Phylodynamic study of the conserved RNA structure encompassing the hemagglutinin cleavage site encoding region of H5 and H7 low pathogenic avian influenza viruses.

Highly pathogenic avian influenza viruses (HPAIVs) evolve from low pathogenic avian influenza viruses (LPAIVs) of the H5 and H7 subtypes. This evolution is characterized by the acquisition of a multi-basic cleavage site (MBCS) motif in the hemagglutinin (HA) that leads to an extended viral tropism and severe disease in poultry. One key unanswered question is whether the risk of transition to HPAIVs is similar for all LPAIVs H5 or H7 strains, or whether specific determinants in the HA sequence of some H5 or H7 LPAIV strains correlate with a higher risk of transition to HPAIVs. Here, we determined if specific features of the conserved RNA stem-loop located at the HA cleavage site-encoding region could be detected along the LPAIV to HPAIV evolutionary pathway. Analysis of the thermodynamic stability of the predicted RNA structures showed no specific patterns common to HA sequences leading to HPAIVs and distinct from those remaining LPAIVs. However, RNA structure clustering analysis revealed that most of the American lineage ancestors leading to H7 emergences via recombination shared the same viral RNA (vRNA) structure topology at the HA1/HA2 boundary region. Our study thus identified predicted secondary RNA structures present in the HA of H7 viruses, which could promote genetic recombination and acquisition of a multibasic cleavage site motif (MBCS).