EPIGENETIC REGULATORY FUNCTION OF BAP1 IN DENTATE GYRUS DEVELOPMENT - ABSTRACT: AS A GROWING POPULATION OF PATIENTS IN THE US HAVE BEEN DIAGNOSED WITH NEURODEVELOPMENTAL DISORDERS (NDDS), EPIGENETIC CAUSES OF THESE DISORDERS CONTINUE TO BE AN ACTIVE AREA OF STUDY. MANY STUDIES HAVE CONNECTED ALTERATIONS IN HISTONE MODIFICATION VIA UBIQUITINATION THROUGH THE POLYCOMB REPRESSIVE COMPLEX 1 (PRC1) AND DEUBIQUITINATION THROUGH THE POLYCOMB REPRESSIVE-DEUBIQUITINASE COMPLEX (PR-DUB) TO NDDS. HOWEVER, THE ROLE OF BRCA1-ASSOCIATED PROTEIN 1 (BAP1), THE CORE COMPONENT OF PR-DUB IS NOT FULLY KNOWN. A CLINICAL STUDY CONDUCTED BY KURY ET AL. INVESTIGATED 11 PATIENTS WITH DE NOVO GERMLINE HETEROZYGOUS MISSENSE BAP1 MUTATIONS WHERE PATIENTS PRESENTED WITH A VARIETY OF NDDS INCLUDING ATTENTION-DEFICIT/HYPERACTIVITY DISORDER, AUTISM SPECTRUM DISORDER, SPEECH DELAY, AND INTELLECTUAL DISABILITY. THIS SUGGESTS THAT BAP1 HAS A SIGNIFICANT ROLE IN BRAIN DEVELOPMENT, BUT UNDERLYING MECHANISMS ARE NOT YET UNDERSTOOD. TO INVESTIGATE THE FUNCTION OF BAP1 IN BRAIN DEVELOPMENT, WE HAVE GENERATED A CONDITIONAL KNOCKOUT MOUSE LACKING BAP1 IN THE DORSAL CORTEX AND HIPPOCAMPUS (BAP1 CKO) USING EMX-1 CRE. STRIKINGLY, WE OBSERVED DEVELOPMENTAL DEFECTS IN THE HIPPOCAMPUS THAT LED TO SIGNIFICANT ALTERATIONS IN THE SIZE, SHAPE, AND GROWTH OF THE DENTATE GYRUS (DG). THE PROGENITOR POPULATION OF THE DG OF BAP1 CKO MICE WAS SIGNIFICANTLY DECREASED IN NEONATES FOLLOWED BY DRAMATIC REDUCTION OF DG GRANULE CELLS (DG-GC) AND OVERALL SIZE THAT PERSISTS THROUGH ADULTHOOD. PREVIOUS STUDIES HAVE DEMONSTRATED THE PERVASIVE ROLE OF BAP1 IN DEUBIQUITINATION OF HISTONE H2AK119UB LEADING TO REDUCED OCCUPANCY AT PRC1 TARGET PROMOTERS IN EMBRYONIC STEM CELLS. WE HAVE FOUND THAT BAP1 CKO SHOWED INCREASED H2AK119UB CONSISTENT WITH KIS PATIENT DATA, SUGGESTING A POTENTIAL MECHANISM FOR BAP1 LOSS OF FUNCTION IN DG DEVELOPMENT. TO EVALUATE CHROMATIN ACCESSIBILITY AND DOWNSTREAM GENE EXPRESSION CHANGES, WE CONDUCTED SINGLE NUCLEI MULTIOMIC SEQUENCING IN THE BAP1 CKO NEONATE AND CONTROL HIPPOCAMPI. IMPORTANTLY, WNT SIGNALING, A KNOWN FUNDAMENTAL CONTRIBUTOR TO INITIATION OF HIPPOCAMPAL DEVELOPMENT AND CELL FATE, WAS DOWNREGULATED IN BAP1 CKO DG-GC AND NEURAL STEM CELLS. INTERESTINGLY, WNT SIGNALING IS KNOWN TO BE AFFECTED BY THE PRC1 COMPLEX THROUGH DECREASED H2AK119UB AND SUBSEQUENT DECREASED H3K27ME3 ENRICHMENT. FURTHERMORE, IN UTERO INTRACEREBROVENTRICULAR INJECTION OF WNT3A LED TO SIGNIFICANT INCREASE IN DG SIZE OF BAP1 CKO MICE. FROM THE ROBUST PRELIMINARY DATA GAINED THROUGH THE BAP1 CKO ANIMAL MODEL, WE HYPOTHESIZE THAT BAP1 PROMOTES DENTATE GYRUS PROGENITOR PROLIFERATION AND LINEAGE PROGRESSION THROUGH DEUBIQUITINATION OF H2AK119UB AND SUBSEQUENT DISINHIBITION OF WNT SIGNALING. WE WILL TEST THIS HYPOTHESIS THROUGH THE FOLLOWING OVERARCHING AIMS: AIM 1) INVESTIGATE THE ROLE OF BAP1 IN HIPPOCAMPAL DG PROGENITOR PROLIFERATION AND LINEAGE PROGRESSION IN THE MOUSE HIPPOCAMPUS AND AIM 2) DETERMINE IF BAP1 LOSS LEADS TO INCREASED H2AK119UB AND REDUCTION OF WNT SIGNALING IN HIPPOCAMPAL DG PROGENITORS.