Mutation in NS5 protein attenuates mouse neurovirulence of yellow fever 17D vaccine virus

This study identifies a molecular basis for attenuation of a medium plaque variant (17D-MP) of the yellow fever 17D vaccine virus. Researchers compared a virulent large plaque variant (17D-LP) with an attenuated medium plaque variant derived from the 17D-204 South African vaccine strain. The 17D-LP virus was highly lethal to mice following intracerebral inoculation (LD50 of 0.2 p.f.u.), while 17D-MP virus was significantly attenuated, with approximately 90% of mice surviving 10,000 p.f.u. inoculation. Viral replication in mouse brains was substantially reduced for 17D-MP compared to 17D-LP. Genetic sequencing revealed a single nucleotide difference at position 8045 (C to U), resulting in a proline-to-serine substitution at residue 137 of the NS5 protein. The 17D-MP virus was also slightly temperature-sensitive at 39.5°C. This is the first report of flavivirus virulence alteration caused by mutation in a non-structural protein gene other than NS1, with the NS5 mutation likely affecting viral RNA capping and protein synthesis functions.

Key findings

• A single mutation at nucleotide 8045 (Pro137Ser in NS5 protein) in 17D-MP virus is responsible for attenuation in mice, reducing LD50 from 0.2 p.f.u. to >10,000 p.f.u.
• 17D-MP virus shows temperature sensitivity at 39.5°C and reduced replication in mouse brains compared to virulent 17D-LP virus
• This is the first documented case of flavivirus virulence attenuation caused by mutation in a non-structural protein gene other than NS1
• The NS5 mutation occurs in the methyltransferase domain, likely impairing viral RNA capping and protein synthesis functions