The Meckel–Gruber Syndrome proteins MKS1 and meckelin interact and are required for primary cilium formation

Abstract

Meckel-Gruber syndrome (MKS) is an autosomal recessive lethal malformation syndrome characterized by renal cystic dysplasia, central nervous system malformations (typically, posterior occipital encephalocele), and hepatic developmental defects. Two MKS genes, MKS1 and MKS3, have been identified recently. The present study describes the cellular, sub-cellular and functional characterization of the novel proteins, MKS1 and meckelin, encoded by these genes. In situ hybridization studies for MKS3 in early human embryos showed transcript localizations in agreement with the tissue phenotype of MKS patients. Both MKS proteins predominantly localized to epithelial cells, including proximal renal tubules and biliary epithelial cells. MKS1 localized to basal bodies, while meckelin localized both to the primary cilium and to the plasma membrane in ciliated cell-lines and primary cells. Meckelin protein with the Q376P missense mutation was unable to localize at the cell membrane. siRNA-mediated reduction of Mks1 and Mks3 expression in a ciliated epithelial cell-line blocked centriole migration to the apical membrane and consequent formation of the primary cilium. Co-immunoprecipitation experiments show that wild-type meckelin and MKS1 interact and, in three-dimensional tissue culture assays, epithelial branching morphogenesis was severely impaired. These results suggest that MKS proteins mediate a fundamental developmental stage of ciliary formation and epithelial morphogenesis.

Aberrant splicing is a common mutational mechanism inMKS1, a key player in Meckel-Gruber syndrome

“…Together with the four previously undescribed mutations presented here, there are now eight different MKS1 mutations known [Kyttälä et al, 2006;Dawe et al, 2007]. Remarkably, all but two of these MKS1 mutations represent splicing defects, among them the Caucasian founder allele c.1408-7_35del.…”

Missense variants in TMEM67 in a patient with Joubert syndrome

“…Mutations in TMEM67 ( MKS3 ) are responsible for the majority of COACH syndrome, although mutations in CC2D2A and RPGRIP1L can also be causative 78, 9, 10, 11. Although mutations in TMEM67/MKS3 are causative for both syndromes (accounting for 9% of JSRD cases and 16% of MKS cases), a correlation between missense mutations in exons 8‐15, particularly in combination with truncating mutations, and Meckel‐Gruber syndrome has been identified 7, 12…”

Genomic analysis of Meckel–Gruber syndrome in Arabs reveals marked genetic heterogeneity and novel candidate genes

“…4 Subsequently, nine additional genes have been identified, all similarly encoding ciliary proteins (TMEM216 (MKS2), TMEM67 (MKS3), CEP290 (MKS4), RPGRIP1L (MKS5), CC2D2A (MKS6), NPHP3 (MKS7), TCTN2 (MKS8), B9D1 (MKS9) and B9D2 (MKS10)). [4][5][6][7][8][9][10][11][12][13][14] The finding that defective ciliary biology is the core molecular pathology of MKS made it possible to dissect the pathogenesis of each of its manifestations. For instance, the primary cilium plays a critical role in SHH signaling that controls anteriorposterior and dorsal-ventral patterning of the developing limb buds and neural tube, respectively, thus explaining the polydactyly and neural tube defects that characterize MKS at the molecular level.…”