Minigene-based splicing analysis and ACMG/AMP-based tentative classification of 56 ATM variants

TP53 minigene analysis of 161 sequence changes provides evidence for role of spatial constraint and regulatory elements on variant-induced splicing impact

We investigated the role of TP53 splicing regulatory elements (SREs) using exons 3 and 6 and their downstream introns as models. Minigene microdeletion assays revealed four SRE-rich intervals: c.573_598, c.618_641, c.653_669 and c.672+14_672 + 36. A diagnostically reported deletion c.655_670del, overlapping an SRE-rich interval, induced an in-frame transcript Δ(E6q21) from new donor site usage. Deletion of at least four intron 6 G-runs led to 100% aberrant transcript expression. Additionally, assay results suggested a donor-to-branchpoint distance <50 nt for complete splicing aberration due to spatial constraint, and >75 nt for low risk of splicing abnormality. Overall, splicing data for 134 single nucleotide variants (SNVs) and 27 deletions in TP53 demonstrated that SRE-disrupting SNVs have weak splicing impact (up to 26% exon skipping), while deletions spanning multiple SREs have profound splicing effects. Our findings may prove relevant for identifying novel germline TP53 variants causing hereditary cancer predisposition and/or somatic variants contributing to tumorigenesis.

Comprehensive functional splicing analysis of non-canonical CNGB3 variants using in vitro minigene splice assays

Variants in the CNGB3 gene, encoding the B3-subunit of the cone photoreceptor cyclic nucleotide gated channel, are a major cause of autosomal recessive achromatopsia, a rare inherited retinal disease. The mutation spectrum of achromatopsia-associated CNGB3 variants comprises all types of mutations, including those that are straightforward to evaluate in molecular genetic diagnostics, such as frame-shifting, nonsense, and canonical splice site variants. Additionally, variants have been identified within splice regions outside the conserved ±1,2 splice site dinucleotides, making their potential impact on disease association challenging to interpret. This poses a major hurdle for clinical interpretation of causality between the patient's genotype and the proposed clinical diagnosis, but also for the inclusion of such patients into clinical trials for gene augmentation therapy, for which only patients with confirmed (likely) pathogenic CNGB3 variants are eligible. We here performed comprehensive genetic functional analysis of 21 candidate spliceogenic CNGB3 variants-15 reported and 6 novel variants-by means of in vitro minigene splice assays and cDNA analysis, and characterization of spliceogenic events by subcloning, Sanger-sequencing, and capillary fragment analysis. For 16 variants, an impact on splicing was confirmed, supporting the reclassification of 86% of variants of uncertain significance as likely pathogenic or pathogenic according to the ACMG/AMP guidelines. This reclassification enables the confirmation of patients' genotypes, both retrospectively and prospectively. © 2025 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Interpreting Variants of Uncertain Significance in PCD: Abnormal Splicing Caused by a Missense Variant of DNAAF3

BACKGROUND

Primary ciliary dyskinesia (PCD) is a rare autosomal recessive disorder characterized by dysfunction of motile cilia. While approximately 50 genes have been identified, around 25% of PCD patients remain genetically unexplained; elucidating the pathogenicity of specific variants remains a challenge.

METHODS

Whole exome sequencing (WES) and Sanger sequencing were conducted to identify potential pathogenic variants of PCD. Minigene assays were performed to evaluate the pathogenicity of variants. Transmission electron microscopy (TEM) and high-speed video analysis (HSVA) were conducted to analyze the function of cilia in respiratory epithelial cells.

RESULTS

We identified two variants of DNAAF3: c.557G>A, p.G186E in exon 5, and c.1364G>A, p.G455D at the terminal nucleotide of exon 10 in a 16-year-old male patient. Through a minigene assay, we demonstrated that the c.1364G>A variant led to a four-nucleotide skipping. The cilia in epithelial ciliary cells of the proband were almost immotile. The absence of outer dynein arms and inner dynein arms was also observed.

CONCLUSIONS

Our study identified two compound heterozygous variants of DNAAF3, a pathogenic gene for PCD, and proved that a novel missense variant c.1364G>A affects splicing. Our findings not only expanded the spectrum of mutations in the DNAAF3 gene but also highlighted the importance of investigating variants of uncertain significance (VUS) for comprehensive genetic diagnoses.

