Studying Familial Bainbridge–Ropers Syndrome Due to a Novel ASXL3 Germline Variant and Expanding the Clinical Spectrum

Background/Objectives: Bainbridge–Ropers syndrome (BRPS) is a rare neurodevelopmental disorder caused by truncating and splicing pathogenic variants in the additional sex combs-like 3 (ASXL3) gene. It is primarily characterized by neurodevelopmental delay and craniofacial dysmorphism. Most reported cases involve de novo mutations in the ASXL3 gene, whereas inherited mutations have been rarely described. The present report aims to describe the clinical and molecular presentation of a familial case of BRPS and to highlight the potential role of parental mosaicism. Methods: We describe the clinical and molecular presentation of a 12-year-old boy and his 20-month-old half-brother, both affected by Bainbridge–Ropers syndrome. Trio-exome sequencing (ES) was performed in the family to identify variants in the ASXL3 gene, and targeted Sanger sequencing was also performed for segregation analysis. Results: Genetic analysis identified a previously unreported heterozygous frameshift variant in the ASXL3 gene (c.1648_1649del; p.Met550Aspfs*5) shared by both siblings. The variant was inherited from their clinically unaffected mother, who carries the mutation in the mosaic state with a variant allele fraction of approximately 15% in peripheral blood DNA. Conclusions: This observation highlights parental mosaicism as a potential mechanism underlying the familial recurrence of BRPS and emphasizes the importance of considering mosaic variants during the genetic evaluation and counseling of affected families.