Deficiency of Phenylethanolamine N-Methyltransferase in Norepinephrine-Producing Pheochromocytoma

Pheochromocytoma is a catecholamine (CA)-producing tumor that is classified into two types: the norepinephrine (NE) and the mixed NE and epinephrine type (E-type) from plasma CA levels. Phenylethanolamine N-methyltransferase (PNMT) is the terminal enzyme in CA synthesis; it catalyzes the synthesis of E from NE. It is not known whether the absence of immunoreactive PNMT is paralleled by a lack of PNMT mRNA. The mRNA of tyrosine hydroxylase (TI-I) and PNMT in seven pheochromocytomas, five NE-type and two E-type tumors, were examined by Northern blot analysis and in situ hybridization (ISH) technique. TH mRNA was detected in all tumors but PNMT mRNA was limited only to the E-type tumors. In addition to our previous immunohistochemical study of 70 pheochromocytomas and paragangliomas in which all pheochromocytomas had cells immunoreactive to TH, but PNMT was expressed only in E-type, we concluded that NE-type pheochromocytoma lacks PNMT both at the mRNA and protein levels, resulting in an inability to produce E. The essential difference between NE-type and E-type pheochromocytoma is that the NE-type lacks PNMT.