Reduced TCOF1 mRNA level in a rhesus macaque with Treacher Collins-like syndrome: further evidence for haploinsufficiency of treacle as the cause of disease

Treacle recruits RNA polymerase I complex to the nucleolus that is independent of UBF

Mutations in treacle lead to Treacher Collins syndrome (TCS), an autosomal dominant disorder of craniofacial development. Treacle associates with upstream binding factor (UBF) to regulate rRNA gene (rDNA) transcription, but the precise mechanisms mediated by treacle remain elusive. Here we show that the central repeated domain of treacle binds with RNA polymerase I (Pol I), while that the treacle C-terminus is involved in rDNA promoter recognition and UBF recruitment. Knockdown of treacle resulted in dispersion of Pol I and UBF away from nucleolus, whereas interactions of treacle with Pol I and rDNA promoter were not disrupted by UBF depletion. These findings indicate that treacle, but not UBF, is essential for nucleolar recruitment of Pol I transcription complex. Furthermore, C-terminally truncated treacle, mimicking TCS-associated mutations, failed to target to the nucleolus, possibly causing loss-of-function in the mutant treacle. Our observations support that TCS results from haploinsufficiency of treacle.

Regulation of the mouse Treacher Collins syndrome homolog (Tcof1) promoter through differential repression of constitutive expression

Treacher Collins syndrome is an autosomal-dominant mandibulofacial dysostosis caused by haploinsufficiency of the TCOF1 gene product treacle. Mouse Tcof1 protein is approximately 61% identical and 71% similar to treacle, and heterozygous knockout of Tcof1 causes craniofacial malformation. Tcof1 expression is high in developing neural crest, but much lower in other tissues. To investigate this dual regulation, highly conserved regions upstream of TCOF1 homologs were tested through deletion and mutation reporter assays, and conserved predicted transcription factor binding sites were assessed through chromatin binding studies. Assays were performed in mouse P19 embryonic carcinoma cells and in HEK293 cells to determine differential activation in cell types at different stages of differentiation. Binding of Cebpb, Zfp161, and Sp1 transcription factors was specific to the Tcof1 regulatory region in P19 cells. The Zfp161 binding site demonstrated P19 cell-specific repression, while the Sp1/Sp3 candidate site demonstrated HEK293 cell-specific activation. Moreover, presence of c-myb and Zfp161 transcripts was specific to P19 cells. A minimal promoter fragment from -253 to +43 bp directs constitutive expression in both cell types, and dual regulation of Tcof1 appears to be through differential repression of this minimal promoter. The CpG island at the transcription start site remains unmethylated in P19 cells, 11.5 dpc mouse embryonic tissue, and adult mouse ear, which supports constitutive activation of the Tcof1 promoter.