Human PRRX1 and PRRX2 genes: cloning, expression, genomic localization, and exclusion as disease genes for Nager syndrome

Abstract

In this study, we extend our examination of the function of the Prrx1 (a.k.a Mhox, Prx1, K-2, and Pmx1) as well as Prrx2 (a.k.a. S8 and Prx2) genes by characterizing the expression of the human orthologs and their potential for causing specific human malformations. The expression pattern of PRRX2 and its close relative, PRRX1, were analyzed in human tissue by RT-PCR. Although the expression of these human genes is similar to their mouse orthologs, there are notable differences in expression. PRRX2 was detected in the human kidney and lung, whereas in mice and chickens neither of these tissues has been reported to express Prrx2. For PRRX1 the expression pattern was quite similar to other vertebrates, but the ratio of the two isoforms was reversed. To begin the search for the gene-disease connection, both genes were mapped to human chromosomes by FISH. The PRRX1 locus maps to 1q23, whereas the PRRX2 locus maps to 9q34.1. This localization, along with the recently described phenotypes of the gene-targeted Prrx1, Prrx2 and double mutant mice, enabled us to search the human disease databases for similar malformations. This examination suggested that mutations at the PRRX1 and/or PRRX2 loci could result in Nager Acrofacial Dysostosis (NAFD) syndrome. We obtained DNA samples from eight patients with NAFD, as well as two patients with Miller syndrome, and analyzed them for mutations in the PRRX1 and PRRX2 genes. The data excludes mutations in the presumed coding sequences of these genes from causing NAFD.

The Identification of Prx1 Transcription Regulatory Domains Provides a Mechanism for Unequal Compensation by thePrx1 and Prx2 Loci

“…Very little is known about the expression pattern of the two isoforms of Prx1. Although the Prx1a and Prx1b transcripts appear to be coexpressed in the limited mice and human tissues examined, there are dramatic tissue-specific differences in the ratio of these two mRNAs in human tissue (5,6). The transcripts of Prx1 and Prx2 are expressed primarily in mesoderm within regions of undifferentiated mesenchyme during embryogenesis, particularly in the ectomesenchyme of the craniofacial region and branchial arches, as well as mesenchymal cells of the developing limbs and cardiac cushions.…”

“…the paired domain) present in other paired related homeoproteins (1,2). Prx1 (also known as mhox, k2, Pmx, PRRX1) (3)(4)(5)(6) and Prx2 (also known as S8 and PRRX2) (6 -8) are 97% identical within their homeodomains and 64% identical over the entire molecule. This high degree of sequence homology as well as similar expression and genomic organization suggest that Prx1 and Prx2 are the result of a gene duplication event (3-6, 8 -10).…”

“…The function of the OAR domain is not well understood; however, three proteins with an OAR domain have been examined. In the Prx2 and Pitx2 homeoproteins, the OAR domain has been defined as inhibiting transactivation (6,11). For the Otp homeoprotein, the OAR domain may either be a transactivator or have an adjacent transactivation domain (12).…”

“…Although Prx1 and Prx2 appear to be down-regulated during chondrogenesis and osteogenesis, expression of both is retained in the perichondrium and the periosteum (9,10,14). Also, Prx1 transcripts are highly expressed in cardiac and skeletal muscle of mouse embryos as well as adults (3,5,6,10). The function of the Prx genes was investigated by creating null alleles in embryonic stem cells.…”

“…A better understanding of how Prx1 and Prx2 proteins regulate transcription should provide insight into their biochemical function as well as this unequal compensation. Recently, we have identified important transcription regulatory regions in the Prx2 homeoprotein (6). To examine similarities and differences between the transcription factors encoded by the Prx1 and Prx2 loci, this study focuses on defining functional domains within the Prx1a and Prx1b homeoproteins.…”