Quantifying selection against synonymous mutations in HIV-1 env evolution.

Area of Science:

• Virology
• Molecular Biology
• Evolutionary Biology

Background:

• Human immunodeficiency virus type 1 (HIV-1) evolves rapidly due to immune system pressure.
• Nonsynonymous mutations allow HIV-1 to escape immune recognition.
• Synonymous mutations, which don't change amino acids, are generally assumed to be neutral.

Purpose of the Study:

• To investigate the evolutionary role of synonymous mutations in HIV-1.
• To determine if synonymous mutations incur a fitness cost for the virus.
• To assess the functional relevance of RNA secondary structures in HIV-1 evolution.

Main Methods:

• Analysis of longitudinal intrapatient HIV-1 env gene sequences (C2-V5 region).
• Tracking the frequency and fate of synonymous variants over time.
• Integration of selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) data.
• Computational modeling to estimate the selective cost of synonymous mutations.

Main Results:

• Most synonymous variants in the HIV-1 env gene are lost, despite sometimes reaching high frequencies.
• Synonymous mutations disrupting RNA stem-loop structures are more likely to be lost.
• Computational models suggest a significant fraction of synonymous mutations are deleterious, with a daily cost of approximately 0.002.
• Weak negative selection against synonymous mutations slows HIV-1 evolution but doesn't create strong conservation patterns.

Conclusions:

• Synonymous mutations in HIV-1 are subject to weak purifying selection.
• RNA secondary structures, particularly stem-loops in the env gene, play a functionally relevant role in HIV-1 evolution.
• The precise base-pairing pattern of synonymous mutations is less critical than the overall RNA structure.