Molecular analysis of Bardet-Biedl syndrome families: report of 21 novel mutations in 10 genes

Abstract

Purpose: Bardet-Biedl syndrome (BBS) is genetically heterogeneous with 15 BBS genes currently identified, accounting for approximately 70% of cases. The aim of our study was to define further the spectrum of BBS mutations in a cohort of 44 European-derived American, 8 Tunisian, 1 Arabic, and 2 Pakistani families (55 families in total) with BBS.

Methods: A total of 142 exons of the first 12 BBS-causing genes were screened by dideoxy sequencing. Cases in which no mutations were found were then screened for BBS13, BBS14, BBS15, RPGRIP1L, CC2D2A, NPHP3, TMEM67, and INPP5E.

Results: Forty-three mutations, including 8 frameshift mutations, 10 nonsense mutations, 4 splice site mutations, 1 deletion, and 20 potentially or probably pathogenic missense variations, were identified in 46 of the 55 families studied (84%). Of these, 21 (2 frameshift mutations, 4 nonsense mutations, 4 splice site mutations, 1 deletion, and 10 missense variations) were novel. The molecular genetic findings raised the possibility of triallelic inheritance in 7 Caucasian families, 1 Arabian family, and 1 Tunisian patient. No mutations were detected for BBS4, BBS11, BBS13, BBS14, BBS15, RPGRIP1L, CC2D2A, NPHP3, TMEM67, or INPP5E.

Conclusions: This mutational analysis extends the spectrum of known BBS mutations. Identification of 21 novel mutations highlights the genetic heterogeneity of this disorder. Differences in European and Tunisian patients, including the high frequency of the M390R mutation in Europeans, emphasize the population specificity of BBS mutations with potential diagnostic implications. The existence of some BBS cases without mutations in any currently identified BBS genes suggests further genetic heterogeneity.

Figure 1.

The 21 novel BBS mutations identified in this study. Three novel mutations were identified in BBS1: a heterozygous c.518+1G>A allele, a homozygous c.1473+4A>G allele, and a heterozygous R512H allele. Two novel mutations were identified in BBS2: a heterozygous R275X allele and a heterozygous c.1659+3A>G allele. A novel homozygous deletion was identified in BBS3: c.123+1118del53985. Two novel mutations were identified in BBS5: a homozygous L50R allele and a homozygous c.619-1G>C allele. One novel mutation was identified in BBS6: a heterozygous R309H allele. Three novel mutations were identified in BBS8: a heterozygous K95R allele, a heterozygous M135I allele, and a heterozygous F337L allele. One novel mutation was identified in BBS9: a heterozygous Q132H allele. Six novel mutations were identified in BBS10: a homozygous L533fsX22 allele, a homozygous A323V allele, a heterozygous L445I allele, a homozygous E499X allele, a heterozygousT516RfsX7 allele, and a heterozygous V602L allele. Two novel mutations were identified in BBS12: a heterozygous W520X allele and a heterozygous R675X allele. Sequence tracings of novel mutations identified in this study compared to normal sequences from controls.

Figure 2.

Amino acid sequence conservation around residues affected by novel missense mutations in BBS1, BBS5, BBS6, BBS8, BBS9, and BBS10 identified in this study. Asterisk: position of the missense mutation. The sequences of BBS proteins or predicted translation products from several species have been compared and aligned. The relevant amino acids are in bold, and they are underlined when different from Homo sapiens reference; other identities are represented by a gray background. Homo, H. sapiens; Mus, Mus musculus; Rattus, Rattus norvegicus; Xenopus, Xenopus (silurana) tropicalis; Danio, Danio rerio; Bos, Bos taurus; Pongo, Pongo abelii; Gallus, gallus; Pan, Pan troglodytes; Equus, Equus caballus.