G C A T T A C G G C A T genes Review Literature Review of BARD1 as a Cancer Predisposing Gene with a Focus on Breast and Ovarian Cancers 1,2,3, 1,2, 1,2 1,2,4, Wejdan M. Alenezi y , Caitlin T. Fierheller y, Neil Recio and Patricia N. Tonin * 1 Department of Human Genetics, McGill University, Montreal, QC H3A 0G4, Canada; [email protected] (W.M.A.); caitlin.fi[email protected] (C.T.F.); [email protected] (N.R.) 2 Cancer Research Program, The Research Institute of the McGill University Health Centre, Montreal, QC H4A 3J1, Canada 3 Department of Medical Laboratory Technology, Taibah University, Medina 42353, Saudi Arabia 4 Department of Medicine, McGill University, Montreal, QC H3A 0G4, Canada * Correspondence: [email protected]; Tel.: +1-514-934-1934 (ext. 44069) Contributed equally to the work. y Received: 30 June 2020; Accepted: 23 July 2020; Published: 27 July 2020 Abstract: Soon after the discovery of BRCA1 and BRCA2 over 20 years ago, it became apparent that not all hereditary breast and/or ovarian cancer syndrome families were explained by germline variants in these cancer predisposing genes, suggesting that other such genes have yet to be discovered. BRCA1-associated ring domain (BARD1), a direct interacting partner of BRCA1, was one of the earliest candidates investigated. Sequencing analyses revealed that potentially pathogenic BARD1 variants likely conferred a low–moderate risk to hereditary breast cancer, but this association is inconsistent. Here, we review studies of BARD1 as a cancer predisposing gene and illustrate the challenge of discovering additional cancer risk genes for hereditary breast and/or ovarian cancer. We selected peer reviewed research articles that focused on three themes: (i) sequence analyses of BARD1 to identify potentially pathogenic germline variants in adult hereditary cancer syndromes; (ii) biological assays of BARD1 variants to assess their effect on protein function; and (iii) association studies of BARD1 variants in family-based and case-control study groups to assess cancer risk. In conclusion, BARD1 is likely to be a low–moderate penetrance breast cancer risk gene. Keywords: BARD1; cancer predisposing gene; hereditary cancer syndromes; breast cancer; ovarian cancer; next-generation sequencing; multi-gene panel testing 1. Introduction After the discovery of BRCA1 and BRCA2, the highly penetrant breast (BC) and ovarian cancer (OC) predisposing genes, it became evident that about 20% of hereditary BC and/or OC syndrome families did not carry pathogenic variants in these genes. Under the assumption that other genes contribute risk to these cancers, research focused on identifying an additional major risk gene, namely “BRCAX”. Since the discovery of BRCA1 in 1994 [1], 27 cancer predisposing genes that confer significant lifetime risk to hereditary adult cancers have been discovered (reviewed in [2,3]). About half of these genes were discovered using genome-wide linkage analysis to identify candidate loci followed by sequencing candidate genes to identify pathogenic variants segregating in cancer families (reviewed in [2]). However, the rarity of large pedigrees of BRCA1 and BRCA2 mutation negative families and availability of family members suitable for linkage analyses posed considerable challenges for gene discovery projects. With knowledge about gene function, sequencing of candidate genes became a favoured strategy for gene discovery. Candidates can be selected based on having similar biological Genes 2020, 11, 856; doi:10.3390/genes11080856 www.mdpi.com/journal/genes Genes 2020, 11, 856 2 of 24 domains,Genes 2020, involvement11, x FOR PEER REVIEW in thesame biological pathway or biochemically interacting with known2 of 26 cancer predisposing gene(s). Using this candidate gene approach over 100 genes have been identified of which the majority have failed to be linked with hereditary cancer syndromes, including BC and/or OC,biological in independent pathway studiesor biochemically (reviewed interacting in [2,4]). with known cancer predisposing gene(s). Using this candidateThe BRCA1-associated gene approach over RING 100domain genes have 1 (BARD1 been identified) gene was of discovered which the majority in 1996 in have an effort failed to to understand be linked thewith biological hereditary function cancer of syndromes, BRCA1 protein including as shown BC and/or in the OC, timeline in independent of major discoveries studies (reviewed of BARD1 in(Figure [2,4]). 