Anne Noreau

Increased exonic de novo mutation rate in individuals with schizophrenia

Schizophrenia is a severe psychiatric disorder that profoundly affects cognitive, behavioral and emotional processes. The wide spectrum of symptoms and clinical variability in schizophrenia suggest a complex genetic etiology, which is consistent with the numerous loci thus far identified by linkage, copy number variation and association studies. Although schizophrenia heritability may be as high as ∼80%, the genes responsible for much of this heritability remain to be identified. Here we sequenced the exomes of 14 schizophrenia probands and their parents. We identified 15 de novo mutations (DNMs) in eight probands, which is significantly more than expected considering the previously reported DNM rate. In addition, 4 of the 15 identified DNMs are nonsense mutations, which is more than what is expected by chance. Our study supports the notion that DNMs may account for some of the heritability reported for schizophrenia while providing a list of genes possibly involved in disease pathogenesis.

De novo mutations in the gene encoding the synaptic scaffolding protein SHANK3 in patients ascertained for schizophrenia

Schizophrenia likely results from poorly understood genetic and environmental factors. We studied the gene encoding the synaptic protein SHANK3 in 285 controls and 185 schizophrenia patients with unaffected parents. Two de novo mutations (R1117X and R536W) were identified in two families, one being found in three affected brothers, suggesting germline mosaicism. Zebrafish and rat hippocampal neuron assays revealed behavior and differentiation defects resulting from the R1117X mutant. As mutations in SHANK3 were previously reported in autism, the occurrence of SHANK3 mutations in subjects with a schizophrenia phenotype suggests a molecular genetic link between these two neurodevelopmental disorders.

Systematic resequencing of X-chromosome synaptic genes in autism spectrum disorder and schizophrenia.

Autism spectrum disorder (ASD) and schizophrenia (SCZ) are two common neurodevelopmental syndromes that result from the combined effects of environmental and genetic factors. We set out to test the hypothesis that rare variants in many different genes, including de novo variants, could predispose to these conditions in a fraction of cases. In addition, for both disorders, males are either more significantly or more severely affected than females, which may be explained in part by X-linked genetic factors. Therefore, we directly sequenced 111 X-linked synaptic genes in individuals with ASD (n=142; 122 males and 20 females) or SCZ (n=143; 95 males and 48 females). We identified >200 non-synonymous variants, with an excess of rare damaging variants, which suggest the presence of disease-causing mutations. Truncating mutations in genes encoding the calcium-related protein IL1RAPL1 (already described in Piton et al. Hum Mol Genet 2008) and the monoamine degradation enzyme monoamine oxidase B were found in ASD and SCZ, respectively. Moreover, several promising non-synonymous rare variants were identified in genes encoding proteins involved in regulation of neurite outgrowth and other various synaptic functions (MECP2, TM4SF2/TSPAN7, PPP1R3F, PSMD10, MCF2, SLITRK2, GPRASP2, and OPHN1).

Mutations in SYNGAP1 in Autosomal Nonsyndromic Mental Retardation

Although autosomal forms of nonsyndromic mental retardation account for the majority of cases of mental retardation, the genes that are involved remain largely unknown. We sequenced the autosomal gene SYNGAP1, which encodes a ras GTPase-activating protein that is critical for cognition and synapse function, in 94 patients with nonsyndromic mental retardation. We identified de novo truncating mutations (K138X, R579X, and L813RfsX22) in three of these patients. In contrast, we observed no de novo or truncating mutations in SYNGAP1 in samples from 142 subjects with autism spectrum disorders, 143 subjects with schizophrenia, and 190 control subjects. These results indicate that SYNGAP1 disruption is a cause of autosomal dominant nonsyndromic mental retardation.

Mutations in FUS cause FALS and SALS in French and French Canadian populations

Background: The identification of mutations in the TARDBP and more recently the identification of mutations in the FUS gene as the cause of amyotrophic lateral sclerosis (ALS) is providing the field with new insight about the mechanisms involved in this severe neurodegenerative disease. Methods: To extend these recent genetic reports, we screened the entire gene in a cohort of 200 patients with ALS. An additional 285 patients with sporadic ALS were screened for variants in exon 15 for which mutations were previously reported. Results: In total, 3 different mutations were identified in 4 different patients, including 1 3-bp deletion in exon 3 of a patient with sporadic ALS and 2 missense mutations in exon 15 of 1 patient with familial ALS and 2 patients with sporadic ALS. Conclusions: Our study identified sporadic patients with mutations in the FUS gene. The accumulation and description of different genes and mutations helps to develop a more comprehensive picture of the genetic events underlying amyotrophic lateral sclerosis.

