Variant effect on splicing regulatory elements,branchpoint usage,and pseudoexonization: Strategies to enhance bioinformatic prediction using hereditary cancer genes as exemplars

It is possible to estimate the prior probability of pathogenicity for germline disease gene variants based on bioinformatic prediction of variant effect/s. However, routinely used approaches have likely led to the underestimation and underreporting of variants located outside donor and acceptor splice site motifs that affect messenger RNA (mRNA) processing. This review presents information about hereditary cancer gene germline variants, outside native splice sites, with experimentally validated splicing effects. We list 95 exonic variants that impact splicing regulatory elements (SREs) in BRCA1, BRCA2, MLH1, MSH2, MSH6, and PMS2. We utilized a pre‐existing large‐scale BRCA1 functional data set to map functional SREs, and assess the relative performance of different tools to predict effects of 283 variants on such elements. We also describe rare examples of intronic variants that impact branchpoint (BP) sites and create pseudoexons. We discuss the challenges in predicting variant effect on BP site usage and pseudoexonization, and suggest strategies to improve the bioinformatic prioritization of such variants for experimental validation. Importantly, our review and analysis highlights the value of considering impact of variants outside donor and acceptor motifs on mRNA splicing and disease causation.     

Next step in molecular genetics of hereditary breast/ovarian cancer: Multigene panel testing in clinical actionably genes and prioritization algorithms in the study of variants of uncertain significance

IntroductionBRCA1 and BRCA2 are the two main genes causing hereditary breast and ovarian cancer (HBOC). However, thanks to the development of Next Generation Sequencing (NGS), other genes linked to this syndrome (CHEK2, BRIP1, ATM and PALB2 among others) can be analysed.Material and methodsan analysis by multigene panel testing was performed in 138 index cases (ICs) from HBOC Spanish families with a previous non-informative result for BRCA1/2. The BRCA Hereditary Cancer Master? Plus kit, including 26 actionable and candidate genes related to HBOC was employed. Once classified, an algorithm was employed to prioritized those variants of unknown significance with a higher risk of having a deleterious effect. Moreover, a mRNA splicing assay was performed for the prioritized VUS c.3402+3A > C in ATM, located at intron 23.ResultsA total of 82 variants were found: 70 VUS and 12 pathogenic or probably pathogenic variants. The diagnostic yield in actionable genes non-BRCA was 7.97% of the total tested ICs. Overall, 19 VUS were prioritized, which meant 27% of the 70 total VUS. RNA analysis of the variant 3402+3A > C confirmed a deleterious impact on splicing.DiscussionThe implementation of a multigene panel in HBOC studied families improved the diagnostic yield, concordant with results obtained in previous publications. Due to the important number of VUS obtained in NGS, the application of a prioritization algorithm is needed in order to select those variants in which it is necessary to conduct further studies.     

Functional Analysis of a Large set of BRCA2 exon 7 Variants Highlights the Predictive Value of Hexamer Scores in Detecting Alterations of Exonic Splicing Regulatory Elements

Exonic variants can alter pre‐mRNA splicing either by changing splice sites or by modifying splicing regulatory elements. Often these effects are difficult to predict and are only detected by performing RNA analyses. Here, we analyzed, in a minigene assay, 26 variants identified in the exon 7 of BRCA2, a cancer predisposition gene. Our results revealed eight new exon skipping mutations in this exon: one directly altering the 5′ splice site and seven affecting potential regulatory elements. This brings the number of splicing regulatory mutations detected in BRCA2 exon 7 to a total of 11, a remarkably high number considering the total number of variants reported in this exon (n = 36), all tested in our minigene assay. We then exploited this large set of splicing data to test the predictive value of splicing regulator hexamers’ scores recently established by Ke et al. ( 2011 ). Comparisons of hexamer‐based predictions with our experimental data revealed high sensitivity in detecting variants that increased exon skipping, an important feature for prescreening variants before RNA analysis. In conclusion, hexamer scores represent a promising tool for predicting the biological consequences of exonic variants and may have important applications for the interpretation of variants detected by high‐throughput sequencing.     

