Novel biallelic variant in BBS9 causative of Bardet–Biedl syndrome: expanding the spectrum of disease-causing genetic alterations

BACKGROUND: Bardet–Biedl syndrome (BBS) is a rare autosomal recessive ciliopathy disorder. Many BBS disease-causing genetic variants have been identified due to the advancement of molecular diagnostic tools. We report on a novel pathogenic variant in a consanguineous Pakistani family with an affected child. CASE PRESENTATION: Clinical exome sequencing was used to search for BBS causing variants in the affected individual and identified a novel homozygous splice-site variant in the BBS9 gene (c.702 + 1del). Sanger sequencing was performed for variant validation and segregation studies. Expression analysis using mRNA levels to assess the functional impact of the novel variant demonstrated skipping of exon 7 in the affected alleles, suggesting a truncating effect. Three-dimensional structural modelling was used to predict pathogenicity of the variant residue and the alteration leads to a partial deletion of the PHTB1_N domain and a total deletion of the PHTB1_C domain. CONCLUSION: The study of this case expands the spectrum of biallelic variants in the BBS9 gene associated with BBS and increased the knowledge on the molecular consequences of splicing variation c.702 + 1del. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12920-021-00943-w.