Laurent Villard

Mutations in a putative global transcriptional regulator cause X-linked mental retardation with alpha-thalassemia (ATR-X syndrome).

The ATR-X syndrome is an X-linked disorder comprising severe psychomotor retardation, characteristic facial features, genital abnormalities, and alpha-thalassemia. We have shown that ATR-X results from diverse mutations of XH2, a member of a subgroup of the helicase superfamily that includes proteins involved in a wide range of cellular functions, including DNA recombination and repair (RAD16, RAD54, and ERCC6) and regulation of transcription (SW12/SNF2, MOT1, and brahma). The complex ATR-X phenotype suggests that XH2, when mutated, down-regulates expression of several genes, including the alpha-globin genes, indicating that it could be a global transcriptional regulator. In addition to its role in the ATR-X syndrome, XH2 may be a good candidate for other forms of X-linked mental retardation mapping to Xq13.

Mutations in GRIN2A and GRIN2B encoding regulatory subunits of NMDA receptors cause variable neurodevelopmental phenotypes

N-methyl-D-aspartate (NMDA) receptors mediate excitatory neurotransmission in the mammalian brain. Two glycine-binding NR1 subunits and two glutamate-binding NR2 subunits each form highly Ca2+-permeable cation channels which are blocked by extracellular Mg2+ in a voltage-dependent manner. Either GRIN2B or GRIN2A, encoding the NMDA receptor subunits NR2B and NR2A, was found to be disrupted by chromosome translocation breakpoints in individuals with mental retardation and/or epilepsy. Sequencing of GRIN2B in 468 individuals with mental retardation revealed four de novo mutations: a frameshift, a missense and two splice-site mutations. In another cohort of 127 individuals with idiopathic epilepsy and/or mental retardation, we discovered a GRIN2A nonsense mutation in a three-generation family. In a girl with early-onset epileptic encephalopathy, we identified the de novo GRIN2A mutation c.1845C>A predicting the amino acid substitution p.N615K. Analysis of NR1-NR2AN615K (NR2A subunit with the p.N615K alteration) receptor currents revealed a loss of the Mg2+ block and a decrease in Ca2+ permeability. Our findings suggest that disturbances in the neuronal electrophysiological balance during development result in variable neurological phenotypes depending on which NR2 subunit of NMDA receptors is affected.

Mecp2 Deficiency Disrupts Norepinephrine and Respiratory Systems in Mice

Rett syndrome is a severe X-linked neurological disorder in which most patients have mutations in the methyl-CpG binding protein 2 (MECP2) gene and suffer from bioaminergic deficiencies and life-threatening breathing disturbances. We used in vivo plethysmography, in vitro electrophysiology, neuropharmacology, immunohistochemistry, and biochemistry to characterize the consequences of the MECP2 mutation on breathing in wild-type (wt) and Mecp2-deficient (Mecp2-/y) mice. At birth, Mecp2-/y mice showed normal breathing and a normal number of medullary neurons that express tyrosine hydroxylase (TH neurons). At ∼1 month of age, most Mecp2-/y mice showed respiratory cycles of variable duration; meanwhile, their medulla contained a significantly reduced number of TH neurons and norepinephrine (NE) content, even in Mecp2-/y mice that showed a normal breathing pattern. Between 1 and 2 months of age, all unanesthetized Mecp2-/y mice showed breathing disturbances that worsened until fatal respiratory arrest at ∼2 months of age. During their last week of life, Mecp2-/y mice had a slow and erratic breathing pattern with a highly variable cycle period and frequent apneas. In addition, their medulla had a drastically reduced number of TH neurons, NE content, and serotonin (5-HT) content. In vitro experiments using transverse brainstem slices of mice between 2 and 3 weeks of age revealed that the rhythm produced by the isolated respiratory network was irregular in Mecp2-/y mice but could be stabilized with exogenous NE. We hypothesize that breathing disturbances in Mecp2-/y mice, and probably Rett patients, originate in part from a deficiency in noradrenergic and serotonergic modulation of the medullary respiratory network.

