Multiexon COL1A2 deletion as a rare mechanism in osteogenesis imperfecta: Case report and literature review.

Osteogenesis imperfecta (OI) is mostly caused by pathogenic variants in COL1A1/COL1A2; while single nucleotide variants predominate, multiexon copy number variants (CNVs) remain under-recognised contributors with incompletely defined phenotypic impacts. We investigated a fetus presenting severe skeletal dysplasia using NGS with CNV calling and MLPA, clinical and radiologic assessments, and transcript analysis of maternal fibroblasts. We also reviewed the literature and curated databases for reported multiexon COL1A2 deletions. The fetus exhibited lethal OI type II features, including progressive long bone shortening and bowing, multiple fractures, diffuse hypomineralisation, and a bell-shaped thorax. Genetic analysis revealed a heterozygous in-frame COL1A2 exons 4-17 deletion. Maternal testing identified low-level mosaicism for this large deletion alongside a germline in-frame deletion of exons 12-17. Detailed breakpoint analysis of the exon 12-17 deletion revealed a complex rearrangement characterised by the insertion of an inverted duplicated segment of exon 3 and intron 3 at the deletion junction. Despite these findings, the mother showed only joint hypermobility and a family history of osteoporosis, without overt features of OI. N-terminal in-frame COL1A2 deletions show variable expressivity, including perinatal lethality. We report a novel, complex and likely unstable genomic rearrangement which probably predisposed to a secondary, larger deletion. Integrated CNV analysis and parental studies are crucial to ensure accurate recurrence risk counselling.