Evidence that the hypermutated M protein of a subacute sclerosing panencephalitis measles virus actively contributes to the chronic progressive CNS disease.

Subacute sclerosing panencephalitis (SSPE) is a progressive degenerative disease of the brain uniformly leading to death. Although caused by measles virus (MV), the virus recovered from patients with SSPE differs from wild-type MV; biologically SSPE virus is defective and its genome displays a variety of mutations among which biased replacements of many uridine by cytidine resides primarily in the matrix (M) gene. To address the question of whether the SSPE MVs with M mutations are passive in that they are not infectious, cannot spread within the CNS, and basically represent an end-stage result of a progressive infection or alternatively SSPE viruses are infectious, and their mutations enable them to persist and thereby cause a prolonged neurodegenerative disease, we utilized reverse genetics to generate an infectious virus in which the M gene of MV was replaced with the M gene of Biken strain SSPE MV and inoculated the recombinant virus into transgenic mice bearing the MV receptor. Our results indicate that despite biased hypermutations in the M gene, the virus is infectious in vivo and produces a protracted progressive infection with death occurring as long as 30 to 50 days after that caused by MV. In primary neuron cultures, the mutated M protein is not essential for MV replication, prevents colocalization of the viral N with membrane glycoproteins, and is associated with accumulation of nucleocapsids in cells' cytoplasm and nucleus.