Distinct Involvement of X-Inactivation in Organogenesis

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. <i>Ofd1</i> is located on the X-chromosome. We found cleft palate in all heterozygous <i>Ofd1</i> mutant mice (<i>Ofd1</i>^fl/WT;<i>Wnt1Cre</i>[HET]), even though some cells should activate the normal X-chromosome due to random X-inactivation. Cells with <i>Ofd1</i> mutant X chromosome activation (<i>Ofd1</i> 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 <i>Ofd1</i>^fl/WT;<i>Wnt1Cre</i>(HET) mice. Unlike the palate, <i>Ofd1</i>^fl/WT;<i>Wnt1Cre</i>(HET) mice showed multiple tooth phenotypes, including extra and absent incisors, while a normal number of incisors was also found. Accumulation of <i>Ofd1</i> mutant cells also occurred in the tooth mesenchyme, and these clusters of <i>Ofd1</i> mutant cells failed to initiate incisor tooth formation due to a lack of Wnt signaling. In contrast to palate development, the location of <i>Ofd1</i> mutant cell clusters differed between <i>Ofd1</i>^fl/WT;<i>Wnt1Cre</i>(HET) mice, likely leading to diversity of tooth phenotypes. Thus, X-inactivation exhibits different involvement between palate and tooth development.