OFD1 is mutated in X-linked Joubert syndrome and interacts with LCA5-encoded lebercilin

Abstract

We ascertained a multi-generation Malaysian family with Joubert syndrome (JS). The presence of asymptomatic obligate carrier females suggested an X-linked recessive inheritance pattern. Affected males presented with mental retardation accompanied by postaxial polydactyly and retinitis pigmentosa. Brain MRIs showed the presence of a "molar tooth sign," which classifies this syndrome as classic JS with retinal involvement. Linkage analysis showed linkage to Xpter-Xp22.2 and a maximum LOD score of 2.06 for marker DXS8022. Mutation analysis revealed a frameshift mutation, p.K948NfsX8, in exon 21 of OFD1. In an isolated male with JS, a second frameshift mutation, p.E923KfsX3, in the same exon was identified. OFD1 has previously been associated with oral-facial-digital type 1 (OFD1) syndrome, a male-lethal X-linked dominant condition, and with X-linked recessive Simpson-Golabi-Behmel syndrome type 2 (SGBS2). In a yeast two-hybrid screen of a retinal cDNA library, we identified OFD1 as an interacting partner of the LCA5-encoded ciliary protein lebercilin. We show that X-linked recessive mutations in OFD1 reduce, but do not eliminate, the interaction with lebercilin, whereas X-linked dominant OFD1 mutations completely abolish binding to lebercilin. In addition, recessive mutations in OFD1 did not affect the pericentriolar localization of the recombinant protein in hTERT-RPE1 cells, whereas this localization was lost for dominant mutations. These findings offer a molecular explanation for the phenotypic spectrum observed for OFD1 mutations; this spectrum now includes OFD1 syndrome, SGBS2, and JS.

Figure 1

Pedigree and Mutation Analysis of Family W07-713 (A) Pedigree of family W07-713. Indicated are the four discerning clinical features of the patients in the family: mental retardation, postaxial polydactyly, juvenile RP, and the presence of a molar tooth sign on an MRI scan. Indicated in gray are the patients who were not investigated by MRI. (B) Chromatograms showing the wild-type sequence in family member III-7 (up) and the c.2841_2847 delAAAAGAC deletion in patient IV-10 (down) in exon 21 of OFD1. (C) Chromatograms showing the wild-type sequence in the healthy father (up) and the c.2767 delG mutation in patient UW87 (down) in exon 21 of OFD1.

Figure 2

Clinical Phenotype of Family W07-713 and Isolated Patient UW87 (A–D) Shown are photographs of (A) the face and (B) hands and foot, as well as (C and D) brain MRI images, of patient IV-10 at the age of 1 year. Note the low-set ears, broad nasal bridge, prominent philtrum and maxillary arch, full lips, and postaxial polydactyly of all four limbs, although central polydactyly can not be excluded. On an axial T1-weighted brain MRI, a clear molar tooth sign is visible. (E–H) Shown for patient III-9, who was 34 yrs-old at the time of examination, are (E) frontal and (F) side views of the face and (G and H) brain MRI images. This patient also has low-set ears, a broad nasal bridge, a prominent philtrum and maxillary arch, and full lips. No polydactyly was present. An axial T1-weighted MRI image showed a molar tooth sign. (I–L) For patient UW87, who was examined at the age of 12 yrs, (I) frontal and (J) side views of the face, as well as (K and L) axial T1-weighted brain MRI images, are shown. This patient showed macrocephaly and obesity. Polydactyly was reported, but the sixth finger was removed shortly after birth. A molar tooth sign was evident from MRI images.