Splicing Dysregulation of Non-Canonical GC-5' Splice Sites of Breast Cancer Susceptibility Genes ATM and PALB2

Background/Objectives: The non-canonical GC-5' splice sites (5'ss) are the most common exception (~1%) to the classical GT/AG splicing rule. They constitute weak 5'ss and can be regulated by splicing factors, so they are especially sensitive to genetic variations inducing the misrecognition of their respective exons. We aimed to investigate the GC-5'ss of the breast/ovarian cancer susceptibility genes, ATM (exon 50), BRIP1 (exon 1), and PALB2 (exon 12), and their dysregulation induced by DNA variants. Methods: Splicing assays of the minigenes, mgATM_49-52, mgBRIP1_1-2, and mgPALB2_5-12, were conducted to study the regulation of the indicated GC-5'ss. Results: A functional map of the splicing regulatory elements (SRE) formed by overlapping exonic microdeletions revealed three essential intervals, ATM c.7335_7344del, PALB2 c.3229_3258del, and c.3293_3322del, which are likely targets for spliceogenic SRE-variants. We then selected 14 ATM and 9 PALB2 variants (Hexplorer score < -40) located at these intervals that were assayed in MCF-7 cells. Nine ATM and three PALB2 variants affected splicing, impairing the recognition of exons 50 and 12, respectively. Therefore, these variants likely disrupt the active SREs involved in the inclusion of both exons in the mature mRNA. DeepCLIP predictions suggested the participation of several splicing factors in exon recognition, including SRSF1, SRSF2, and SRSF7, involved in the recognition of other GC sites. The ATM spliceogenic variants c.7336G>T (p.(Glu2446Ter)) and c.7340T>A (p.(Leu2447Ter)) produced significant amounts of full-length transcripts (55-59%), which include premature termination stop codons, so they would inactivate ATM through both splicing disruption and protein truncation mechanisms. Conclusions: ATM exon 50 and PALB2 exon 12 require specific sequences for efficient recognition by the splicing machinery. The mapping of SRE-rich intervals in minigenes is a valuable approach for the identification of spliceogenic variants that outperforms any prediction software. Indeed, 12 spliceogenic SRE-variants were identified in the critical intervals.

Unraveling noncoding DNA variants and epimutations: a paradigm shift in hereditary cancer research

Exhaustive efforts have been dedicated to uncovering genomic aberrations linked to cancer susceptibility. Noncoding sequence variants and epigenetic alterations significantly influence gene regulation and could contribute to cancer development. However, exploring noncoding regions in hereditary cancer susceptibility demands cutting-edge methodologies for functionally characterizing genomic discoveries. Additionally, comprehending the impact on cancer development of variants in noncoding DNA and the epigenome necessitates integrating diverse data through bioinformatic analyses. As novel technologies and analytical methods continue to advance, this realm of research is rapidly gaining traction. Within this mini-review, we delve into future research domains concerning aberrations in noncoding DNA regions, such as pseudoexons, promoter variants and cis-epimutations.

Comprehensive splicing analysis of the alternatively spliced CHEK2 exons 8 and 10 reveals three enhancer/silencer-rich regions and 38 spliceogenic variants

Splicing is controlled by a large set of regulatory elements (SREs) including splicing enhancers and silencers, which are involved in exon recognition. Variants at these motifs may dysregulate splicing and trigger loss-of-function transcripts associated with disease. Our goal here was to study the alternatively spliced exons 8 and 10 of the breast cancer susceptibility gene CHEK2. For this purpose, we used a previously published minigene with exons 6-10 that produced the expected minigene full-length transcript and replicated the naturally occurring events of exon 8 [Δ(E8)] and exon 10 [Δ(E10)] skipping. We then introduced 12 internal microdeletions of exons 8 and 10 by mutagenesis in order to map SRE-rich intervals by splicing assays in MCF-7 cells. We identified three minimal (10-, 11-, 15-nt) regions essential for exon recognition: c.863_877del [ex8, Δ(E8): 75%] and c.1073_1083del and c.1083_1092del [ex10, Δ(E10): 97% and 62%, respectively]. Then 87 variants found within these intervals were introduced into the wild-type minigene and tested functionally. Thirty-eight of them (44%) impaired splicing, four of which (c.883G>A, c.883G>T, c.884A>T, and c.1080G>T) induced negligible amounts (<5%) of the minigene full-length transcript. Another six variants (c.886G>A, c.886G>T, c.1075G>A, c.1075G>T, c.1076A>T, and c.1078G>T) showed significantly strong impacts (20-50% of the minigene full-length transcript). Thirty-three of the 38 spliceogenic variants were annotated as missense, three as nonsense, and two as synonymous, underlying the fact that any exonic change is capable of disrupting splicing. Moreover, c.883G>A, c.883G>T, and c.884A>T were classified as pathogenic/likely pathogenic variants according to ACMG/AMP (American College of Medical Genetics and Genomics/Association for Molecular Pathology)-based criteria. © 2024 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Systematic Minigene-Based Splicing Analysis and Tentative Clinical Classification of 52 CHEK2 Splice-Site Variants

BACKGROUND

Disrupted pre-mRNA splicing is a frequent deleterious mechanism in hereditary cancer. We aimed to functionally analyze candidate spliceogenic variants of the breast cancer susceptibility gene CHEK2 by splicing reporter minigenes.