The role of targeted BRCA1/BRCA2 mutation analysis in hereditary breast/ovarian cancer families of Portuguese ancestry

We report the analysis of altogether 1050 suspected hereditary breast/ovarian cancer (HBOC) families, 524 fully screened for BRCA1/BRCA2 mutations and 526 tested only for the most common mutations. Of the 119 families with pathogenic mutations, 40 (33.6%) had the BRCA2 c.156_157insAlu rearrangement and 15 (12.6%) the BRCA1 c.3331_3334del mutation, the former being specific of Portuguese ancestry and the latter showing a founder effect in Portugal. Interestingly, the two most common mutations were found in a significant proportion of the HBOC families with an a priori BRCAPRO mutation probability <10%. We recommend that all suspected HBOC families from Portugal or with Portuguese ancestry, even those fulfilling moderately stringent clinical‐criteria for genetic testing, should be specifically analyzed for the two most common BRCA1/BRCA2 founder mutations, and we here present a simple method for this first tier test. Screening of the entire coding regions of BRCA1 and BRCA2 should subsequently be offered to those families with a mutation probability ≥10% if none of those founder mutations are found.     

Functional Analysis of BARD1 Missense Variants in Homology‐Directed Repair of DNA Double Strand Breaks

Genes associated with hereditary breast and ovarian cancer (HBOC) are often sequenced in search of mutations that are predictive of susceptibility to these cancer types, but the sequence results are frequently ambiguous because of the detection of missense substitutions for which the clinical impact is unknown. The BARD1 protein is the heterodimeric partner of BRCA1 and is included on clinical gene panels for testing for susceptibility to HBOC. Like BRCA1, it is required for homology‐directed DNA repair (HDR). We measured the HDR function of 29 BARD1 missense variants, 27 culled from clinical test results and two synthetic variants. Twenty‐three of the assayed variants were functional for HDR; of these, four are known neutral variants. Three variants showed intermediate function, and three others were defective in HDR. When mapped to BARD1 domains, residues crucial for HDR were located in the N‐ and C‐ termini of BARD1. In the BARD1 RING domain, critical residues mapped to the zinc‐coordinating amino acids and to the BRCA1‐BARD1 binding interface, highlighting the importance of interaction between BRCA1 and BARD1 for HDR activity. Based on these results, we propose that the HDR assay is a useful complement to genetic analyses to classify BARD1 variants of unknown clinical significance.     

Next-generation sequencing for the diagnosis of hereditary breast and ovarian cancer using genomic capture targeting multiple candidate genes

To optimize the molecular diagnosis of hereditary breast and ovarian cancer (HBOC), we developed a next-generation sequencing (NGS)-based screening based on the capture of a panel of genes involved, or suspected to be involved in HBOC, on pooling of indexed DNA and on paired-end sequencing in an Illumina GAIIx platform, followed by confirmation by Sanger sequencing or MLPA/QMPSF. The bioinformatic pipeline included CASAVA, NextGENe, CNVseq and Alamut-HT. We validated this procedure by the analysis of 59 patients'' DNAs harbouring SNVs, indels or large genomic rearrangements of BRCA1 or BRCA2. We also conducted a blind study in 168 patients comparing NGS versus Sanger sequencing or MLPA analyses of BRCA1 and BRCA2. All mutations detected by conventional procedures were detected by NGS. We then screened, using three different versions of the capture set, a large series of 708 consecutive patients. We detected in these patients 69 germline deleterious alterations within BRCA1 and BRCA2, and 4 TP53 mutations in 468 patients also tested for this gene. We also found 36 variations inducing either a premature codon stop or a splicing defect among other genes: 5/708 in CHEK2, 3/708 in RAD51C, 1/708 in RAD50, 7/708 in PALB2, 3/708 in MRE11A, 5/708 in ATM, 3/708 in NBS1, 1/708 in CDH1, 3/468 in MSH2, 2/468 in PMS2, 1/708 in BARD1, 1/468 in PMS1 and 1/468 in MLH3. These results demonstrate the efficiency of NGS in performing molecular diagnosis of HBOC. Detection of mutations within other genes than BRCA1 and BRCA2 highlights the genetic heterogeneity of HBOC.     