Two affected boys in a Rett syndrome family Clinical and molecular findings

Background: The authors report a family in which two boys had severe neonatal encephalopathy of unknown origin. They both presented with the same condition and died of severe apnea before they were 1 year old. Their sister has a classic form of Rett syndrome. Methods: Because mutations in the methyl-CpG-binding protein 2 (MECP2) gene have been identified in 70 to 80% of the sporadic cases of Rett syndrome, the authors looked for a mutation in the MECP2 gene in this family. Results: The authors identified a missense mutation (T158M) in the affected girl and subsequently showed that one of her affected brothers, for whom DNA was available, carried the same mutation. The mother of the patients is a carrier of the T158M mutation. X-chromosome inactivation studies showed that the mother has a completely skewed X-chromosome inactivation pattern that favors the expression of the normal allele; this explains why she does not exhibit any phenotypic manifestation. In addition, the MECP2 mutation appeared on the grandpaternal X chromosome in this family. Conclusions: An MECP2 mutation can be identified in boys, even though they do not present a Rett syndrome phenotype.

Treatment with desipramine improves breathing and survival in a mouse model for Rett syndrome.

Rett syndrome (RS) is a severe X‐linked neurological disorder in which most patients have mutations in the methyl‐CpG binding protein2 (MECP2) gene. No effective treatment exists. We previously showed that the Mecp2‐deficient mice, a mouse model of RS, have highly variable respiratory rhythm and frequent apneas due to reduced norepinephrine (NE) content, and a drastic decrease of tyrosine hydroxylase (TH)‐expressing neurons in the medulla. We showed here that treating these mice with desipramine (DMI), which specifically inhibits NE reuptake, significantly improved their respiratory rhythm during several weeks. In addition, the treatment significantly extended their lifespan. At the cellular level, we showed that the reduced number of TH‐expressing neurons before treatment in the mutant animals was not due to apoptosis. Conversely, we found that DMI treatment increased the number of TH‐expressing neurons in the mutant brainstem to reach wild‐type levels. We showed that this increase was not due to cellular proliferation. We propose that the Mecp2‐deficient TH‐expressing neurons lose their ability to synthesize TH at some point during their postnatal development. Our results suggest that a pharmacological stimulation of the noradrenergic system could be a promising approach for the treatment of the respiratory dysfunction which causes a significant proportion of death in RS patients.

MECP2 mutations in males

Rett syndrome (RS; MIM 312750) is a severe neurological disorder affecting exclusively females. Its prevalence is about 1 in 10 000 female births, and it is a prominent cause of profound mental handicap in women. RS is caused by mutations in the X-linked methyl CpG-binding protein 2 (MECP2) gene. These mutations were initially thought to be lethal in males. However, MECP2 mutations are now frequently identified in mentally retarded male patients. The frequency of disease-causing MECP2 mutations in this population is between 1.3% and 1.7%. Surprisingly, MECP2 mutations in males are responsible for a wide spectrum of neurological disorders, ranging from mild mental retardation to severe neonatal encephalopathy. The aim of this review is to describe the nature of the MECP2 mutations identified in male patients to date and their associated phenotypes.

Mutations in the oligophrenin-1 gene (OPHN1) cause X linked congenital cerebellar hypoplasia

A number of apparently non-syndromic X linked mental retardation syndromes are associated with subtle but characteristic phenotypic manifestations. Such manifestations can be dysmorphic features but they potentially also extend to abnormal brain morphology. In this latter field, progress in neuroimaging has aided the approach to brain malformations associated with mental retardation hence allowing a new classification of conditions previously described as non-syndromic. This classification is based on very similar brain malformations in affected subjects. Among the many brain malformations that can be associated with mental retardation in affected children, rhombencephalic anomalies are being recognised with increasing frequency. Accordingly, the classification of malformations of the posterior fossa has evolved considerably during the last decade.1–3 The cerebellum is known to be involved in movement coordination. However, besides its role in the control and integration of motor activity, the cerebellum also represents an essential node in the neural network subserving higher order behaviour.4,5 An abundant circuitry links the cerebellum with associative and paralimbic areas of the cerebral cortex and cerebellar lesions are known to underlie a cognitive syndrome combining impaired affective regulation, fine motor coordination, language fluency, verbal memory, and the ability to plan.4,5 These fascinating characteristics have led researchers to search for genetic determinants controlling cerebellar development. One way of addressing the genetics of cerebellar development in humans is to study families in which this brain region is abnormally developed. A number of families with X linked congenital cerebellar hypoplasia (CCH) have been reported,6–8 but no disease causing gene has been identified so far. We have studied several families with X linked congenital cerebellar hypoplasia (CCH) and mental retardation and we have found different mutations in the oligophrenin-1 ( OPHN1 ) gene. Carrier females are mildly affected and, accordingly, we found that they have a random …

