett syndrome (RTT), a leading cause of mental retardation with autistic features in females, is caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). RTT is characterized by a diverse set of neurological features that includes cognitive, motor, behavioral and autonomic disturbances. The diverse features suggest that specific neurons contribute to particular phenotypes and raise the question whether restoring MeCP2 function in a cell-specific manner will rescue some of the phenotypes seen in RTT. To address this, we generated transgenic mice expressing inducible MeCP2 under the control of the brain-specific promoters calcium/calmodulin-dependent protein kinase II (CamKII) or neuron-specific enolase (Eno2) and bred them onto mouse models lacking functional MeCP2. Expression of normal MeCP2 in either CamKII or Eno2 distribution was unable to prevent the appearance of most of the phenotypes of the RTT mouse models. These results suggest that most RTT phenotypes are caused either by disruption of complex neural networks involving neurons throughout the brain or by disruption of the function of specific neurons outside of the broad CamKII or Eno2 distribution.