Medulloblastoma is the most common
malignant brain tumor in children. The presence of
microsatellite instability (MSI) in
brain tumors, particularly
medulloblastomas, has not been properly addressed. The aim of the present study was to evaluate the role of MSI in
medulloblastoma carcinogenesis. MSI status was determined in 36 patients using a pentaplex
PCR of quasimonomorphic markers (NR27, NR21, NR24, BAT25, and BAT26).
Methylation status of
mismatch repair (MMR) genes was achieved by methylation-specific
multiplex ligation-dependent probe amplification (MLPA). In addition,
MutS homolog 6 (
MSH6) expression was determined by
immunohistochemistry.
Mutations of 10 MSI target genes (
TCF4,
XRCC2,
MBD4, MRE11, ATR,
MSH3,
TGFBR2, RAD50,
MSH6, and BAX) were studied by pentaplex
PCR followed by analysis with GeneScan 3.7 software.
Mutation analysis of hotspot regions of
beta-catenin (
CTNNB1) and BRAF (v-raf murine
sarcoma viral oncogene homolog B1)
oncogenes was performed by
PCR single-strand conformation polymorphism analysis followed by direct
sequencing. Among the 36
tumors, we found four (11%) cases with
instability, one with high MSI and three with low MSI.
Methylation analysis of MMR genes in cases presenting shifts on the MSI markers revealed mild hypermethylation of
MSH6 in 75% of cases, yet
MSH6 was expressed in all the
tumors. The MSI target genes
MBD4 (methyl-CpG
binding domain protein 4) and MRE11 (
meiotic recombination 11 homolog A) were
mutated in two different
tumors. No
CTNNB1 or BRAF
mutations were found. This study is the most comprehensive analysis of MSI in
medulloblastomas to date. We observed the presence of MSI together with
mutations of MSI target genes in a small fraction of cases, suggesting a new
genetic pathway for a role in
medulloblastoma development.
