Failure of central nervous system myelination in MBP/c-myc transgenic mice: evidence for c-myc cytotoxicity.

c-myc is a member of the helix-loop-helix/leucine zipper family of proteins that modulate the transcriptional activity of specific target genes. Although aberrant c-myc expression has been reported to play a role in multistage carcinogenesis in astrocytic gliomas, little is known about the effects of the expression of c-myc on oligodendrocytes. Using transgenic animals expressing a human c-myc oncogene under transcriptional control of the myelin basic protein gene, we investigated the effect of overexpression of this oncogene in oligodendrocytes. The MBP/c-myc transgenic mice developed severe neurological disturbances characterized by action tremors and recurrent seizures, and premature death during postnatal weeks three to five. Affected transgenic mice of various strains had severely hypomyelinated central nervous systems and expressed low levels of c-myc, myelin basic protein (MBP) and proteolipid protein (PLP) mRNAs in the brain. These c-myc transgenic mice also exhibited an increased number of TUNEL positive nuclei, which in most cases were located in cells that expressed c-myc, as judged by double immunohistochemistry. There was no evidence of brain tumors in the c-myc transgenic mice, including heterozygous mice from two strains that had normal lifespans. These observations indicate that the myelin deficiency observed in the MBP/c-myc transgenic animals results from a cytotoxic effect of the c-myc transgene.