Murine leukemia virus


 * "MMLV" redirects here. It is also the Roman numeral for 2055.

The murine leukemia viruses (MLVs or MuLVs) are retroviruses named for their ability to cause cancer in murine (mouse) hosts. Some MLVs may infect other vertebrates. MLVs include both exogenous and endogenous viruses. Replicating MLVs have a positive sense, single-stranded RNA (ssRNA) genome that replicates through a DNA intermediate via the process of reverse transcription.

Classification
The murine leukemia viruses are Type VI retroviruses belonging to the gammaretroviral genus of the Retroviridae family. The viral particles of replicating MLVs have C-type morphology as determined by electron microscopy.

The MLVs include both exogenous and endogenous viruses. Exogenous forms are transmitted as new infections from one host to another. The Moloney, Rauscher, Abelson and Friend MLVs, named for their discoverers, are used in cancer research.

Endogenous MLVs are integrated into the host's germ line and are passed from one generation to the next. Stoye and Coffin have classified them into four categories by host specificity, determined by the genomic sequence of their envelope region. The ecotropic MLVs (from eco, "house") are capable of infecting mouse cells in culture. Non-ecotropic MLVs may be xenotropic (from xeno, "foreign", infecting non-mouse species), polytropic or modified polytropic (infecting a range of hosts including mice). Different strains of mice may have different numbers of endogenous retroviruses, and new viruses may arise as the result of recombination of endogenous sequences.

Virion structure
As Type C retroviruses, replicating murine leukemia viruses produce a virion containing a spherical nucleocapsid (the viral genome in complex with viral proteins) surrounded by a lipid bilayer derived from the host cell membrane. The lipid bilayer contains integrated host and viral proteins studded with carbohydrate molecules. The viral particle is approximately 90 nanometres (nm) in diameter.

Genome
The genomes of exogenous and endogenous murine leukemia viruses have been fully sequenced. The viral genome is a single stranded, linear, positive-sense RNA molecule of around 8000 nucleotides. From 5' to 3' (typically displayed as "left" to "right"), the genome contains gag, pol, and env regions, coding for structural proteins, enzymes including the RNA-dependent DNA polymerase (reverse transcriptase), and coat proteins, respectively. The genomic molecule contains a 5' methylated cap structure and a 3' poly-adenosine tail.

The genome includes a conserved RNA structural element called a core encapsidation signal that directs packaging of RNA into the virion; the tertiary structure of this element has been solved using nuclear magnetic resonance spectroscopy.

Viral evolution
As with other retroviruses, the MLVs replicate their genomes with relatively low fidelity. Thus, divergent viral sequences may be found in a single host organism. MLV reverse transcriptases are thought to have a slightly higher fidelity than the HIV-1 RT.

Applications of MLVs

 * Gene therapy: MLV-derived particles can deliver therapeutic genes to target cells.
 * Cancer studies: MLVs are used to study cancer development.
 * As a model retrovirus in viral clearance studies
 * Reverse Transcriptase from MMLV used in Biotechnology