The genetic causes of primary antibody deficiencies and autism spectrum disorder (ASD) are largely unknown. Here, we report a patient with hypogammaglobulinemia and ASD who carries biallelic mutations in the transcription factor PAX5. A patient-specific Pax5 mutant mouse revealed an early B cell developmental block and impaired immune responses as the cause of hypogammaglobulinemia. Pax5 mutant mice displayed behavioral deficits in all ASD domains. The patient and the mouse model showed aberrant cerebellar foliation and severely impaired sensorimotor learning. PAX5 deficiency also caused profound hypoplasia of the substantia nigra and ventral tegmental area due to loss of GABAergic neurons, thus affecting two midbrain hubs, controlling motor function and reward processing, respectively. Heterozygous Pax5 mutant mice exhibited similar anatomic and behavioral abnormalities. Lineage tracing identified Pax5 as a crucial regulator of cerebellar morphogenesis and midbrain GABAergic neurogenesis. These findings reveal new roles of Pax5 in brain development and unravel the underlying mechanism of a novel immunological and neurodevelopmental syndrome.

The functional and molecular sources of behavioral variability in mice are not fully understood. As a consequence, the predominant use of male mice has become a standard in animal reseach, under the assumption that males are less variable than females. Similarly, to homogenize genetic background, neuroscience studies have almost exclusively used the C57BL/6 (B6) strain. Here, we examined individual differences in performance in the context of associative learning. We performed delayed eyeblink conditioning while recording locomotor activity in mice from both sexes in two strains (B6 and B6CBAF1). Further, we used a C-FOS immunostaining approach to explore brain areas involved in eyeblink conditioning across subjects, and correlate them with behavioral performance. We found that B6 male and female mice show comparable variability in this task and that females reach higher learning scores. We found a strong positive correlation across sexes between learning scores and voluntary locomotion. C-FOS immunostainings revealed positive correlations between C-FOS positive cell density and learning in the cerebellar cortex as well as multiple, previously unreported extra-cerebellar areas. We found consistent and comparable correlations in eyeblink performance and C-fos expression in B6 and B6CBAF1 females and males. Taken together, we show that differences in motor behavior and activity across brain areas correlate with learning scores during eyeblink conditioning across strains and sexes.

Delay eyeblink conditioning has been extensively used to study associative learning and the cerebellar circuits underlying this task have been largely identified. However, there is a little knowledge on how factors such as strain, sex and innate behaviour influence performance during this type of learning. In this study, we used male and female mice of C57BL/6J (B6) and B6CBAF1 strains to investigate the effect of sex, strain and locomotion in delay eyeblink conditioning. We performed a short and a long delay eyeblink conditioning paradigm and used a c-Fos immunostaining approach to explore the involvement of different brain areas in this task. We found that both B6 and B6CBAF1 females reach higher learning scores compared to males in the initial stages of learning. This sex-dependent difference was no longer present as the learning progressed. Moreover, we found a strong positive correlation between learning scores and voluntary locomotion irrespective of the training duration. c-Fos immunostainings after the short paradigm showed positive correlations between c-Fos expression and learning scores in the cerebellar cortex and brainstem, as well as previously unreported areas. By contrast, after the long paradigm, c-Fos expression was only significantly elevated in the brainstem. Taken together, we show that differences in voluntary locomotion and activity across brain areas correlate with performance in delay eyeblink conditioning across strains and sexes.