Paired Box Gene 9

Alternative Names

  • PAX9
  • Paired Domain Gene 9
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OMIM Number

167416

Gene Map Locus
14q13.3

Description

PAX9 gene is a transcription factor and member of the paired box (PAX) family, a class of genes that code for transcription factors that play major roles in fetal development and organogenesis.  The PAX9 protein has been shown to interact with MSX1 to form a heterodimeric protein complex, which in turn activates the transcription of other genes, such as BMP4, that are involved in tooth development.  Results obtained from animal studies show that PAX9 knocked-out mice exhibit impaired development of organs, musculature and the skeleton, including absent and abnormally developed teeth, and cleft secondary palate.  

Mutations in the human PAX9 gene are associated with a condition known as Selective Tooth Agenesis-3, characterized by teeth agenesis and missing secondary dentition.  PAX9 mutations tend to affect primarily molar development, although lateral incisors and premolars are also known to be affected.

Molecular Genetics

The PAX9 gene is a 22kb stretch of DNA located on the long arm of chromosome 14 and contains five exons.  The encoded protein called Paired box protein PAX9 is made up of 341 amino acids with a molecular mass of 36 kDa.  Like other members of the PAX family, the PAX9 protein also consists of a DNA binding domain called the paired box domain, an octapeptide, and a paired-type homeodomain.  PAX9 is expressed mostly in neural crest derived mesenchyme, which is involved in craniofacial and tooth development. 

Most mutations in the PAX9 gene have been found to cluster in exons 1, 2, and 4.  These include two categories of mutations.  The first category includes in-frame, missense, insertions and deletions that result in loss of function and defects in BMP4 activation.  The second category of mutations includes nonsense and frame shift mutations that cause defects in MSX1 transactivation.  The latter category of mutations results in a more severe phenotype.

Epidemiology in the Arab World

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Other Reports

Saudi Arabia

Shahid et al., (2016) enrolled 21 Saudi Arabian families having 41 patients with non-syndromic form of tooth agenesis.  DNA was isolated from peripheral blood samples and sent for Sanger sequencing for MSX1, PAX9, MMP20 genes.  Shahid et al., (2016) identified a novel (g.10672A > T; p.Asn16Ile) mutation in the PAX9 gene in two patients with negative family history of teeth agenesis.  This novel change was predicted to be damaging by several algorithms such as Poly-phen2 and SIFT.  This was the first mutation to be identified in exon 3 of the PAX9 gene.

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