METHODS

A total of 128 CHEK2 splice-site variants identified in the Breast Cancer After Diagnostic Gene Sequencing (BRIDGES) project (https://cordis.europa.eu/project/id/634935) were analyzed with MaxEntScan and subsetted to 52 variants predicted to impact splicing. Three CHEK2 minigenes, which span all 15 exons, were constructed and validated. The 52 selected variants were then genetically engineered into the minigenes and assayed in MCF-7 (human breast adenocarcinoma) cells.

RESULTS

Of 52 variants, 46 (88.5%) impaired splicing. Some of them led to complex splicing patterns with up to 11 different transcripts. Thirty-four variants induced splicing anomalies without any trace or negligible amounts of the full-length transcript. A total of 89 different transcripts were annotated, which derived from different events: single- or multi-exon skipping, alternative site-usage, mutually exclusive exon inclusion, intron retention or combinations of the abovementioned events. Fifty-nine transcripts were predicted to introduce premature termination codons, 7 kept the original open-reading frame, 5 removed the translation start codon, 6 affected the 5'UTR (Untranslated Region), and 2 included missense variations. Analysis of variant c.684-2A > G revealed the activation of a non-canonical TG-acceptor site and exon 6 sequences critical for its recognition.

CONCLUSIONS

Incorporation of minigene read-outs into an ACMG/AMP (American College of Medical Genetics and Genomics/Association for Molecular Pathology)-based classification scheme allowed us to classify 32 CHEK2 variants (27 pathogenic/likely pathogenic and 5 likely benign). However, 20 variants (38%) remained of uncertain significance, reflecting in part the complex splicing patterns of this gene.

Minigene Assay as an Effective Molecular Diagnostic Strategy in Determining the Pathogenicity of Noncanonical Splice-Site Variants in FLCN

Primary spontaneous pneumothorax (PSP) or pulmonary cyst is one of the manifestations of Birt-Hogg-Dubé syndrome, which is caused by pathogenic variants in FLCN gene. Genetic testing in patients with PSP identifies a certain number of missense or intronic variants. These variants are usually considered as variants of uncertain significance, whose functional interpretations pose a challenge in clinical genetics. To improve recognition of pathogenic splice-altering variants in FLCN gene, computational tools are used to prioritize potential splice-altering variants and then a hybrid minigene assay is performed to verify the RNA splicing pattern. Herein, variants in FLCN exon 11 and its flanking sequence are focused. Eight variants detected in 11 patients with PSP are evaluated, and six variants are prioritized by in silico tools as potential splice-altering variants of uncertain significance. Four variants (c.1177-5_1177-3delCTC, c.1292_1300+4del, c.1300+4C>T, and c.1300+5G>A) are demonstrated by minigene assay to alter RNA splicing of FLCN, and the last three of them are novel. RT-PCR of patient-derived RNA gives consistent results. Genotype-phenotype correlation analysis in patients with PSP with these variants demonstrates good concordance. Our results underline the importance of RNA analysis, which could provide molecular evidence for pathogenicity of a variant, and provide essential information for the clinical interpretation of variants. Combining the clinical information, a definitive diagnosis could be made.

Splicing Analysis of 16 PALB2 ClinVar Variants by Minigene Assays: Identification of Six Likely Pathogenic Variants

PALB2 loss-of-function variants are associated with significant increased risk of breast cancer as well as other types of tumors. Likewise, splicing disruptions are a common mechanism of disease susceptibility. Indeed, we previously showed, by minigene assays, that 35 out of 42 PALB2 variants impaired splicing. Taking advantage of one of these constructs (mgPALB2_ex1-3), we proceeded to analyze other variants at exons 1 to 3 reported at the ClinVar database. Thirty-one variants were bioinformatically analyzed with MaxEntScan and SpliceAI. Then, 16 variants were selected for subsequent RNA assays. We identified a total of 12 spliceogenic variants, 11 of which did not produce any trace of the expected minigene full-length transcript. Interestingly, variant c.49-1G > A mimicked previous outcomes in patient RNA (transcript ∆(E2p6)), supporting the reproducibility of the minigene approach. A total of eight variant-induced transcripts were characterized, three of which (∆(E1q17), ∆(E3p11), and ∆(E3)) were predicted to introduce a premature termination codon and to undergo nonsense-mediated decay, and five (▼(E1q9), ∆(E2p6), ∆(E2), ▼(E3q48)-a, and ▼(E3q48)-b) maintained the reading frame. According to an ACMG/AMP (American College of Medical Genetics and Genomics/Association for Molecular Pathology)-based classification scheme, which integrates mgPALB2 data, six PALB2 variants were classified as pathogenic/likely pathogenic, five as VUS, and five as likely benign. Furthermore, five ±1,2 variants were catalogued as VUS because they produced significant proportions of in-frame transcripts of unknown impact on protein function.