title: Premature termination of SMARCB1 translation may be followed by reinitiation in schwannomatosis-associated schwannomas, but results in absence of SMARCB1 expression in rhabdoid tumors.
abstract: In schwannomatosis, germline SMARCB1 mutations predispose to the development of multiple schwannomas, but not vestibular schwannomas. Many of these are missense or splice-site mutations or in-frame deletions, which are presumed to result in the synthesis of altered SMARCB1 proteins. However, also nonsense and frameshift mutations, which are characteristic for rhabdoid tumors and are predicted to result in the absence of SMARCB1 protein via nonsense-mediated mRNA decay, have been reported in schwannomatosis patients. We investigated the consequences of four of the latter mutations, i.e. c.30delC, c.34C>T, c.38delA, and c.46A>T, all in SMARCB1-exon 1. We could demonstrate for the c.30delC and c.34C>T mutations that the respective mRNAs were still present in the schwannomas of the patients. We hypothesized that these were prevented from degradation by translation reinitiation at the AUG codon encoding methionine at position 27 of the SMARCB1 protein. To test this, we expressed the mutations in MON cells, rhabdoid cells without endogenous SMARCB1 protein, and found that all four resulted in synthesis of the N-terminally truncated protein. Mutation of the reinitiation methionine codon into a valine codon prevented synthesis of the truncated protein, thereby confirming its identity. Immunohistochemistry with a SMARCB1 antibody revealed a mosaic staining pattern in schwannomas of the patients with the c.30delC and c.34C>T mutations. Our findings support the concept that, in contrast to the complete absence of SMARCB1 expression in rhabdoid tumors, altered SMARCB1 proteins with modified activity and reduced (mosaic) expression are formed in the schwannomas of schwannomatosis patients with a germline SMARCB1 mutation.
authors.affiliation: Department of Genome Analysis, Academic Medical Center, Room K2-216, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands, t.j.hulsebos@amc.uva.nl.
authors.forename: Theo J M
authors.initials: TJ
authors.lastname: Hulsebos
authors.forename: Susan
authors.initials: S
authors.lastname: Kenter
authors.forename: Wim I M
authors.initials: WI
authors.lastname: Verhagen
authors.forename: Frank
authors.initials: F
authors.lastname: Baas
authors.forename: Uta
authors.initials: U
authors.lastname: Flucke
authors.forename: Pieter
authors.initials: P
authors.lastname: Wesseling
mesh_terms: D045744:Cell Line, Tumor; D002868:Chromosomal Proteins, Non-Histone / Q000235:genetics* / Q000378:metabolism*; D004252:DNA Mutational Analysis; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000235:genetics; D006801:Humans; D008297:Male; D008875:Middle Aged; D016297:Mutagenesis, Site-Directed; D009154:Mutation / Q000235:genetics*; D009442:Neurilemmoma / Q000235:genetics*; D012333:RNA, Messenger / Q000378:metabolism; D018335:Rhabdoid Tumor / Q000235:genetics* / Q000473:pathology; D000071796:SMARCB1 Protein; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism*; D014162:Transfection
chemical_list: D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins; D012333:RNA, Messenger; D000071796:SMARCB1 Protein; C513266:SMARCB1 protein, human; D014157:Transcription Factors