Comparison of the virulent Asibi strain of yellow fever virus with the 17D vaccine strain derived from it.

Abstract

We have sequenced the virulent Asibi strain of yellow fever virus and compared this sequence to that of the 17D vaccine strain, which was derived from it. These two strains of viruses differ by more than 240 passages. We found that the two RNAs, 10,862 nucleotides long, differ at 68 nucleotide positions; these changes result in 32 amino acid differences. Overall, this corresponds to 0.63% nucleotide sequence divergence, and the changes are scattered throughout the genome. The overall divergence at the level of amino acid substitution is 0.94%, but these changes are not randomly distributed among the virus proteins. The capsid protein is unchanged, while proteins NS1, NS3, and NS5 contain 0.5% amino acid substitutions, and proteins ns4a and ns4b average 0.8% substitutions. In contrast, proteins ns2a and ns2b have 3.0 and 2.3% amino acid divergence, respectively. The envelope protein also has a relatively high rate of amino acid change of 2.4% (a total of 12 amino acid substitutions). The large number of changes in ns2a and ns2b, which are largely conservative in nature, may result from lowered selective pressure against alteration in this region; among flaviviruses, these polypeptides are much less highly conserved than NS1, NS3, and NS5. However, many of the amino acid substitutions in the E protein are not conservative. It seems likely that at least some of the difference in virulence between the two strains of yellow fever virus results from changes in the envelope protein that affect virus binding to host receptors. Such differences in receptor binding could result in the reduced neurotropism and vicerotropism exhibited by the vaccine strain.

Geographic distribution and evolution of yellow fever viruses based on direct sequencing of genomic cDNA fragments

“…Asibi and FV strains differed by only one nucleotide at position 1119 confirming that the two viruses are closely related and were presumably responsible for all the outbreaks occurring along the West African coast in 1927 (Deubel et al, 1986b;Jennings et al, 1993). The published sequence of the Asibi strain (Hahn et al, 1987) shows two additional nucleotide changes at positions 992 and 1169 in the E protein gene that were not observed in the strain analysed here. This heterogeneity may be due to variation in the passage history of the Asibi strain or to clonal differences (Hahn et aL, 1987).…”

“…changes in the corresponding region. Two amino acid changes, threonine to arginine at position 380 related to RGD sequence, and proline to histidine at position 390, may alter flavivirus attachment or entry (Hahn et al, 1987). (iv) The gene fragment contained an Nglycosylation site of the form asparagine prolinethreonine at amino acid position 309 which could serve as a carbohydrate attachment site (Schlesinger et al, 1983;Rice et al, 1985;Deubel et al, 1987).…”

The phylogeny of yellow fever virus 17D vaccines

“…Previous studies that dealt with full genome analyses of yellow fever vaccine strains focused either on the comparison between wildtype and vaccine strains and by this on possible mutations leading to attenuation [12,21,22], or discussed the comparison of only few vaccine strains [9,22,23]. In this work we sequenced three more vaccine strains whose sequence has not been published until now and together with all other known full genomes of YFV-17D viruses we compared the sequences of overall 13 17D vaccine strains.…”

Insect-transmitted vertebrate viruses: Flaviviridae

“…Somewhere between in vitro Passages 89 and 114, a marked change in monkey virulence occurred, eventually leading to the present 17D vaccine (3,45). Differences in the deduced amino acid sequences of the Asibi and 17D yellow fever viruses concentrate in the E, the M, NS2a, and NS2b proteins (44,45). Amino acids in the NS2 proteins are not highly conserved among flaviviruses, and substitutions within these proteins are thought not to have a significant effect on their function (44).…”

“…Differences in the deduced amino acid sequences of the Asibi and 17D yellow fever viruses concentrate in the E, the M, NS2a, and NS2b proteins (44,45). Amino acids in the NS2 proteins are not highly conserved among flaviviruses, and substitutions within these proteins are thought not to have a significant effect on their function (44). Identification of critical regions of the flavivirus genome responsible for attenuation has yet to be absolutely identified, but these observations suggest that structural coding regions are probably important.…”