If so, one would expect that increased Nodal might relieve the inhibitory effect of BMP on APS gene expression

If so, one would expect that increased Nodal might relieve the inhibitory effect of BMP on APS gene expression. that the patterning of forebrain and medial craniofacial elements requires a fine balance between BMP and Nodal signaling during primitive streak development, and provide a potential mechanistic basis for a new multigenic model of HPE. INTRODUCTION Holoprosencephaly (HPE), a partial or complete failure of forebrain bifurcation, is the most common anomaly of forebrain development in humans. HPE is often associated with other anomalies, including variable medial RCAN1 craniofacial deficiencies and occasional laterality defects (1). Heterozygosity for mutations in any of several loci has been associated with HPE, often in genes functioning in the INH1 and intercellular signaling pathways (2). However, the extreme phenotypic variability in HPE patients with particular gene mutations cannot be explained by single-gene haploinsufficiency. Such considerations have led to a multiple hit hypothesis for HPE pathogenesis, in which HPE might frequently result from two or more independent genetic lesions impacting common or interacting developmental pathways during forebrain formation (3). An understanding of the cellular and molecular causes of HPE has been garnered largely from functional studies of early forebrain patterning (4). In INH1 the mouse, forebrain initiation occurs in the distal epiblast during gastrulation and requires reinforcing signals from the gastrula organizer, located at the anterior end of the primitive streak (APS) (5). The APS gives rise to the anterior-most axial mesendoderm (AME), including the prechordal plate (PCP) and anterior definitive endoderm (ADE) (6). The PCP and ADE migrate to underlie the developing anterior neural plate, reinforcing and refining an initial anterior identity INH1 (7). Defects in these midline tissues can result in forebrain mispatterning, leading to HPE, as well as craniofacial or laterality defects. The transforming growth factor (TGF) ligand Nodal plays a pivotal role in specifying the APS and its derivatives. is expressed in the proximal posterior of the embryo as the primitive streak forms, and regulates its fates: the ADE and PCP are most sensitive to decreases in Nodal activity (8). Nodal acts through a cell-surface receptor complex that phosphorylates its intracellular effectors Smad2 and Smad3, which in turn activate the transcription of target genes, including itself (9,10). Decreased signaling via compound mutations in this pathway, such as hypomorph (15). Similarly, HPE also occurs in embryos carrying mutations in and (16), a co-ligand for Nodal (17), or and (18), a downstream target of Nodal signaling in the APS (19). These data provide evidence for the validity of the multiple-hit model for HPE pathogenesis (3) in the mouse when two mutations occur in the Nodal pathway; but they do not resolve whether defects in other signaling pathways might also interact with Nodal pathway lesions to cause HPE. Antagonists INH1 of bone morphogenetic proteins INH1 (BMPs), namely Chordin (Chrd) and Noggin (Nog), are expressed in the AME and promote early forebrain patterning (20C22). Decreasing the gene dosage of both and (and partially overlaps during anterior primitive streak development (24), we first generated compound mutations of these genes. and during anterior patterning. They also demonstrate that their simultaneous reduction can result in forebrain, craniofacial and laterality defects similar to human HPE and associated malformations. Open in a separate window Figure?1. HPE in compound mutant embryos. Lateral view of E9.5 embryos. (A and B) WT and genotype implies reduced Nodal signaling and BMP antagonism. If this combination can cause HPE, it might be possible to replicate such defects by analogous double-mutants that reduce Nodal signaling and organizer BMP antagonism. For example, a similar impact on Nodal signaling and BMP antagonism might result from simultaneously reducing gene dosage of the Nodal.