Refinement of cortical dysgeneses spectrum associated with TUBA1A mutations

Objective: We have recently shown that de novo mutations in the TUBA1A gene are responsible for a wide spectrum of neuronal migration disorders. To better define the range of these abnormalities, we searched for additional mutations in a cohort of 100 patients with lissencephaly spectrum for whom no mutation was identified in DCX, LIS1 and ARX genes and compared these data to five previously described patients with TUBA1A mutations. Results: We detected de novo TUBA1A mutations in six patients and highlight the existence of a prominent form of TUBA1A related lissencephaly. In four patients, the mutations identified, c.1190T>C (p.L397P), c.1265G>A (p.R422H), c.1264C>T (p.R422C), c.1306G>T (p.G436R), have not been reported before and in two others, the mutation corresponds to a recurrent missense mutation, c.790C>T (p.R264C), likely to be a hot spot of mutation. All together, it emerges that the TUBA1A related lissencephaly spectrum ranges from perisylvian pachygyria, in the less severe form, to posteriorly predominant pachygyria in the most severe, associated with dysgenesis of the anterior limb of the internal capsule and mild to severe cerebellar hypoplasia. When compared with a large series of lissencephaly of other origins (ILS17, ILSX or unknown origin), these features appear to be specific to TUBA1A related lissencephaly. In addition, TUBA1A mutated patients share a common clinical phenotype that consists of congenital microcephaly, mental retardation and diplegia/tetraplegia. Conclusions: Our data highlight the presence of consistent and specific abnormalities that should allow the differentiation of TUBA1A related lissencephalies from those related to LIS1, DCX and ARX genes.

Periventricular heterotopia, mental retardation, and epilepsy associated with 5q14.3-q15 deletion

Background: Periventricular heterotopia (PH) is an etiologically heterogeneous disorder characterized by nodules of neurons ectopically placed along the lateral ventricles. Most affected patients have seizures and their cognitive level varies from normal to severely impaired. At present, two genes have been identified to cause PH when mutated. Mutations in FLNA (Xq28) and ARFGEF2 (20q13) are responsible for X-linked bilateral PH and a rare autosomal recessive form of PH with microcephaly. Chromosomal rearrangements involving the 1p36, 5p15, and 7q11 regions have also been reported in association with PH but the genes implicated remain unknown. Fourteen additional distinct anatomoclinical PH syndromes have been described, but no genetic insights into their causes have been gleaned. Methods: We report the clinical and imaging features of three unrelated patients with epilepsy, mental retardation, and bilateral PH in the walls of the temporal horns of the lateral ventricles, associated with a de novo deletion of the 5q14.3-15 region. We used microarray-based comparative genomic hybridization to define the boundaries of the deletions. Results: The three patients shared a common deleted region spanning 5.8 Mb and containing 14 candidate genes. Conclusion: We identified a new syndrome featuring bilateral periventricular heterotopia (PH), mental retardation, and epilepsy, mapping to chromosome 5q14.3-q15. This observation reinforces the extreme clinical and genetic heterogeneity of PH. Array comparative genomic hybridization is a powerful diagnostic tool for characterizing causative chromosomal rearrangements of limited size, identifying potential candidate genes for, and improving genetic counseling in, malformations of cortical development.

Epileptic and nonepileptic features in patients with early onset epileptic encephalopathy and STXBP1 mutations

Purpose:  STXBP1 (MUNC18‐1) mutations have been associated with various types of epilepsies, mostly beginning early in life. To refine the phenotype associated with STXBP1 aberrations in early onset epileptic syndromes, we studied this gene in a cohort of patients with early onset epileptic encephalopathy.