Common arterial trunk associated with a homeodomain mutation of NKX2.6.

Persistent truncus arteriosus (PTA) is a failure of septation of the cardiac outflow tract (OFT) into the pulmonary artery and the aorta. A common arterial trunk (CAT) is often diagnosed as PTA in the absence of evidence of embryological mechanism. We have used autozygosity mapping of a large consanguineous family segregating CAT to map the causative locus to chromosome 8p21. An F151L mutation was identified in the homeodomain of NKX2.6, a transcription factor expressed in murine pharyngeal endoderm and embryonic OFT myocardium. Although expression of Nkx2.6 during murine embryogenesis is strongly suggestive of a role for this gene in heart development, mice homozygous for a targeted mutation of Nkx2.6 are normal. However, in these mice, it has been shown that Nkx2.5 expression expands into regions lacking Nkx2.6, suggesting functional complementation. As transcriptional targets of NKX2.6 are unknown, we investigated functional effects of the mutation in transcriptional and protein interaction assays using NKX2.5 as a surrogate. Introduction of F157L into human NKX2.5 substantially reduced its transcription activating function, its synergism with partners at the atrial natriuretic factor (ANF) and connexin-40 (Cx40) promoters and its specific DNA binding. We tested NKX2.5 target promoters for NKX2.6 activity. NKX2.6 was inactive at ANF but weakly activated transcription of a Cx40 promoter, whereas the F151L mutant lacked this activity. These findings indicate a previously unsuspected role for NKX2.6 in heart development, which should be re-evaluated in more sophisticated model systems.

Craniofacial expression of human and murine TBX22 correlates with the cleft palate and ankyloglossia phenotype observed in CPX patients.

Cleft palate with ankyloglossia (CPX; MIM 303400) is inherited as a Mendelian, semidominant X-linked disorder and has been described in several large families from different ethnic origins. It is a useful genetic model for non-syndromic cleft palate, a common congenital disorder. Recently, the underlying genetic defect in CPX was identified, where unique mutations were found in the T-box-containing transcription factor TBX22. Here we report two new familial cases with novel missense and insertion mutations, each occurring within the T-box domain and highlighting the functional significance of this DNA-binding motif. We describe TBX22 expression in early human development, where expression is found in the palatal shelves and is highest prior to elevation to a horizontal position above the tongue. mRNA is also detected in the base of the tongue in the region of the frenulum that corresponds to the ankyloglossia seen in CPX patients. Other sites of expression include the inferior portion of the nasal septum that fuses to the palatal shelves, the mesenchyme from which tooth buds develop, and the tooth buds themselves. We have also identified the orthologous mouse Tbx22 gene and performed expression analysis in E12.5-E17.5 mouse embryos. The location of mRNA expression closely correlates between mouse and human, while at later stages of development, we also detected expression in mouse lung and whisker follicles. We conclude that expression of TBX22 is entirely consistent with the CPX phenotype and that the mouse should provide a useful model for elucidating its role in craniofacial development.

Cloning and characterization of Igsf9 in mouse and human: a new member of the immunoglobulin superfamily expressed in the developing nervous system.

We describe the cloning and characterization of a novel member of the immunoglobulin superfamily, Igsf9. The predicted protein structure of IGSF9 closely matches that of the neural cell-adhesion molecule (NCAM) subfamily, consisting of an extracellular region containing five immunoglobulin domains and two fibronectin type III (FnIII) repeats, a transmembrane region, and a cytoplasmic tail. We have also characterized the orthologous human IGSF9 gene at 1q22-q23, revealing a highly conserved sequence and genomic organization. Expression of Igsf9 was detected by RT-PCR in mouse embryonic RNA from embryonic day (E) 7.5 to E16.5, while whole-mount in situ hybridization at E10.5 shows intense expression within the dorsal root ganglia, trigeminal ganglia, and olfactory epithelium, and less intense expression in the neuroepithelium, retina, and hindgut. In the human, transcription was detected in a wide variety of fetal tissues at both 8 and 14 weeks. Protein homology of IGSF9 is most similar to the Drosophila melanogaster Turtle protein that functions in coordinated motor output in complex behaviors.