ABSTRACT
Epilepsy and mental retardation limited to females (EFMR) is a disorder with an X-linked mode of inheritance and an unusual expression pattern. Disorders arising from mutations on the X chromosome are typically characterized by affected males and unaffected carrier females. In contrast, EFMR spares transmitting males and affects only carrier females. Aided by systematic resequencing of 737 X chromosome genes, we identified different protocadherin 19 (PCDH19) gene mutations in seven families with EFMR. Five mutations resulted in the introduction of a premature termination codon. Study of two of these demonstrated nonsense-mediated decay of PCDH19 mRNA. The two missense mutations were predicted to affect adhesiveness of PCDH19 through impaired calcium binding. PCDH19 is expressed in developing brains of human and mouse and is the first member of the cadherin superfamily to be directly implicated in epilepsy or mental retardation.
Subject(s)
Cadherins/genetics , Chromosomes, Human, X , Codon, Nonsense/genetics , Cognition Disorders/genetics , Epilepsy/genetics , Genomic Imprinting , Mutation, Missense/genetics , Animals , Brain/growth & development , Brain/metabolism , Brain/pathology , Case-Control Studies , Cognition Disorders/pathology , Epilepsy/pathology , Female , Fibroblasts/cytology , Fibroblasts/metabolism , Gene Expression Regulation, Developmental , Genes, X-Linked/genetics , Humans , In Situ Hybridization , Male , Mental Retardation, X-Linked/genetics , Mental Retardation, X-Linked/pathology , Mice/embryology , Pedigree , Phenotype , Protocadherins , RNA, Messenger/genetics , RNA, Messenger/metabolism , Reverse Transcriptase Polymerase Chain Reaction , Skin/cytology , Skin/metabolismABSTRACT
Nonsense-mediated mRNA decay (NMD) is of universal biological significance. It has emerged as an important global RNA, DNA and translation regulatory pathway. By systematically sequencing 737 genes (annotated in the Vertebrate Genome Annotation database) on the human X chromosome in 250 families with X-linked mental retardation, we identified mutations in the UPF3 regulator of nonsense transcripts homolog B (yeast) (UPF3B) leading to protein truncations in three families: two with the Lujan-Fryns phenotype and one with the FG phenotype. We also identified a missense mutation in another family with nonsyndromic mental retardation. Three mutations lead to the introduction of a premature termination codon and subsequent NMD of mutant UPF3B mRNA. Protein blot analysis using lymphoblastoid cell lines from affected individuals showed an absence of the UPF3B protein in two families. The UPF3B protein is an important component of the NMD surveillance machinery. Our results directly implicate abnormalities of NMD in human disease and suggest at least partial redundancy of NMD pathways.
