Identification and characterization of a novel intronic splicing mutation in CSF1R-related leukoencephalopathy

AIMS

Colony stimulating factor 1 receptor (CSF1R)-related leukoencephalopathy is a rapidly progressing neurodegenerative disease caused by CSF1R gene mutations. This study aimed to identify and investigate the effect of a novel intronic mutation (c.1754-3C>G) of CSF1R on splicing.

METHODS

A novel intronic mutation was identified using whole-exome sequencing. To investigate the impact of this mutation, we employed various bioinformatics tools to analyze the transcription of the CSF1R gene and the three-dimensional structure of its encoded protein. Furthermore, reverse transcription polymerase chain reaction (RT-PCR) was performed to validate the findings.

RESULTS

A novel mutation (c.1754-3C>G) in CSF1R was identified, which results in exon 13 skipping due to the disruption of the 3' splice site consensus sequence NYAG/G. This exon skipping event was further validated in the peripheral blood of the mutation carrier through RT-PCR and Sanger sequencing. Protein structure prediction indicated a disruption in the tyrosine kinase domain, with the truncated protein showing significant structural alterations.

CONCLUSIONS

Our findings underscore the importance of intronic mis-splicing mutations in the diagnosis and management of CSF1R-related leukoencephalopathy.

Predicted deleterious variants in ABCA1, LPL, LPA and KIF6 are associated with statin response and adverse events in patients with familial hypercholesterolemia and disturb protein structure and stability

OBJECTIVES

This study explored the association of deleterious variants in pharmacodynamics (PD) genes with statin response and adverse effects in patients with familial hypercholesterolemia (FH) and analyzed their potential effects on protein structure and stability.

METHODS

Clinical and laboratory data were obtained from 144 adult FH patients treated with statins. A panel of 32 PD genes was analyzed by exon-targeted gene sequencing. Deleterious variants were identified using prediction algorithms and their structural effects were analyzed by molecular modeling studies.

RESULTS

A total of 102 variants were predicted as deleterious (83 missense, 8 stop-gain, 4 frameshift, 1 indel, 6 splicing). The variants ABCA1 rs769705621 (indel), LPA rs41267807 (p.Tyr2023Cys) and KIF6 rs20455 (p.Trp719Arg) were associated with reduced low-density lipoprotein cholesterol (LDLc) response to statins, and the LPL rs1801177 (p.Asp36Asn) with increased LDLc response (P < 0.05). LPA rs3124784 (p.Arg2016Cys) was predicted to increase statin response (P = 0.022), and ABCA1 rs769705621 to increase the risk of statin-related adverse events (SRAE) (P = 0.027). LPA p.Arg2016Cys and LPL p.Asn36Asp maintained interactions with solvent, LPA p.Tyr2023Cys reduced intramolecular interaction with Gln1987, and KIF6 p.Trp719Arg did not affect intramolecular interactions. DDMut analysis showed that LPA p.Arg2016Cys and p.Tyr2023Cys and LPL p.Asp36Asn caused energetically favorable changes, and KIF6 p.Trp719Arg resulted in unfavorable energetic changes, affecting protein stability.

CONCLUSION

Deleterious variants in ABCA1, LPA, LPL and KIF6 are associated with variability in LDLc response to statins, and ABCA1 rs769705621 is associated with SRAE risk in FH patients. Molecular modeling studies suggest that LPA p.Tyr2023Cys and KIF6 p.Trp719Arg disturb protein conformational structure and stability.

All exons are not created equal-exon vulnerability determines the effect of exonic mutations on splicing

It is now widely accepted that aberrant splicing of constitutive exons is often caused by mutations affecting cis-acting splicing regulatory elements (SREs), but there is a misconception that all exons have an equal dependency on SREs and thus a similar vulnerability to aberrant splicing. We demonstrate that some exons are more likely to be affected by exonic splicing mutations (ESMs) due to an inherent vulnerability, which is context dependent and influenced by the strength of exon definition. We have developed VulExMap, a tool which is based on empirical data that can designate whether a constitutive exon is vulnerable. Using VulExMap, we find that only 25% of all exons can be categorized as vulnerable, whereas two-thirds of 359 previously reported ESMs in 75 disease genes are located in vulnerable exons. Because VulExMap analysis is based on empirical data on splicing of exons in their endogenous context, it includes all features important in determining the vulnerability. We believe that VulExMap will be an important tool when assessing the effect of exonic mutations by pinpointing whether they are located in exons vulnerable to ESMs.

Expanding the genetic and phenotypic spectrum of congenital myasthenic syndrome: new homozygous VAMP1 splicing variants in 2 novel individuals

We report the cases of two Spanish pediatric patients with hypotonia, muscle weakness and feeding difficulties at birth. Whole-exome sequencing (WES) uncovered two new homozygous VAMP1 (Vesicle Associated Membrane Protein 1) splicing variants, NM_014231.5:c.129+5 G > A in the boy patient (P1) and c.341-24_341-16delinsAGAAAA in the girl patient (P2). This gene encodes the vesicle-associated membrane protein 1 (VAMP1) that is a component of a protein complex involved in the fusion of synaptic vesicles with the presynaptic membrane. VAMP1 has a highly variable C-terminus generated by alternative splicing that gives rise to three main isoforms (A, B and D), being VAMP1A the only isoform expressed in the nervous system. In order to assess the pathogenicity of these variants, expression experiments of RNA for VAMP1 were carried out. The c.129+5 G > A and c.341-24_341-16delinsAGAAAA variants induced aberrant splicing events resulting in the deletion of exon 2 (r.5_131del; p.Ser2TrpfsTer7) in the three isoforms in the first case, and the retention of the last 14 nucleotides of the 3' of intron 4 (r.340_341ins341-14_341-1; p.Ile114AsnfsTer77) in the VAMP1A isoform in the second case. Pathogenic VAMP1 variants have been associated with autosomal dominant spastic ataxia 1 (SPAX1) and with autosomal recessive presynaptic congenital myasthenic syndrome (CMS). Our patients share the clinical manifestations of CMS patients with two important differences: they do not show the typical electrophysiological pattern that suggests pathology of pre-synaptic neuromuscular junction, and their muscular biopsies present hypertrophic fibers type 1. In conclusion, our data expand both genetic and phenotypic spectrum associated with VAMP1 variants.

Variants in the circadian clock genes PER2 and PER3 associate with familial sleep phase disorders

Delayed sleep phase disorder and advanced sleep phase disorder cause disruption of the circadian clock and present with extreme morning/evening chronotype with unclear role of the genetic etiology, especially for delayed sleep phase disorder. To assess if genotyping can aid in clinical diagnosis, we examined the presence of genetic variants in circadian clock genes previously linked to both sleep disorders in Slovenian patient cohort. Based on Morning-evening questionnaire, we found 15 patients with extreme chronotypes, 13 evening and 2 morning, and 28 controls. Sanger sequencing was used to determine the presence of carefully selected candidate SNPs in regions of the CSNK1D, PER2/3 and CRY1 genes. In a patient with an extreme morning chronotype and a family history of circadian sleep disorder we identified two heterozygous missense variants in PER3 gene, c.1243C>G (NM_001377275.1 (p.Pro415Ala)) and c.1250A>G (NM_001377275.1 (p.His417Arg)). The variants were significantly linked to Advanced sleep phase disorder and were also found in proband's father with extreme morningness. Additionally, a rare SNP was found in PER2 gene in a patient with clinical picture of Delayed sleep phase disorder. The novel variant in PER2 (NM_022817.3):c.1901-218 G>T was found in proband's parent with eveningness, indicating an autosomal dominant inheritance. We identified a family with autosomal dominant inheritance of two PER3 heterozygous variants that can be linked to Advanced sleep phase disorder. We revealed also a rare hereditary form of Delayed sleep phase disorder with a new PER2 variant with autosomal dominant inheritance, shedding the light into the genetic causality.

Assessing the reliability of point mutation as data augmentation for deep learning with genomic data

BACKGROUND

Deep neural networks (DNNs) have the potential to revolutionize our understanding and treatment of genetic diseases. An inherent limitation of deep neural networks, however, is their high demand for data during training. To overcome this challenge, other fields, such as computer vision, use various data augmentation techniques to artificially increase the available training data for DNNs. Unfortunately, most data augmentation techniques used in other domains do not transfer well to genomic data.

