De novo mutations in the gene encoding the synaptic scaffolding proteinSHANK3in patients ascertained for schizophrenia

Abstract

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. S chizophrenia (SCZ) is a chronic psychiatric disorder characterized by a profound disruption in cognition, behavior, and emotion which begins in adolescence or early adulthood. There is significant clinical variability among SCZ patients, suggesting that it is etiologically heterogeneous. There are several hypotheses to explain genetic factors underlying SCZ, such as polygenic inheritance (1) or, in a fraction of cases, variably penetrant de novo mutations. The de novo hypothesis is based on several observations. One is that relatives of an individual with SCZ have a higher risk of being affected (parents 6%, offspring 13%, and siblings 9% compared with 1% for the general population) (2). The greater frequency in offspring than in parents may occur if new mutations account for a fraction of SCZ cases. Also, there is a significantly increased risk of SCZ with increasing paternal age (3), which could result from the age-related increase in paternal de novo mutations. Furthermore, despite reduced reproductive fitness (4) and extremely variable environmental factors, the incidence of SCZ is maintained at ∼1% worldwide. Interestingly, recent studies reported de novo copy-number variants in SCZ, providing further support for the de novo mutation hypothesis (5, 6).As part of the Synapse to Disease (S2D) project aimed at exploring the de novo mutation hypothesis in brain diseases, we are sequencing synaptic genes in individuals with SCZ and autism spectrum disorder (ASD), two neurodevelopmental disorders. Recently, mutations in the SHANK3 (SH3 and multiple ankyrin repeat domains 3) gene, encoding a scaffolding protein abundant in the postsynaptic density of excitatory synapses on dendritic spines, were found in patients with ASD (7-9). Considering that ASD and SCZ share some features, we decided to screen the SHANK3 gene in our cohort of SCZ probands. Given our hypothesis that a significant fraction of SCZ cases are the result of new mutations, we selected SCZ cases with unaffected parents and screened for de novo mutations.

Prevalence of SHANK3 variants in patients with different subtypes of autism spectrum disorders

“…39 Several studies suggest that SHANK3 has a critical role also in other neurodevelopmental disorders such as schizophrenia and intellectual disability. 10,[40][41][42] In the present study, we screened two cohorts of ASD patients for SHANK3 mutations: 133 patients from SCAP 33 and 88 from Italy. We found five potentially pathogenic alterations (Table 1), resulting in a mutation rate of 2.3%, which is twice as high as the frequency of deleterious SHANK3 mutations reported in previous studies.…”

Genetic studies of schizophrenia: an update

“…This hypothesis did not receive much attention in genetic studies until recently, when high-throughput DNA sequencing of SCZ families provided direct evidence that affected offspring have excess DNMs across the genome [72,73] . Further studies of DNMs in individual genes [74,75] , a particular set of genes [76,77] , or the exome [78][79][80] , also provided evidence that DNMs are enriched in SCZ patients. Direct measurement also indicates that SCZ patients have a higher mutation rate [76] .…”

Synapse development in health and disease

“…SHANK proteins localize to the excitatory postsynaptic density and contain multiple protein-protein interaction domains. Mutations in SHANK2 and SHANK3 have been identified in patients with autism, mental retardation, and schizophrenia [4,5]. In conjunction with HOMER, SHANKs form a mesh-like matrix at the postsynaptic density which regulates the morphology of dendritic spines and recruits postsynaptic density proteins [27].…”

“…For example, deletions of neurexin-1α, a synaptic cell adhesion protein, were initially identified by large-scale genetic screens in patients with autism and schizophrenia, and subsequently found in patients with severe ID and epilepsy [3]. Mutations in genes encoding the SHANK postsynaptic scaffolding protein family were first identified in a patient with ID, and later associated with autism and schizophrenia [4,5]. As the number of genes contributing to neuropsychiatric disorders has grown, it has become increasingly clear that pathways contributing to synaptic development and activity-dependent growth are important in their etiology.…”