A Mouse Model of Börjeson-Forssman-Lehmann Syndrome reveals a potential link with Autism Spectrum Disorder

ABSTRACT OF THE DISSERTATION A Mouse Model of Börjeson-Forssman-Lehmann Syndrome reveals a potential link with Autism Spectrum Disorder By Cheng Cheng Doctor of Philosophy in Biology and Biomedical Sciences Developmental, Regenerative and Stem Cell Biology Washington University in St. Louis, 2018 Dr. Azad Bonni, Chair Intellectual disability (ID) is a prevalent neurodevelopmental disorder that affects 1% to 3% of the general population. ID is characterized by developmental deficiencies in cognitive function and adaptive behaviors. Lacking effective treatments, ID currently presents an immense burden to affected families and the economy. Therefore, there is an urgent need to elucidate the pathogenesis of ID. Human genetic studies have associated ID with a number of gene mutations. ID can be divided into two major groups: a non-syndromic form, characterized by intellectual impairment manifesting alone, and a syndromic form, characterized by both cognitive deficiencies and other anomalies, including biochemical disorders, skeletal abnormalities, facial dysmorphisms and neurological disorders. However, scientists have limited their studies to relatively few syndromic forms of ID, such as Rett, Angelman and Fragile X syndrome. In addition to these commonly studied syndromes, Börjeson-Forssman-Lehman syndrome (BFLS) was identified over five decades ago as a cause of X-linked ID and characterized as a syndromic form of moderately to severely impaired cognitive function associated with early developmental delay, truncal obesity, small genitalia, facial dysmorphism and seizures. Whether BFLS patients display other neurological manifestations besides cognitive impairment and seizures remains unexplored. In particular, whereas some genetic forms of ID are accompanied by manifestations of autism spectrum disorders (ASD), whether BFLS also features symptoms and signs of ASD is unknown. Forty years after the first description of BFLS, mutations in the gene encoding plant homeofinger protein 6 (PHF6) were discovered to be causative for BFLS. These mutations are distributed throughout the entire gene in distinct domains of PHF6, and are composed of missense, nonsense, truncation, duplication and frameshift. Accumulating evidence suggests that PHF6 plays a role in transcriptional regulation. PHF6 contains nuclear localization sequences and PHD domains and can interact with transcriptional elongation complex, PAF1 to regulate cortical neural migration. Furthermore, other studies also suggest that PHF6 may regulate transcription through association with the nucleosome remodeling complex NuRD and upstream binding factor UBF1. The discovery of PHF6 interactors and their functions during neural development have raised additional interesting questions in understanding BFLS pathogenesis. How does PHF6 regulation of gene expression at the genome-wide level impact BFLS pathogenesis? How do specific PHF6 mutations trigger the pathogenesis of BFLS? Within this thesis, I provide detailed characterization of a mouse containing a patient-specific mutation of PHF6 as a novel BFLS model. Mice with the patient-specific PHF6 C99F mutation display deficits in cognitive function, emotionality and social behaviors, and are more susceptible to seizures, suggesting that PHF6 C99F mice feature phenotypes relevant to BFLS. In electrophysiological studies in acute slices, the intrinsic excitability of entorhinal cortical layer II stellate neurons is increased, providing mechanistic basis for susceptibility of BFLS mice to seizures. Genome-wide RNA-Seq analyses of the cerebral cortex show that PHF6 promotes the expression of neurogenesis genes and concomitantly suppresses the expression of synaptic genes, suggesting that PHF6 promotes an immature state of neurons. Strikingly, bioinformatics analyses reveal that PHF6-regulated genes are overrepresented by ASD relevant gene signatures including modules that are misregulated in single gene causes of ASD. Consistent with these findings, we have identified an ASD patient with an underlying PHF6 missense mutation. Collectively, our findings in the novel BFLS mouse model broaden our understanding of the clinical features of BFLS patients and elucidate cellular and molecular underpinnings of BFLS pathogenesis