RESULTS

Most genomic data possesses peculiar properties and data augmentations may significantly alter the intrinsic properties of the data. In this work, we propose a novel data augmentation technique for genomic data inspired by biology: point mutations. By employing point mutations as substitutes for codons, we demonstrate that our newly proposed data augmentation technique enhances the performance of DNNs across various genomic tasks that involve coding regions, such as translation initiation and splice site detection.

CONCLUSION

Silent and missense mutations are found to positively influence effectiveness, while nonsense mutations and random mutations in non-coding regions generally lead to degradation. Overall, point mutation-based augmentations in genomic datasets present valuable opportunities for improving the accuracy and reliability of predictive models for DNA sequences.

Tumor Antigens beyond the Human Exome

With the advent of immunotherapeutics, a new era in the combat against cancer has begun. Particularly promising are neo-epitope-targeted therapies as the expression of neo-antigens is tumor-specific. In turn, this allows the selective targeting and killing of cancer cells whilst healthy cells remain largely unaffected. So far, many advances have been made in the development of treatment options which are tailored to the individual neo-epitope repertoire. The next big step is the achievement of efficacious "off-the-shelf" immunotherapies. For this, shared neo-epitopes propose an optimal target. Given the tremendous potential, a thorough understanding of the underlying mechanisms which lead to the formation of neo-antigens is of fundamental importance. Here, we review the various processes which result in the formation of neo-epitopes. Broadly, the origin of neo-epitopes can be categorized into three groups: canonical, noncanonical, and viral neo-epitopes. For the canonical neo-antigens that arise in direct consequence of somatic mutations, we summarize past and recent findings. Beyond that, our main focus is put on the discussion of noncanonical and viral neo-epitopes as we believe that targeting those provides an encouraging perspective to shape the future of cancer immunotherapeutics.

Escaping from CRISPR-Cas-mediated knockout: the facts, mechanisms, and applications

Clustered regularly interspaced short palindromic repeats and associated Cas protein (CRISPR-Cas), a powerful genome editing tool, has revolutionized gene function investigation and exhibits huge potential for clinical applications. CRISPR-Cas-mediated gene knockout has already become a routine method in research laboratories. However, in the last few years, accumulating evidences have demonstrated that genes knocked out by CRISPR-Cas may not be truly silenced. Functional residual proteins could be generated in such knockout organisms to compensate the putative loss of function, termed herein knockout escaping. In line with this, several CRISPR-Cas-mediated knockout screenings have discovered much less abnormal phenotypes than expected. How does knockout escaping happen and how often does it happen have not been systematically reviewed yet. Without knowing this, knockout results could easily be misinterpreted. In this review, we summarize these evidences and propose two main mechanisms allowing knockout escaping. To avoid the confusion caused by knockout escaping, several strategies are discussed as well as their advantages and disadvantages. On the other hand, knockout escaping also provides convenient tools for studying essential genes and treating monogenic disorders such as Duchenne muscular dystrophy, which are discussed in the end.

Understanding and Rescuing the Splicing Defect Caused by the Frequent ABCA4 Variant c.4253+43G>A Underlying Stargardt Disease

Pathogenic variants in ABCA4 are the underlying molecular cause of Stargardt disease (STGD1), an autosomal recessive macular dystrophy characterized by a progressive loss of central vision. Among intronic ABCA4 variants, c.4253+43G>A is frequently detected in STGD1 cases and is classified as a hypomorphic allele, generally associated with late-onset cases. This variant was previously reported to alter splicing regulatory sequences, but the splicing outcome is not fully understood yet. In this study, we attempted to better understand its effect on splicing and to rescue the aberrant splicing via antisense oligonucleotides (AONs). Wild-type and c.4253+43G>A variant-harboring maxigene vectors revealed additional skipping events, which were not previously detected upon transfection in HEK293T cells. To restore exon inclusion, we designed a set of 27 AONs targeting either splicing silencer motifs or the variant region and screened these in maxigene-transfected HEK293T cells. Candidate AONs able to promote exon inclusion were selected for further testing in patient-derived photoreceptor precursor cells. Surprisingly, no robust splicing modulation was observed in this model system. Overall, this research helped to adequately characterize the splicing alteration caused by the c.4253+43G>A variant, although future development of AON-mediated exon inclusion therapy for ABCA4 is needed.

Alternative splicing patterns of hnrnp genes in gill tissues of rainbow trout (Oncorhynchus mykiss) during salinity changes

Alternative splicing (AS) plays an important role in various physiological processes in eukaryotes, such as the stress response. However, patterns of AS events remain largely unexplored during salinity acclimation in fishes. In this study, we conducted AS analysis using RNA-seq datasets to explore splicing patterns in the gill tissues of rainbow trout exposed to altered salinity environments, ranging from 0 ‰ (T0) to 30 ‰ (T30). The results revealed 1441, 351, 483, 1051 and 1049 differentially alternatively spliced (DAS) events in 5 pairwise comparisons, including T6 vs. T0, T12 vs. T0, T18 vs. T0, T24 vs. T0, and T30 vs. T0, respectively. These DAS events were derived from 1290, 328, 444, 963 and 948 genes. Enrichment analysis indicated that these DAS genes were related to RNA splicing and processing. Among these, 14 DAS genes were identified as members of the large heterogeneous nuclear RNP (hnRNP) gene family. Alternative 3' splice site (A3SS), exon skipping (SE) and intron retention (RI) events resulted in the fragmentation or even loss of the functional RNA recognition motif (RRM) domains in hnrnpa0, hnrnp1a, hnrnp1b and hnrnpc genes. The incomplete RRM domains would hinder the interactions between hnRNP genes and pre-mRNAs. It would in turn influence the splicing patterns and mRNA stability of downstream target genes in response to salinity changes. The study provides insights into salinity acclimation in gill tissues of rainbow trout and serves as a significant reference on the osmoregulation mechanisms at post-transcription regulation levels in fish.

A dual-subcellular localized β-glucosidase confers pathogen and insect resistance without a yield penalty in maize

Maize is one of the most important crops for food, cattle feed and energy production. However, maize is frequently attacked by various pathogens and pests, which pose a significant threat to maize yield and quality. Identification of quantitative trait loci and genes for resistance to pests will provide the basis for resistance breeding in maize. Here, a β-glucosidase ZmBGLU17 was identified as a resistance gene against Pythium aphanidermatum, one of the causal agents of corn stalk rot, by genome-wide association analysis. Genetic analysis showed that both structural variations at the promoter and a single nucleotide polymorphism at the fifth intron distinguish the two ZmBGLU17 alleles. The causative polymorphism near the GT-AG splice site activates cryptic alternative splicing and intron retention of ZmBGLU17 mRNA, leading to the downregulation of functional ZmBGLU17 transcripts. ZmBGLU17 localizes in both the extracellular matrix and vacuole and contribute to the accumulation of two defence metabolites lignin and DIMBOA. Silencing of ZmBGLU17 reduces maize resistance against P. aphanidermatum, while overexpression significantly enhances resistance of maize against both the oomycete pathogen P. aphanidermatum and the Asian corn borer Ostrinia furnacalis. Notably, ZmBGLU17 overexpression lines exhibited normal growth and yield phenotype in the field. Taken together, our findings reveal that the apoplastic and vacuolar localized ZmBGLU17 confers resistance to both pathogens and insect pests in maize without a yield penalty, by fine-tuning the accumulation of lignin and DIMBOA.

ABCA4 c.6480-35A>G, a novel branchpoint variant associated with Stargardt disease

Introduction: Inherited retinal dystrophies (IRDs) can be caused by variants in more than 280 genes. The ATP-binding cassette transporter type A4 (ABCA4) gene is one of these genes and has been linked to Stargardt disease type 1 (STGD1), fundus flavimaculatus, cone-rod dystrophy (CRD), and pan-retinal CRD. Approximately 25% of the reported ABCA4 variants affect RNA splicing. In most cases, it is necessary to perform a functional assay to determine the effect of these variants. Methods: Whole genome sequencing (WGS) was performed in one Spanish proband with Stargardt disease. The putative pathogenicity of c.6480-35A>G on splicing was investigated both in silico and in vitro. The in silico approach was based on the deep-learning tool SpliceAI. For the in vitro approach we used a midigene splice assay in HEK293T cells, based on a previously established wild-type midigene (BA29) containing ABCA4 exons 46 to 48. Results: Through the analysis of WGS data, we identified two candidate variants in ABCA4 in one proband: a previously described deletion, c.699_768+342del (p.(Gln234Phefs*5)), and a novel branchpoint variant, c.6480-35A>G. Segregation analysis confirmed that the variants were in trans. For the branchpoint variant, SpliceAI predicted an acceptor gain with a high score (0.47) at position c.6480-47. A midigene splice assay in HEK293T cells revealed the inclusion of the last 47 nucleotides of intron 47 creating a premature stop codon and allowed to categorize the variant as moderately severe. Subsequent analysis revealed the presence of this variant as a second allele besides c.1958G>A p.(Arg653His) in an additional Spanish proband in a large cohort of IRD cases. Conclusion: A splice-altering effect of the branchpoint variant, confirmed by the midigene splice assay, along with the identification of this variant in a second unrelated individual affected with STGD, provides sufficient evidence to classify the variant as likely pathogenic. In addition, this research highlights the importance of studying non-coding regions and performing functional assays to provide a conclusive molecular diagnosis.

