Fragile X syndrome patient-derived neurons developing in the mouse brain show FMR1 -dependent 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 transplanted cells populated the brain and a proportion differentiated into neurons and glial cells. 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 after an initial delay. Additionally, increased percentages of Arc- and Egr1-positive FXS neurons and wider dendritic protrusions of mature FXS striatal medium spiny neurons pointed to an increase in synaptic activity and synaptic strength as compared to control. This transplantation approach provides new insights into the alterations of neuronal development in FXS by facilitating physiological development of cells in a 3D context, and could be used to test new therapeutic compounds correcting neuronal development defects in FXS. ### Competing Interest Statement R.J. is a co-founder of Fate, Fulcrum, and Omega Therapeutics and an advisor to Camp4 and Dewpoint Therapeutics. H.W, A.N.C., A.C. and O.W. were full time employees of Fulcrum Therapeutics at the time of the study. The study was funded by Fulcrum Therapeutics and NIH grant 5R01MH104610-21 to R.J. The other authors have no competing interests to declare.