New perspectives on the evolutionary history of hepatitis B Virus Genotype F and its subgenotypes

TORRES, C.; MBAYED, V.A.; CAMPOS, R.H.

Punta del Este, Maldonado

Simposio; 150 YEARS OF DARWIN’S EVOLUTIONARY THEORY: A SOUTH AMERICAN CELEBRATION; 2009

Hepatitis B virus (HBV) is a globally distributed human pathogen. The eight genotypes of HBV (A – H) described so far show a distinctive geographic distribution. Genotype F is indigenous to America and is classified into four different subgenotypes (F1-F4). The evolutionary history of HBV remains controversial. The aim of our work was to analyze evolutionary issues of HBV genotype F (HBV/F), emphasizing the study of subgenotypes F1b (HBV/F1b) and F4 (HBV/F4) which are prevalent in our region. HBV/F complete genomes from 37 serum samples from Argentina and Chile were sequenced and phylogenetically analyzed by Maximum Likelihood, Maximum Parsimony and Neighbor Joining methods. Congruent topology and grouping support was found between the phylogenetic trees obtained by different methods. HBV/F showed a split into two main groups, one corresponded to subgenotype F1 which was formed by Central America isolates (F1a) and South America isolates (F1b), and the other one corresponded to subgenotypes F2-F4, exclusively found in constrained areas of South America. Subclusters were identified within each subgenotype. Bayesian coalescent analyses were carried out to estimate an overall substitution rate and the time to the most recent common ancestor (tMRCA) for HBV/F and its subgenotypes. Different molecular clocks and demographic scenarios were considered. Analyses were performed on complete genomes and nonoverlapping regions. Using time-stamped complete genomes, the tMRCA for HBV/F was estimated in 245 years (HPD95%=93-487), whereas for HBV/F1b and HBV/F4 was 23 (HPD95%=15-33) and 68 years (HPD95%=38-112), respectively. Besides, the substitution rate for HBV/F was estimated in 1.97x10-4 s/s/y (HPD95%=1.03x10-4-2.95x10-4). However, when fixed substitution rates were assayed, HBV/F showed tMRCA values from 8985 (0.5x10-5 s/s/y) to 196 years (2.5x10-4 s/s/y), whereas HBV/F1b and F4 HBV/F4 displayed values from 1420 and 1584 years (0.5x10-5 s/s/y) to 29 and 31 years (2.5x10-4 s/s/y), respectively. Finally, the results obtained using time-stamped data failed to explain the wide geographical distribution and the clustering observed in HBV/F. Conversely, a rate of substitution of ~1x10-5 s/s/y would be in agreement with a HBV/F ancestor of ~4700 years and HBV/F1b and HBV/F4 ancestors of ~700 and ~850 years respectively, which might fit better with epidemiological, geographical and anthropological data.