FexSplice: A LightGBM-Based Model for Predicting the Splicing Effect of a Single Nucleotide Variant Affecting the First Nucleotide G of an Exon

Single nucleotide variants (SNVs) affecting the first nucleotide G of an exon (Fex-SNVs) identified in various diseases are mostly recognized as missense or nonsense variants. Their effect on pre-mRNA splicing has been seldom analyzed, and no curated database is available. We previously reported that Fex-SNVs affect splicing when the length of the polypyrimidine tract is short or degenerate. However, we cannot readily predict the splicing effects of Fex-SNVs. We here scrutinized the available literature and identified 106 splicing-affecting Fex-SNVs based on experimental evidence. We similarly identified 106 neutral Fex-SNVs in the dbSNP database with a global minor allele frequency (MAF) of more than 0.01 and less than 0.50. We extracted 115 features representing the strength of splicing cis-elements and developed machine-learning models with support vector machine, random forest, and gradient boosting to discriminate splicing-affecting and neutral Fex-SNVs. Gradient boosting-based LightGBM outperformed the other two models, and the length and nucleotide compositions of the polypyrimidine tract played critical roles in the discrimination. Recursive feature elimination showed that the LightGBM model using 15 features achieved the best performance with an accuracy of 0.80 ± 0.12 (mean and SD), a Matthews Correlation Coefficient (MCC) of 0.57 ± 0.15, an area under the curve of the receiver operating characteristics curve (AUROC) of 0.86 ± 0.08, and an area under the curve of the precision-recall curve (AUPRC) of 0.87 ± 0.09 using a 10-fold cross-validation. We developed a web service program, named FexSplice that accepts a genomic coordinate either on GRCh37/hg19 or GRCh38/hg38 and returns a predicted probability of aberrant splicing of A, C, and T variants.

Applications of artificial intelligence in clinical laboratory genomics

The transition from analog to digital technologies in clinical laboratory genomics is ushering in an era of "big data" in ways that will exceed human capacity to rapidly and reproducibly analyze those data using conventional approaches. Accurately evaluating complex molecular data to facilitate timely diagnosis and management of genomic disorders will require supportive artificial intelligence methods. These are already being introduced into clinical laboratory genomics to identify variants in DNA sequencing data, predict the effects of DNA variants on protein structure and function to inform clinical interpretation of pathogenicity, link phenotype ontologies to genetic variants identified through exome or genome sequencing to help clinicians reach diagnostic answers faster, correlate genomic data with tumor staging and treatment approaches, utilize natural language processing to identify critical published medical literature during analysis of genomic data, and use interactive chatbots to identify individuals who qualify for genetic testing or to provide pre-test and post-test education. With careful and ethical development and validation of artificial intelligence for clinical laboratory genomics, these advances are expected to significantly enhance the abilities of geneticists to translate complex data into clearly synthesized information for clinicians to use in managing the care of their patients at scale.

Introme accurately predicts the impact of coding and noncoding variants on gene splicing, with clinical applications

Predicting the impact of coding and noncoding variants on splicing is challenging, particularly in non-canonical splice sites, leading to missed diagnoses in patients. Existing splice prediction tools are complementary but knowing which to use for each splicing context remains difficult. Here, we describe Introme, which uses machine learning to integrate predictions from several splice detection tools, additional splicing rules, and gene architecture features to comprehensively evaluate the likelihood of a variant impacting splicing. Through extensive benchmarking across 21,000 splice-altering variants, Introme outperformed all tools (auPRC: 0.98) for the detection of clinically significant splice variants. Introme is available at https://github.com/CCICB/introme .

Molecular Basis of Unequal Alternative Splicing of Human SCD5 and Its Alteration by Natural Genetic Variations

Alternative splicing (AS) is a major means of post-transcriptional control of gene expression, and provides a dynamic versatility of protein isoforms. Cancer-related AS disorders have diagnostic, prognostic and therapeutic values. Changes in the expression and AS of human stearoyl-CoA desaturase-5 (SCD5) are promising specific tumor markers, although the transcript variants (TVs) of the gene have not yet been confirmed. Our in silico, in vitro and in vivo study focuses on the distribution of SCD5 TVs (A and B) in human tissues, the functionality of the relevant splice sites, and their modulation by certain single-nucleotide variations (SNVs). An order of magnitude higher SCD5A expression was found compared with SCD5B. This unequal splicing is attributed to a weaker recognition of the SCD5B-specific splicing acceptor site, based on predictions confirmed by an optimized minigene assay. The pronounced dominance of SCD5A was largely modified (rs1430176385_A, rs1011850309_A) or even inverted (rs1011850309_C) by natural SNVs at the TV-specific splice sites. Our results provide long missing data on the proportion of SCD5 TVs in human tissues and reveal mutation-driven changes in SCD5 AS, potentially affecting tumor-associated reprogramming of lipid metabolism, thus having prognostic significance, which may be utilized for novel and personalized therapeutic approaches.

Genetic risk variants for multiple sclerosis are linked to differences in alternative pre-mRNA splicing

BACKGROUND

Multiple sclerosis (MS) is a chronic immune-mediated disease of the central nervous system to which a genetic predisposition contributes. Over 200 genetic regions have been associated with increased disease risk, but the disease-causing variants and their functional impact at the molecular level are mostly poorly defined. We hypothesized that single-nucleotide polymorphisms (SNPs) have an impact on pre-mRNA splicing in MS.

METHODS

Our study focused on 10 bioinformatically prioritized SNP-gene pairs, in which the SNP has a high potential to alter alternative splicing events (ASEs). We tested for differential gene expression and differential alternative splicing in B cells from MS patients and healthy controls. We further examined the impact of the SNP genotypes on ASEs and on splice isoform expression levels. Novel genotype-dependent effects on splicing were verified with splicing reporter minigene assays.

RESULTS

We were able to confirm previously described findings regarding the relation of MS-associated SNPs with the ASEs of the pre-mRNAs from GSDMB and SP140. We also observed an increased IL7R exon 6 skipping when comparing relapsing and progressive MS patients to healthy subjects. Moreover, we found evidence that the MS risk alleles of the SNPs rs3851808 (EFCAB13), rs1131123 (HLA-C), rs10783847 (TSFM), and rs2014886 (TSFM) may contribute to a differential splicing pattern. Of particular interest is the genotype-dependent exon skipping of TSFM due to the SNP rs2014886. The minor allele T creates a donor splice site, resulting in the expression of the exon 3 and 4 of a short TSFM transcript isoform, whereas in the presence of the MS risk allele C, this donor site is absent, and thus the short transcript isoform is not expressed.

CONCLUSION

In summary, we found that genetic variants from MS risk loci affect pre-mRNA splicing. Our findings substantiate the role of ASEs with respect to the genetics of MS. Further studies on how disease-causing genetic variants may modify the interactions between splicing regulatory sequence elements and RNA-binding proteins can help to deepen our understanding of the genetic susceptibility to MS.

Computational approaches for predicting variant impact: An overview from resources, principles to applications

One objective of human genetics is to unveil the variants that contribute to human diseases. With the rapid development and wide use of next-generation sequencing (NGS), massive genomic sequence data have been created, making personal genetic information available. Conventional experimental evidence is critical in establishing the relationship between sequence variants and phenotype but with low efficiency. Due to the lack of comprehensive databases and resources which present clinical and experimental evidence on genotype-phenotype relationship, as well as accumulating variants found from NGS, different computational tools that can predict the impact of the variants on phenotype have been greatly developed to bridge the gap. In this review, we present a brief introduction and discussion about the computational approaches for variant impact prediction. Following an innovative manner, we mainly focus on approaches for non-synonymous variants (nsSNVs) impact prediction and categorize them into six classes. Their underlying rationale and constraints, together with the concerns and remedies raised from comparative studies are discussed. We also present how the predictive approaches employed in different research. Although diverse constraints exist, the computational predictive approaches are indispensable in exploring genotype-phenotype relationship.

