NLGN2 Haploinsufficiency Causes a Distinctive Neurobehavioral Phenotype Characterized By Anxiety, Autism, Intellectual Disability, and Obesity

Background: Dysregulation of synaptic function is considered a common pathophysiological mechanism for a variety of neurobehavioral phenotypes and mutations in several synaptic proteins have been implicated in these disorders. Animal and in vitro studies have shown that neuroligin 2 (NL2) clusters in inhibitory postsynaptic membranes and induces, through the mediation of collybistin and gephyrin, an inhibitory postsynaptic specialization by localization of the glycine and GABA_A receptors.  These functions are essential for the maintenance of excitatory/inhibitory balance in the brain. Deletion of nlgn2 in mice leads to changes in the molecular make-up of inhibitory synapses and inhibitory synaptic transmission, and increases anxiety-like behaviors. Conditional knockout (KO) of nlgn2 in the medial prefrontal cortex of mice causes deficits in anxiety, fear memory and social interaction. The conditional nlgn2 KO in the basal amygdala, a brain region with prominent effect on fear and anxiety behaviors, leads to a robust anxiety phenotype upon exposure to an anxiogenic environment. In humans, missense variants in NLGN2 have been implicated in a few cases of schizophrenia.

Objectives: Investigation of the molecular basis of a distinctive neurobehavioral phenotype in a 14-year-old Caucasian male. 

Methods: The proband exhibited global developmental delay since early infancy. He was diagnosed with PDD-NOS at age 3 years but later his phenotype has been dominated by short attention span, severe anxiety and obsessive-compulsive behaviors. His other problems included hypotonia, some dysmorphic features, macrocephaly and obesity, which was associated with hyperphagia, food-seeking behavior, and diet-related obsession. His previous work-up has been negative and included chromosomal microarray analysis, molecular testing for fragile X syndrome, and methylation studies for Prader Willi syndrome. Exome sequencing was performed using Agilent Clinical Research Exome kit and Illumina HiSeq 2000 100 bp paired-end reads. GeneDx's XomeAnalyzer was used to evaluate sequence changes between the proband, parental samples and reference. Sanger sequencing was used to confirm positive exome findings.

Results: Exome sequencing revealed a heterozygous de novo variant in NLGN2designated as c.441C>A (p.Y147X; p.Tyr147Ter), which has not been reported previously as a disease causing mutation nor as a benign polymorphism. It is predicted to cause loss of normal protein function either through protein truncation or nonsense-mediated mRNA decay. 

Conclusions: This is the first report of a truncating variant in NLGN2 that recapitulates the anxiety phenotype in mice. We provide evidence that the de-novo loss-of-function variant in our proband is pathogenic. Our data add to the accumulating evidence implicating synaptic proteins in the etiologies of a spectrum of neurodevelopmental phenotypes and demonstrate the power of exome sequencing in psychogenetics. The genetic data may have implications for the treatment of the psychological findings in our proband.