Retroviruses and mouse embryos: a model system in which to study gene expression in development and differentiation

Area of Science:

• Developmental Biology
• Virology
• Epigenetics

Background:

• Moloney leukaemia virus (M-MuLV) can be transmitted genetically through mouse substrains (Mov-1 to Mov-14).
• Viral genome activation in some substrains suggests developmental regulation of M-MuLV expression.

Purpose of the Study:

• To investigate the impact of cellular differentiation on M-MuLV expression.
• To determine the methylation status and infectivity of M-MuLV genomes in different embryonic stages and cell types.

Main Methods:

• Introduction of M-MuLV into preimplantation and postimplantation mouse embryos, and embryonal carcinoma (EC) cells.
• Analysis of viral genome methylation and infectivity using transfection assays.
• Assessment of M-MuLV replication in differentiated versus undifferentiated cells.

Main Results:

• M-MuLV did not express in preimplantation embryos or EC cells, but replicated efficiently in postimplantation embryos and differentiated cells.
• Viral genomes in early embryos were methylated and non-infectious, while those in postimplantation embryos remained unmethylated and infectious.
• De novo methylation repressed viral genes in undifferentiated EC cells, but this was absent in differentiated cells and postimplantation embryos.

Conclusions:

• Efficient de novo methylation in pluripotent embryonic cells represses introduced genes, including viral genomes.
• This methylation activity is crucial for gene regulation during early embryogenesis and is absent in differentiated cells.
• Germline integration of M-MuLV can cause recessive lethal mutations, as observed in the Mov-13 substrain, indicating critical developmental roles for the integration site.