INTRODUCTION: The MECP2 duplication syndrome is a severe neurodevelopmental disorder with core symptoms of autism spectrum disorders (ASD) ^1,2, and various rodent and primate models have been established to explore the possible pathological basis associated to atypical MECP2 level^3-5. However, how the autistic behaviors caused by atypical MECP2 duplication associate with brain structural alternations in rats, and how the developmental trajectories affect the gene- and behavior-associated brain alterations are rarely investigated.

METHODS: A human MECP2 duplication (MECP2-DP) rat model was created by the bacterial artificial chromosome transgenic method (Fig. 1A). The model efficacy was evaluated by biochemical analysis (Fig. 1A) and behavioral tests (Fig. 1B-C). Longitudinal DTI was performed in MECP2-DP(n=30) and control animals(n=25) at postnatal 14 days, 21 days, 28 days, 42 days and 56 days respectively. To identify robust DTI signatures, we performed three-way (FA: factional anisotropy; AD: axonal diffusivity; RD: radial diffusivity) autistic-behavior-guided fusion analysis⁶ (Fig. 1D) on the rat longitudinal DTI data. Linear mixed-effect (LME) model was used to test the gene effect at five age stages. Further, we analyzed the associations between the identified patterns and social recognition deficits (Fig. 1F).

RESULTS: Western blot analysis showed global increase of MECP2 protein across the rat brain, especially in cerebral cortex and hippocampus with almost doubled expression (Fig. 2A-B). MECP2-DP rats showed slower body weight (Fig. 2C) increase and lower survival rate (Fig. 2D). Typical autistic phenotypes were observed in MECP2-DP rats, including significantly less locomotor distance and slower speed (Fig. 2E). Three-chamber test showed MECP2-DP rats exhibited decreased preference of interacting with novel animals (Fig. 2F). A FA-AD-RD joint component was identified (Fig.2G). Atypical MECP2 level significantly altered the AD/RD in dorsal lateral septum (LSD) area, and the FA in olfactory area (OA) and preoptic area (POA) was also significantly disrupted. With age growing, the abnormal structural alterations became more discriminative. In addition, each pattern was significantly correlated with the social novelty time during P28 to P56, which indicated the identified abnormal structural alterations might be a possible neurological underpinning of social recognition deficits induced by atypical MECP2 level.

CONCLUSIONS: A typical MECP2 level can result in multiple autistic behaviors in rats. And disrupted FA in OA/POA, decreased RD/AD in LSD were also observed in MECP2-DP rats, and showed significant associations with social recognition deficits. The identified core patterns may help us to better understand the underlying mechanisms involved in the autistic behavior induced by atypical MECP2 level from a multimodal brain imaging perspective.