Male with an apparently normal phenotype carrying a BRCA1 exon 20 duplication in trans to a BRCA1 frameshift variant

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  • Ines Block
  • Àngels Mateu-Regué
  • Thi Tuyet Nhu Do
  • Ieva Miceikaite
  • Daniel Sdogati
  • Martin J. Larsen
  • Qin Hao
  • Henriette Roed Nielsen
  • Susanne E. Boonen
  • Anne Bine Skytte
  • Uffe Birk Jensen
  • Louise K. Høffding
  • Arcangela De Nicolo
  • Alessandra Viel
  • Emma Tudini
  • Michael T. Parsons
  • Torben A. Kruse
  • Amanda B. Spurdle
  • Mads Thomassen

Background: Reports of dual carriers of pathogenic BRCA1 variants in trans are extremely rare, and so far, most individuals have been associated with a Fanconi Anemia-like phenotype. Methods: We identified two families with a BRCA1 in-frame exon 20 duplication (Ex20dup). In one male individual, the variant was in trans with the BRCA1 frameshift variant c.2475delC p.(Asp825Glufs*21). We performed splicing analysis and used a transcription activation domain (TAD) assay to assess the functional impact of Ex20dup. We collected pedigrees and mapped the breakpoints of the duplication by long- and short-read genome sequencing. In addition, we performed a mitomycin C (MMC) assay from the dual carrier using cultured lymphoblastoid cells. Results: Genome sequencing and RNA analysis revealed the BRCA1 exon 20 duplication to be in tandem. The duplication was expressed without skipping any one of the two exon 20 copies, resulting in a lack of wild-type transcripts from this allele. TAD assay indicated that the Ex20dup variant has a functional level similar to the well-known moderate penetrant pathogenic BRCA1 variant c.5096G > A p.(Arg1699Gln). MMC assay of the dual carrier indicated a slightly impaired chromosomal repair ability. Conclusions: This is the first reported case where two BRCA1 variants with demonstrated functional impact are identified in trans in a male patient with an apparently normal clinical phenotype and no BRCA1-associated cancer. The results pinpoint a minimum necessary BRCA1 protein activity to avoid a Fanconi Anemia-like phenotype in compound heterozygous status and yet still predispose carriers to hormone-related cancers. These findings urge caution when counseling families regarding potential Fanconi Anemia risk. Furthermore, prudence should be taken when classifying individual variants as benign based on co-occurrence in trans with well-established pathogenic variants.

Original languageEnglish
Article number6
JournalBreast Cancer Research
Issue number1
Number of pages14
Publication statusPublished - 2024

Bibliographical note

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© 2024, The Author(s).

    Research areas

  • BRCA1, Dual carrier, Exon duplication, Fanconi Anemia, Transcription activation domain assay, Variant classification

ID: 379655683