In vivo dissection of human NRXN1 isoforms reveals gain-of-function pathogenicity of schizophrenia-associated 3' deletions.

Heterozygous deletions in NRXN1, encoding the presynaptic adhesion molecule Neurexin 1, are among the most frequently identified rare variants in schizophrenia and other neuropsychiatric disorders. Patient sequencing has revealed that 3' deletions within NRXN1 generate novel isoforms not produced from the intact locus, yet whether these isoforms are functional, passively non-functional, or actively pathogenic in vivo is still not clear. To address this, we expressed eight human NRXN1 isoforms in C. elegans neurons including four control isoforms and four 3' deletion variant isoforms identified in schizophrenia patient cell lines, and characterized their effects on protein localization and two independent behaviors: a food deprivation response and social feeding behaviors. Most human isoforms showed expression and localization within the nerve ring and neurons similar to the C. elegans ortholog, NRX-1; however, several isoforms, particularly among the 3' deletion variants, displayed aberrant accumulation in neuronal cell bodies or as puncta in neuropil. Functionally, isoforms fell into one of three categories: no effect on nrx-1 loss-of-function behavioral phenotypes, partial rescue, or gain of function, with multiple isoforms showing differences between the behaviors. Strikingly, two 3' deletion isoforms produced gain-of-function behavioral phenotypes more severe than the nrx-1 null mutant, demonstrating that these patient-derived variants can be actively pathogenic. These results establish C. elegans as a tractable in vivo platform for dissecting the isoform-specific functional consequences of NRXN1 variants and suggest that strategies for NRXN1-associated neuropsychiatric diseases must account for both loss-of-function and gain-of-function isoform mechanisms.