Diagnostic mRNA splicing assay for variants in BRCA1 and BRCA2 identified two novel pathogenic splicing aberrations

Cancer predisposition in pediatric neuro-oncology-practical approaches and ethical considerations

A genetic predisposition to tumor development can be identified in up to 10% of pediatric patients with central nervous system (CNS) tumors. For some entities, the rate of an underlying predisposition is even considerably higher. In recent years, population-based approaches have helped to further delineate the role of cancer predisposition in pediatric oncology. Investigations for cancer predisposition syndrome (CPS) can be guided by clinical signs and family history leading to directed testing of specific genes. The increasingly adopted molecular analysis of tumor and often parallel blood samples with multi-gene panel, whole-exome, or whole-genome sequencing identifies additional patients with or without clinical signs. Diagnosis of a genetic predisposition may put an additional burden on affected families. However, information on a given cancer predisposition may be critical for the patient as potentially influences treatment decisions and may offer the patient and healthy carriers the chance to take part in intensified surveillance programs aiming at early tumor detection. In this review, we discuss some of the practical and ethical challenges resulting from the widespread use of new diagnostic techniques and the most important CPS that may manifest with brain tumors in childhood.

Imprecise Medicine: BRCA2 Variants of Uncertain Significance (VUS), the Challenges and Benefits to Integrate a Functional Assay Workflow with Clinical Decision Rules

Pathological mutations in homology-directed repair (HDR) genes impact both future cancer risk and therapeutic options for patients. HDR is a high-fidelity DNA repair pathway for resolving DNA double-strand breaks throughout the genome. BRCA2 is an essential protein that mediates the loading of RAD51 onto resected DNA breaks, a key step in HDR. Germline mutations in BRCA2 are associated with an increased risk for breast, ovarian, prostate, and pancreatic cancer. Clinical findings of germline or somatic BRCA2 mutations in tumors suggest treatment with platinum agents or PARP inhibitors. However, when genetic analysis reveals a variant of uncertain significance (VUS) in the BRCA2 gene, precision medicine-based decisions become complex. VUS are genetic changes with unknown pathological impact. Current statistics indicate that between 10–20% of BRCA sequencing results are VUS, and of these, more than 50% are missense mutations. Functional assays to determine the pathological outcome of VUS are urgently needed to provide clinical guidance regarding cancer risk and treatment options. In this review, we provide a brief overview of BRCA2 functions in HDR, describe how BRCA2 VUS are currently assessed in the clinic, and how genetic and biochemical functional assays could be integrated into the clinical decision process. We suggest a multi-step workflow composed of robust and accurate functional assays to correctly evaluate the potential pathogenic or benign nature of BRCA2 VUS. Success in this precision medicine endeavor will offer actionable information to patients and their physicians.

Interpretation of BRCA2 Splicing Variants: A Case Series of Challenging Variant Interpretations and the Importance of Functional RNA Analysis

A substantial proportion of pathogenic variants associated with an increased risk of hereditary cancer are sequence variants affecting RNA splicing. The classification of these variants can be complex when both non-functional and functional transcripts are produced from the variant allele. We present four BRCA2 splice site variants with complex variant interpretations (BRCA2 c.68-3T>G, c.68-2A>G, c.425G>T, c.8331+2T>C). Evidence supporting a pathogenic classification is available for each variant, including in silico models, absence in population databases, and published functional data. However, comprehensive RNA analysis showed that some functional transcript may be produced by each variant. BRCA2 c.68-3T>G results in a partial splice defect. For BRCA2 c.68-2A>G and c.425G>T, aberrant splicing was shown to produce a potentially functional, in-frame transcript. BRCA2 c.8331+2T>C may utilize a functional GC donor in place of the wild-type GT donor. The severity of cancer history for carriers of these variants was also assessed using a history weighting algorithm and was not consistent with pathogenic controls (carriers of known pathogenic variants in BRCA2). Due to the conflicting evidence, our laboratory classifies these BRCA2 variants as variants of uncertain significance. This highlights the importance of evaluating new and existing evidence to ensure accurate variant classification and appropriate patient care.

Functional Categorization of BRCA1 Variants of Uncertain Clinical Significance in Homologous Recombination Repair Complementation Assays

PURPOSE

Because BRCA1 is a high-risk breast/ovarian cancer susceptibility gene, BRCA1 sequence variants of uncertain clinical significance (VUS) complicate genetic counseling. As most VUS are rare, reliable classification based on clinical and genetic data is often impossible. However, all pathogenic BRCA1 variants analyzed result in defective homologous recombination DNA repair (HRR). Thus, BRCA1 VUS may be categorized based on their functional impact on this pathway.

EXPERIMENTAL DESIGN

Two hundred thirty-eight BRCA1 VUS-comprising most BRCA1 VUS known in the Netherlands and Belgium-were tested for their ability to complement Brca1-deficient mouse embryonic stem cells in HRR, using cisplatin and olaparib sensitivity assays and a direct repeat GFP (DR-GFP) HRR assay. Assays were validated using 25 known benign and 25 known pathogenic BRCA1 variants. For assessment of pathogenicity by a multifactorial likelihood analysis method, we collected clinical and genetic data for functionally deleterious VUS and VUS occurring in three or more families.

RESULTS

All three assays showed 100% sensitivity and specificity (95% confidence interval, 83%-100%). Out of 238 VUS, 45 showed functional defects, 26 of which were deleterious in all three assays. For 13 of these 26 variants, we could calculate the probability of pathogenicity using clinical and genetic data, resulting in the identification of 7 (likely) pathogenic variants.

CONCLUSIONS

We have functionally categorized 238 BRCA1 VUS using three different HRR-related assays. Classification based on clinical and genetic data alone for a subset of these variants confirmed the high sensitivity and specificity of our functional assays.

The intronic BRCA1 c.5407-25T>A variant causing partly skipping of exon 23-a likely pathogenic variant with reduced penetrance?

Rare sequence variants in the non-coding part of the BRCA genes are often reported as variants of uncertain significance (VUS), which leave patients and doctors in a challenging position. The aim of this study was to determine the pathogenicity of the BRCA1 c.5407-25T>A variant found in 20 families from Norway, France and United States with suspected hereditary breast and ovarian cancer. This was done by combining clinical and family information with allele frequency data, and assessment of the variant’s effect on mRNA splicing. Mean age at breast (n = 12) and ovarian (n = 11) cancer diagnosis in female carriers was 49.9 and 60.4 years, respectively. The mean Manchester score in the 20 families was 16.4. The allele frequency of BRCA1 c.5407-25T>A was 1/64,566 in non-Finnish Europeans (gnomAD database v2.1.1). We found the variant in 1/400 anonymous Norwegian blood donors and 0/784 in-house exomes. Sequencing of patient-derived cDNA from blood, normal breast and ovarian tissue showed that BRCA1 c.5407-25T>A leads to skipping of exon 23, resulting in frameshift and protein truncation: p.(Gly1803GlnfsTer11). Western blot analysis of transiently expressed BRCA1 proteins in HeLa cells showed a reduced amount of the truncated protein compared with wild type. Noteworthily, we found that a small amount of full-length transcript was also generated from the c.5407-25T>A allele, potentially explaining the intermediate cancer burden in families carrying this variant. In summary, our results show that BRCA1 c.5407-25T>A leads to partial skipping of exon 23, and could represent a likely pathogenic variant with reduced penetrance.