The Role of Alternative Splicing in Cancer: Regulatory Mechanism, Therapeutic Strategy, and Bioinformatics Application

[Formula: see text] Alternative splicing (AS) can generate distinct transcripts and subsequent isoforms that play differential functions from the same pre-mRNA. Recently, increasing numbers of studies have emerged, unmasking the association between AS and cancer. In this review, we arranged AS events that are closely related to cancer progression and presented promising treatments based on AS for cancer therapy. Obtaining proliferative capacity, acquiring invasive properties, gaining angiogenic features, shifting metabolic ability, and getting immune escape inclination are all splicing events involved in biological processes. Spliceosome-targeted and antisense oligonucleotide technologies are two novel strategies that are hopeful in tumor therapy. In addition, bioinformatics applications based on AS were summarized for better prediction and elucidation of regulatory routines mingled in. Together, we aimed to provide a better understanding of complicated AS events associated with cancer biology and reveal AS a promising target of cancer treatment in the future.

Mutation analysis reveals novel and known mutations in SAG gene in first two Egyptian families with Oguchi disease

Background

Oguchi disease is a rare type of congenital stationary night blindness associated with an abnormal fundus appearance. It is inherited in an autosomal recessive manner where two types exist according to the gene affected; type 1 associated with S-antigen (SAG) gene mutations and type 2 associated with rhodopsin kinase (GRK1) gene mutations.

Purpose

The aim of this work was to describe the clinical and genetic findings of the first two reported families of Oguchi disease in Egypt and African region.

Methods

Four members of two consanguineous Egyptian families with history of night blindness since childhood underwent complete ophthalmological examination, standard automated static perimetry, fundus color photography, fundus autofluorescence (FAF), fundus fluorescein angiography (FFA) in light-adapted state and spectral-domain optical coherence tomography (SD-OCT) of both the macula and the optic nerve head as well as central corneal thickness with repeated fundus photography following prolonged dark adaptation. Mutation screening of 7 coding exons of GRK1 gene and 15 coding exons of SAG gene as well as some flanking regions were performed using Sanger sequencing technique. The variants were tested for pathogenicity using different in silico functional analysis tools.

Results

The clinical examination and investigations confirmed Oguchi disease phenotype. One patient showed p.R193* (c.577C > T) which is a previously reported SAG gene mutation in a homozygous form. The other three patients from a different family showed (c.649–1 G > C), a novel canonical splice site SAG gene mutation in a homozygous form.

Conclusion

The identification of the novel canonical splice site SAG gene variant in three members of the same family with clinically confirmed Oguchi disease reinforces its pathogenicity. A fourth patient from another family carried a previously reported mutation in the same gene. SAG gene variants may be the underlying genetic cause for Oguchi disease in Egypt. Our findings have expanded the spectrum of Oguchi disease-associated mutations in SAG gene and may serve as a basis for genetic diagnosis for Oguchi disease.

Presumed COL4A3/COL4A4 Missense/Synonymous Variants Induce Aberrant Splicing

Background

The incorrect interpretation of missense and synonymous variants can lead to improper molecular diagnosis and subsequent faulty genetic counselling. The aim of this study was to evaluate the pathogenicity of presumed COL4A3/COL4A4 missense and synonymous variants detected by next-generation sequencing to provide evidence for diagnosis and genetic counselling.

Methods

Patients' clinical findings and genetic data were analysed retrospectively. An in vitro minigene assay was conducted to assess the effect of presumed COL4A3/COL4A4 missense and synonymous variants on RNA splicing.

Results

Five unclassified COL4A3/COL4A4 variants, which were detected in five of 343 patients with hereditary kidney diseases, were analysed. All of them were predicted to affect splicing by Human Splicing Finder. The presumed COL4A3 missense variant c.4793T > G [p. (Leu1598Arg)] resulted in a loss of alternative full-length transcript during the splicing process. The COL4A3 transcript carried synonymous variant c.765G > A [p. (Thr255Thr)], led to an in-frame deletion of exon 13. Nevertheless, variants c.3566G > A [p. (Gly1189Glu)] in COL4A3 and c.3990G > A [p. (Pro1330Pro)], c.4766C > T [p. (Pro1589Leu)] in COL4A4 exhibited no deleterious effect on splicing. Among the five patients harbouring the abovementioned COL4A3/COL4A4 variants, three patients were genetically diagnosed with autosomal recessive Alport syndrome, one patient was highly suspected of having thin basement membrane nephropathy, and the other patient was clinically diagnosed with Alport syndrome.

Conclusions

COL4A3 presumed missense variant p. (Leu1598Arg) and synonymous variant p. (Thr255Thr) affect RNA splicing, which highlights the prime importance of transcript analysis of unclassified exonic sequence variants for better molecular diagnosis and genetic counselling. Meanwhile, the reliability of splicing predictions by predictive tools for exonic substitutions needs to be improved.

Identification of Novel Mutations in Chinese Infants With Citrullinemia

Citrullinemia is a rare autosomal recessive disorder characterized by elevated concentrations of citrulline in the blood resulting from malfunction of the urea cycle. It is categorized into two types, types I and II, which are caused by argininosuccinate synthase 1 (ASS1), and citrin (SLC25A13) gene mutations, respectively. In this study, we performed genetic analysis on nine Chinese infants with citrullinemia using next-generation sequencing, which identified a novel mutation (p.Leu313Met) and a rare mutation (p.Thr323Ile, rs1250895424) of ASS1. We also found a novel splicing mutation of SLC25A13: c.1311 + 4_+7del. Functional analysis of the ASS1 missense mutations showed that both significantly impaired the enzyme activity of ASS1, with the p. Thr323Ile mutation clearly affecting the interaction between ASS1 and protein arginine methyltransferase 7 (PRMT7). These findings expand the mutational spectrum of ASS1 and SLC25A13, and further our understanding of the molecular genetic mechanism of citrullinemia in the Chinese population.

Background splicing as a predictor of aberrant splicing in genetic disease

Mutations of splice sites, auxiliary splicing elements and the splicing machinery cause a wide range of genetic disease. Here we report that many of the complex effects of splicing mutations can be predicted from background splicing information, with emphasis on BRCA1, BRCA2 and DMD. Background splicing arises from very low level splicing between rarely used background splice sites and from low-level exon skipping between intron splice sites. We show how this information can be downloaded from the Snaptron database of spliced RNA, which we then compared with databases of human splice site mutations. We report that inactivating mutations of intron splice sites typically caused the non-mutated partner splice site to splice to a known background splice site in over 90% of cases and to the strongest background splice site in the large majority of cases. Consequently, background splicing information can usefully predict the effects of splice site mutations, which include cryptic splice activation and single or multiple exon skipping. In addition, de novo splice sites and splice sites involved in pseudoexon formation, recursive splicing and aberrant splicing in cancer show a 90% match to background splice sites, so establishing that the enhancement of background splicing causes a wide range of splicing aberrations. We also discuss how background splicing information can identify cryptic splice sites that might be usefully targeted by antisense oligonucleotides (ASOs) and how it might indicate possible multiple exon skipping side effects of ASOs designed to induce single exon skipping.

Postzygotic mosaicism of a novel PTPN11 mutation in monozygotic twins discordant for metachondromatosis

Genetic mosaicism caused by postzygotic mutations is of a great interest due to its role in human disease. Monozygotic twins arising from a single zygote are considered as genetically identical, and any differences likely to be caused by postzygotic events. Thus, phenotypically discordant monozygotic twins offer a unique opportunity to study genotype-phenotype correlation. Here, we present a three-generation family starting from a pair of monozygotic twins discordant for metachondromatosis due to postzygotic p.(Gln175His) variant in the PTPN11 gene. Both phenotypically discordant monozygotic twins harbor p.(Gln175His), however significant differences in mosaic ratio is observed not only between twins, but also within different tissue types within one individual. Phenotypic manifestation of p.(Gln175His) in examined family clearly depends on allele variant fraction (VAF). Individuals harboring constitutional mutation (VAF 50%) present typical metachondromatosis. Milder phenotype is observed in twin harboring high-level mosaicism in the tissue of ectodermal origin (VAF 45%), but not in a blood (VAF 5%). Finally, her twin sister harboring low-level mosaicism in blood (VAF 2%) and nonblood (VAF 12%) tissues is phenotypically normal. Our results provide insights into biological role of mosaicism in disease and further support the usefulness of nonblood tissues as an optimal source of DNA for the identification of postzygotic mutations in phenotypically discordant monozygotic twins.

