Compound heterozygous IFT81 variations in a skeletal ciliopathy patient cause Bardet-Biedl syndrome-like ciliary defects

Owing to their crucial roles in development and homeostasis, defects in cilia cause ciliopathies with diverse clinical manifestations. The intraflagellar transport (IFT) machinery, containing the IFT-A and IFT-B complexes, mediates not only the intraciliary bidirectional trafficking but also import and export of ciliary proteins together with the kinesin-2 and dynein-2 motor complexes. The BBSome, containing eight subunits encoded by causative genes of Bardet-Biedl syndrome (BBS), connects the IFT machinery to ciliary membrane proteins to mediate their export from cilia. Although mutations in subunits of the IFT-A and dynein-2 complexes cause skeletal ciliopathies, mutations in some IFT-B subunits are also known to cause skeletal ciliopathies. We here show that compound heterozygous variations of an IFT-B subunit, IFT81, found in a patient with skeletal ciliopathy cause defects in its interactions with other IFT-B subunits, and in ciliogenesis and ciliary protein trafficking when one of the two variants was expressed in IFT81-knockout (KO) cells. Notably, we found that IFT81-KO cells expressing IFT81(& UDelta;490-519), which lacks the binding site for the IFT25-IFT27 dimer, causes ciliary defects reminiscent of those found in BBS cells and those in IFT74-KO cells expressing a BBS variant of IFT74, which forms a heterodimer with IFT81. In addition, IFT81-KO cells expressing IFT81(& UDelta;490-519) in combination with the other variant, IFT81 (L645*), which mimics the cellular conditions of the above skeletal ciliopathy patient, demonstrated essentially the same phenotype as those expressing only IFT81(& UDelta;490-519). Thus, our data indicate that BBS-like defects can be caused by skeletal ciliopathy variants of IFT81.

Automated summary

Due to their important roles in development and homeostasis, defects in cilia lead to ciliopathies with various clinical manifestations. The intraflagellar transport (IFT) machinery, consisting of IFT-A and IFT-B complexes, is responsible for intraciliary trafficking as well as import and export of ciliary proteins. The BBSome, composed of eight subunits encoded by causative genes of Bardet-Biedl syndrome (BBS), links the IFT machinery to ciliary membrane proteins to facilitate their export from cilia. Mutations in IFT-B subunits, including IFT81, can cause skeletal ciliopathies by disrupting the interactions among IFT-B subunits and impairing ciliogenesis and ciliary protein trafficking.