P673: RNA analysis and long read sequencing identify causative APC complex rearrangement in unsolved FAP case

transcripts in blood-derived RNA sequencing data by visual inspection in the integrated genome viewer.We finally Sanger-sequenced a suggestive region and used in silico prediction to interpret the identified variant.All wet lab analyses were done from dried blood spot filter cards.Results: The clinical information (11-year-old boy presenting with short stature, joint stiffness, dysostosis multiplex, a coarse face, umbilical and inguinal hernia, hypertrophic adenoids, and recurrent infections) led us to suspect mucopolysaccharidosis (MPS) or a related metabolic disorder.Enzyme activities were normal for ARSB, GUSB, IDUA, GALNS, GLB1, MAN2B1, but below the level of detection for IDS.The transcripts for most candidate genes showed no abnormalities, but insertion of an intron 6-derived stretch of 46 nucleotides was observed in the IDS transcript.DNA sequencing of this intronic region (46 +/-80 nucleotides) revealed a hemizygous IDS variant (c.880-1180A>T; NM_000202.5),which is predicted to create a strong splice donor.Together with a nearby splice acceptor from a rarely used alternative exon, this motif suggests that the 46-nucleotide inclusion is due to aberrant splicing.The variant was classified as likely pathogenic based on ACMG/AMP/ClinGen guidelines for observed impact on splicing. Conclusion:Our study demonstrates the significant potential of non-DNA primary screening approaches, particularly enzyme activity assays and RNAsequencing, in improving the efficiency and cost-effectiveness of genetic diagnostics for rare (metabolic) disorders.It also suggests that such a multi-omics concept can be based on dried blood spot filter cards as the single sample type.To establish the utility of our approach in routine clinical settings, further research involving systematic, automated protocols and large-scale validation studies is necessary.