Targeting Alternative Splicing for Therapeutic Interventions

Targeting of pre-mRNA splicing has yielded a rich variety of strategies for altering gene expression as a treatment for disease. The search for therapeutics that can modulate splicing has been dominated by antisense oligonucleotides (ASOs) and small molecule compounds, with each platform achieving remarkably effective results in the clinic. The success of RNA-targeting drugs has led to the exploration of new strategies to expand the repertoire of this type of therapeutic. Here, we discuss some of the more common causes of faulty gene expression and provide examples of approaches that have been developed to target and correct these defects for therapeutic value.

Unraveling the molecular basis underlying nine putative splice site variants of von Willebrand factor

Approximately 10% of von Willebrand factor (VWF) gene variants are suspected to disrupt messenger RNA (mRNA) processing, the number of which might be underestimated due to the lack of transcript assays. In the present study, we provided a detailed strategy to evaluate the effects of nine putative splice site variants (PSSVs) of VWF on mRNA processing as well as protein properties and establish their genotype-phenotype relationships. Eight of nine PSSVs affected VWF splicing: c.322A>T, c.1534-13_1551delinsCA, and c.8116-2del caused exon skipping; c.221-2A>C, c.323+1G>T, and c.2547-13T>A resulted in the activation of cryptic splice sites; c.2684A>G led to exon skipping and activation of a cryptic splice site; c.2968-14A>G created a new splice site. The remaining c.5171-9del was likely benign. The efficiency of nonsense-mediated mRNA decay (NMD) was much higher in platelets compared to leukocytes, impairing the identification of aberrant transcripts in 4 of 8 PSSVs. The nonsense variant c.322A>T partially impaired mRNA processing, leaking a small amount of correct transcripts with c.322T (p.Arg108*), while the missense variant c.2684A>G totally disrupted normal splicing of VWF, rather than produced mutant protein with the substitution of Gln895Arg. The results of this study would certainly add novel insights into the molecular events behind von Willebrand disease.

Pseudoexon activation in disease by non-splice site deep intronic sequence variation - wild type pseudoexons constitute high-risk sites in the human genome

Accuracy of pre-messenger RNA (pre-mRNA) splicing is crucial for normal gene expression. Complex regulation supports the spliceosomal distinction between authentic exons and the many seemingly functional splice sites delimiting pseudoexons. Pseudoexons are nonfunctional intronic sequences that can be activated for aberrant inclusion in mRNA, which may cause disease. Pseudoexon activation is very challenging to predict, in particular when activation occurs by sequence variants that alter the splicing regulatory environment without directly affecting splice sites. As pseudoexon inclusion often evades detection due to activation of nonsense-mediated mRNA decay, and because conventional diagnostic procedures miss deep intronic sequence variation, pseudoexon activation is a heavily underreported disease mechanism. Pseudoexon characteristics have mainly been studied based on in silico predicted sequences. Moreover, because recognition of sequence variants that create or strengthen splice sites is possible by comparison with well-established consensus sequences, this type of pseudoexon activation is by far the most frequently reported. Here we review all known human disease-associated pseudoexons that carry functional splice sites and are activated by deep intronic sequence variants located outside splice site sequences. We delineate common characteristics that make this type of wild type pseudoexons distinct high-risk sites in the human genome.

New SHH and Known SIX3 Variants in a Series of Latin American Patients with Holoprosencephaly

Holoprosencephaly (HPE) is the failure of the embryonic forebrain to develop into 2 hemispheres promoting midline cerebral and facial defects. The wide phenotypic variability and causal heterogeneity make genetic counseling difficult. Heterozygous variants with incomplete penetrance and variable expressivity in the SHH, SIX3, ZIC2, and TGIF1 genes explain ∼25% of the known causes of nonchromosomal HPE. We studied these 4 genes and clinically described 27 Latin American families presenting with nonchromosomal HPE. Three new SHH variants and a third known SIX3 likely pathogenic variant found by Sanger sequencing explained 15% of our cases. Genotype-phenotype correlation in these 4 families and published families with identical or similar driver gene, mutated domain, conservation of residue in other species, and the type of variant explain the pathogenicity but not the phenotypic variability. Nine patients, including 2 with SHH pathogenic variants, presented benign variants of the SHH, SIX3, ZIC2, and TGIF1 genes with potential alteration of splicing, a causal proposition in need of further studies. Finding more families with the same SIX3 variant may allow further identification of genetic or environmental modifiers explaining its variable phenotypic expression.

IntSplice2: Prediction of the Splicing Effects of Intronic Single-Nucleotide Variants Using LightGBM Modeling

Prediction of the effect of a single-nucleotide variant (SNV) in an intronic region on aberrant pre-mRNA splicing is challenging except for an SNV affecting the canonical GU/AG splice sites (ss). To predict pathogenicity of SNVs at intronic positions −50 (Int-50) to −3 (Int-3) close to the 3’ ss, we developed light gradient boosting machine (LightGBM)-based IntSplice2 models using pathogenic SNVs in the human gene mutation database (HGMD) and ClinVar and common SNVs in dbSNP with 0.01 ≤ minor allelic frequency (MAF) < 0.50. The LightGBM models were generated using features representing splicing cis-elements. The average recall/sensitivity and specificity of IntSplice2 by fivefold cross-validation (CV) of the training dataset were 0.764 and 0.884, respectively. The recall/sensitivity of IntSplice2 was lower than the average recall/sensitivity of 0.800 of IntSplice that we previously made with support vector machine (SVM) modeling for the same intronic positions. In contrast, the specificity of IntSplice2 was higher than the average specificity of 0.849 of IntSplice. For benchmarking (BM) of IntSplice2 with IntSplice, we made a test dataset that was not used to train IntSplice. After excluding the test dataset from the training dataset, we generated IntSplice2-BM and compared it with IntSplice using the test dataset. IntSplice2-BM was superior to IntSplice in all of the seven statistical measures of accuracy, precision, recall/sensitivity, specificity, F1 score, negative predictive value (NPV), and matthews correlation coefficient (MCC). We made the IntSplice2 web service at https://www.med.nagoya-u.ac.jp/neurogenetics/IntSplice2.

Detection of Germline Variants in 450 Breast/Ovarian Cancer Families with a Multi-Gene Panel Including Coding and Regulatory Regions

With the progress of sequencing technologies, an ever-increasing number of variants of unknown functional and clinical significance (VUS) have been identified in both coding and non-coding regions of the main Breast Cancer (BC) predisposition genes. The aim of this study is to identify a mutational profile of coding and intron-exon junction regions of 12 moderate penetrance genes (ATM, BRIP1, CDH1, CHEK2, NBN, PALB2, PTEN, RAD50, RAD51C, RAD51D, STK11, TP53) in a cohort of 450 Italian patients with Hereditary Breast/Ovarian Cancer Syndrome, wild type for germline mutation in BRCA1/2 genes. The analysis was extended to 5′UTR and 3′UTR of all the genes listed above and to the BRCA1 and BRCA2 known regulatory regions in a subset of 120 patients. The screening was performed through NGS target resequencing on the Illumina platform MiSeq. 8.7% of the patients analyzed is carriers of class 5/4 coding variants in the ATM (3.6%), BRIP1 (1.6%), CHEK2 (1.8%), PALB2 (0.7%), RAD51C (0.4%), RAD51D (0.4%), and TP53 (0.2%) genes, while variants of uncertain pathological significance (VUSs)/class 3 were identified in 9.1% of the samples. In intron-exon junctions and in regulatory regions, variants were detected respectively in 5.1% and in 32.5% of the cases analyzed. The average age of disease onset of 44.4 in non-coding variant carriers is absolutely similar to the average age of disease onset in coding variant carriers for each proband’s group with the same cancer type. Furthermore, there is not a statistically significant difference in the proportion of cases with a tumor onset under age of 40 between the two groups, but the presence of multiple non-coding variants in the same patient may affect the aggressiveness of the tumor and it is worth underlining that 25% of patients with an aggressive tumor are carriers of a PTEN 3′UTR-variant. This data provides initial information on how important it might be to extend mutational screening to the regulatory regions in clinical practice.