Exome Sequencing Identifies FUS Mutations as a Cause of Essential Tremor

Essential tremor (ET) is a common neurodegenerative disorder that is characterized by a postural or motion tremor. Despite a strong genetic basis, a gene with rare pathogenic mutations that cause ET has not yet been reported. We used exome sequencing to implement a simple approach to control for misdiagnosis of ET, as well as phenocopies involving sporadic and senile ET cases. We studied a large ET-affected family and identified a FUS p.Gln290(∗) mutation as the cause of ET in this family. Further screening of 270 ET cases identified two additional rare missense FUS variants. Functional considerations suggest that the pathogenic effects of ET-specific FUS mutations are different from the effects observed when FUS is mutated in amyotrophic lateral sclerosis cases; we have shown that the ET FUS nonsense mutation is degraded by the nonsense-mediated-decay pathway, whereas amyotrophic lateral sclerosis FUS mutant transcripts are not.

De novo STXBP1 mutations in mental retardation and nonsyndromic epilepsy

We sequenced genes coding for components of the SNARE complex (STX1A, VAMP2, SNAP25) and their regulatory proteins (STXBP1/Munc18‐1, SYT1), which are essential for neurotransmission, in 95 patients with idiopathic mental retardation. We identified de novo mutations in STXBP1 (nonsense, p.R388X; splicing, c.169+1G>A) in two patients with severe mental retardation and nonsyndromic epilepsy. Reverse transcriptase polymerase chain reaction and sequencing showed that the splicing mutation creates a stop codon downstream of exon‐3. No de novo or deleterious mutations in STXBP1 were found in 190 control subjects, or in 142 autistic patients. These results suggest that STXBP1 disruption is associated with autosomal dominant mental retardation and nonsyndromic epilepsy. Ann Neurol 2009;65:748–753

Alteration of Ganglioside Biosynthesis Responsible for Complex Hereditary Spastic Paraplegia

Hereditary spastic paraplegias (HSPs) form a heterogeneous group of neurological disorders. A whole-genome linkage mapping effort was made with three HSP-affected families from Spain, Portugal, and Tunisia and it allowed us to reduce the SPG26 locus interval from 34 to 9 Mb. Subsequently, a targeted capture was made to sequence the entire exome of affected individuals from these three families, as well as from two additional autosomal-recessive HSP-affected families of German and Brazilian origins. Five homozygous truncating (n = 3) and missense (n = 2) mutations were identified in B4GALNT1. After this finding, we analyzed the entire coding region of this gene in 65 additional cases, and three mutations were identified in two subjects. All mutated cases presented an early-onset spastic paraplegia, with frequent intellectual disability, cerebellar ataxia, and peripheral neuropathy as well as cortical atrophy and white matter hyperintensities on brain imaging. B4GALNT1 encodes β-1,4-N-acetyl-galactosaminyl transferase 1 (B4GALNT1), involved in ganglioside biosynthesis. These findings confirm the increasing interest of lipid metabolism in HSPs. Interestingly, although the catabolism of gangliosides is implicated in a variety of neurological diseases, SPG26 is only the second human disease involving defects of their biosynthesis.

Deleterious mutations in the essential mRNA metabolism factor, hGle1, in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the selective death of motor neurons. Causative mutations in the global RNA-processing proteins TDP-43 and FUS among others, as well as their aggregation in ALS patients, have identified defects in RNA metabolism as an important feature in this disease. Lethal congenital contracture syndrome 1 and lethal arthrogryposis with anterior horn cell disease are autosomal recessive fetal motor neuron diseases that are caused by mutations in another global RNA-processing protein, hGle1. In this study, we carried out the first screening of GLE1 in ALS patients (173 familial and 760 sporadic) and identified 2 deleterious mutations (1 splice site and 1 nonsense mutation) and 1 missense mutation. Functional analysis of the deleterious mutants revealed them to be unable to rescue motor neuron pathology in zebrafish morphants lacking Gle1. Furthermore, in HeLa cells, both mutations caused a depletion of hGle1 at the nuclear pore where it carries out an essential role in nuclear export of mRNA. These results suggest a haploinsufficiency mechanism and point to a causative role for GLE1 mutations in ALS patients. This further supports the involvement of global defects in RNA metabolism in ALS.

Exome sequencing reveals SPG11 mutations causing juvenile ALS.

We report here the description of a nonconsanguineous family with 2 affected individuals with a recessively inherited juvenile motor neuron disease. Exome sequencing of these 2 affected individuals led us to identify 2 compound heterozygous deletions leading to a frameshift and a premature stop codon in the SPG11 gene. One of these deletions, c.5199delA in exon 30, has not been previously reported. Interestingly, these deletions are associated with an intrafamilial phenotypic heterogeneity as one affected has atypical juvenile amyotrophic lateral sclerosis (ALS) and the other has classical hereditary spastic paraplegia with thin corpus callosum. Our findings confirm SPG11 as a genetic cause of juvenile amyotrophic lateral sclerosis and indicate that SPG11 mutations could be associated with 2 different clinical phenotypes within the same family.