Exceptions to the rule: Case studies in the prediction of pathogenicity for genetic variants in hereditary cancer genes

Based on current consensus guidelines and standard practice, many genetic variants detected in clinical testing are classified as disease causing based on their predicted impact on the normal expression or function of the gene in the absence of additional data. However, our laboratory has identified a subset of such variants in hereditary cancer genes for which compelling contradictory evidence emerged after the initial evaluation following the first observation of the variant. Three representative examples of variants in BRCA1, BRCA2 and MSH2 that are predicted to disrupt splicing, prematurely truncate the protein, or remove the start codon were evaluated for pathogenicity by analyzing clinical data with multiple classification algorithms. Available clinical data for all three variants contradicts the expected pathogenic classification. These variants illustrate potential pitfalls associated with standard approaches to variant classification as well as the challenges associated with monitoring data, updating classifications, and reporting potentially contradictory interpretations to the clinicians responsible for translating test outcomes to appropriate clinical action. It is important to address these challenges now as the model for clinical testing moves toward the use of large multi‐gene panels and whole exome/genome analysis, which will dramatically increase the number of genetic variants identified.     

Gene‐panel testing of breast and ovarian cancer patients identifies a recurrent RAD51C duplication

Gene‐panel sequencing allows comprehensive analysis of multiple genes simultaneously and is now routinely used in clinical mutation testing of high‐risk breast and ovarian cancer patients. However, only BRCA1 and BRCA2 are often analyzed also for large genomic changes. Here, we have analyzed 10 clinically relevant susceptibility genes in 95 breast or ovarian cancer patients with gene‐panel sequencing including also copy number variants (CNV) analysis for genomic changes. We identified 12 different pathogenic BRCA1, BRCA2, TP53, PTEN, CHEK2, or RAD51C mutations in 18 of 95 patients (19%). BRCA1/2 mutations were observed in 8 patients (8.4%) and CHEK2 protein‐truncating mutations in 7 patients (7.4%). In addition, we identified a novel duplication encompassing most of the RAD51C gene. We further genotyped the duplication in breast or ovarian cancer families (n = 1149), in unselected breast (n = 1729) and ovarian cancer cohorts (n = 553), and in population controls (n = 1273). Seven additional duplication carries were observed among cases but none among controls. The duplication associated with ovarian cancer risk (3/590 of all ovarian cancer patients, 0.5%, P = .032 compared with controls) and was found to represent a large fraction of all identified RAD51C mutations in the Finnish population. Our data emphasizes the importance of comprehensive mutation analysis including CNV detection in all the relevant genes.     

Biochemical and structural characterization of two variants of uncertain significance in the PMS2 gene

Lynch syndrome (LS) is an autosomal dominant inherited disorder that is associated with an increased predisposition to certain cancers caused by loss‐of‐function mutations in one of four DNA mismatch repair (MMR) genes (MLH1, MSH2, MSH6, or PMS2). The diagnosis of LS is often challenged by the identification of missense mutations where the functional effects are not known. These are termed variants of uncertain significance (VUSs) and account for 20%–30% of noncoding and missense mutations. VUSs cause ambiguity during clinical diagnosis and hinder implementation of appropriate medical management. In the current study, we focus on the functional and biological consequences of two nonsynonymous VUSs in PMS2. These variants, c.620G>A and c.123_131delGTTAGTAGA, result in the alteration of glycine 207 to glutamate (p.Gly207Glu) and the deletion of amino acid residues 42–44 (p.Leu42_Glu44del), respectively. While the PMS2 p.Gly207Glu variant retains in vitro MMR and ATPase activities, PMS2 p.Leu42_Glu44del appears to lack such capabilities. Structural and biophysical characterization using circular dichroism, small‐angle X‐ray scattering, and X‐ray crystallography of the N‐terminal domain of the PMS2 variants indicate that the p.Gly207Glu variant is properly folded similar to the wild‐type enzyme, whereas p.Leu42_Glu44del is disordered and prone to aggregation.     

