Clinical consequences of BRCA2 hypomorphism

Prognostic significance of mutation type and chromosome fragility in Fanconi anemia

Fanconi anemia (FA) is a rare genetic disease characterized by high phenotypic and genotypic heterogeneity, and extreme chromosome fragility. To better understand the natural history of FA, identify genetic risk and prognostic factors, and develop novel therapeutic strategies, the Spanish Registry of Patients with FA collects data on clinical features, chromosome fragility, genetic subtypes, and DNA sequencing with informed consent of participating individuals. In this article, we describe the clinical evolution of 227 patients followed up for up to 30 years, for whom our data indicate a cumulative cancer incidence of 86% by age 50. We found that patients with lower chromosome fragility had a milder malformation spectrum and better outcomes in terms of later-onset hematologic impairment, less severe bone marrow failure, and lower cancer risk. We also found that outcomes were better for patients with mutations leading to mutant FANCA protein expression (genetic hypomorphism) than for patients lacking this protein. Likewise, prognosis was consistently better for patients with biallelic mutations in FANCD2 (mainly hypomorphic mutations) than for patients with biallelic mutations in FANCA and FANCG, with the lack of the mutant protein in patients with biallelic mutations in FANCG contributing to their poorer outcomes. Our results regarding the clinical impact of chromosome fragility and genetic hypomorphism suggest that mutant FA proteins retain residual activity. This finding should encourage the development of novel therapeutic strategies aimed at partially or fully enhancing mutant FA function, thereby preventing or delaying bone marrow failure and cancer in patients with FA. Clinical Trial Registration number: NCT06490510.

Finding significance: New perspectives in variant classification of the RAD51 regulators, BRCA2 and beyond

For many individuals harboring a variant of uncertain functional significance (VUS) in a homologous recombination (HR) gene, their risk of developing breast and ovarian cancer is unknown. Integral to the process of HR are BRCA1 and regulators of the central HR protein, RAD51, including BRCA2, PALB2, RAD51C and RAD51D. Due to advancements in sequencing technology and the continued expansion of cancer screening panels, the number of VUS identified in these genes has risen significantly. Standard practices for variant classification utilize different types of predictive, population, phenotypic, allelic and functional evidence. While variant analysis is improving, there remains a struggle to keep up with demand. Understanding the effects of an HR variant can aid in preventative care and is critical for developing an effective cancer treatment plan. In this review, we discuss current perspectives in the classification of variants in the breast and ovarian cancer genes BRCA1, BRCA2, PALB2, RAD51C and RAD51D.

BRCA2 Haploinsufficiency in Telomere Maintenance

Our previous studies showed an association between monoallelic BRCA2 germline mutations and dysfunctional telomeres in epithelial mammary cell lines and increased risk of breast cancer diagnosis for women with BRCA2 999del5 germline mutation and short telomeres in blood cells. In the current study, we analyzed telomere dysfunction in lymphoid cell lines from five BRCA2 999del5 mutation carriers and three Fanconi Anemia D1 patients by fluorescence in situ hybridization (FISH). Metaphase chromosomes were harvested from ten lymphoid cell lines of different BRCA2 genotype origin and analyzed for telomere loss (TL), multitelomeric signals (MTS), interstitial telomere signals (ITS) and extra chromosomal telomere signals (ECTS). TL, ITS and ECTS were separately found to be significantly increased gradually between the BRCA2^+/+, BRCA2^+/- and BRCA2^-/- lymphoid cell lines. MTS were found to be significantly increased between the BRCA2^+/+ and the BRCA2^+/- heterozygous (p < 0.0001) and the BRCA2^-/- lymphoid cell lines (p < 0.0001) but not between the BRCA2 mutated genotypes. Dysfunctional telomeres were found to be significantly increased in a stepwise manner between the BRCA2 genotypes indicating an effect of BRCA2 haploinsufficiency on telomere maintenance.