Subacute sclerosing panencephalitis (
SSPE), a very rare but lethal disease caused by
measles viruses (MV) persisting in the human central nervous system (CNS) is characterized by lack of viral budding, reduced expression of the
viral envelope proteins and spread of MV genomes through the CNS despite massive immune responses. The five major MV genes from several
SSPE cases were cloned and sequenced, the two transmembrane envelope
glycoproteins hemagglutinin (H) and fusion
protein (F) were expressed and their maturation, cellular localization and functionality analyzed. We conclude that 1) mutations in the MV genes arise not only individually, by errors of the MV polymerase, but also in clusters as hypermutations, presumably due to
RNA unwinding/modifying activity altering accidentally formed
double-stranded RNA regions, 2) MVs spread in
SSPE brains after clonal selection, 3) the MV
matrix (M) gene is most heavily mutated and dispensable, 4) the two genes encoding envelope transmembrane
proteins give rise to functional but altered
proteins (typically F is heavily altered in its cytoplasmic domain), 5) H
protein is transported poorly to the cell surface, 6) F and H
proteins maintain tightly interdepending fusion functions, presumably to allow local cell fusion and MV
ribonucleoprotein (RNP) spread through the CNS.