FRalpha, BMP4 und humanes T-Genals Kandidatengene für Neuralrohrdefekte

Neural tube defects (NTDs) are likely to result from an interaction of several genes and environmental factors. A role for genetic factors is supported by the finding that first degree relatives of NTD patients have a significantly increased recurrence risk. Since periconceptional folic acid supplementation of the maternal diet has been shown to have a preventive effect on occurrence and recurrence of the malformation, one contributing factor is believed to involve an altered folate metabolism. Other potential candidate genes for human NTDs are offered by animal models. NTDs have been observed in a variety of mutant mice, spontaneously arisen or genetically engineered. The secreted polypeptide Bmp4 was chosen from the candidate lists as it is a neurulation gene: Bmp4 is expressed in the dorsal epidermal ectoderm, posterior mesoderm and the presumptive neural crest cells. Targeted inactivation in mice leads to abnormal phenotypes: Bmp4 knockout mice exhibit little mesoderm differentiation causing disorganised structures (including a small neural plate and cranially open folds Our study with spina bifida aperta (SBA) patients revealed single point mutations in BMP 4 that may have acted as susceptibility factors in the individual cases. Interestingly, an association study provides evidence for a genotype disequilibrium between SBA patients and controls for the BMP4 polymorphism 455T>C (V152A). The frequency of the heterozygous 455T>C genotype is lower in cases than in controls. A possible explanation is that BMP4 heterozygosity at this site is a protective factor in the normal population, although this hypothesis cannot be proven to date. One of the candidate genes for human NTDs, the murine Brachyury gene (T gene), was recognised by virtue of its mutant phenotype. Mice that are homozygous for null mutations in Brachyury die in midgestation because of abnormalities in notochord and defects in other mesoderm derived structures. Heterozygous mice have short tails, notochord anomalies, and fusion of neural tube and gut in the caudal region. Studies of T gene expression pattern show that it is maintained in those cells that are absent in mutant mouse embryos, early stage mesoderm, tailbud, and notochord. Brachyury encodes a DNA binding protein that functions as a transcription activator. Genotyping for the TIVS7 three allele polymorphism was performed using primers and single strand conformation analysis (SSCA) as previously described. We detected the two common alleles corresponding to a T (allele 1) to C (allele 2) single base exchange at position 79 bp downstream from the 5¢ end of intron 7. The rare third allele was not detected in our population. Genotypes resulting from the nucleotide exchanges were confirmed by sequencing several independent subjects. We found TIVS7 heterozygosity (homozygosity of allele 2) in 44.3% (7.6%) of cases and in 40.2% (8.3%) of controls (table 1). Genotype frequencies in cases and controls were compared using Cochran- Armitage trend tests. There was no significant difference in TIVS7 genotype distribution in our sample (p=0.70). Analysis of 101 trios (father, mother, SBA child) showed no transmission disequilibrium for the TIVS7-2 allele (transmitted:nontransmitted 45:49). A p value of 0.68 was calculated using the exact version of McNemar’s test.17 In conclusion, our results do not indicate an increased risk for sporadic SBA among homozygotes or heterozygotes for the human T TIVS7 genotypes. These data are supported by previous studies and therefore it is questionable if this polymorphism plays an important role in the susceptibility for sporadic NTD.