Thm2interacts with paralog,Thm1, and sensitizes to Hedgehog signaling in postnatal skeletogenesis

Ciliopathies are genetic syndromes that link osteochondrodysplasias to dysfunction of primary cilia. Primary cilia extend from the surface of bone and cartilage cells, to receive extracellular cues and mediate signaling pathways. Mutations in several genes that encode components of the intraflagellar transport-A ciliary protein complex have been identified in skeletal ciliopathies, including THM1 . Here, we report a role for genetic interaction between Thm1 and its paralog, Thm2, in skeletogenesis. THM2 localizes to the ciliary axoneme, but unlike its paralog, Thm2 deficiency does not affect ciliogenesis and Thm2- null mice survive into adulthood. Since paralogs often have redundant functions, we crossed a Thm1 null ( aln ) allele into the Thm2 colony. After 5 generations of backcrossing the colony onto a C57BL6/J background, we observed that by postnatal day 14, Thm2 -/- ; Thm1 aln/+ mice are smaller than control littermates. Thm2 -/- ; Thm1 aln/+ mice exhibit shortened long bones, narrow ribcage, shortened cranium and mandibular defects. Mutant mice also show aberrant architecture of the tibial growth plate, with an expanded proliferation zone and diminished hypertrophic zone, indicating impaired chondrocyte differentiation. Using microcomputed tomography, Thm2 -/- ; Thm1 aln/+ tibia were revealed to have reduced cortical and trabecular bone mineral density. Deletion of one allele of Gli2 , a major transcriptional activator of the Hedgehog (Hh) pathway, exacerbated the small phenotype of Thm2 -/- ; Thm1 aln/+ mice and caused small stature in Thm2 -null mice. Together, these data reveal Thm2 as a novel locus that sensitizes to Hh signaling in skeletal development. Further, Thm2 -/- ; Thm1 aln/+ mice present a new postnatal ciliopathy model of osteochondrodysplasia.