Expression patterns and immunohistochemical localization of PITX2B transcription factor in the developing mouse heart.

The Pitx2 gene is involved in the establishment of vertebrate left-right axis with an important role in subsequent heart organogenesis. Mutations in the Pitx2 gene have been associated with Axenfeld-Rieger syndrome, which is characterized by ocular, craniofacial, and umbilical anomalies, as well as cardiac defects. In addition, recent data have unravelled a molecular link between PITX2 loss of function and atrial fibrillation (AF), supporting an important role of Pitx2 not only in development but also in heart homeostasis. Three PITX2 isoforms have been described in mice: PITX2A, PITX2B, and PITX2C. During heart organogenesis, PITX2C seems to play a determinant role in left-right signalling from early somitogenesis onwards. However the participation of the PITX2A and/or PITX2B isoforms during cardiogenesis is controversial. Here we report for the first time that the Pitx2a and Pitx2b isoforms are jointly expressed with the Pitx2c isoform during heart development. Interestingly, in terms of relative quantification of mRNA, the Pitx2b and Pitx2c isoforms display similar expression profiles during cardiogenesis, decreasing with further development but maintaining their expression until adult stages. Moreover, a detailed analysis of PITX2B protein during cardiac development shows that PITX2B is dynamically expressed in the developing ventricular septum and asymmetrically expressed in the tricuspid valve primordia, suggesting a putative role of the PITX2B isoform during ventricular septation as well as in the maturation of the right portion of the atrioventricular canal.