Human ciliopathies arise from defects in the primary cilium and can lead to obesity, retinal degeneration and cystic kidney disease, and are also associated with a wide array of morphological abnormalities. Although most of the characterized ciliopathies are single gene recessive disorders, there is evidence that mutations in more than one cilia-associated gene can have additive or synergistic effects in disease 1-5
. It has been estimated that there are more than 100 cilia-associated human diseases 6
and that hundreds of genes are required for the construction of cilia and the centrioles that template cilia 7,8
, making ciliopathies a model for the complex genetic interactions seen in human genetic disease.
Mutations in human DYNC2H1
, which encodes the heavy chain of the cytoplasmic dynein-2 motor required for trafficking cargo from the tip to the base of the cilium, have been associated with short rib-polydactyly syndrome (SRP) type III and Jeune asphyxiating thoracic dystrophy (JATD) 9,10
, two related skeletal dysplasias characterized by shortened long bones, a narrow rib cage and polydactyly, and other features of ciliopathies. Assembly of cilia depends on the process of intraflagellar transport (IFT) 11
. IFT particles, composed of IFT-A and IFT-B protein complexes, are transported to the tip of the cilium (anterograde transport) by the heterotrimeric kinesin-2, and transport of products back to the base of the cilium (retrograde transport) is powered by cytoplasmic dynein-2. Chondrocyte cilia from individuals with DYNC2H1
mutations are shortened with bulbous distal ends, similar to the phenotypes of IFT-dynein mutant cilia in other species 10
. The human syndromes show a range of severity, from lethality during gestation to adult survival in affected individuals, with no apparent relationship between the nature of the mutation and the severity of the disease 9
. The presence of both SRP type III and the less severe JATD within the same family also suggests that the human phenotypes can be modified by other genetic or environmental factors 12
Many of the morphological abnormalities seen in human ciliopathies are likely to be caused by disruption of the Hedgehog (Hh) signaling pathway 13,14
. Genetic analysis in the mouse and zebrafish has shown that primary cilia are essential for Hh signal transduction in vertebrate embryos 13
. Mutations in all of the IFT genes that have been studied disrupt Hh signaling. For example, mouse mutants that lack IFT-B complex proteins lack cilia and fail to respond to Hh signals; these mutants can neither activate Hh target genes nor produce the Gli repressors that keep target genes off in the absence of ligand 15,16
The proteins that mediate Hedgehog signal transduction are enriched in wild-type primary cilia. Patched1 (Ptch1), the Hh receptor, is present in cilia in the absence of ligand, but moves out in response to Hh ligand 17
. The transmembrane protein Smoothened (Smo), which acts downstream of Ptch1 moves into cilia in response to Shh, and cilia localization of Smo is required to activate downstream signaling 17,18
. The Gli2 and Gli3 transcription factors that implement Hh signals are enriched at the tips of cilia 19
, and the level of Gli2 and Gli3 at cilia tips increases in response to ligand 20,21
. It is, however, unclear how or whether IFT directly regulates trafficking of specific components of the Hh signal transduction pathway.
mutants show a loss of Shh-dependent signaling in the neural tube and die at midgestation (~e10.5) 15,22
. Here we define the genetic relationships between Dync2h1
and other genes required for ciliogenesis. Unexpectedly, we find that both the cilia morphology and Shh phenotypes of Dync2h1
homozygotes are strongly suppressed when the level of either the IFT-A or IFT-B proteins is reduced. The results indicate that the balance of anterograde and retrograde IFT controls ciliary architecture, which in turn controls Shh signaling and the developmental processes that are disrupted in ciliopathies.