Beyond genomics: using RNA-seq from dried blood spots to unlock the clinical relevance of splicing variation in a diagnostic setting

We aimed to assess the impact of splicing variants reported in our laboratory to gain insight into their clinical relevance. A total of 108 consecutive individuals, for whom 113 splicing variants had been reported, were selected for RNA-sequencing (RNA-seq), considering the gene expression in blood. A protocol was developed to perform RNA extraction and sequencing using the same sample (dried blood spots, DBS) provided for the DNA analysis, including library preparation and bioinformatic pipeline analysis. Splicing in genes of interest was inspected using IGV, with at least three unaffected individuals as controls. From the 113 variants, we confirmed an abnormal splicing in 64 variants (57%). In 15 variants (13%), we did not observe a splicing alteration. In the remaining 34 variants, no decision could be made on the splicing effect due to insufficient sample quality (21%) or a low number of reads (9%). The most common event leading to aberrant splicing was exon skipping, identified in 31 variants (48%). Other events included cryptic donor/acceptor site usage (n = 25; 39%), intron retention (n = 4; 6%), and other complex events (n = 4; 6%). In three patients, pathologically reduced enzymatic activity (measured using the same DBS) served as additional confirmation of the abnormal splicing caused by variants in HEXA, GAA, and GLA. We implemented an RNA-seq pipeline using the same sample provided for genomic testing. This multiomic approach, as implemented in our routine diagnostic processes, clarifies the clinical relevance of most of the analyzed variants and delivers more comprehensive genetic testing.