Discovery of a pre-mRNA structural scaffold as a contributor to the mammalian splicing code

The specific recognition of splice signals at or near exon-intron junctions is not explained by their weak conservation and instead is postulated to require a multitude of features embedded in the pre-mRNA strand. We explored the possibility of 3D structural scaffold of AdML-a model pre-mRNA substrate-guiding early spliceosomal components to the splice signal sequences. We find that mutations in the non-cognate splice signal sequences impede recruitment of early spliceosomal components due to disruption of the global structure of the pre-mRNA. We further find that the pre-mRNA segments potentially interacting with the early spliceosomal component U1 snRNP are distributed across the intron, that there is a spatial proximity of 5' and 3' splice sites within the pre-mRNA scaffold, and that an interplay exists between the structural scaffold and splicing regulatory elements in recruiting early spliceosomal components. These results suggest that early spliceosomal components can recognize a 3D structural scaffold beyond the short splice signal sequences, and that in our model pre-mRNA, this scaffold is formed across the intron involving the major splice signals. This provides a conceptual basis to analyze the contribution of recognizable 3D structural scaffolds to the splicing code across the mammalian transcriptome.

Functional analysis of novel genetic variants of NKX2-5 associated with nonsyndromic congenital heart disease

NKX2-5, a master cardiac regulatory transcription factor was the first known genetic cause of congenital heart diseases (CHDs). To further investigate its role in CHD pathogenesis, we performed mutational screening of 285 CHD probands and 200 healthy controls. Five coding sequence variants were identified in six CHD cases (2.1%), including three in the N-terminal region (p.A61G, p.R95L, and p.E131K) and one each in homeodomain (HD) (p.A148E) and tyrosine-rich domain (p.P247A). Variant-p.A148E showed tertiary structure changes and differential DNA binding affinity of mutant compared to wild type. Two N-terminal variants-p.A61G and p.E131K along with HD variant p.A148E demonstrated significantly reduced transcriptional activity of Nppa and Actc1 promoters in dual luciferase promoter assay supported by their reduced expression in qRT-PCR. Nonetheless, variant p.R95L affected the synergy of NKX2-5 with serum response factor and TBX5 leading to significantly decreased Actc1 promoter activity depicting a distinctive role of this region. The aberrant expression of other target genes-Irx4, Mef2c, Bmp10, Myh6, Myh7, and Myocd is also observed in response to NKX2-5 variants, possibly due to the defective gene regulatory network. Severely impaired downstream promoter activities and abnormal expression of target genes due to N-terminal variants supports the emerging role of this region during cardiac-developmental pathways.

Benchmarking deep learning splice prediction tools using functional splice assays

Hereditary disorders are frequently caused by genetic variants that affect pre-messenger RNA splicing. Though genetic variants in the canonical splice motifs are almost always disrupting splicing, the pathogenicity of variants in the noncanonical splice sites (NCSS) and deep intronic (DI) regions are difficult to predict. Multiple splice prediction tools have been developed for this purpose, with the latest tools employing deep learning algorithms. We benchmarked established and deep learning splice prediction tools on published gold standard sets of 71 NCSS and 81 DI variants in the ABCA4 gene and 61 NCSS variants in the MYBPC3 gene with functional assessment in midigene and minigene splice assays. The selection of splice prediction tools included CADD, DSSP, GeneSplicer, MaxEntScan, MMSplice, NNSPLICE, SPIDEX, SpliceAI, SpliceRover, and SpliceSiteFinder-like. The best-performing splice prediction tool for the different variants was SpliceRover for ABCA4 NCSS variants, SpliceAI for ABCA4 DI variants, and the Alamut 3/4 consensus approach (GeneSplicer, MaxEntScacn, NNSPLICE and SpliceSiteFinder-like) for NCSS variants in MYBPC3 based on the area under the receiver operator curve. Overall, the performance in a real-time clinical setting is much more modest than reported by the developers of the tools.

Molecular characterization of colorectal cancer: A five-gene prognostic signature based on RNA-binding proteins

BACKGROUND

Colorectal cancer (CRC) is one of the most common cancers worldwide. RNA-binding proteins (RBPs) regulate essential biological processes and play essential roles in a variety of cancers. The present study screened differentially expressed RBPs, analyzed their function and constructed a prognostic model to predict the overall survival of patients with CRC.

METHODS

We downloaded CRC RNA-sequencing data from the Cancer Genome Atlas (TCGA) portal and screened differentially expressed RBPs. Then, functional analyses of these genes were performed, and a risk model was established by multivariate Cox regression.

RESULTS

We obtained 132 differentially expressed RBPs, including 66 upregulated and 66 downregulated RBPs. Functional analysis revealed that these genes were significantly enriched in RNA processing, modification and binding, ribosome biogenesis, post-transcriptional regulation, ribonuclease and nuclease activity. Additionally, some RBPs were significantly related to interferon (IFN)-alpha and IFN-beta biosynthetic processes and the Toll-like receptor signaling pathway. A prognostic model was constructed and included insulin like growth factor 2 messenger ribonucleic acid binding protein 3 (IGF2BP3), poly (A) binding protein cytoplasmic 1 like (PABPC1L), peroxisome proliferator activated receptor gamma coactivator 1 alpha (PPARGC1A), peptidyl- transfer ribonucleic acid hydrolase 1 homolog (PTRH1) and tudor domain containing 7 (TDRD7). The model is an independent risk factor for clinicopathological characteristics.

CONCLUSION

Our study provided novel insights into the pathogenesis of CRC and constructed a prognostic gene model, which may be helpful for determining the prognosis of CRC.

A Missense Mutation rs781536408 (c.2395G>A) of TYK2 Affects Splicing and Causes Skipping of Exon18 in vivo

TYK2 variants can impact disease onset or progression. In our previous study, we identified abnormal splicing that happened near rs781536408 in the TYK2 gene. The purpose of this research was to examine the effect of the mutation on alternative splicing in vivo and in vitro. Whole exome sequencing was performed to identify the mutations followed by bidirectional Sanger sequencing. Then the minigene analysis was carried out based on HeLa and HEK293T cell lines. The results showed that rs781536408 (c.2395G>A, p.G799R) was homozygous in the patient, but heterozygous in parents. PCR amplification confirmed the abnormal splicing in the somatic cells of the patients, but not in the parents. Sanger sequencing results showed that there was a skipping of exon18 near the mutation. For minigene analysis, there was no difference between the wild-type and the mutant type in the two minigene construction strategies, indicating that mutation c.2395G>A had no effect on splicing in vitro. Combining the results of in vivo, we speculated that the effect of the mutation on splicing was not absolute, but rather in degree.

Principles and Practical Considerations for the Analysis of Disease-Associated Alternative Splicing Events Using the Gateway Cloning-Based Minigene Vectors pDESTsplice and pSpliceExpress

Splicing is an important RNA processing step. Genetic variations can alter the splicing process and thereby contribute to the development of various diseases. Alterations of the splicing pattern can be examined by gene expression analyses, by computational tools for predicting the effects of genetic variants on splicing, and by splicing reporter minigene assays for studying alternative splicing events under defined conditions. The minigene assay is based on transient transfection of cells with a vector containing a genomic region of interest cloned between two constitutive exons. Cloning can be accomplished by the use of restriction enzymes or by site-specific recombination using Gateway cloning. The vectors pDESTsplice and pSpliceExpress represent two minigene systems based on Gateway cloning, which are available through the Addgene plasmid repository. In this review, we describe the features of these two splicing reporter minigene systems. Moreover, we provide an overview of studies in which determinants of alternative splicing were investigated by using pDESTsplice or pSpliceExpress. The studies were reviewed with regard to the investigated splicing regulatory events and the experimental strategy to construct and perform a splicing reporter minigene assay. We further elaborate on how analyses on the regulation of RNA splicing offer promising prospects for gaining important insights into disease mechanisms.

Clinical Utility of Functional RNA Analysis for the Reclassification of Splicing Gene Variants in Hereditary Cancer

BACKGROUND

Classification of splicing variants (SVs) in genes associated with hereditary cancer is often challenging. The aim of this study was to investigate the occurrence of SVs in hereditary cancer genes and the clinical utility of RNA analysis.

