No preferential parent of origin for the isochromosome 17q in childhood primitive neuroectodermal tumor (medulloblastoma).

We have shown that an i(17q) is the most frequent abnormality in central nervous system primitive neuroectodermal tumors (PNETs; medulloblastoma), implicating the presence of a tumor suppressor gene which maps to 17p. In the present study, we investigated whether the deletion of chromosome arm 17p that results from the formation of the i(17q) is preferentially of maternal or paternal origin. Eight cases of primary PNETs of the posterior fossa were examined at five polymorphic loci which map to 17p13. Two or three informative loci were detected in each family. Of the eight cases, four tumors evidenced loss of the paternal allele for loci on 17p13 and four tumors demonstrated maternal deletions. Although the number of cases is relatively small, these studies do not implicate the loss of an imprinted gene as a mechanism for tumorigenesis in children with central nervous system PNETs/medulloblastoma.