The genetic basis of DOORS syndrome: an exome-sequencing study

Abstract

Background: Deafness, onychodystrophy, osteodystrophy, mental retardation, and seizures (DOORS) syndrome is a rare autosomal recessive disorder of unknown cause. We aimed to identify the genetic basis of this syndrome by sequencing most coding exons in affected individuals.

Methods: Through a search of available case studies and communication with collaborators, we identified families that included at least one individual with at least three of the five main features of the DOORS syndrome: deafness, onychodystrophy, osteodystrophy, intellectual disability, and seizures. Participants were recruited from 26 centres in 17 countries. Families described in this study were enrolled between Dec 1, 2010, and March 1, 2013. Collaborating physicians enrolling participants obtained clinical information and DNA samples from the affected child and both parents if possible. We did whole-exome sequencing in affected individuals as they were enrolled, until we identified a candidate gene, and Sanger sequencing to confirm mutations. We did expression studies in human fibroblasts from one individual by real-time PCR and western blot analysis, and in mouse tissues by immunohistochemistry and real-time PCR.

Findings: 26 families were included in the study. We did exome sequencing in the first 17 enrolled families; we screened for TBC1D24 by Sanger sequencing in subsequent families. We identified TBC1D24 mutations in 11 individuals from nine families (by exome sequencing in seven families, and Sanger sequencing in two families). 18 families had individuals with all five main features of DOORS syndrome, and TBC1D24 mutations were identified in half of these families. The seizure types in individuals with TBC1D24 mutations included generalised tonic-clonic, complex partial, focal clonic, and infantile spasms. Of the 18 individuals with DOORS syndrome from 17 families without TBC1D24 mutations, eight did not have seizures and three did not have deafness. In expression studies, some mutations abrogated TBC1D24 mRNA stability. We also detected Tbc1d24 expression in mouse phalangeal chondrocytes and calvaria, which suggests a role of TBC1D24 in skeletogenesis.

Interpretation: Our findings suggest that mutations in TBC1D24 seem to be an important cause of DOORS syndrome and can cause diverse phenotypes. Thus, individuals with DOORS syndrome without deafness and seizures but with the other features should still be screened for TBC1D24 mutations. More information is needed to understand the cellular roles of TBC1D24 and identify the genes responsible for DOORS phenotypes in individuals who do not have a mutation in TBC1D24.

Funding: US National Institutes of Health, the CIHR (Canada), the NIHR (UK), the Wellcome Trust, the Henry Smith Charity, and Action Medical Research.

Figure 1

Study profile DOORS=deafness, onychodystrophy, osteodystrophy, mental retardation, and seizures. DDOD=dominant deafness-onychodystrophy.

Figure 2

Physical features of participants with TBC1D24 mutations (A–F) Some individuals have a wide base of the nose and bulbous end of the nose (individual numbers from tables corresponding to each panel: A=3, B=6, C=5a, D=2b, E=2a, F=7). (G–L) Hands in individuals with TBC1D24 mutations have triphalangeal thumbs, brachydactyly, short terminal phalanges, and hypoplasia or aplasia of the nails (G=3, H=6, I=5a, J=2b, K=2a, I=7). (M–O) Feet with short terminal phalanges and hypoplasia or aplasia of the nails (M=3, N=6, O=5a). (P–S) Radiographs of the hands in individuals with TBC1D24 mutations. Note the triphalangeal thumbs and the short terminal phalanges (P=4, Q=5a, R=6, S=1). (T,U) Radiographs of the feet in individuals with TBC1D24 mutations, showing short terminal phalanges (T=6, U=5a).

Figure 3

Mutations in TBC1D24 (A) Location of the mutations identified in DOORS syndrome and in other epilepsy syndromes. Phe251Leu (blue; homozygous mutation, affecting four siblings) causes focal epilepsy, dysarthria, intellectual disability, cortical thickening, cerebellar atrophy., Asp147His and Ala515Val (red; compound heterozygous mutation, affecting seven individuals in one family) causes familial infantile myoclonic epilepsy. Ser324Thrfs*3 (purple; homozygous mutation, affecting five individuals in one family) causes myoclonic epilepsy, dystonia, hemiparesis, autonomic signs, lethargy, progressive diffuse cerebral atrophy. Phe229Ser and Cys156* (green; compound heterozygous mutation affecting two siblings)—causes familial malignant migrating partial seizures of infancy, progressive diffuse cerebral atrophy. The diagram also shows the exonic structure of TBC1D24, with the introns not drawn to scale. (B) Real-time PCR of TBC1D24 in fibroblasts from the individual with a frameshift deletion and a splicing mutation, showing substantial reduction of TBC1D24 mRNA in affected fibroblasts. (C) Western blot analysis of the cells used in panel B, showing that TBC1D24 protein is undetectable by this method in affected fibroblasts. (D) Structural model of TBC1D24 with the TBC domain coloured in blue and the TBC, LysM, Domain catalytic (TLDc) domain coloured in grey. The N terminal and C termini are labelled, and the red spheres show the alpha carbon atoms of the residues aligning with the arginine and glutamine fingers interacting with the GTP of Rab proteins in other TBC proteins, based on the structure of Gyp1p in complex with Rab33. The carbon atoms of residues substituted in DOORS syndrome are shown with purple spheres and those substituted in other epilepsy syndromes are shown with black spheres.