Rare Variants in Tissue Inhibitor of Metalloproteinase 2 as a Risk Factor for Schizophrenia: Evidence From Familial and Cohort Analysis

Candidate gene and genome-wide association study based common risk variant identification is being complemented by whole exome sequencing (WES)/whole genome sequencing based rare variant discovery in elucidation of genetic landscape of schizophrenia (SZ), a common neuropsychiatric disorder. WES findings of de novo mutations in case-parent trios have further implied genetic etiology, but do not explain the high genetic risk in general populations. Conversely, WES in multiplex families may be an insightful strategy for the identification of highly penetrant rare variants in SZ and possibly enhance our understanding of disease biology. In this study, we analyzed a 5-generation Indian family with multiple members affected with SZ by WES. We identified a rare heterozygous missense variant (NM_003255: c.506C>T; p.Pro169Leu; MAF = 0.0001) in Tissue Inhibitor of Metalloproteinase 2 (TIMP2, 17q25.3) segregating with all 6 affected individuals but not with unaffected members. Linkage analysis indicated a maximum logarithm of the odds score of 1.8, θ = 0 at this locus. The variant was predicted to be damaging by various in silico tools and also disrupt the structural integrity by molecular dynamics simulations. WES based screening of an independent SZ cohort (n = 370) identified 4 additional rare missense variants (p.Leu20Met, p.Ala26Ser, p.Lys48Arg and p. Ile217Leu) and a splice variant rs540397728 (NM_003255:c.232-5T>C), also predicted to be damaging, increasing the likelihood of contribution of this gene to SZ risk. Extensive biochemical and knockout mouse studies suggesting involvement of TIMP2 in neurodevelopmental and behavioral deficits, together with genetic evidence for TIMP2 conferring SZ risk from this study may have possible implications for new therapeutics.

Extracellular proteolysis in structural and functional plasticity of mossy fiber synapses in hippocampus

Brain is continuously altered in response to experience and environmental changes. One of the underlying mechanisms is synaptic plasticity, which is manifested by modification of synapse structure and function. It is becoming clear that regulated extracellular proteolysis plays a pivotal role in the structural and functional remodeling of synapses during brain development, learning and memory formation. Clearly, plasticity mechanisms may substantially differ between projections. Mossy fiber synapses onto CA3 pyramidal cells display several unique functional features, including pronounced short-term facilitation, a presynaptically expressed long-term potentiation (LTP) that is independent of NMDAR activation, and NMDA-dependent metaplasticity. Moreover, structural plasticity at mossy fiber synapses ranges from the reorganization of projection topology after hippocampus-dependent learning, through intrinsically different dynamic properties of synaptic boutons to pre- and postsynaptic structural changes accompanying LTP induction. Although concomitant functional and structural plasticity in this pathway strongly suggests a role of extracellular proteolysis, its impact only starts to be investigated in this projection. In the present report, we review the role of extracellular proteolysis in various aspects of synaptic plasticity in hippocampal mossy fiber synapses. A growing body of evidence demonstrates that among perisynaptic proteases, tissue plasminogen activator (tPA)/plasmin system, β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) and metalloproteinases play a crucial role in shaping plastic changes in this projection. We discuss recent advances and emerging hypotheses on the roles of proteases in mechanisms underlying mossy fiber target specific synaptic plasticity and memory formation.