Distinct Involvement of X-Inactivation in Organogenesis

Abstract

X-inactivation is the process of dosage compensation to balance X-linked gene expression levels between females (XX) and males (XY). One of the 2 X-chromosomes is randomly selected for inactivation in each cell during embryogenesis and remains inactive throughout life. Therefore, females have 2 types of cells: those with paternal X-chromosome activation and those with maternal X-chromosome activation. However, it remains unclear how X-inactivation is involved in palate and tooth development. Ofd1 is located on the X-chromosome. We found cleft palate in all heterozygous Ofd1 mutant mice (Ofd1^fl/WT;Wnt1Cre[HET]), even though some cells should activate the normal X-chromosome due to random X-inactivation. Cells with Ofd1 mutant X chromosome activation (Ofd1 mutant cells) accumulated at the tip of the palatal shelves due to cell segregation caused by a lack of EFNB1 expression, resulting in a cleft palate in all Ofd1^fl/WT;Wnt1Cre(HET) mice. Unlike the palate, Ofd1^fl/WT;Wnt1Cre(HET) mice showed multiple tooth phenotypes, including extra and absent incisors, while a normal number of incisors was also found. Accumulation of Ofd1 mutant cells also occurred in the tooth mesenchyme, and these clusters of Ofd1 mutant cells failed to initiate incisor tooth formation due to a lack of Wnt signaling. In contrast to palate development, the location of Ofd1 mutant cell clusters differed between Ofd1^fl/WT;Wnt1Cre(HET) mice, likely leading to diversity of tooth phenotypes. Thus, X-inactivation exhibits different involvement between palate and tooth development.