Abstracts

Rationale: IQSEC2 is located on the X chromosome and encodes a postsynaptic excitatory synapse protein. Mutations in IQSEC2 are associated with multiple cases of syndromic and non-syndromic intellectual-disability and epileptic encephalopathy, with pharmacoresistant seizures as a predominant clinical feature. The majority (88%) of male and female patients have loss-of-function IQSEC2 variants including from premature termination codons (Mignot et al., 2018). To investigate IQSEC2 function in a disease model, we developed Iqsec2 knock-out mice. Here, we employ whole cell patch clamp electrophysiology and seizure threshold testing in initial efforts to understand how Iqsec2 loss leads to disease. Methods: Only male mice were examined to avoid the confounding effects of X-inactivation in female heterozygous mice. Whole cell patch clamp recording was performed on DIV14 dissociated hippocampal neurons derived from newborn Iqsec2+/Y and Iqsec2-/Y mice. For examining spontaneous seizure activity and interictal epileptiform abnormalities, mice were implanted with cranial electrodes and subjected to video-EEG monitoring. For examining network excitability differences between the Iqsec2+/Y and Iqsec2-/Y mice, seizure activity was induced by transcorneal high frequency and 6 Hz electroconvulsive stimulation, or by subcutaneous injection of pentylenetetrazol (PTZ), a GABAA receptor antagonist. Results: Adult Iqsec2-/Y mice display increased adulthood mortality with cause of death a single lethal seizure. Video-EEG was recorded from 2 month old Iqsec2-/Y (n=3) and Iqsec2+/Y (n=2) mice for 48 hours. We also evaluated longer-term recordings in four Iqsec2-/Y mice between 60-110 days of age for a total of 96 hours. Overall, no electroclinical seizure or obvious epileptiform activity was captured in the mice recorded on video-EEG. Although no difference between genotypes (Iqsec2-/Y, n=7 Iqsec2+/Y n=14), was observed for high frequency electroshock, Iqsec2-/Y (n=8) mice had a significantly increased threshold (p< 0.001) to 6 Hz stimulus compared to age-matched wildtype controls (n=8). Furthermore, in preliminary assessments using convulsive doses of PTZ, only 27% of the Iqsec2-/Y mice reached the generalized tonic-clonic seizure endpoint compared to 85% of Iqsec2+/Y mice (Iqsec2 -/Y, n=8 Iqsec2+/Y n=8), and those that did had a longer latency. Additionally, voltage clamp recordings of hippocampal primary neuron culture derived from mutant male mice show a significant increase in mEPSC frequency (p=0.02) and amplitude (p<0.001) selectively onto interneurons, with no change onto excitatory neurons. Conclusions: Iqsec2 loss preferentially increases excitatory drive onto interneurons. Despite the fact that all knockout males (median survival age 95 days) eventually succumb to a lethal seizure, they actually have decreased susceptibility to electrically induced partial seizures and to a GABAA antagonist. Together these data shed new light on IQSEC2’s role in maintaining neuronal function and normal network excitability while offering a new potentially valuable animal model for preclinical studies on IQSEC2 disorder. Funding: This research is supported by NIH grant R37 NS031348