The Screening Of Causative Gene And The Functional Effects Of Genetic Mutation Of Frontotemporal Dementia In The Chinese Han Population

Background: Frontotemporal dementia(FTD)is a autosomal dominantly inherited neurodegenerative disorder characterized by progressive deficits in behavior,cognition,personality or language impairment.And it is the second most common presenile dementia only than Alzheimer disease.According to the typical clinical manifestitation,FTD can be classified into behavioral variant FTD(bv FTD),progressive nonfluent aphasia(PNFA),and semantic dementia(SD).Up to 15% of patients have comorbid with motor neuron disease.Approximately 40% of FTD patients have a definite dementia family history,only 10% have a clear autosomal dominant inheritance mode.More than eight genes have been recognized to be the causative gene of FTD,including MAPT,C9orf72,GRN,TBK1,CHMP2 B,FUS,VCP,TARDBP and so on.The MAPT gene was the first discovered causative gene of FTD.To date,approximately 160 families around the world had reported 57 pathogenic variants in the MAPT gene(https://www.alzforum.org).Wheares,there are a few reports about the causative gene variants in the Chinese population.So it is necessary to further clarify the disease-causative gene mutation of FTD patients in the Chinese Han population,especially the MAPT gene variant spectrum.Aim: To depict the genetic spectrum and clinical profiles of FTD patients in the Chinese population.Methods: A total of 30(6 familial and 24 sporadic)FTD probands were enrolled from April 2015 to March 2019.Whole exon sequencing was used to screen the causative genes of FTD and those variants screened were verified by Sanger sequencing.Combined with co-segregation analysis and functional experiments to further classify the pathogenicity of variants.Results: Two novel variants(p.D430 H and p.R406Q)and a pathogenic variant(p.P301L)in the MAPT gene,and a novel GRN splicing varaint c.350+2A>G were identified in three sporadic and a familiar FTD patients.According to the ACMG guidelines and standards,the p.R406 Q variant in MAPT gene and the c.350+2A>G variant in GRN gene were classified as likely pathogenic variants,whereas,the p.D430 H variant in the MAPT gene was classified as an uncertain significance variant.In the cell culture transfection studies,the total phosphorylation levels of two variants(p.R406 Q and p.D430H)in the MAPT gene were significantly increased,specifically at the Thr205,Thr231 and Ser396 epitopes.The recombinant tau protein experiments showed that the p.R406 Q and p.D430 H variants reduced the ability of tau to bind to the microtubules and remarkably increased the aggregation of tau in the detection of thioflavin S.Furthermore,the real-time PCR showed that the m RNA level of c.350+2A>G variant was slightly higher than the normal control,and no statistically difference was found.The Sanger sequencing showed that this variant did not lead to the exon 5 skipping.Conclusion: Not only did our findings expand the causative gene variant spectrum but also characterized the genotype-phenotype associations of MAPT gene in the Chinese population.