# PITX2

## Overview
PITX2 is a gene that encodes the paired-like homeodomain transcription factor 2, a protein integral to various developmental processes in vertebrates. This transcription factor is characterized by its homeodomain, a conserved DNA-binding domain that facilitates its role in regulating gene expression during embryogenesis. The PITX2 protein is involved in the development of asymmetrical organs, such as the heart and lungs, and plays a critical role in establishing left-right axis formation. It is activated by the Wnt/β-catenin signaling pathway, which is essential for cell proliferation and differentiation. The protein exists in multiple isoforms due to alternative splicing, each contributing to its diverse functional roles across different tissues. Mutations in the PITX2 gene are associated with several developmental disorders, including Axenfeld-Rieger syndrome, highlighting its clinical significance in human health (Espinoza2005Protein; Tran2021Pitx; Tümer2009Axenfeld–Rieger).

## Structure
The PITX2 protein is a transcription factor characterized by a homeodomain, a highly conserved DNA-binding domain with a helix-turn-helix motif. This domain is crucial for its role in developmental processes, including body plan specification and cell fate determination (BanerjeeBasu1999Threading). The homeodomain consists of three helical regions connected by short loops, forming a canonical fold stabilized by hydrophobic interactions (BanerjeeBasu1999Threading; Doerdelmann2012Structural). 

The PITX2 protein also features a C-terminal tail, which plays a multifunctional role in modulating transcriptional activity through specific protein interactions. This region is involved in autoregulation and protein-protein interactions, crucial for its function in development (Amendt1999Multifunctional). Phosphorylation by protein kinase C (PKC) is a common post-translational modification of PITX2, affecting its transcriptional activities. Phosphorylation at the C-terminal region enhances transcriptional activation, while N-terminal phosphorylation inhibits it (Espinoza2005Protein).

PITX2 exists in multiple isoforms (A, B, and C) due to alternative splicing, each with different N-terminal domains that influence their transcriptional activities (Espinoza2005Protein). These isoforms contribute to the protein's diverse functional roles in various tissues.

## Function
The PITX2 gene encodes a transcription factor that plays a crucial role in various developmental processes in healthy human cells. It is involved in the regulation of gene expression necessary for the proper development of asymmetrical organs, such as the heart and lungs, and is essential for establishing left-right axis formation during embryogenesis (Tran2021Pitx). PITX2 is activated by the Wnt/Dvl/ß-catenin signaling pathway, which is critical for cell-type-specific proliferation by activating growth-regulating genes like Ccnd2, Ccdn1, cjun, and cmyc (Tran2021Pitx). 

In the heart, PITX2 is the only member of the Pitx family expressed and is vital for correct positioning, morphogenesis, and function. It regulates the development of the heart's outflow tract, left-right specification of the atria, and the identity of left atrial cardiomyocytes (Tran2021Pitx). PITX2 also plays a role in maintaining the homeostasis of the left atrium by regulating ion transport and intercalated disc genes, which are essential for proper cardiac function and electrical stability (Tao2014Pitx2). 

In the eye, PITX2 is involved in morphogenesis and is crucial for maintaining normal eye function, as evidenced by its regulation of genes involved in stress response and visual function (Strungaru2011PITX2).

## Clinical Significance
Mutations in the PITX2 gene are primarily associated with Axenfeld-Rieger syndrome (ARS), an autosomal dominant disorder characterized by developmental abnormalities of the anterior segment of the eye, leading to conditions such as iris hypoplasia, iridogoniodysgenesis syndrome, and an increased risk of early-onset glaucoma (Kozlowski2000Variation; Tümer2009Axenfeld–Rieger). ARS can also present with systemic features, including craniofacial dysmorphism, dental anomalies, and umbilical defects (Hendee2018PITX2). The severity of these disorders correlates with the residual activity of the PITX2 protein, with mutations affecting the homeodomain and C-terminal regions leading to partial or complete loss of function (Hendee2018PITX2; Priston2001Functional).

PITX2 mutations have also been implicated in other conditions, such as Peters anomaly, ring dermoid of the cornea, and cardiac phenotypes like atrial fibrillation and Wolff-Parkinson-White syndrome (Hendee2018PITX2; Tümer2009Axenfeld–Rieger). In zebrafish models, PITX2 gene lesions result in congenital defects similar to human phenotypes, including abnormal cornea and craniofacial development, highlighting the gene's role in the Wnt pathway and collagen gene expression regulation (Hendee2018PITX2). Additionally, PITX2 mutations can lead to tetralogy of Fallot, a congenital heart defect, due to impaired transcriptional activity (Sun2016PITX2).

