Background:  Progressive multifocal leukoencephalopathy (PML) is currently the second most frequent cause of AIDS-related deaths. Unlike other opportunistic infections, it also occurs in HAART-treated patients, either shortly after starting or during chronic successful treatment. Following primary infection, the causative agent, JC virus (JCV), establishes a persistent benign infection in the urinary tract and is excreted in urine in 30% of healthy persons. The mechanisms leading to JCV reactivation and PML are unclear, but it is known that the major JCV capsid protein, VP1, is involved in cell entry, through binding with cell sialic acid residues. And, recently, VP1 aminoacid substitutions have been reported in PML. Thus, the objective was to investigate whether JCV from PML patients carries genetic VP1 changes that might associate with neurotropism and neurovirulence.

Methods:  The entire JCV-VP1 region was amplified, cloned (2 to 48 clones per sample, median 23) and sequenced from the cerebrospinal fluid (CSF) of 26 PML patients (20 with HIV infection), and 11 paired plasma and 6 paired urine samples. From 9 patients, sequential CSF (n = 7) or plasma (n = 2) samples were also analyzed. JCV DNA was measured by real-time PCR. 3D modeling was used to map the mutations on VP1 structure.

Results:  Compared to wild-type virus, i.e., that present in urine of healthy persons, 1 of 8 specific single mutations or deletions was identified in almost all CSF clones from each of 24 of 26 patients (92%). These conferred substitutions or deletions in 1 of the 3 outer loops of VP1, most frequently involving residues 55 (55F, 7 patients, 27%) and 269 (269F, 6 patients, 23%). Paired plasma always showed the same CSF mutation, but no mutations were identified in urines. Mutations were maintained in sequential samples from 7 patients with progressive disease and stable or increasing JCV DNA in CSF. They were lost in 2 patients:  1 undergoing PML remission and relapse, with onset of a new mutation; and 1 with decreasing CSF JCV DNA, with emergence of a previously minor different mutant variant. By 3D modeling, all mutated residues clustered within or in the immediate proximity to the the sialic acid cell receptor binding site on VP1.

Conclusions:  In PML, JCV from CSF and plasma, but not urine, carries VP1 substitutions at critical sites for cell binding, which are maintained during the disease. These findings support a model whereby JCV acquires adaptive changes during transition from sites of persistence to the brain, eventually leading to PML.