<scp>CRISPR</scp> Activation Reveals the Spliceogenicity of an Intronic <scp> <i>NEB</i> </scp> Variant in Fetuses With Arthrogryposis Multiplex Congenita 6

Whole-genome sequencing identifies intronic variants whose pathogenicity can be predicted with tools like SpliceAI. However, an actionable classification of such variants may require RNA-based validation, which can be limited by low expression in clinically accessible tissues. We report two fetuses from one family with Arthrogryposis multiplex congenita 6 (AMC6 [OMIM # 619334]) and biallelic NEB variants: a paternally inherited likely pathogenic frameshift variant, Chr2(GRCh38):g.151579391del; NM_001164508.2:c.16653del; NP_001157980.2:p.(Asp5552ThrfsTer5), and a maternally inherited intronic variant of uncertain clinical significance, Chr2(GRCh38):g.151496267G>A; NM_001164508.2:c.24486+9C>T; NP_001157980.2:p.(?). Because NEB is poorly expressed in fibroblasts, we used CRISPR activation to induce its expression in fibroblasts from the heterozygous mother. RNA-sequencing subsequently confirmed that the intronic variant generated a novel splice donor site associated with inferred loss of splicing at the canonical donor site. After NMD-inhibition, we could thus identify 45.5% of NEB transcripts with a 7 bp exon extension, predicted to result in a protein-coding frameshift. The intronic variant was classified as likely pathogenic, allowing a genetic diagnosis.