## Interactions
PITX2 interacts with several proteins and nucleic acids, playing a crucial role in various cellular processes. It forms complexes with proteins such as YB-1, hnRNP K, nucleolin, and hnRNP U, which were identified through mass spectrometry and confirmed by immunoblotting (Huang2009Proteomic). The interaction between PITX2 and YB-1 was validated at endogenous levels, suggesting a significant role in regulating gene expression (Huang2009Proteomic).

PITX2 also interacts with PAWR, a protein that modulates its transcriptional activity in ocular cells. This interaction occurs through the homeodomain and adjacent C-terminal inhibitory domain of PITX2, as confirmed by various assays (Acharya2009Human). PAWR inhibits PITX2-mediated transcription activation, impacting processes like apoptosis and tumorigenesis (Acharya2009Human).

In the context of the Wnt signaling pathway, PITX2 interacts with β-catenin and LEF-1. It can bind independently to both proteins, with β-catenin interacting with the homeodomain and LEF-1 with the C-terminal region of PITX2. This interaction is crucial for the regulation of LEF-1 isoform expression, particularly in tooth development (Amen2007PITX2). These interactions highlight PITX2's role in transcriptional regulation and developmental processes.


## References


[1. (Huang2009Proteomic) Yue Huang, Kan Huang, Goran Boskovic, Yulia Dementieva, James Denvir, Donald A. Primerano, and Guo-Zhang Zhu. Proteomic and genomic analysis of pitx2 interacting and regulating networks. FEBS Letters, 583(4):638–642, January 2009. URL: http://dx.doi.org/10.1016/j.febslet.2009.01.028, doi:10.1016/j.febslet.2009.01.028. This article has 12 citations and is from a peer-reviewed journal.](https://doi.org/10.1016/j.febslet.2009.01.028)

[2. (Tao2014Pitx2) Ye Tao, Min Zhang, Lele Li, Yan Bai, Yuefang Zhou, Anne M. Moon, Henry J. Kaminski, and James F. Martin. Pitx2 , an atrial fibrillation predisposition gene, directly regulates ion transport and intercalated disc genes. Circulation: Cardiovascular Genetics, 7(1):23–32, February 2014. URL: http://dx.doi.org/10.1161/CIRCGENETICS.113.000259, doi:10.1161/circgenetics.113.000259. This article has 121 citations and is from a peer-reviewed journal.](https://doi.org/10.1161/CIRCGENETICS.113.000259)

[3. (Tümer2009Axenfeld–Rieger) Zeynep Tümer and Daniella Bach-Holm. Axenfeld–rieger syndrome and spectrum of pitx2 and foxc1 mutations. European Journal of Human Genetics, 17(12):1527–1539, June 2009. URL: http://dx.doi.org/10.1038/ejhg.2009.93, doi:10.1038/ejhg.2009.93. This article has 301 citations and is from a domain leading peer-reviewed journal.](https://doi.org/10.1038/ejhg.2009.93)

[4. (Doerdelmann2012Structural) Thomas Doerdelmann, Douglas J. Kojetin, Jamie M. Baird-Titus, Laura A. Solt, Thomas P. Burris, and Mark Rance. Structural and biophysical insights into the ligand-free pitx2 homeodomain and a ring dermoid of the cornea inducing homeodomain mutant. Biochemistry, 51(2):665–676, January 2012. URL: http://dx.doi.org/10.1021/bi201639x, doi:10.1021/bi201639x. This article has 7 citations and is from a peer-reviewed journal.](https://doi.org/10.1021/bi201639x)

[5. (Espinoza2005Protein) Herbert M. Espinoza, Mrudula Ganga, Usha Vadlamudi, Donna M. Martin, Brian P. Brooks, Elena V. Semina, Jeffrey C. Murray, and Brad A. Amendt. Protein kinase c phosphorylation modulates n- and c-terminal regulatory activities of the pitx2 homeodomain protein. Biochemistry, 44(10):3942–3954, February 2005. URL: http://dx.doi.org/10.1021/bi048362x, doi:10.1021/bi048362x. This article has 17 citations and is from a peer-reviewed journal.](https://doi.org/10.1021/bi048362x)

[6. (Hendee2018PITX2) Kathryn E Hendee, Elena A Sorokina, Sanaa S Muheisen, Linda M Reis, Rebecca C Tyler, Vujica Markovic, Goran Cuturilo, Brian A Link, and Elena V Semina. Pitx2 deficiency and associated human disease: insights from the zebrafish model. Human Molecular Genetics, 27(10):1675–1695, March 2018. URL: http://dx.doi.org/10.1093/hmg/ddy074, doi:10.1093/hmg/ddy074. This article has 47 citations and is from a domain leading peer-reviewed journal.](https://doi.org/10.1093/hmg/ddy074)

