Developmental Trajectory of Pathological Brain Growth in a Mouse Model of Pten Haploinsufficiency: An MRI Study

Background: Macrocephaly/autism syndrome is caused by mutations in the gene PTEN (Phosphatase and tensin homolog), which encodes a negative regulator of the PI3K-Akt-mTOR pathway. Some individuals with autism spectrum disorder (ASD) and/or PTEN mutations show brain enlargement and white matter abnormalities. Mice haploinsufficient for Pten (Pten^+/-) display brain overgrowth and ASD-like behavioral phenotypes.

Objectives: While Pten^+/- mice show macroscale brain overgrowth, it is unknown whether this is region-specific or consistent across the brain. We used magnetic resonance imaging (MRI) to analyze the volume of the brain and individual structures in Pten^+/- and wild-type littermate control (WT) mice at postnatal day 7 (P7; equivalent to human birth) and in adulthood (P60).

Methods: MRI was performed on brains of male WT and Pten^+/- mice collected at P7 (WT: n=10; Pten^+/-: n=10) and P60 (WT: n=10; Pten^+/-: n=9) to measure the volume of different structures. Immunohistochemistry, immunocytochemistry, isotropic fractionator, and flow cytometry were used to confirm findings and to examine glial overgrowth in the Pten^+/- mice. All experiments were performed blind to genotype.

Results: Pten^+/- mice showed similar patterns of regional overgrowth to humans with PTEN mutations and ASD. At P7 and P60, virtually all individual regions, collapsed grey matter (GM), collapsed white matter (WM), and total brain volume were increased. Examining relative volume across brain areas, we found that WM was increased, and GM was decreased, in adulthood, similar to human patients. Juvenile Pten^+/- mice showed no relative differences in WM or GM. This may be due to increased inter-subject variability at P7 relative to P60, which is more pronounced in Pten^+/- mice. Previously, we found hyperplasia in neuronal but not glial populations at P0. At P7, however, we found increased glia, but not neurons, indicating that the adult pattern of hyperplasia was already established. This increased proliferation of glia likely contributes to increased WM volume.

Conclusions: Pten^+/- mice recapitulate the increased brain volume, WM overgrowth, and regional relative volume increases found in individuals with macrocephaly/autism syndrome. Increased variability among P7 Pten^+/- mice in comparison to age-matched controls and Pten^+/- adults suggests that specific brain region growth may be desynchronized across animals and during early developmental stages before stabilizing by adulthood. These findings indicate that Pten^+/- mice are a useful model for investigating the mechanistic basis of brain structure abnormalities found in humans with PTEN haploinsufficiency. Furthermore, this model may help illuminate the relationship between abnormal scaling across brain areas and the behavioral and cognitive symptoms of ASD and intellectual disability.