1). BARD1The directly BRCA1-associated interacts with RING BRCA1 domain via its 1 N-terminal (BARD1) RINGgene was (really discovered interesting in new1996gene) in an domain effort to [5 ]. understand the biological function of BRCA1 protein as shown in the timeline of major discoveries of As pathogenic BRCA1 variants located in the RING domain disrupt BARD1-BRCA1 interaction, it was BARD1 (Figure 1). BARD1 directly interacts with BRCA1 via its N-terminal RING (really interesting new hypothesized that variants in BARD1 may also affect this interaction, thereby making BARD1 an attractive gene) domain [5]. As pathogenic BRCA1 variants located in the RING domain disrupt BARD1-BRCA1 candidateinteraction, to pursueit was hypothesized in BRCA1 mutation that variants negative in cancerBARD1 families. may also Although affect this potentially interaction, pathogenic thereby makingBARD1 variantsBARD1 havean attractive been reported, candidate the to role pursue of BARD1 in BRCA1in cancer mutation predisposition negative cancer remains families. inconsistent. Although Here, potentially we aim topathogenic review what BARD1 is known variants about haveBARD1 been asreported, a candidate the role cancer of predisposingBARD1 in cancer gene predisposition and discuss challenges remains facinginconsistent. researchers Here, aiming we aim to to discover review what additional is known cancer about predisposing BARD1 as a candidate genes. We cancer focused predisposing on publications gene that:and (i)discuss described challenges the analyses facing researchers of potentially aiming pathogenic to discover germline additionalBARD1 cancervariants predisposing in adult genes. hereditary We cancerfocused syndromes; on publications (ii) BARD1 that: (i)in described vitro and thein cellulo analysesassays of potentially that assessed pathogenic the biological germline effect BARD1 of potentially variants pathogenicin adult hereditary variantson cancer protein syndromes; function; ( andii) BARD1 (iii) analyzed in vitro cancer and in risk cellulo associated assays that with assessed potentially the pathogenicbiological BARD1effect ofvariants potentially in family-based pathogenic variants and case-control on protein study function; groups. and (iii) analyzed cancer risk associated with potentially pathogenic BARD1 variants in family-based and case-control study groups. FigureFigure 1.1. TimelineTimeline ofof thethe majormajor discoveriesdiscoveries associated with BARD1 functionfunction and and risk risk to to breast breast and and ovarian ovarian cancercancer[ [1,5–13].1,5–13]. 2.2.BARD1 BARD1 asas a Candidate Cancer Cancer Predisposing Predisposing Gene Gene LittleLittle waswas knownknown aboutabout thethe biologicalbiological function of of BRCA1BRCA1 whenwhen it it was was first first reported reported as as a a cancer cancer predisposingpredisposing genegene otherother thanthan thethe predictedpredicted loss-of-functionloss-of-function variants variants suggested suggested that that it it may may behave behave as as tumourtumour suppressor suppressor genegene [[1]1].. ToTo learnlearn aboutabout BRCA1BRCA1 function,function, a yeastyeast two-hybrid screening screening system system was was usedused to to identify identify proteins proteins thatthat directlydirectly interactinteract withwith BRCA1BRCA1 and a proteinprotein was identifiedidentified that that interacts interacts directlydirectly withwith its N-terminal RING RING domain domain [5]. [ 5Repeat]. Repeatinging the assay the assay using using a human a human cell line, cell a similar line, a protein similar proteinwhich whichcontained contained a C-terminal a C-terminal motif with motif significan with significantt homology homology to the conserved to the conservedC-terminal C-terminalBRCA1 C terminus (BRCT) domains in BRCA1 and three tandem ankyrin (ANK) domains adjacent to the BRCT BRCA1 C terminus (BRCT) domains in BRCA1 and three tandem ankyrin (ANK) domains adjacent domains was identified. The BRCA1–BARD1 interaction was shown to be disrupted by BRCA1 missense to the BRCT domains was identified. The BRCA1–BARD1 interaction was shown to be disrupted by variants [5]. The formation of a stable BRCA1–BARD1 complex was considered to be critical for BRCA1 BRCA1 missense variants [5]. The formation of a stable BRCA1–BARD1 complex was considered to be function and thus BARD1 may play a role as an effector or regulator of BRCA1 [5]. This led to the criticalhypothesis for BRCA1 that variants function in BARD1 and thus could BARD1 encode may an aberrant play a role protein as an affecting effector the or interaction regulatorof with BRCA1 BRCA1 [5]. This led to the hypothesis that variants in BARD1 could encode an aberrant protein affecting the interaction with BRCA1 and predispose to BC and/or OC, especially in hereditary cancer syndrome familiesGenes 2020 not, 11,accounted x; doi: FOR PEER for REVIEW by BRCA1 or BRCA2 [6]. This study was the firstwww.mdpi.com/journal/genes report of molecular genetic analyses of BARD1 in cancer samples in 1998 [6] (Figure1). In a targeted