ORIGINE, EPIDEMIOLOGIA E FILOGEOGRAFIA DEI GENOTIPI D E A DEL VIRUS DELL'EPATITE B

Hepatitis B virus (HBV) is the leading cause of liver disease and infects an estimated 240 million people worldwide. It is characterized by an high degree of genetic heterogeneity because of the use of a reverse transcriptase during viral replication. The ten genotypes (A-J) that have been described so far further segregate into a number of subgenotypes which have distinct ethno-geographic distribution. Genotypes A and D are ubiquitous and the most prevalent genotypes in Europe (mainly represented by subgenotypes D1-3 and A2); genotypes B and C are restricted to eastern Asia and Oceania; genotype E to central and western Africa; and genotypes H and F (classified into 4 subgenotypes) to Latin America and Alaska. The aim of this study was to determine the HBV genotypes/subgenotypes circulating in Italy and to study their distribution in relation to demographic and risk factors. To this aim, we analyzed the P gene sequences of a total 230 HBsAg-positive Italian patients. Our study showed that the HBV genotype prevalent in Italy was D (72.2%) followed by A (18.7%). In particular in patients infected with HBV-D the subgenotype prevalent was D3 (80.1%); in patients with HBV-A the subgenotype prevalent was A2 (93%). The subgenotype D3 was common among subjects who used injecting drug (69.2%) while the subgenotype A2 was common among subjects having sex with men (66.7%). Furthermore, to reconstruct the spatio-temporal dynamics (origin and geographic dispersion) of the genotypes/subgenotypes widespread in Italy and in Europe, the Italian HBV sequences characterized in this study were aligned with reference sequences obtained in different countries, retrieved from public databases. The study was performed using new methods of phylogenetic analysis based on molecular clocks coalescent theory and the phylogeographic approach. The phylogeographical analysis of HBV-D showed that India had the highest posterior probability of being the location of the tree root, whereas central Asia was the most probable location of the common ancestor of subgenotypes D1-D3. The time of the most recent common ancestor (tMRCA) of the tree root was 128 years ago, which suggests that the common ancestor of the currently circulating subgenotypes existed in the second half of the XIX century. The mean tMRCA of subgenotypes D1-D3 was between the 1940s and the 1950-60s. On the basis of our phylogeographic reconstruction, it seems that HBV-D reached the Mediterranean area in the middle of the XX century by means of at least two routes: the first pathway (mainly due to the spread of subgenotype D1) crossing the Middle East and reaching north Africa and the eastern Mediterranean, and the second pathway (closely associated with D2) that crossed the former Soviet Union and reached eastern Europe and the Mediterranean through Albania. The phylogeographical analysis of HBV-A showed that the common ancestor of the currently circulating A subgenotypes was placed in west-central Africa a mean 1057 years ago. The genotype diverged into two main clades at the beginning of the 13th century: one including all of the west-central African quasi-subgenotypes and the other corresponding to subgenotype A1, originating in east Africa and further segregating into two main subclades: an “African” and a “Cosmopolitan” clade. It is likely that the slave trade was the main source the spread of Cosmopolitan HBV-A1, which was exported to Asia in the 17th century as a result of Arab or Portuguese trade, and to Latin America in the 18th centuries through the trans-Atlantic slave trade. The origin of the currently circulating A2 strains dates back to the first decades of the 20th century, and the evolutionary demography analysis suggests an exponential growth of infections, between 1970s and the mid-1990s. This study suggests that the HBV genotypes were dispersed in Europe in different times and through different transmission routes. In particular, some genotypes (such as HBV-D or HBV-A2) spread recently (in the last 100 years), mainly through parenteral or sexual exposure; on the contrary other strains (such as HBV-A1) have been dispersed in more ancient times and are now linked to individuals of particular geographic origin or ethnicity.