Identification and Characterization of Known Biallelic Mutations in the IFT27 (BBS19) Gene in a Novel Family With Bardet-Biedl Syndrome

Abstract

Bardet-Biedl syndrome (BBS; MIM 209900) is a rare ciliopathy characterized by retinitis pigmentosa, postaxial polydactyly, obesity, hypogonadism, cognitive impairment and kidney dysfunction. Mutations in 22 BBS genes have been identified to cause the disease. We report a family with typical BBS features (retinitis pigmentosa, postaxial polydactyly, obesity, cognitive impairment, and atrioventricular septal defect) mutated in IFT27/BBS19. IFT27 is part of the Intraflagellar transport (IFT), a bidirectional mechanism allowing the protein motility within the cilia. Using whole exome sequencing, two compound heterozygous mutations were found in the proband (NM_006860.4:c.[104A > G];[349+1G > T], p.[Tyr35Cys];[?]) consistent with the expected autosomal recessive inheritance mode. These two mutations have already been reported but independently in other families and lacking either familial segregation or functional validation. This is the third report of IFT27 mutations in BBS patients confirming IFT27 as a BBS gene (BBS19). Mutations in IFT genes (IFT27, IFT172 and IFT74) confirm the IFT-pathway as a pathomechanism for BBS.

Keywords: BBS19; Bardet-Biedl syndrome; IFT27 gene; ciliopathy; whole exome sequencing.

FIGURE 1

Identification of two mutations in IFT27. (A) Pedigree of the reported family with one affected individual and segregation analysis of the two IFT27 mutations. Example of Sanger sequencing profiles for the heterozygous individuals. (B) PCR amplification was performed on RNA extracted from blood of individual II.1 and a healthy unrelated control amplified between exon 2 and exon 7. (C) IFT27 cDNA scheme representing the obtained fragments with size and expected composition. PCR primers are positioned. (D) Sanger sequencing of normal F1 in a healthy unrelated control (left side showing each exon boundaries from exon 3 to 7) and cut and eluted F3 band in individual II.1 demonstrating the absence of exons 4 to 6 (right side).

FIGURE 2

IFT27 gene and protein. (A) Schematic of the IFT27 locus and (B) IFT27 protein and the position and/or nature of known mutations. Nomenclature is given according to the following RefSeq identifiers: NM_006860.4 and NP_006851.1. (C) Multiple sequence alignment of IFT27 proteins from different species and of the human RAB8a and RAB10 proteins. The secondary structures derived from the IFT27 structure are shown in black above the sequences (Bhogaraju et al., 2011). The mutation Tyr35 is highlighted using a black “^∗” and the Switch I region is shown using a black line below the sequences. Residues are presented and colored according to the Jalview program (Waterhouse et al., 2009). Hs, Homo sapiens; Mm, Mus musculus; Xl, Xenopus laevis; Dr, Danio rerio; Cr, Chlamydomonas reinhardtii.