Expanding the spectrum of SMAD3-related phenotypes to agnathia-otocephaly

Abstract

Background: Agnathia-otocephaly is a rare and lethal anomaly affecting craniofacial structures derived from the first pharyngeal arch. It is characterized by agnathia, microstomia, aglossia, and abnormally positioned auricles with or without associated anomalies. Variants affecting function of OTX2 and PRRX1, which together regulate the neural crest cells and the patterning of the first pharyngeal arch as well as skeletal and limb development, were identified to be causal for the anomaly in a few patients.

Methods: Family-based exome sequencing (ES) on a fetus with severe agnathia-otocephaly, cheilognathopalatoschisis, laryngeal hypoplasia, fused lung lobes and other organ abnormalities and mRNA expression analysis were performed.

Results: Exome sequencing detected a de novo SMAD3 missense variant in exon 6 (c.860G>A) associated with decreased mRNA expression. Variants in SMAD3 cause Loeys-Dietz syndrome 3 presenting with craniofacial anomalies such as mandibular hypoplasia, micro- or retro-gnathia, bifid uvula and cleft palate as well as skeletal anomalies and arterial tortuosity. The SMAD3 protein acts as a transcriptional regulator in the transforming growth factor β (TGFB) and bone morphogenetic (BMP) signaling pathways, which play a key role in the development of craniofacial structures originating from the pharyngeal arches.

Conclusion: Agnathia-otocephaly with or without associated anomalies may represent the severe end of a phenotypic spectrum related to variants in genes in the interacting SMAD/TGFB/BMP/SHH/FGF developmental pathways.

Keywords: SMAD3; agnathia-otocephaly; exome sequencing; prenatal.

Figure 1

(a–e) The affected fetus at 19 + 2 weeks of gestation in prenatal ultrasound (a and b) and post mortem (c–e) showing cheilognathopalatoschisis, microstomia, absent mandible and ventral median positioned auricles. (f) Relative SMAD3 mRNA expression in kidney and liver tissue of an age‐matched control fetus and the affected fetus (f). RNA was extracted from FFPE tissue, the expression of SMAD3 was normalized to GUSB expression. FFPE, formalin fixed paraffin embedded

Figure 2

Roles of SMAD3 in the TGF‐β and the BMP pathway. The SMAD3 protein gets activated through TGF‐β signaling and can act in different downstream pathways. If SMAD3 binds to TRIM33, a chromatin reading and remodeling protein, the complex opens the histone to allow other TFs to enter the DNA. Complexes of SMAD3 and SMAD4 get recruited to different genes, and the transcription of a specific gene is determined by varying partner TFs. In those effector pathways SMAD3 is involved in chondrocyte and osteoblast maturation (Massagué, 2012). If SMAD3 binds to DROSHA it regulates the processing of different miRNA precursors. SMAD3 forming a complex with other SMAD proteins driven by BMP signaling also activates transcription via other TFs, regulating various bone formation processes (Rahman et al., 2015). TGF‐β, transforming growth factor‐β; TFs, transcription factors