Precise data on the incidence of transforming ras oncogenes in pediatric
tumors and the correlations with the histopathological properties of the
tumors are very limited. Additionally the presence of ras activation in
medulloblastomas has not been investigated so far. Using a combination of techniques including in vitro gene amplification by polymerase chain reaction (PCR) and detection of single base mutations by sequence-specific
oligonucleotides we studied N-ras activation (mutations at
codon 12, 13, and 61) in 32
medulloblastomas.
DNA was isolated from 20 microns sections of
formalin-fixed
paraffin-embedded tissue. Mutations were found in 3 out of 32 examined
medulloblastomas. In all cases only mutations of
codon 61 were found: two of three mutations were C to A mutations at position 1 of the
codon 61 (leading to a substitution of a
glutamine residue for a
lysine) and one was A to T mutation at position 3 in the same
codon (
glutamine-
histidine). Our results indicate 10% incidence N-ras mutation in
medulloblastoma, higher than in other CNS
tumors studied so far. The main advantages of the procedure described are its greatly improved sensitivity, the increased speed with which
tumor samples can be analyzed, and the possibility of using
paraffin-embedded sections to analyze various rare
tumors in retrospect.