An unusual class of PITX2 mutations in Axenfeld-Rieger syndrome.
BACKGROUND
Mutations in the PITX2 homeobox gene are known to contribute to Axenfeld-Rieger syndrome (ARS), an autosomal-dominant developmental disorder. Although most mutations are in the homeodomain and result in a loss of function, there is a growing subset in the C-terminal domain that has not yet been characterized. These mutations are of particular interest because the C-terminus has both inhibitory and stimulatory activities.


METHODS
In this study we used a combination of in vitro DNA binding and transfection reporter assays to investigate the fundamental issue of whether C-terminal mutations result in gain or loss of function at a cellular level.


RESULTS
We report a new frameshift mutation in the PITX2 allele that predicts a truncated protein lacking most of the C-terminal domain (D122FS). This newly reported mutant and another ARS C-terminal mutant (W133Stop) both have greater binding than wild-type to the bicoid element. Of interest, the mutants yielded approximately 5-fold greater activation of the prolactin promoter in CHO cells, even though the truncated proteins were expressed at lower levels than the wild-type protein. The truncated proteins also had greater than wild-type activity in 2 other cell lines, including the LS8 oral epithelial line that expresses the endogenous Pitx2 gene.


CONCLUSIONS
The results indicate that the PITX2 C-terminal domain has inhibitory activity and support the notion that ARS may also be caused by gain-of-function mutations.
Genetics Program, University of Iowa, Iowa City, Iowa 52242-1109, USA.
Irfan
I
Saadi
Rafael
R
Toro
Adisa
A
Kuburas
Elena
E
Semina
Jeffrey C
JC
Murray
Andrew F
AF
Russo
D000015:Abnormalities, Multiple / Q000235:genetics* / Q000378:metabolism; D000818:Animals; D000869:Anterior Eye Segment / Q000002:abnormalities*; D016466:CHO Cells; D002675:Child, Preschool; D006224:Cricetinae; D003412:Cricetulus; D004252:DNA Mutational Analysis; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000378:metabolism; D019281:Dimerization; D005260:Female; D018398:Homeodomain Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D006801:Humans; D008297:Male; D051379:Mice; D009154:Mutation; D013577:Syndrome; D014071:Tooth Abnormalities / Q000235:genetics* / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism; D015533:Transcriptional Activation / Q000502:physiology; D014470:Umbilical Cord / Q000002:abnormalities*; D000097577:Homeobox Protein PITX2
D004268:DNA-Binding Proteins; D018398:Homeodomain Proteins; D014157:Transcription Factors; D000097577:Homeobox Protein PITX2