RNA-sequencing first approach generates new diagnostic candidates in Mendelian disorders

Background RNA-sequencing is increasingly being used as a complementary tool to DNA sequencing in diagnostics where DNA analysis has been uninformative. RNA-sequencing allows us to identify alternative splicing and aberrant gene expression allowing for improved interpretation of variants of unknown significance (VUS). Additionally, RNA-sequencing provides the opportunity not only to look at the splicing effects of known VUSs but also to scan the transcriptome for abnormal splicing events and expression abnormalities in other relevant genes that may be the cause of a patient’s phenotype. Methods Using RNA from patient blood, we have systematically assessed transcriptomic profiles of 87 patients with suspected Mendelian disorders, 38% of which did not have a candidate sequence variant. Cases with VUSs and known events were assessed first followed by assessment of cases with no VUS. Each VUS was visually inspected using the Integrative Genomics Viewer (IGV) to search for splicing abnormalities. Once aberrant splicing was identified in cases with VUS, multiple open-source alternative splicing tools (MAJIQ, rMATS-turbo, FRASER2 and LeafCutterMD) were used to investigate if they would identify what was observed in IGV. Expression outliers were detected using OUTRIDER. To find diagnoses in cases without a VUS or gene of interest, two separate strategies were used. The first was a genotype to phenotype approach using variant calls obtained from the RNA-sequencing and overlapping those calls with results from splicing tools. The second strategy involved using phenotype information available to filter results from splicing tools. Results Using RNA-sequencing only, we were able to assess 71% of VUSs and detect aberrant splicing in 14/48 patients with a VUS. Furthermore, we identified four new diagnoses by detecting novel aberrant splicing events in patients with no candidate sequence variants from prior genomic DNA testing (n=33) or those in which the candidate VUS did not affect splicing (n=23) and identified one additional diagnosis through detection of skewed X-inactivation. Conclusion We demonstrate the identification of novel diagnoses using an RNA-sequencing first approach in patients without candidate VUSs. Furthermore, we demonstrate the utility of blood-based RNA analysis in improving diagnostic yields and highlight optimal approaches for such analysis. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work was funded by a National Institute for Health Research (NIHR) Research Professorship grant (RP-2016-07-011) awarded to Diana Baralle. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethics committee of University of Southampton gave ethical approval for this work I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors [https://github.com/carojoquendo/RNA\_splicing\_and_disease][1] [1]: https://github.com/carojoquendo/RNA_splicing_and_disease