Of note, we found another individual (patient 9) to harbor a nucleotide transversion at nucleotide position 774, but in this case a C→G transversion, causing the predicted missense change P64R. This particular change has not been reported yet. Experimental data show that mutant PITX2 proteins that harbor missense mutations in their homeodomain demonstrated reduced or even abolished DNA binding.
41 Amino acid position 64 is located in the loop between helices 1 and 2 of the homeodomain
(Fig. 2B)and is conserved considerably between homeodomain-containing proteins.
42 Amino acid changes at this particular residue have been shown to have a disease-causing effect in the case of the homeodomain transcription factors ARX (x-linked myoclonic epilepsy),
43 TGIF (holoprosencephaly),
44 and PIT1 (pituitary hormone deficiency).
45 Because of their rigid structure, proline residues provoke the orientation of helices and are therefore often found in loops. It may be that a replacement of this proline residue with leucine or arginine changes the orientation of the first and third helices and subsequently alters the stability of the homeodomain.
46 Because the P64L missense mutation has been reported before in a pedigree with Axenfeld-Rieger and was not present in 100 ethnically matched control subjects, we consider it to be pathogenic, as well as the P64R mutation. It was striking that all our patients harboring
PITX2 mutations exhibited a dental phenotype, but none of the patients who carried a
FOXC1 mutation had dental anomalies. This result is consistent with those of previous studies showing that mutations in the
PITX2 gene appear to be more strongly associated with dental findings than mutations in the
FOXC1 gene.
3 4 Earlier studies have shown that PITX2 activates the
DLX2 gene which is required for tooth and craniofacial development.
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