Background: An estimated 10–15% of all genetic diseases are attributable to variants in noncanonical splice sites, auxiliary splice sites and deep-intronic variants. Most of these unstudied variants are classified as variants of uncertain significance (VUS), which are not clinically actionable. This study investigated two novel splice-altering variants, CHM NM_000390.4:c.941-11T>G and CACNA1F NM_005183.4:c.2576+4_2576+5del implicated in choroideremia and cone dystrophy (COD), respectively, resulting in significant visual loss. Methods: Next-generation sequencing was employed to identify the candidate variants in CHM and CACNA1F, which were confirmed using Sanger sequencing. Cascade analysis was undertaken when additional family members were available. Functional analysis was conducted by cloning genomic regions of interest into gateway expression vectors, creating variant and wildtype midigenes, which were subsequently transfected into HEK293 cells. RNA was harvested and amplified by RT-PCR to investigate the splicing profile for each variant compared to the wildtype. Novel variants were reclassified according to ACMG/AMP and ClinGen SVI guidelines. Results: Midigene functional analysis confirmed that both variants disrupted splicing. The CHM NM_000390.4:c.941-11T>G variant caused exon 8 skipping, leading to a frameshift and the CACNA1F NM_005183.4:c.2576+4_2576+5del variant caused a multimodal splice defect leading to an in-frame insertion of seven amino acids and a frameshift. With this evidence, the former was upgraded to likely pathogenic and the latter to a hot VUS. Conclusions: This study adds to the mutational spectrum of splicing defects implicated in retinal degenerations by identifying and characterising two novel variants in CHM and CACNA1F. Our results highlight the importance of conducting functional analysis to investigate the consequences of intronic splice-altering variants and the significance of reclassifying VUS to confirm a genetic diagnosis.

Variants in pre-messenger RNA (mRNA) processing genes account for ∼15% to 20% of autosomal dominant retinitis pigmentosa, causing severe vision loss. We provide an in-depth characterization of pre-mRNA-processing variants in the population, with a focus on genotype-phenotype correlations. Participants underwent next-generation sequencing, variant filtering and interpretation. Impact(s) on protein structure/function were predicted through AlphaFold, ColabFold, among others. Clinical information was collated and segregation analysis undertaken. Variants were classified according to American College of Medical Genetics and Genomics/Association for Molecular Pathology guidelines. A total of 635 patients with retinitis pigmentosa (RP) were screened. Notably, 36 different variants were identified in PRPF31, PRPF8, SNRNP200, PRPF3, RP9, and PRPF 6; 19 were classified as likely pathogenic/pathogenic. Thirteen of 36 variants were absent from PubMed, gnomAD, LOVD, and ClinVar, with an additional 10 not previously associated with retinal degeneration. Genotype-phenotypes correlations were illuminated; variants in PRPF genes caused more severe retinal degeneration than SNRNP200 variants. We provide a detailed overview of the genetic landscape of variants in pre-mRNA-processing factor genes in Ireland, accounting for 14% of genetically solved autosomal dominant retinitis pigmentosa cases. Results highlight significant genetic diversity between populations, mutational diversity inherent in inherited retinal degenerations and the role of variants in mRNA-processing genes as causative of devastating ocular disorders.

Objectives: This study investigated a variant, RPGR NM_001034853.2 c.1307G>A, p.[Gly436Asp, p?], in a large Irish pedigree with severe X-Linked Retinitis Pigmentosa (XLRP). The effect of the variant on RNA splicing was interrogated using in vitro functional analysis to provide evidence of disease causality. Methods: Three related individuals presenting with XLRP underwent target-capture sequencing, together with confirmatory Sanger sequencing and cascade analyses, to identify candidate variants. In silico investigations were undertaken using SpliceAI (version 1.3.1) and Alamut Visual software (version 2.13), among others. Functional analyses using in vitro midigene splice assays employing gateway expression vectors were undertaken. Variant and wildtype RNA were amplified by RT-PCR to investigate effects on splicing. RPGR c.1307G>A was subsequently reclassified using ACMG/AMP and ClinGen SVI recommendations. Results: Midigene investigation confirmed a cryptic acceptor site is being utilised together with the cryptic branchpoint motif to excise intron 10 and 90 bases of exon 11, leading to a frameshift and the creation of a premature stop codon. No functional RPGR transcript is predicted to remain. Given evidence of aberrant splicing, the variant classification was upgraded to pathogenic. Conclusions: RPGR c.1307G>A leads to creation of a cryptic branchpoint within an exon, resulting in protein truncation with deleterious effect(s). To the best of our knowledge, this is the first variant that leads to creation of a cryptic branchpoint within an exon associated with any IRD. The results illustrate the importance of investigating the functional consequences of both coding and non-coding variants with a predicted impact on splicing to understand their pathogenicity.