The expression of
hydroxyindole-O-methyltransferase (
HIOMT), an
enzyme catalyzing the final step of
melatonin biosynthesis, was examined in three
pineoblastomas and five
pineocytomas by in situ hybridization analysis. Distinct hybridization signals for
HIOMT mRNA, though weaker than in normal pineal gland pinealocytes, were detected in two of the three
pineoblastoma and three of the five
pineocytoma cases. Of the
pineoblastomas, hybridization signals were observed in most
tumor cells of one case, while in another, signals were detected in occasional cells clustered or scattered throughout the neoplastic field. Of the
pineocytomas, signals were detected in most
tumor cells of two cases, while in one case, signals were detected only in occasional cells. Among these specimens, one
pineoblastoma and one
pineocytoma were also analyzed using northern blot and reverse transcription polymerase chain reaction (RT-PCR) analyses. In the northern blot analysis, an apparently single band corresponding to the size of
HIOMT mRNA was detected in both
pineoblastoma and
pineocytoma RNA blots. In the RT-PCR analysis, three species of
HIOMT mRNA generated via alternative splicing were detected in both
tumors. These results suggest that the neoplastic cells of
pineoblastomas and
pineocytomas often retain the ability to express
HIOMT mRNA, as in normal pinealocytes, and that
HIOMT is a useful
tumor marker for the diagnosis of
pineal parenchymal tumors.