BACKGROUND: Clinicians often deal with copy-number variants of unknown significance (CNVUS) when managing neurodevelopmental disorders (NDDs). Variant classification is often complemented with textual comments, while the American College of Medical Genetics and Genomics (ACMG)/Clinical Genome Resource (ClinGen) numerical scores are rarely reported. Our aim was to determine if the application of ACMG/ClinGen scoring and inheritance/segregation studies are relevant for the reclassification of CNVUS. METHODS: We retrieved 167 CNVUS (112 duplications, 55 heterozygous deletions) from test reports of 141 patients with NDD in a 5-year period. None of those testing reports included ACMG/ClinGen scoring information for the CNVUS. One clinical and one laboratorial geneticist independently applied the ACMG/ClinGen scoring system for CNVs. Final scores/categories were assessed for potential modification when adding inheritance/segregation criteria. RESULTS: 138 (83%) of the CNVUS retained the VUS classification, 14 (8%) changed to benign and 15 (9%) to (likely) pathogenic. Variants deemed benign (11 duplications, 3 deletions) mostly overlapped with ClinGen-established benign regions or were common in the general population
variants deemed (likely) pathogenic (all deletions) were either associated with unrelated autosomal recessive/later-onset autosomal dominant (AD) conditions, or with an AD NDD phenotype in a single case. Inheritance studies were available for 20 (12%) variants (17 inherited, 3 de novo), and none led to a change in classification. A simulation showed that adding inheritance information would also not change the classification of any other variant. CONCLUSION: Application of the ACMG/ClinGen scoring system led by itself to reclassification of 17% of VUS, despite a very low increase in diagnostic yield (1/141, 0.7%). Additionally, segregation/inheritance studies in CNVUS were mostly irrelevant in most NDD cases, challenging their routine broad application in clinical practice.