Our analysis of the krc allele of Mks1 provides a better understanding of the developmental defects of MKS. Our findings demonstrate that Mks1 is required in vivo for normal cilia formation in multiple tissues during development. Those defects in ciliogenesis are responsible for disruption of Hh signaling in a range of different cell types, which leads to a variety of developmental abnormalities.
In contrast to siRNA knockdown experiments in cell culture, Mks1 is not required for apical localization of basal bodies, but rather for cilia length and morphology. This role for Mks1 is consistent with findings for several other mouse mutants that disrupt basal body proteins, including Mks4–6, Oro-Facial-Digital syndrome 1 and most Bardet-Biedl syndrome proteins (9
). The mechanisms by which proteins localized to the basal body affect cilia are not understood but may include regulation of trafficking to the cilium or transport within the cilium itself (43
The cilia abnormalities in Mks1krc
mutants cause developmental defects in a variety of tissues and organs. The neural tube and limb phenotypes can be explained as disruptions in cilia-dependent Shh signaling. However, the defects seen in these tissues do not correspond to the simple loss of Hh signaling seen in other mutants with reduced cilia number (21
). In the Mks1krc
neural tube, there is both a reduced response to high level Shh activity accompanied by an expansion of the domain of Shh signaling; a similar expansion of the domain of Shh activity is also seen in the limb. The abnormalities in the skull, lungs and ribcage in Mks1krc
mutants are similar to phenotypes in mouse loss of function mutants in the Hh pathway (44
), suggesting that these defects in Mks1krc
mutant mice and in MKS patients may also be due to altered Hh signaling.
However, MKS phenotypes cannot be solely explained by altered Hh signaling. Reversal of left–right asymmetry, ductal plate malformations and cystic kidneys have not been reported in Hh pathway mutants, but do occur as a result of ciliary or basal body defects (13
). While the left–right asymmetry defects are likely the result of disrupted motile cilia-dependent flow in the Mks1krc
node, the cause of the kidney cysts and extra bile ducts requires further examination. Together, our findings suggest that the ciliogenesis defects in Mks1krc
mutants likely underlie both the Hh-dependent and -independent phenotypes in MKS. Interestingly, a mutation in the C. elegans
homolog of Mks1
) does not affect cilia formation or morphology on its own but acts with nephrocystin genes to regulate cilia formation, suggesting that the role of Mks1 may have diverged between the nematode and vertebrate lineages (24
). The differences between the cell culture, nematode and mouse studies highlight the importance of developing vertebrate models to better define the basis of some human disorders.
Five of six genomic loci that are linked to Meckel syndrome have been identified and studied to varying extents (2
). Although no mouse models for MKS types 2 or 4 exist, Mks3
mutants have recently been reported, but the full spectrum of MKS characters has not been analyzed in these models. Similar to Mks1, Mks5 is also required for left–right asymmetry and Shh signaling in the limb and neural tube; however, the mutants have not been assayed for additional skeletal phenotypes or cyst formation (47
mutant mice develop polycystic kidneys but lack other MKS characteristics, whereas Mks3 mutant rats (wpk
) show cilia defects in kidneys, eyes and sperm (11
), The milder phenotypes in Mks3
animals compared with Mks1
mutants may reflect the relatively milder defects in Mks3
patients compared with those with mutations in Mks1
). Our analysis reveals that Mks1krc
mutants currently provide the most complete parallel to the human syndrome and identifies defects in both ciliogenesis and Hh signaling as key underlying defects in several tissues. Because Mks1krc
mutants provide a faithful model of MKS, this mutant provides the means to further assess the molecular mechanisms underlying the broad spectrum of developmental defects in Meckel syndrome.