[Autism, genetics and synaptic function alterations]. / Autisme, génétique et anomalies de la fonction synaptique.

Autism is a neurodevelopmental disorder characterized by a deficit of language and communication both associated with a restricted repertoire of activities and interests. The current prevalence of autistic disorder stricto sensu is estimated at 1/500 whereas autism spectrum disorders (ASD) increases up to 1/150 to 1/200. Mental deficiency (MD) and epilepsy are present in numerous autistic individuals. Consequently, autism is as a major public health issue. Autism was first considered as a non biological disease; however various rational approaches for analysing epidemiological data suggested the possibility of the influence of genetic factors. In 2003, this hypothesis was clearly illustrated by the characterization of genetic mutations transmitted through a mendelian manner. Subsequently, the glutamate synapse appeared as a preferential causal target in autism because the identified genes encoded proteins present in this structure. Strikingly, the findings that an identical genetic dysfunction of the synapse might also explain some MD suggested the possibility of a genetic comorbidity between these neurodevelopmental conditions. To date, various identified genes are considered indifferently as "autism" or "MD" genes. The characterization of mutations in the NLGN4X gene in patients with Asperger syndrome, autism without MD, or MD without autism, was the first example. It appears that a genetic continuum between ASD on one hand, and between autism and MD on the other hand, is present. Consequently, it is likely that genes already involved in MD will be found mutated in autistic patients and will represent future target for finding new factors in autism.

Spectrum of HLXB9 gene mutations in Currarino syndrome and genotype-phenotype correlation.

Currarino syndrome (CS) is a rare congenital malformation described in 1981 as the association of three main features: typical sacral malformation (sickle-shaped sacrum or total sacral agenesis below S2), hindgut anomaly, and presacral tumor. In addition to the triad, tethered cord and/or lipoma of the conus are also frequent and must be sought, as they may lead to severe complications if not treated. The HLXB9 gene, located at 7q36, is disease-causing. It encodes the HB9 transcription factor and interacts with DNA through a highly evolutionarily conserved homeodomain early in embryological development. Thus far, 43 different heterozygous mutations have been reported in patients fulfilling CS criteria. Mutation detection rate is about 50%, and reaches 90% in familial cases. Here, we report 23 novel mutations in 26 patients among a series of 50 index cases with CS, and review mutational reports published since the identification of the causative gene. Three cytogenetic anomalies encompassing the HLXB9 gene are described for the first time. Truncating mutations (frameshifts or nonsense mutations) represent 57% of those identified, suggesting that haploinsufficiency is the basis of CS. No obvious genotype-phenotype correlation can be drawn thus far. Genetic heterogeneity is suspected, since at least 19 of the 24 patients without HLXB9 gene mutation harbor subtle phenotypic variations.

Autosomal dominant anhidrotic ectodermal dysplasias at the EDARADD locus.

Anhidrotic ectodermal dysplasia (EDA) is a disorder of ectodermal differentiation characterized by sparse hair, abnormal or missing teeth, and inability to sweat. X-linked EDA is the most common form, caused by mutations in the EDA gene, which encodes ectodysplasin, a member of the tumor necrosis factor (TNF) family. Autosomal dominant and recessive forms of EDA have been also described and are accounted for by two genes. Mutations in EDAR, encoding a TNF receptor (EDAR) cause both dominant and recessive forms. In addition, mutations in a recently identified gene, EDARADD, encoding EDAR-associated death domain (EDARADD) have been shown to cause autosomal recessive EDA. Here, we report a large Moroccan family with an autosomal dominant EDA. We mapped the disease gene to chromosome 1q42.2-q43, and identified a novel missense mutation in the EDARADD gene (c.335T>G, p.Leu112Arg). Thus, the EDARADD gene accounts for both recessive and dominant EDA. EDAR is activated by its ligand, ectodysplasin, and uses EDARADD to build an intracellular complex and activate nuclear factor kappa B (NF-kB). We compared the functional consequences of the dominant (p.Leu112Arg) and recessive mutation (p.Glu142Lys), which both occurred in the death domain (DD) of EDARADD. We demonstrated that the p.Leu112Arg mutation completely abrogated NF-kB activation, whereas the p.Glu142Lys retained the ability to significantly activate the NF-kB pathway. The p.Leu112Arg mutation is probably a dominant negative form as its cotransfection impaired the wild-type EDARADD's ability to activate NF-kB. Our results confirm that NF-kB activation is impaired in EDA and support the role of EDARADD DD as a downstream effector of EDAR signaling.

Autosomal recessive anhidrotic ectodermal dysplasia in a large Moroccan family.

We studied a large Moroccan family in which anhidrotic ectodermal dysplasia is transmitted as an autosomal recessive trait. Fourteen family members, both males and females, were affected and they all had a common ancestor. Linkage analysis by homozygosity mapping in this family will permit the gene localisation of this rare form of anhidrotic ectodermal dysplasia.