Objective: To explore the genotype-phenotype relationship in a child with Opitz G/BBB syndrome (OS) with mild clinical phenotype.

Methods: A child with motor developmental delay as the initial symptom admitted to Xi'an Children's Hospital on June 10, 2021 was selected for this study. Clinical data were collected, and peripheral blood samples were obtained from the child and his mother. Whole exome sequencing (WES) was performed to identify genetic variant in the child. Candidate variant were verified by Sanger sequencing to assess inheritance patterns and pathogenicity. Real-time fluorescence quantitative PCR (RT-qPCR) and Western blot (WB) analyses were conducted to evaluate the effects of the variant on mRNA and protein expression, respectively, using recombinant expression plasmids generated in vitro. This study was approved by the Medical Ethics Committee of Xi'an Children's Hospital (Ethics No. 20240045).

Results: The child, a 9-month-and-7-day-old boy, presented with a low nasal bridge, hypertelorism, and difficulty sitting independently. Echocardiography revealed an atrial septal defect. WES identified a homozygous variant in the MIDI gene, c.1483C>T (p.R495X), which was confirmed by Sanger sequencing and found to be inherited from the mother.Recombinant expression plasmids were successfully constructed. RT-qPCR analysis showed that the variant significantly reduced MIDI gene mRNA expression, while WB results indicated that the variant led to the production of a truncated protein.

Conclusions: The mild clinical phenotype of OS in this child may be attributed to the mRNA degradation escape mechanism induced by the nonsense variant c.1483C>T (p.R495X) in the MIDI gene. These findings provide valuable diagnostic insights for this pedigree and contribute to the understanding of the genotype-phenotype correlation in OS.