Absence of 9q22-9qter in trisomy 9 does not prevent a Dandy-Walker phenotype.

We report on a female fetus with partial trisomy 9 due to a reciprocal translocation in the mother. Routine ultrasound examination at 23 weeks showed hypoplasia of the cerebellar vermis, dilated foramen Magendii, and dilatation of the cisterna magna. Due to the poor prognosis, the parents opted for termination of pregnancy. A postmortem examination confirmed caudal hypoplasia and dysplasia of the cerebellar vermis, resulting in a massively dilated foramen Magendii through which the enlarged cisterna magna communicated with the fourth ventricle. There was also micropolygyria indicating migration disorder. Cytogenetic studies showed a 47,XX,+der(9)t(7;9) (q35;q22.2)mat karyotype. Investigation of the parents revealed a translocation (7;9) (q35;q22.2) in the mother and a normal male karyotype in the father. We systematically searched the chromosome 9 gene map for genes that were trisomic in our fetus and genes that were located on the regions that had the normal two copies of genes. Genes that could potentially be involved in the formation of the Dandy-Walker phenotype are transcription factors or genes responsible for the regulation of normal in particular cerebral development but also adhesion molecules. We conclude that one cause for Dandy-Walker malformation could be a gene dosage effect of genes located on 9pter-9q22. In addition, it seems that absence of trisomy 9 in q22-pter does not prevent abnormal cerebellar development.