CNTNAP2 and NRXN1 Are Mutated in Autosomal-Recessive Pitt-Hopkins-like Mental Retardation and Determine the Level of a Common Synaptic Protein in Drosophila

Abstract

Heterozygous copy-number variants and SNPs of CNTNAP2 and NRXN1, two distantly related members of the neurexin superfamily, have been repeatedly associated with a wide spectrum of neuropsychiatric disorders, such as developmental language disorders, autism spectrum disorders, epilepsy, and schizophrenia. We now identified homozygous and compound-heterozygous deletions and mutations via molecular karyotyping and mutational screening in CNTNAP2 and NRXN1 in four patients with severe mental retardation (MR) and variable features, such as autistic behavior, epilepsy, and breathing anomalies, phenotypically overlapping with Pitt-Hopkins syndrome. With a frequency of at least 1% in our cohort of 179 patients, recessive defects in CNTNAP2 appear to significantly contribute to severe MR. Whereas the established synaptic role of NRXN1 suggests that synaptic defects contribute to the associated neuropsychiatric disorders and to severe MR as reported here, evidence for a synaptic role of the CNTNAP2-encoded protein CASPR2 has so far been lacking. Using Drosophila as a model, we now show that, as known for fly Nrx-I, the CASPR2 ortholog Nrx-IV might also localize to synapses. Overexpression of either protein can reorganize synaptic morphology and induce increased density of active zones, the synaptic domains of neurotransmitter release. Moreover, both Nrx-I and Nrx-IV determine the level of the presynaptic active-zone protein bruchpilot, indicating a possible common molecular mechanism in Nrx-I and Nrx-IV mutant conditions. We therefore propose that an analogous shared synaptic mechanism contributes to the similar clinical phenotypes resulting from defects in human NRXN1 and CNTNAP2.

Shining a light on CNTNAP2: complex functions to complex disorders

“…10 Thus, the type of mutation present or the genetic background on which the mutations occur may be a major factor in whether a heterozygous disruption can cause disorder. So far only three different homozygous mutations have been identified in patients, all of which cause severe disorder (either CDFE or PittHopkins-like ID), 14,17 thus it is possible that complete loss of CNTNAP2 produces a more severe effect on cognitive function. However, it should be noted that there is evidence that the CDFE mutation may not be a null allele, because it produces a potentially deleterious protein (see section 'Understanding CASPR2 function'); thus it will be necessary to identify further patients to clarify this.…”

“…49 Mutations in TCF4 have been shown to cause PittHopkins syndrome (PTHS) and three rare TCF4 SNPs are associated with schizophrenia. 17,[49][50][51] PTHS is characterised by severe intellectual disability, absent or severely impaired speech, characteristic facial features and epilepsy. 52 Many of these features are shared with patients carrying CNTNAP2 mutations, leading researchers to test patients with PTHS-like features for CNTNAP2 mutations.…”

“…52 Many of these features are shared with patients carrying CNTNAP2 mutations, leading researchers to test patients with PTHS-like features for CNTNAP2 mutations. 17 Two mutations affecting the CNTNAP2 locus (one homozygous and one compound heterozygote) were identified in two independent pedigrees (Table 1). This suggested that disruption of the TCF4-CNTNAP2 pathway could be related to intellectual disability, seizures, and/or behavioural abnormalities.…”

Synapse development in health and disease

“…Likewise, mutations in neuroligin, dnlg1, decrease synaptic growth due to deficits in bouton addition and postsynaptic differentiation [15]. In mammals, a synaptogenic role for neurexins is suggested by the observation that neurexins bind the postsynaptic glutamate receptor δ2 via an adaptor protein, which is essential for excitatory synaptogenesis in the cerebellum [3]. Another recently recognized synaptic cell adhesion molecule is amyloid precursor protein (APP), a type I membrane protein involved in Alzheimer's disease pathogenesis.…”

“…In recent years, clinically diverse disorders such as autism spectrum disorders (ASDs), schizophrenia, epilepsy, and intellectual disability (ID) have been linked to dysfunction of a number of proteins implicated in synaptic development [1,2]. 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].…”