Figure 3

OFD1 Interaction with Lebercilin (A) OFD1 and lebercilin protein structures. The protein domains of the two OFD1 isoforms (OFD1^FL and OFD1^Δex10) and the OFD1 fragments identified in the yeast two-hybrid screen are depicted, along with the protein domains of the lebercilin fragments expressed by the deletion constructs that were tested for interaction. (B) Yeast two-hybrid assays confirmed binding of both OFD1 fragments, spanning amino acid residues 240–1012 and 356–1012, to full-length lebercilin and the region containing the first two predicted coiled-coil domains of lebercilin. OFD1 fragments containing coiled-coil domains two/three and four (amino acids 240–614 Δex10 [Δaa313–353]) and coiled-coil domains five and six (amino acids 615–1012) both interacted with full-length lebercilin and with the lebercilin fragment spanning amino acids 96–305, although the OFD1 fragment containing coiled-coil domains two/three and four showed the highest affinity. NA = not analyzed; activation of the HIS3 and ADE2 reporter genes is qualitatively indicated by – (no growth on selective media), +/− (slow growth), + (growth), or ++ (fast growth). The activation of the MEL1 (α-galactosidase activity) and the LacZ (β-galactosidase activity) reporter genes was in line with these results. (C) Both GST-OFD1 ^Δex10 and the GST-fused OFD1 fragments spanning amino acid residues 240–1012 and 356–1012 (10% input of the assay shown in panel 3) efficiently pulled down 3xFLAG-tagged full-length lebercilin (10% input shown in panel 1; pull-down assay shown in panel 2). Unfused GST failed to pull down lebercilin at all, whereas, in a positive control experiment, GST-nephrocystin-4-I pulled down RPGRIP1^C2-end. (D) The immunoblot (panel 4) shows that 3xFLAG-tagged OFD1 fragments coimmunoprecipitated with HA-lebercilin, whereas the unrelated 3xFLAG-LRRK2 protein did not. Five percent of the cell lysate inputs used in the assays are shown in panels 1 and 2, and the HA immunoprecipitates are shown in panel 3. As a positive control, 3xFLAG-RPGRIP1^C2-end coimmunoprecipitated with HA-fused nephrocystin-4-I. (E) When anti-FLAG immunoprecipitation was performed, HA-lebercilin coimmunoprecipitated with all 3xFLAG-tagged OFD1 fragments, as shown in the immunoblot in panel 4. Panels 1 and 2 show five percent of the cell lysate inputs used in the assays, and panel 3 depicts FLAG immunoprecipitates.

Figure 4

OFD1 and Lebercilin Colocalization in hTERT-RPE1 Cells and Rat Retina (A–C) Colocalization of eCFP-OFD1^Δex10 (A, green) and the basal-body and cilium marker GT335 (B, red), a mouse monoclonal antibody against polyglutamylated tubulin. eCFP-OFD1^Δex10 localizes to the pericentriolar region. (C) is overlay of (A) and (B). (D–F) eCFP-OFD1^FL (D, green) also shows similar pericentriolar localization, as seen from costaining with GT335 (E, red). (F) is an overlay of (D) and (E). (G–I) Colocalization of eCFP-OFD1^Δex10 (G, green) and mRFP-lebercilin (H, red). Colocalization is observed at the basal-body region. (I) is an overlay of (G) and (H). (J–O) Colocalization of lebercilin (J, green) and OFD1 (M, green) with centrin (K and N, red), a marker of the connecting cilium region, in rat photoreceptor cells. (L) and (O) are overlays. In all pictures, nuclei are stained with DAPI (blue). Abbreviations are as follows: RPE, retinal pigment epithelium; OS, outer segment; CC, connecting cilium; IS, inner segment; ONL. outer nuclear layer; OPL, outer plexiform layer; and INL, inner nuclear layer.