Calibration of Multiple In Silico Tools for Predicting Pathogenicity of Mismatch Repair Gene Missense Substitutions

Classification of rare missense substitutions observed during genetic testing for patient management is a considerable problem in clinical genetics. The Bayesian integrated evaluation of unclassified variants is a solution originally developed for BRCA1/2. Here, we take a step toward an analogous system for the mismatch repair (MMR) genes (MLH1, MSH2, MSH6, and PMS2) that confer colon cancer susceptibility in Lynch syndrome by calibrating in silico tools to estimate prior probabilities of pathogenicity for MMR gene missense substitutions. A qualitative five‐class classification system was developed and applied to 143 MMR missense variants. This identified 74 missense substitutions suitable for calibration. These substitutions were scored using six different in silico tools (Align‐Grantham Variation Grantham Deviation, multivariate analysis of protein polymorphisms [MAPP], MutPred, PolyPhen‐2.1, Sorting Intolerant From Tolerant, and Xvar), using curated MMR multiple sequence alignments where possible. The output from each tool was calibrated by regression against the classifications of the 74 missense substitutions; these calibrated outputs are interpretable as prior probabilities of pathogenicity. MAPP was the most accurate tool and MAPP + PolyPhen‐2.1 provided the best‐combined model (R² = 0.62 and area under receiver operating characteristic = 0.93). The MAPP + PolyPhen‐2.1 output is sufficiently predictive to feed as a continuous variable into the quantitative Bayesian integrated evaluation for clinical classification of MMR gene missense substitutions.     

Landscape of pathogenic variations in a panel of 34 genes and cancer risk estimation from 5131 HBOC families

PurposeIntegration of gene panels in the diagnosis of hereditary breast and ovarian cancer (HBOC) requires a careful evaluation of the risk associated with pathogenic or likely pathogenic variants (PVs) detected in each gene. Here we analyzed 34 genes in 5131 suspected HBOC index cases by next-generation sequencing.MethodsUsing the Exome Aggregation Consortium data sets plus 571 individuals from the French Exome Project, we simulated the probability that an individual from the Exome Aggregation Consortium carries a PV and compared it to the estimated frequency within the HBOC population.ResultsOdds ratio conferred by PVs within BRCA1, BRCA2, PALB2, RAD51C, RAD51D, ATM, BRIP1, CHEK2, and MSH6 were estimated at 13.22 [10.01–17.22], 8.61 [6.78–10.82], 8.22 [4.91–13.05], 4.54 [2.55–7.48], 5.23 [1.46–13.17], 3.20 [2.14–4.53], 2.49 [1.42–3.97], 1.67 [1.18–2.27], and 2.50 [1.12–4.67], respectively. PVs within RAD51C, RAD51D, and BRIP1 were associated with ovarian cancer family history (OR = 11.36 [5.78–19.59], 12.44 [2.94–33.30] and 3.82 [1.66–7.11]). PALB2 PVs were associated with bilateral breast cancer (OR = 16.17 [5.48–34.10]) and BARD1 PVs with triple-negative breast cancer (OR = 11.27 [3.37–25.01]). Burden tests performed in both patients and the French Exome Project population confirmed the association of PVs of BRCA1, BRCA2, PALB2, and RAD51C with HBOC.ConclusionOur results validate the integration of PALB2, RAD51C, and RAD51D in the diagnosis of HBOC and suggest that the other genes are involved in an oligogenic determinism.