Identification of JC virus variants in multiple tissues of pediatric and adult PML patients

The transcriptional control region (TCR) of JC virus (JCV), the causative agent of progressive multifocal leukoencephalopathy (PML), undergoes rearrangement during replication of the virus in its human host. The mechanism by which viral promoter/enhancer sequences are deleted and duplicated within the TCR of the archetype form of JCV is not understood, but it is hypothesized that the generation of JCV variants with rearranged TCRs contributes to the virus's pathogenic potential. In a recent study of a pediatric PML patient, we detected extensive rearrangement of the JCV TCR in multiple tissues, and the archetype TCR was amplified from sites other than the kidney. These findings differed from those of previous studies that had examined tissues from adult PML patients. Since exposure to JCV usually occurs early in life, it is likely that some pediatric cases of PML arise as the result of a primary infection, whereas adult cases of PML are thought to result from the reactivation of an infection suffered as an immunocompetent child. To investigate further whether rearrangement of the JCV TCR is affected by the host's age and immune status at the time of exposure, a second pediatric patient and two adult PML patients were examined. As in our first study, multiple tissues were found to contain JCV DNA; however, fewer rearranged variants were detected. In one adult patient, related rearranged variants were detected in the brain, while archetype JCV was found in the other tissues. Based on differences in their VP1 sequences, these two forms represented different JCV genotypes, indicating that this patient had suffered a dual infection. The relevance of these findings to the rearrangement process that alters the JCV TCR is discussed.