Functional variants of POC5 identified in patients with idiopathic scoliosis

Abstract

Idiopathic scoliosis (IS) is a spine deformity that affects approximately 3% of the population. The underlying causes of IS are not well understood, although there is clear evidence that there is a genetic component to the disease. Genetic mapping studies suggest high genetic heterogeneity, but no IS disease-causing gene has yet been identified. Here, genetic linkage analyses combined with exome sequencing identified a rare missense variant (p.A446T) in the centriolar protein gene POC5 that cosegregated with the disease in a large family with multiple members affected with IS. Subsequently, the p.A446T variant was found in an additional set of families with IS and in an additional 3 cases of IS. Moreover, POC5 variant p.A455P was present and linked to IS in one family and another rare POC5 variant (p.A429V) was identified in an additional 5 cases of IS. In a zebrafish model, expression of any of the 3 human IS-associated POC5 variant mRNAs resulted in spine deformity, without affecting other skeletal structures. Together, these findings indicate that mutations in the POC5 gene contribute to the occurrence of IS.

Figure 3. Expression of human POC5 SNVs leads to scoliosis in zebrafish.

Zebrafish expressing mutant POC5 mRNAs, but not WT POC5 mRNAs, showed an overt axial phenotype after 72 hpf, ranging from mild to severe curvature of the body axis in approximately 50% of injected embryos (P < 0.001, χ² test). (A) Left panel: mutant zebrafish phenotypes (3–5 dpf). Images illustrating the various degrees of severity of axial spine deformities. Axial spine deformities are indicated by arrows. Right panel: after 3 dpf, embryos injected with 50 ng/μl, mut-POC5 mRNAs c.G1336A (n = 153), c.G1363C (n = 149), and c.C1286T (n = 130) exhibited mild to severe curvature of the body axis compared with noninjected WT (n = 234), and WT POC5–injected (n = 122) embryos. (B) Lateral view (left panels) of WT and mut-POC5 juvenile zebrafish. MicroCT images (right panels) of WT fish showed a fully mineralized normal spine, while mut-POC5 fish present spine curvature and rotation reminiscent of IS.

Figure 2. IS families and cases with exon 10 POC5 SNVs.

Forty additional IS families and 150 IS cases were studied. (A) IS families F19, F35, and F41 and IS cases C39, C58, and C83 show the same c.1336G>A POC5 rare missense SNV listed in dbSNP138 (rs34678567). Sample chromatogram for an IS patient with this SNV is shown. The position of the SNV is indicated by an arrow above the chromatogram. Pedigrees of families and cases are shown. (B) IS family F31 harboring the c.1363G>C POC5 novel missense SNV and corresponding chromatogram. (C) IS cases C1, C77, C137, C149, and C150 harboring the c.1286C>T POC5 rare missense SNV listed in dbSNP138 (rs146984380) and corresponding sequence chromatograms. Cobb’s angle and genotype at the mutated position are indicated below symbols corresponding to individuals.

Figure 1. Identification of a rare POC5 SNV in family F2.

Exome sequencing identifies a rare missense SNV in POC5, c.1336G>A, listed in dbSNP138 (rs34678567). Sample chromatograms for a healthy individual without this SNV (upper left panel) and for an affected individual with this SNV (lower left panel) are shown. The pedigree of family F2 is shown. Cobb’s angle and genotype at the mutated position are indicated below symbols corresponding to individuals. U, uncertain status; ND, status not determined.