Amino acid changes in the attachment G glycoprotein of human respiratory syncytial viruses (subgroup A) isolated in Italy over several epidemics (1997-2006)

The human respiratory syncytial virus (HRSV) is the most important cause of admission to hospital for acute lower respiratory tract infections in infants and young children worldwide. Only few studies have investigated the molecular evolution of HRSV, and none has been conducted in Italy. The genetic diversity of the G glycoprotein of 59 subgroup A strains obtained from two clinical centers located in Northern and Central Italy was studied, during seven nonconsecutive epidemic seasons (1997-2006). The nucleotide sequences encompassing 624 bp, at the carboxy terminus of the G glycoprotein gene, were compared to sequences representative of previously defined HRSV genotypes. Phylogenetic analysis indicated that most Italian group A isolates clustered into two different lineages (GA2 and GA5), whereas only few isolates grouped into the other known lineages. Eight positively selected sites were found and it was predicted that serine and threonine of positively selected sites 117 and 262 (respectively) are O-glycosilated. The presence of multiple identical sequences in three lineages (GA1, GA5, and BE/A1) suggests that certain strains are predominant in a given epidemic season. Although most of the,,sites of the G glycoprotein gene of HRSV-A strains seem invariable because of strong purifying selection, some evolutionary hot spots may be present. Since the G glycoprotein is a major target (together with the F glycoprotein) of the HRSV humoral immune response, it is important to provide information about its genetic heterogeneity in order to address better both therapeutic and vaccine strategy.