Involvement of host ADAR in biased hypermutation of intracelebral measles virus causing SSPE

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It has been clarified that SSPE (subacute sclerosing panencephalitis) viruses, measles virus defective variants isolated from brains of SSPE patients, acquire biased hypermutation in their M genes, in which frequent replacements from adenine to guanine in genome sense are found. Although in recent years adenosine deaminases that act on RNAs (ADARs) might be involved in its phenomenon, there has not yet been any report showing direct evidences that ADAR causes biased hypermutation of measles virus M gene. Therefore, we examined if ADAR constitutively expressed in neural cells from the externally introduced gene can induce biased hypermutation in the measles virus infected cells.We established A172/ADAR+ cells, an ADAR constitutively expressed cell line using A172 cells, derived from human glioblastoma. These cells were infected rMV323 virus (derived from the Ichinose B strain cDNA inserted with GFP gene) or Edm-B virus (derived from the Edmonston-B strain cDNA) and incubated at 35℃ for 1 week. One half of the infected A172/ADAR+ cells were cocultured with 5 x 10^5 fresh A172/ADAR+ cells and further incubated. The other half of the infected cells were stocked at -80℃ for viral RNA analysis (RT-PCR and direct sequencing of the M gene). This way of passage were repeated 10 times. Amplification of the M gene cDNA was detected in samples of the passage 1 to 2 of rMV323 virus and of the passage 1 to 7 of Edm-B virus. At present, however, positive mutation is not suggested in the M gene.