Human-mouse chimeric Fragile X syndrome model reveals FMR1-dependent neuronal phenotypes

Abnormal neuronal development in Fragile X syndrome (FXS) is poorly understood. Data on FXS patients remain scarce and FXS animal models have failed to yield successful therapies. In vitro models do not fully recapitulate the morphology and function of human neurons. Here, we co-injected neural precursor cells (NPCs) from FXS patient-derived and corrected isogenic control induced pluripotent stem cells into the brain of neonatal immune-deprived mice. The cells populated the brain and differentiated into neurons and astrocytes. Single-cell RNA sequencing of transplanted cells revealed upregulated excitatory synaptic transmission and neuronal differentiation pathways in FXS neurons. Immunofluorescence analyses showed accelerated maturation of FXS neurons, an increased proportion of Arc-positive FXS neurons and increased dendritic protrusion width of FXS striatal medium spiny neurons. Our data show faster maturation and suggest increased synaptic activity and synaptic strength of FXS transplanted neurons. This model provides new insights into the alterations in FXS neuronal development.