PAX5 DEFICIENCY DISRUPTS MIDBRAIN GABAERGIC DEVELOPMENT IN A MOUSE MODEL OF AUTISM SPECTRUM DISORDER

The transcription factor PAX5 has been identified as a high-risk gene for autism spectrum disorder (ASD), a neurodevelopmental condition characterized by repetitive behaviours and social and communication deficits. We previously generated a mouse model with patient-specific PAX5 mutations, which displayed behavioural deficits in all ASD-related domains. We observed hypoplasia of the substantia nigra (SN) and ventral tegmental area (VTA) in both the index patient and the mouse model, resulting from a loss of GABAergic neurons in both regions.
While PAX5 is a known regulator of B-lymphocyte development, its role in neurodevelopment remains poorly defined. The GABAergic, progenitors which populate the SN and VTA, originate in rhombomere 1 at E11.5; notably, PAX5 is expressed in a specific progenitor lineage in this region, which also expresses the transcription factor Tal1. We hypothesized that PAX5 regulates gene networks critical for midbrain GABAergic maturation. To this end, we performed single-cell RNA sequencing (scRNA-seq) of Pax5^+/+, Pax5^+/−, and Pax5^−/− embryonic midbrain regions at E13.5.
We observed a significant reduction in a specific GABAergic progenitor population in mutant embryos and trajectory inference unveiled that this population emerges from uncommitted progenitors, which eventually populate the SN and VTA. We found differentially expressed genes that are associated with GABAergic neurogenesis in this cluster and validated our findings by mRNA in situ hybridization. Our findings suggest that PAX5 commits progenitor neurons to the GABAergic fate during midbrain development and provides a basis for the GABAergic cell depletion observed in the adult brain due to PAX5 deficiency.