GENETIC-DETERMINANTS RESPONSIBLE FOR ACQUISITION OF DENGUE TYPE-2 VIRUS MOUSE NEUROVIRULENCE

Studies conducted some 50 years ago showed that serial intracerebral p assage of dengue viruses in mice selected for neurovirulent mutants th at also exhibited significant attenuation for humans. We investigated the genetic basis of mouse neurovirulence of dengue virus because it m ight be directly or indirectly associated with attenuation for humans, Analysis of the sequence in the C-PreM-E-NS1 region of the parental d engue type 2 virus (DEN2) New Guinea C (NGC) strain and its mouse-adap ted, neurovirulent mutant revealed that 10 nucleotide changes occurred during serial passage in mice. Seven of these changes resulted in ami no acid substitutions, i.e., Leu(55)-Phe and Arg(57)-Lys in PreM, Glu( 71)-Asp, Glu(126)-Lys, Phe(402)-Ile, and Thr(454)-IIe in E, and Arg(10 5)-Gln in NS1. The sequence of C was fully conserved between the paren tal and mutant DEN2. We constructed intertypic chimeric dengue viruses that contained the PreM-E genes or only the NS1 gene of neurovirulent DEN2 NGC substituting for the corresponding genes of DEN4. The DEN2 ( PreM-E)/DEN4 chimera was neurovirulent for mice, whereas DEN2 (NS1)/DE N4 was not, The mutations present in the neurovirulent DEN2 PreM-E gen es were then substituted singly or in Combination into the sequence of the nonneurovirulent, parental DEN2. Intracerebral titration of the v arious mutant chimeras so produced identified two amino acid changes, namely, Glu(71)-Asp and Glu(126)-Lys, in DEN2 E as being responsible f or mouse neurovirulence. The conservative amino acid change of Glu(71) -Asp probably had a minor effect, if any. The Glu(126)-Lys substitutio n in DEN2 E, representing a change from a negatively charged amino aci d to a positively charged amino acid, most likely plays an important r ole in conferring mouse neurovirulence.