[7. (Sun2016PITX2) Yu-Min Sun, Jun Wang, Xing-Biao Qiu, Fang Yuan, Ying-Jia Xu, Ruo-Gu Li, Xin-Kai Qu, Ri-Tai Huang, Song Xue, and Yi-Qing Yang. Pitx2 loss-of-function mutation contributes to tetralogy of fallot. Gene, 577(2):258–264, February 2016. URL: http://dx.doi.org/10.1016/j.gene.2015.12.001, doi:10.1016/j.gene.2015.12.001. This article has 23 citations and is from a peer-reviewed journal.](https://doi.org/10.1016/j.gene.2015.12.001)

[8. (Tran2021Pitx) Thai Q Tran and Chrissa Kioussi. Pitx genes in development and disease. Cellular and Molecular Life Sciences, 78(11):4921–4938, April 2021. URL: http://dx.doi.org/10.1007/s00018-021-03833-7, doi:10.1007/s00018-021-03833-7. This article has 21 citations and is from a domain leading peer-reviewed journal.](https://doi.org/10.1007/s00018-021-03833-7)

[9. (BanerjeeBasu1999Threading) Sharmila Banerjee-Basu and Andreas D. Baxevanis. Threading analysis of the pitx2 homeodomain: predicted structural effects of mutations causing rieger syndrome and iridogoniodysgenesis. Human Mutation, 14(4):312–319, October 1999. URL: http://dx.doi.org/10.1002/(SICI)1098-1004(199910)14:4<312::AID-HUMU6>3.0.CO;2-S, doi:10.1002/(sici)1098-1004(199910)14:4<312::aid-humu6>3.0.co;2-s. This article has 15 citations and is from a domain leading peer-reviewed journal.](https://doi.org/10.1002/(SICI)1098-1004(199910)14:4)

[10. (Strungaru2011PITX2) M. Hermina Strungaru, Tim Footz, Yi Liu, Fred B. Berry, Pascal Belleau, Elena V. Semina, Vincent Raymond, and Michael A. Walter. Pitx2 is involved in stress response in cultured human trabecular meshwork cells through regulation of slc13a3. Investigative Opthalmology &amp; Visual Science, 52(10):7625, September 2011. URL: http://dx.doi.org/10.1167/iovs.10-6967, doi:10.1167/iovs.10-6967. This article has 24 citations.](https://doi.org/10.1167/iovs.10-6967)

[11. (Kozlowski2000Variation) K. Kozlowski. Variation in residual pitx2 activity underlies the phenotypic spectrum of anterior segment developmental disorders. Human Molecular Genetics, 9(14):2131–2139, September 2000. URL: http://dx.doi.org/10.1093/hmg/9.14.2131, doi:10.1093/hmg/9.14.2131. This article has 86 citations and is from a domain leading peer-reviewed journal.](https://doi.org/10.1093/hmg/9.14.2131)

[12. (Acharya2009Human) Moulinath Acharya, David J. Lingenfelter, LiJia Huang, Philip J. Gage, and Michael A. Walter. Human prkc apoptosis wt1 regulator is a novel pitx2-interacting protein that regulates pitx2 transcriptional activity in ocular cells. Journal of Biological Chemistry, 284(50):34829–34838, December 2009. URL: http://dx.doi.org/10.1074/jbc.M109.006684, doi:10.1074/jbc.m109.006684. This article has 26 citations and is from a domain leading peer-reviewed journal.](https://doi.org/10.1074/jbc.M109.006684)

[13. (Amen2007PITX2) Melanie Amen, Xiaoming Liu, Usha Vadlamudi, Gabriela Elizondo, Evan Diamond, John F. Engelhardt, and Brad A. Amendt. Pitx2 and β-catenin interactions regulate lef-1 isoform expression. Molecular and Cellular Biology, 27(21):7560–7573, November 2007. URL: http://dx.doi.org/10.1128/mcb.00315-07, doi:10.1128/mcb.00315-07. This article has 60 citations and is from a domain leading peer-reviewed journal.](https://doi.org/10.1128/mcb.00315-07)

[14. (Priston2001Functional) M. Priston. Functional analyses of two newly identified pitx2 mutants reveal a novel molecular mechanism for axenfeld-rieger syndrome. Human Molecular Genetics, 10(16):1631–1638, August 2001. URL: http://dx.doi.org/10.1093/hmg/10.16.1631, doi:10.1093/hmg/10.16.1631. This article has 71 citations and is from a domain leading peer-reviewed journal.](https://doi.org/10.1093/hmg/10.16.1631)

[15. (Amendt1999Multifunctional) Brad A. Amendt, Lillian B. Sutherland, and Andrew F. Russo. Multifunctional role of the pitx2 homeodomain protein c-terminal tail. Molecular and Cellular Biology, 19(10):7001–7010, October 1999. URL: http://dx.doi.org/10.1128/mcb.19.10.7001, doi:10.1128/mcb.19.10.7001. This article has 90 citations and is from a domain leading peer-reviewed journal.](https://doi.org/10.1128/mcb.19.10.7001)