MATERIAL AND METHODS

1518 individuals were tested for cancer predisposition, using a Next Generation Sequencing (NGS) panel of 36 genes. Splicing variant analysis was performed using RT-PCR and Sanger Sequencing.

RESULTS

In total, 34 different SVs were identified, 53% of which were classified as pathogenic or likely pathogenic. The remaining 16 variants were initially classified as Variant of Uncertain Significance (VUS). RNA analysis was performed for 3 novel variants.

CONCLUSION

The RNA analysis assisted in the reclassification of 20% of splicing variants from VUS to pathogenic. RNA analysis is essential in the case of uncharacterized splicing variants, for proper classification and personalized management of these patients.

Genetic analysis of 39 erythrocytosis and hereditary hemochromatosis-associated genes in the Slovenian family with idiopathic erythrocytosis

BACKGROUND

Erythrocytosis is a condition with an excessive number of erythrocytes, accompanied by an elevated haemoglobin and/or haematocrit value. Congenital erythrocytosis has a diverse genetic background with several genes involved in erythropoiesis. In clinical practice, nine genes are usually examined, but in approximately 70% of patients, no causative mutation can be identified. In this study, we screened 39 genes, aiming to identify potential disease-driving variants in the family with erythrocytosis of unknown cause.

PATIENTS AND METHODS

Two affected family members with elevated haemoglobin and/or haematocrit and negative for acquired causes and one healthy relative from the same family were selected for molecular-genetic analysis of 24 erythrocytosis and 15 hereditary haemochromatosis-associated genes with targeted NGS. The identified variants were further analysed for pathogenicity using various bioinformatic tools and review of the literature.

RESULTS

Of the 12 identified variants, two heterozygous variants, the missense variant c.471G>C (NM_022051.2) (p.(Gln157His)) in the EGLN1 gene and the intron variant c.2572-13A>G (NM_004972.3) in the JAK2 gene, were classified as low-frequency variants in European population. None of the two variants were present in a healthy family member. Variant c.2572-13A>G has potential impact on splicing by one prediction tool.

CONCLUSION

For the first time, we included 39 genes in the erythrocytosis clinical panel and identified two potential disease-driving variants in the Slovene family studied. Based on the reported functional in vitro studies combined with our bioinformatics analysis, we suggest further functional analysis of variant in the JAK2 gene and evaluation of a cumulative effect of both variants.

Hybrid Minigene Assay: An Efficient Tool to Characterize mRNA Splicing Profiles of NF1 Variants

Neurofibromatosis type 1 (NF1) is caused by heterozygous loss of function mutations in the NF1 gene. Although patients are diagnosed according to clinical criteria and few genotype-phenotype correlations are known, molecular analysis remains important. NF1 displays allelic heterogeneity, with a high proportion of variants affecting splicing, including deep intronic alleles and changes outside the canonical splice sites, making validation problematic. Next Generation Sequencing (NGS) technologies integrated with multiplex ligation-dependent probe amplification (MLPA) have largely overcome RNA-based techniques but do not detect splicing defects. A rapid minigene-based system was set up to test the effects of NF1 variants on splicing. We investigated 29 intronic and exonic NF1 variants identified in patients during the diagnostic process. The minigene assay showed the coexistence of multiple mechanisms of splicing alterations for seven variants. A leaky effect on splicing was documented in one de novo substitution detected in a sporadic patient with a specific phenotype without neurofibromas. Our splicing assay proved to be a reliable and fast method to validate novel NF1 variants potentially affecting splicing and to detect hypomorphic effects that might have phenotypic consequences, avoiding the requirement of patient's RNA.

Alternative splicing: An important regulatory mechanism in colorectal carcinoma

Alternative splicing (AS) is a process that produces various mRNA splicing isoforms via different splicing patterns of mRNA precursors (pre-mRNAs). AS is the primary mechanism for increasing the types and quantities of proteins to improve biodiversity and influence multiple biological processes, including chromatin modification, signal transduction, and protein expression. It has been reported that AS is involved in the tumorigenesis and development of colorectal carcinoma (CRC). In this review, we delineate the concept, types, regulatory processes, and technical advances of AS and focus on the role of AS in CRC initiation, progression, treatment, and prognosis. This summary of the current knowledge about AS will contribute to our understanding of CRC initiation and development. This study will help in the discovery of novel biomarkers and therapeutic targets for CRC prognosis and treatment.

Use of RNA‑sequencing to detect abnormal transcription of the collagen α‑2 (VI) chain gene that can lead to Bethlem myopathy

Bethlem myopathy (BM) is an autosomal dominant or autosomal recessive disorder and is usually associated with mutations in the collagen VI genes. In the present study, the pathogenicity of a novel splice-site mutation was explored using RNA-sequencing in a family with suspected BM, and a myopathy panel was performed in the proband. The genetic status of all family members was confirmed using Sanger sequencing. Clinical data and magnetic resonance imaging (MRI) features were also documented. In silico analysis was performed to predict the effects of the splice mutation. RNA-sequencing and reverse transcription (RT)-PCR were used to assess aberrant splicing. Immunocytochemistry was conducted to measure collagen VI protein levels within the gastrocnemius and in cultured skin fibroblasts. The results revealed that three patients in the family shared a similar classic BM presentation. MRI revealed distinct patterns of fatty infiltration in the lower extremities. A novel splicing mutation c.736-1G>C in the collagen α-2 (VI) chain (COL6A2) gene was found in all three patients. In silico analysis predicted that the mutation would destroy the normal splice acceptor site. RNA-sequencing detected two abnormal splicing variants adjacent to the mutation site, and RT-PCR confirmed the RNA-sequencing findings. Furthermore, a defect in the collagen protein within cultured fibroblasts was detected using immunocytochemistry. The mutation c.736-1G>C in the COL6A2 gene caused aberrant splicing and led to premature termination of protein translation. In conclusion, these findings may improve our knowledge of mutations of the COL6A2 gene associated with BM and demonstrated that RNA-sequencing can be a powerful tool for finding the underlying mechanism of a disease-causing mutations at a splice site.

Molecular analysis of GALT gene in Argentinian population: Correlation with enzyme activity and characterization of a novel Duarte-like allele

Background

Classical galactosemia is an autosomal recessive inherited metabolic disorder caused by mutations in the galactose-1-phosphate uridyltransferase (GALT) gene. GALT enzyme deficiency leads to the accumulation of galactose-1-phosphate in various organs, causing hepatic, renal and cerebral impairment. Over 300 mutations have been reported in the GALT gene. The aim of this study was to describe molecular characterization of GALT gene in Argentinian patients with decreased GALT activity, and to correlate molecular results with enzyme activity.

Methods

37 patients with enzyme activity below 6.3 μmol/h/g Hb (35% of normal value) were included. GALT activity was measured on red blood cells. DNA was extracted from peripheral blood. p.Gln188Arg mutation was studied by PCR-RFLP and, on samples negative or heterozygous, GALT gene was sequenced. In vivo splicing analysis of the GALT gene was performed on RNA extracted from leukocytes of one patient.

Results

14 different sequence variations were identified among 72 unrelated alleles. The two most common disease-causing mutations were p.Gln188Arg (24/72) and p.Lys285Asn (9/72). Three novel mutations were detected. One of them, c.688G>A, caused partial skipping of exon 9 of the GALT gene. Enzyme activity correlated with GALT genotype in 36 of the 37 patients.

Conclusion

This is the first report of sequence variations in the GALT gene in the Argentinian population. This study highlights the contribution of the molecular analysis to the diagnosis of Galactosemia and reveals c.688G>A as a novel Duarte-like variant, with a high prevalence in our population.

ESI mutagenesis: a one-step method for introducing mutations into bacterial artificial chromosomes

A simple and efficient recombineering-based method for introducing point mutations into bacterial artificial chromosomes using an artificial intron cassette.

Bacterial artificial chromosome (BAC)–based transgenes have emerged as a powerful tool for controlled and conditional interrogation of protein function in higher eukaryotes. Although homologous recombination-based recombineering methods have streamlined the efficient integration of protein tags onto BAC transgenes, generating precise point mutations has remained less efficient and time-consuming. Here, we present a simplified method for inserting point mutations into BAC transgenes requiring a single recombineering step followed by antibiotic selection. This technique, which we call exogenous/synthetic intronization (ESI) mutagenesis, relies on co-integration of a mutation of interest along with a selectable marker gene, the latter of which is harboured in an artificial intron adjacent to the mutation site. Cell lines generated from ESI-mutated BACs express the transgenes equivalently to the endogenous gene, and all cells efficiently splice out the synthetic intron. Thus, ESI mutagenesis provides a robust and effective single-step method with high precision and high efficiency for mutating BAC transgenes.