Figure 5

Effect of OFD1 Mutations on Interaction with Lebercilin (A) Interaction of wild-type (WT) OFD1 and OFD1 mutants with lebercilin in the yeast two-hybrid assay. Expression constructs encoding the amino acid fragment 356–1012 of OFD1 as GAL4-AD-fusion protein, either wild-type or containing the p.K948fs, p.K923fs, p.I784fs, p.E709fs, p.N630fs, or p.S586fs mutation, were cotransformed with pBD-lebercilin^CC1+2 constructs in PJ694α. As a negative control, the empty pAD vector was cotransformed with pBD-lebercilin^CC1+2. Cotransformation of GAL4-AD- and GAL4-BD-expressing plasmids from the Hybrizap kit was used as positive control. Media lacking the amino acids Leu and Trp selected for cotransformants (−LW panel). Additional omission of His and Ade from the media selected for activation of associated HIS3 and ADE2 reporter genes (−LWHA panel). Colonies growing under this selection were transferred to a filter for a β-galactosidase assay (−LWHA β-galactosidase panel). Blue staining indicates β-galactosidase activity by activation of the LacZ reporter gene. The interaction of lebercilin^CC1+2 with wild-type OFD1, as well as with the OFD1 mutants p.K948fs, p.E923fs, and p.E709fs, was detected by growth on −LWHA-selective media and a positive LacZ assay. The OFD1 proteins containing the p.I784fs, p.N630fs, and p.S586fs mutations did not interact with lebercilin, as evidenced by the fact that neither growth nor β-galactosidase activity could be detected in the assays. (B) Liquid β-galactosidase assays revealed that the OFD1 p.K948fs mutation decreased the LacZ reporter gene activity to 24% as compared to that of the wild-type (Student t test, p value = 0.0005), indicating a significant reduction in interaction with lebercilin. The p.E923fs mutant retained only 18% of reporter gene activity (p value = 0.0003). For the p.E709fs mutant, 60% of activity remained; this activity level was not significantly different from that of OFD1 wild-type at a p value threshold of 0.005 (p value = 0.03). For the calculation of relative remaining reporter gene activity, values were corrected for background activity in the assay. Error bars represent the standard error of the mean.

Figure 6

Localization of OFD1 Mutants upon Overexpression in hTERT-RPE1 Cells (A–C) Colocalization of eCFP-OFD1 p.K948fs (A, green) with the basal body and cilium marker GT335 (B, red). The mutant protein shows pericentriolar localization comparable to that observed for wild-type eCFP-OFD1. (C) is an overlay of (A) and (B). (D–F) Colocalization of eCFP-OFD1 p.K923fs (D, green) with GT335 (E, red), showing pericentriolar localization. (F) is an overlay of (D) and (E). (G–I) eCFP-OFD1 p.I784fs (G, green) is scattered throughout the cytoplasm but concentrates at both centrioles, which are indicated by GT335 staining (H, red). (I) is an overlay of (G) and (H). (J–L) Colocalization of eCFP-OFD1 p.E709fs (J, green) with GT335 (K, red), indicating pericentriolar localization. (L) is an overlay of (J) and (K). (M–O) eCFP-OFD1 p.N630fs (M, green) localizes diffusely throughout the cytoplasm, and only limited colocalization with GT335 (N, red) is observed. (O) is an overlay of (M) and (N). (P–R) eCFP-OFD1 p.S586fs (P, green) also shows cytoplasmic distribution and limited colocalization with GT335 (Q, red). (R) is an overlay of (P) and (Q). In all pictures, nuclei are stained with DAPI (blue).

Figure 7

Genotype-Phenotype Correlation of OFD1 Mutations with OFD1, SGBS2, and XL-JS Mutations thus far reported as occurring before amino acid residue 631 are all embryonic lethal for males. In females, these cause OFD1 syndrome. The SGBS2-associated p.E709fs mutation causes early death in males, but females are not affected. Male patients with the p.E923fs and p.K948fs mutations that cause XL-JS may live beyond the age of 30 years. We have also indicated the position of the exon 17 p.I784fs mutation that results in OFD1 syndrome in females and is lethal to males to show that the phenotype boundaries are variable (dotted lines). However, the overall severity of the phenotype inversely correlates with the size of the remaining OFD1 protein.