Long-read sequencing reveals a hidden Alu-mediated splice defect in CPLANE1, causing orofaciodigital syndrome type VI.

Orofaciodigital syndrome type VI (OFD VI) is a recessive ciliopathy characterized by excessive polydactyly, molar tooth sign, cleft lip, and developmental delay, caused by pathogenic variants in CPLANE1. Here, we present a patient with OFD VI that remained genetically unexplained after routine genetic testing, including short-read whole genome sequencing (WGS). Using long-read sequencing, we found two biallelic splice-site variants in CPLANE1, c.8633-4_8633-3del, and an Alu element insertion close to an exon-intron boundary. Transcript analysis showed that each variant independently resulted in exon skipping, and quantitative expression studies revealed reduced total CPLANE1 mRNA levels in patient-derived fibroblasts. Based on these findings, we were able to re-classify the c.8633-4_8633-3del variant from a variant of uncertain significance (VUS) to likely pathogenic. The identification of an Alu element insertion missed by short-read WGS highlights the added diagnostic value of long-read sequencing in uncovering cryptic, transposable element-associated pathogenic variants.