Disrupted IRF6-NME1/2 Complexes as a Cause of Cleft Lip/Palate

Mutations and common polymorphisms in interferon regulatory factor 6 ( IRF6) are associated with both syndromic and nonsyndromic forms of cleft lip/palate (CLP). To date, much of the focus on this transcription factor has been on identifying its direct targets and the gene regulatory network in which it operates. Notably, however, IRF6 is found predominantly in the cytoplasm, with its import into the nucleus tightly regulated like other members of the IRF family. To provide further insight into the role of IRF6 in the pathogenesis of CLP, we sought to identify direct IRF6 protein interactors using a combination of yeast 2-hybrid screens and co-immunoprecipitation assays. Using this approach, we identified NME1 and NME2, well-known regulators of Rho-type GTPases, E-cadherin endocytosis, and epithelial junctional remodeling, as bona fide IRF6 partner proteins. The NME proteins co-localize with IRF6 in the cytoplasm of primary palatal epithelial cells in vivo, and their interaction with IRF6 is significantly enhanced by phosphorylation of key serine residues in the IRF6 C-terminus. Furthermore, CLP associated IRF6 missense mutations disrupt the ability of IRF6 to bind the NME proteins and result in elevated activation of Rac1 and RhoA, compared to wild-type IRF6, when ectopically expressed in 293T epithelial cells. Significantly, we also report the identification of 2 unique missense mutations in the NME proteins in patients with CLP (NME1 R18Q in an IRF6 and GRHL3 mutation-negative patient with van der Woude syndrome and NME2 G71V in a patient with nonsyndromic CLP). Both variants disrupted the ability of the respective proteins to interact with IRF6. The data presented suggest an important role for cytoplasmic IRF6 in regulating the availability or localization of the NME1/2 complex and thus the dynamic behavior of epithelia during lip/palate development.

A landmark-free framework for the detection and description of shape differences in embryos

This paper introduces a new method to quantify and characterize shape changes during early facial development without the use of landmarks. Landmarks are traditionally used in morphometric analysis, but very few can be identified reliably across all stages of embryonic development. This method uses deformable registration to produce a dense vector field describing the point correspondences between two images. Low and mid-level features are extracted from the deformable vector field to find regions of organized differences that are biologically relevant. These methods are shown to detect regions of difference when evaluated on chick embryo images warped with small magnitude deformations in regions critical to midfacial development.

An X-linked GFP transgene reveals unexpected paternal X-chromosome activity in trophoblastic giant cells of the mouse placenta

A GFP transgene has been integrated on the proximal part of the mouse X chromosome just distal of Timp and Syn1. During development, this X-linked GFP transgene exhibits widespread green fluorescence throughout the embryonic and adult life of male mice but displays mosaic expression in tissues as a result of X-inactivation in females. In living female embryos, inactivation of the transgene is imprinted in extraembryonic regions and random in the embryo proper, demonstrating that this reporter is behaving in a similar fashion to the majority of X-linked loci, and so provides a vital readout of X chromosome activity. This is observation is further supported in T16H/X female mice harboring the GFP transgene on the normal X chromosome where reporter inactivation is observed in somatic cells. The differential expression of GFP activity facilitates fluorescence activated cell sorting for the purification of GFP+ vs. GFP- cells from female embryonic tissues, thereby allowing access to populations of cells that have kept active a particular X chromosome. By tracking the activity of this X-linked GFP transgene, we discovered that the primary and secondary giant cells of the X/X placenta maintain an active paternal copy of this transgene on the presumed silenced paternal X-chromosome. This finding implies that the imprint on the paternal X chromosome may be relaxed in these trophectodermal derivatives.

New mutations in MID1 provide support for loss of function as the cause of X-linked Opitz syndrome

Opitz syndrome (OS) is a genetically heterogeneous malformation disorder. Patients with OS may present with a variable array of malformations that are indicative of a disturbance of the primary midline developmental field. Mutations in the C-terminal half of MID1, an RBCC (RING, B-box and coiled-coil) protein, have recently been shown to underlie the X-linked form of OS. Here we show that the MID1 gene spans at least 400 kb, almost twice the distance originally reported and has a minimum of six mRNA isoforms as a result of the alternative use of 5' untranslated exons. In addition, our detailed mutational analysis of MID1 in a cohort of 15 patients with OS has resulted in the identification of seven novel mutations, two of which disrupt the N-terminus of the protein. The most severe of these (E115X) is predicted to truncate the protein before the B-box motifs. In a separate patient, a missense change (L626P) was found that also represents the most C-terminal alteration reported to date. As noted with other C-terminal mutations, GFP fusion constructs demonstrated that the L626P mutant formed cytoplasmic clumps in contrast to the microtubular distribution seen with the wild-type sequence. Notably, however, both N-terminal mutants showed no evidence of cytoplasmic aggregation, inferring that this feature is not pathognomonic for X-linked OS. These new data and the finding of linkage to MID1 in the absence of a demonstrable open reading frame mutation in a further family support the conclusion that X-linked OS results from loss of function of MID1.

A very high density microsatellite map (1 STR/41 kb) of 1.7 Mb on Xp22 spanning the microphthalmia with linear skin defects (MLS) syndrome critical region

Microphthalmia with linear skin defects (MLS) syndrome is an X-linked disorder presenting only in XX individuals. It is characterised by dysmorphic features such as microphthalmia, sclerocornea, and linear streaks of erythematous and hypoplastic skin restricted to the head and neck. Karyotype analyses have so far revealed a terminal deletion or translocation causing monosomy for the distal Xp region (Xp22.3) in all patients. We have used existing cosmid clones from the region to perform a saturation screen for AC-type microsatellites with the goal of facilitating analysis of five novel patients with features of MLS. Three of these cases had an Xp22.3 abnormality, while the other two showed some characteristic features of MLS but had apparently normal karyotypes. Forty-two novel microsatellite markers have now been developed from the 1.7 Mb cloned interval. Ninety-three percent of the novel markers exhibited allelic variation, representing an average of one polymorphic PCR-based marker (STR) every 41 kb.