Splicing analysis of SLC40A1 missense variations and contribution to hemochromatosis type 4 phenotypes

Hemochromatosis type 4, or ferroportin disease, is considered as the second leading cause of primary iron overload after HFE-related hemochromatosis. The disease, which is predominantly associated with missense variations in the SLC40A1 gene, is characterized by wide clinical heterogeneity. We tested the possibility that some of the reported missense mutations, despite their positions within exons, cause splicing defects. Fifty-eight genetic variants were selected from the literature based on two criteria: a precise description of the nucleotide change and individual evidence of iron overload. The selected variants were investigated by different in silico prediction tools and prioritized for midigene splicing assays. Of the 15 variations tested in vitro, only two were associated with splicing changes. We confirm that the c.1402G>A transition (p.Gly468Ser) disrupts the exon 7 donor site, leading to the use of an exonic cryptic splicing site and the generation of a truncated reading frame. We observed, for the first time, that the p.Gly468Ser substitution has no effect on the ferroportin iron export function. We demonstrate alternative splicing of exon 5 in different cell lines and show that the c.430A>G (p.Asn144Asp) variant promotes exon 5 inclusion. This could be part of a gain-of-function mechanism. We conclude that splicing mutations rarely contribute to hemochromatosis type 4 phenotypes. An in-depth investigation of exon 5 auxiliary splicing sequences may help to elucidate the mechanism by which splicing regulatory proteins regulate the production of the full length SLC40A1 transcript and to clarify its physiological importance.

Impact of a Gap Junction Protein Alpha 4 Variant on Clinical Disease Phenotype in F508del Homozygous Patients With Cystic Fibrosis

Background

Lung disease phenotype varies widely even in the F508del (homozygous) genotype. Leukocyte-driven inflammation is important for pulmonary disease pathogenesis in cystic fibrosis (CF). Blood cytokines correlate negatively with pulmonary function in F508del homozygous patients, and gap junction proteins (GJA) might be related to the influx of blood cells into the lung and influence disease course. We aimed to assess the relationship between GJA1/GJA4 genotypes and the clinical disease phenotype.

Methods

One-hundred-and-sixteen homozygous F508del patients (mean age 27 years, m/f 66/50) were recruited from the CF centers of Bonn, Frankfurt, and Amsterdam. Sequence analysis was performed for GJA1 and GJA4. The clinical disease course was assessed over 3 years using pulmonary function tests, body mass index, Pseudomonas aeruginosa colonization, diabetes mellitus, survival to end-stage lung disease, blood and sputum inflammatory markers.

Results

Sequence analysis revealed one clinically relevant single nucleotide polymorphism. In this GJA4 variant (rs41266431), homozygous G variant carriers (n = 84/116; 72.4%) had poorer pulmonary function (FVC% pred: mean 78/86, p < 0.040) and survival to end-stage lung disease was lower (p < 0.029). The frequency of P. aeruginosa colonization was not influenced by the genotype, but in those chronically colonized, those with the G/G genotype had reduced pulmonary function (FVC% pred: mean 67/80, p < 0.049). Serum interleukin-8 (median: 12.4/6.7 pg/ml, p < 0.052) and sputum leukocytes (2305/437.5 pg/ml, p < 0.025) were higher for the G/G genotype.

Conclusions

In carriers of the A allele (27.6%) the GJA4 variant is associated with significantly better protection against end-stage lung disease and superior pulmonary function test results in F508del homozygous patients. This SNP has the potential of a modifier gene for phenotyping severity of CF lung disease, in addition to the CFTR genotype.

Clinical Trial Registration

The study was registered with ClinicalTrials.gov, number NCT04242420, retrospectively on January 24th, 2020.

The novel function of miR-3195 for mutant PROK2 (c.223-4C>A) degradation

Kallmann syndrome (KS) is a rare human genetic disorder characterized by hypogonadotropic hypogonadism with the reduction or absence of olfactory sense. Mutations in multiple genes, including chemokine prokineticin-2 (PROK2), are considered to contribute to the abnormal migration of gonadotropin-releasing hormone neurons in the embryonic stage. However, the mechanisms of the different inheritance modes of KS have not been comprehensively determined. In this article, we present the case of one KS patient with the same mutation in PROK2 (c.223-4C>A) as his mother. RNA sequencing analysis of his leukocytes showed a new transcript of PROK2, which contained a partial intron (192 bp) compared to those of his parents. Furthermore, we observed that hsa-miR-3195 was expressed at low levels in his and his father's sera compared to his mother's. Unexpectedly, hsa-miR-3195 was also identified to specifically target the 192 bp intron of the aberrant PROK2 transcript of this patient. We determined that high expression of hsa-miR-3195 could efficiently target aberrant PROK2 and stabilize the normal function of PROK2 in vitro, which provided a probable explanation for the different phenotypes of the patient and his mother with the same genotype.

The emergence of new trends in clinical laboratory diagnosis

Diagnostic processes typically rely on traditional and laborious methods, that are prone to human error, resulting in frequent misdiagnosis of diseases. Computational approaches are being increasingly used for more precise diagnosis of the clinical pathology, diagnosis of genetic and microbial diseases, and analysis of clinical chemistry data. These approaches are progressively used for improving the reliability of testing, resulting in reduced diagnostic errors. Artificial intelligence (AI)-based computational approaches mostly rely on training sets obtained from patient data stored in clinical databases. However, the use of AI is associated with several ethical issues, including patient privacy and data ownership. The capacity of AI-based mathematical models to interpret complex clinical data frequently leads to data bias and reporting of erroneous results based on patient data. In order to improve the reliability of computational approaches in clinical diagnostics, strategies to reduce data bias and analyzing real-life patient data need to be further refined.

Enigmatic rhodopsin mutation creates an exceptionally strong splice acceptor site

The c.620 T > G mutation in rhodopsin found in the first mapped autosomal dominant retinitis pigmentosa (adRP) locus is associated with severe, early-onset RP. Intriguingly, another mutation affecting the same nucleotide (c.620 T > A) is related to a mild, late-onset RP. Assuming that both mutations are missense mutations (Met207Arg and Met207Lys) hampering the ligand-binding pocket, previous work addressed how they might differentially impair rhodopsin function. Here, we investigated the impact of both mutations at the mRNA and protein level in HEK293 cells and in the mouse retina. We show that, in contrast to c.620 T > A, c.620 T > G is a splicing mutation, which generates an exceptionally strong splice acceptor site (SAS) resulting in a 90 bp in-frame deletion and protein mislocalization in vitro and in vivo. Moreover, we identified the core element underlying the c.620 T > G SAS strength. Finally, we demonstrate that the c.620 T > G SAS is very flexible in branch point choice, which might explain its remarkable performance. Based on these results, we suggest that (i) point mutations should be routinely tested for mRNA splicing to avoid dispensable analysis of mutations on protein level, which do not naturally exist. (ii) Puzzling disease courses of mutations in other genes might also correlate with their effects on mRNA splicing. (iii) Flexibility in branch point choice might be another factor influencing the SAS strength. (iv) The core splice element identified in this study could be useful for biotechnological applications requiring effective SAS.

Systematics for types and effects of RNA variations

Systematics is described for annotation of variations in RNA molecules. The conceptual framework is part of Variation Ontology (VariO) and facilitates depiction of types of variations, their functional and structural effects and other consequences in any RNA molecule in any organism. There are more than 150 RNA related VariO terms in seven levels, which can be further combined to generate even more complicated and detailed annotations. The terms are described together with examples, usually for variations and effects in human and in diseases. RNA variation type has two subcategories: variation classification and origin with subterms. Altogether six terms are available for function description. Several terms are available for affected RNA properties. The ontology contains also terms for structural description for affected RNA type, post-transcriptional RNA modifications, secondary and tertiary structure effects and RNA sugar variations. Together with the DNA and protein concepts and annotations, RNA terms allow comprehensive description of variations of genetic and non-genetic origin at all possible levels. The VariO annotations are readable both for humans and computer programs for advanced data integration and mining.