Isolation and characterization of retrovirus-like elements from normal human fetuses

Area of Science:

• Virology
• Molecular Biology
• Developmental Biology

Background:

• Retrovirus-like particles (RLPs) have been detected in human fetal samples.
• Understanding the nature and origin of these particles is crucial for human developmental biology.

Purpose of the Study:

• To characterize RLPs isolated from human fetal tissues.
• To investigate the relationship between RLP expression and embryonic differentiation.

Main Methods:

• Isolation and density gradient centrifugation of particles from fetal plasma and organs.
• Electron microscopy for particle morphology.
• Enzymatic assays for reverse transcriptase (RT) activity using various template-primers.
• Inhibition assays using antisera against known RTs.
• RNA-DNA hybridization to determine nucleic acid origin and integration.

Main Results:

• RLPs were isolated from fetal plasma and embryonic organs, with densities of 1.19-1.22 g/ml and some lighter particles (1.15-1.17 g/ml).
• Electron microscopy revealed retrovirus-like morphology. Particles contained high molecular weight RNAs (70S and 35S) and RT activity.
• RT showed partial inhibition (40%) by antiserum against feline endogenous virus (RD114) RT, but no inhibition by antisera against AMV, SNV, or GALV RT.
• Complementary DNA hybridized to adult human DNA, indicating endogenous origin.
• RLPs were exclusively found in embryonic organs during differentiation and could not be induced.

Conclusions:

• The isolated particles are endogenous retroviral elements, not infectious retroviruses, due to lack of induced expression and demonstrated infectivity.
• Retroviral gene expression appears correlated with embryonic differentiation in humans.
• These findings contribute to understanding endogenous retroviral elements in human development.