Molecular Pathogenesis Of Renpenning Syndrome Caused By PQBP1 Mutations

Renpenning syndrome is a group of X-linked intellectual disability?XLID?syndromes caused by mutations in human polyglutamine-binding protein 1?PQBP1?gene.A series of mutation sites have been reported in this gene among different families with this disease.Although the patients share the common clinical symptoms such as intellectual disability,microcephaly,short stature with lean body build and microrchidia,variability among different families is also profound in more detailed clinical examinations.Until now,little is known about the molecular pathogenesis of the various mutations in patients.In this dissertation,we used the SH-SY5Y cell line,primary cultured hippocampal neurons and Drosophila neuromuscular junction?NMJ?model,combined with genetics,biochemistry,cell biology and electrophysiology approaches to examine the cellular and synaptic functions of the most common mutations found in the patients:c.461₄62delAG,c.459₄62delAGAG and c.463₄64dupAG in PQBP1 exon 4 and to analyze the pathogenic mechanism in Renpenning syndrome.All three mutations result in the frame-shifts and premature termination.First,we examined the interaction and binding domain between PQBP1 mutations and fragile X mental retardation protein?FMRP?.GST pull-down assay shows that the frame-shifted sequences of PQBP1 c.459₄62delAGAG and c.463₄64dupAG mutations encode a specific C-terminal epitope and bind the RGG domain of FMRP directly,while PQBP1 c.461₄62delAG mutant does not.In this domain,there is a conserved phosphorylation sites to keep the balance between phosphorylation and non-phosphorylation status of FMRP in vivo.According to biochemistry assays,we found that PQBP1-delAGAG and dupAG mutants preferentially bind non-phosphorylated FMRP compared with PQBP1-WT,and promote its ubiquitin-mediated degradation.In cultured cells,PQBP1 mutants impair the normal function of FMRP,such as up-regulating its target MAP 1B and disrupting homeostatic synaptic scaling in primary hippocampal neurons.Next,we chose Drosophila NMJ model and generated the c.463₄64dupAG transgenic flies to analyze the synaptic function of PQBP1-dupAG mutation.Then,a remarkable synaptic over-growth in the NMJ was be found,which can be rescued by exogenously expressing dFMRP.Our evidence strongly supports a gain-of-function pathogenic mechanism of the prevailing mutations of Renpenning syndrome PQBP1 c.459₄62delAGAG and c.463₄64dupAG.In addition,we reveal that the pathology of Renpenning syndrome is directly linked to Fragile X syndrome.These findings will reshape our oppinion of therapeutic strategies for X-linked intellectual disability.