Altered regulation of platelet-derived growth factor receptor-a gene-transcription in vitro by spina bifida-associated mutant Pax1 proteins (neural tube defectsyembryonal carcinomayosteosarcoma)

Mouse models show that congenital neural tube defects (NTDs) can occur as a result of mutations in the platelet-derived growth factor receptor-a gene (PDGFRa). Mice heterozygous for the PDGFRa-mutation Patch, and at the same time homozygous for the undulated mutation in the Pax1 gene, exhibit a high incidence of lumbar spina bifida occulta, suggesting a functional relation between PDGFRa and Pax1. Using the human PDGFRa promoter linked to a luciferase reporter, we show in the present paper that Pax1 acts as a transcriptional activator of the PDGFRa gene in differentiated Tera-2 human embryonal carcinoma cells. Two mutant Pax1 proteins carrying either the undulated-mutation or the Gln3 His mutation previously identified by us in the PAX1 gene of a patient with spina bifida, were not or less effective, respectively. Surprisingly, Pax1 mutant proteins appear to have opposing transcriptional activities in undif- ferentiated Tera-2 cells as well as in the U-2 OS osteosarcoma cell line. In these cells, the mutant Pax1 proteins enhance PDGFRa-promoter activity whereas the wild-type protein does not. The apparent up-regulation of PDGFRa expression in these cells clearly demonstrates a gain-of-function phenom- enon associated with mutations in Pax genes. The altered transcriptional activation properties correlate with altered protein-DNA interaction in band-shift assays. Our data pro- vide additional evidence that mutations in Pax1 can act as a risk factor for NTDs and suggest that the PDGFRa gene is a direct target of Pax1. In addition, the results support the hypothesis that deregulated PDGFRa expression may be causally related to NTDs.