Orodental malformations associated with human MSX1 sequence variants

Background MSX1 sequence variants have been known to cause human tooth agenesis (TA) with or without orofacial clefts. However, their roles during the whole processes of tooth development are not fully understood. This study aimed to characterize a 4-membered family with TA carrying a novel MSX1 pathogenic variant and investigate the disease mechanism. Methods The authors conducted whole exome analysis to define the disease-causing sequence variant. They performed microcomputed tomography, morphometric analyses, transcriptome profiling, and molecular characterization to study the affected teeth and the gene variant. Results The authors identified an MSX1 pathogenic variant, p.Glu232∗, in affected family members with TA and concomitant orodental anomalies, namely, prominent maxillary labial frenum, central incisor diastema, median maxillary anterior alveolar cleft, tooth fusion, mandibular molar dysmorphology, thin dentin layer, and slender dental roots. MSX1-defective teeth were not apparently microdontic but had thin dentin layers. The mandibular molars showed a homeotic transformation to maxillary counterparts. Genes involved in extracellular matrix organization and dentinogenesis, such as DMP1 and MMP20, were downregulated in dental pulp tissues of MSX1-defective teeth. The p.Glu232∗-truncated MSX1 properly localized to the nucleus but partially lost its transactivation ability. Analyzing reported cases indicated that truncation sequence variants within the homeobox domain of MSX1 caused a more severe TA phenotype than those outside of the homeobox domain, probably due to dominant negativity compared with haploinsufficiency. Conclusions This study provides in vivo evidence that MSX1 contributes to developmental processes of various orodental tissues in humans. Practical Implications Clinically, hypertrophic labial frenum, incisor diastema, and median maxillary anterior alveolar cleft might be considered diagnostic for MSX1-associated TA.