Processed Transcript Insertion as a Novel Germline Mutational Mechanism in BRCA1-Associated Hereditary Breast Cancer.

Area of Science:

• Genetics
• Molecular Biology
• Oncology

Background:

• Germline BRCA1 mutations are key drivers of hereditary breast and ovarian cancer (HBOC).
• Pathogenic variants include small sequence changes and structural rearrangements.
• Current diagnostic methods often fail to detect complex structural variants.

Purpose of the Study:

• To identify and characterize novel pathogenic mechanisms in hereditary breast cancer.
• To expand the known mutational spectrum of BRCA1.
• To highlight limitations in conventional diagnostic workflows.

Main Methods:

• Next-generation sequencing (NGS) with a custom hereditary cancer panel.
• Validation using orthogonal sequencing and multiplex ligation-dependent probe amplification (MLPA).
• RNA-level functional assays (e.g., nonsense-mediated decay inhibition) and constitutional origin confirmation.

Main Results:

• A 700 bp insertion of an RPL18A processed transcript was identified in BRCA1 exon 16.
• The insertion resulted in a frameshift, premature stop codon, and transcript degradation.
• The variant was confirmed as heritable and showed loss of heterozygosity (LOH) in tumor tissue.

Conclusions:

• This study reports the first instance of a heritable processed transcript insertion as a pathogenic BRCA1 event.
• Such complex variants are undetectable by standard genetic testing lacking structural variant analysis.
• Comprehensive diagnostic approaches are crucial for accurate genetic counseling in hereditary cancer syndromes.