Mason-Pfizer monkey virus (M-PMV), formerly Simian retrovirus (SRV), is a species of retroviruses that usually infect and cause a fatal immune deficiency in Asian macaques. The ssRNA virus appears sporadically in mammary carcinoma of captive macaques at breeding facilities which expected as the natural host, but the prevalence of this virus in feral macaques remains unknown. M-PMV was transmitted naturally by virus-containing body fluids (saliva, urine, blood, etc.), via biting, scratching, grooming, and fighting. Cross contaminated instruments or equipment (fomite) can also spread this virus among animals. Some clinical and pathological symptoms of M-PMV-infected newborn rhesus macaques are diarrhea, weight loss, splenomegaly, lymphadenopathy, anemia, neutropenia, and neoplastic diseases (retroperitoneal fibromatosis or rare B-cell lymphomas). Infected new-born Rhesus monkeys may develop immunodeficiency disease accompanied by opportunistic infections. To prevent the infection of this virus, two vaccines have been developed: a formalin-inactivated vaccine SRV-1 and a recombinant vaccine expressing M-PMV envelope glycoprotein gp70 and gp22. M-PMV-based vector is a candidate for delivering therapeutic genes in human gene transfer. Based on the M-PMV 1) promoter region remain transcriptionally active in human cells and 2) the constitutive transport element (CTE) expression in the target cells aids the facilitation of the nuclear export for the gene therapy.
History Mason-Pfizer monkey virus (M-PMV) derived from breast tumor tissue of an 8 years-old female rhesus macaque (Macaca mulatta) in 1970 by Dr. Harish C. Chopra and Marcus M. Mason. Initial discovery suspected the virus particles to be an oncogenic virus due to its resemblance to known oncogenic RNA virus (MMTV). Shortly after its discovery, M-PMV was considered to induce simian AIDS (SAIDs). However, current studies have shown that M-PMV is unrelated to simian immunodeficiency virus (SIV), which is currently recognized as the simian counterpart of the human immunodeficiency virus. M-PMV now belongs to SRV-3. SRV-1 serotype was identified in the early 1980s in rhesus macaque, M. cyclopis, and M. fascicularis at National Primate Research Center (NPRC), California and New England. The SRV serotype-2 was found in endemic infections of pig-tailed monkey (M. nemestrina), cynomolgus macaques, a Japanese macaque (M. fuscata), at Washington NPRC, and in rhesus and Celebes black macaques (M. nigra) at Oregon NPRC. SRV-3 is present at Wisconsin Primate Center, while SRV-4 and SRV-5 have been identified at University of California and Beijing Primate Center. In 2010, a Japanese research group reported two SRV isolates from seropositive cynomolgus macaques and tentatively designated them as SRV/D-Tsukuba (SRV/D-T). In 2011, players of Foldit helped to decipher the crystal structure of the M-PMV retroviral protease. While the puzzle was available to play for three weeks, players produced an accurate 3D model of the enzyme in just ten days, which was then used to solve the structure with molecular replacement. The problem of how to configure the structure of the enzyme had stumped scientists for 15 years. Until 2015, seven serotype of M-PMV have been identified.
Classification Mason-Pfizer monkey viruses are group VI retrovirus belongs to betaretrovirus genus of orthoretroviridae subfamily. M-PMV was classified based on viral serotype as simian retrovirus type 3 (SRV-3). Distinguished from other orthoretroviruses for its accumulation of A-type (immature particles) intracellular particles morphology in the cytoplasm and spherical nucleocapsid. Once assemble is complete in the cytosol, particles are then transported to the plasma membrane to complete the maturation process by producing exogenous mature particles (D-type morphology). D-type particles contain fewer dense surface spikes and contain icosahedral capsids.
Morphology and genetic structure M-PMV is an enveloped RNA retrovirus with an icosahedral capsid (20 triangular faces and 12 vertices). The nucleic acid is encapsulated inside the spherical core. The enveloped virus is made up of lipid bilayer derived from host cell and virus-specific proteins. The matrix protein binds with nucleocapsid while lining the inner surface of the envelope to facilitate the viral genome assembly and budding process. The retroviral replication process steps include Gag particle formation, transport to the membrane (attachment), entry into the cell, uncoating of the viral capsid, release the genome, synthesis of new viral proteins and nucleic acids, assemble of progeny virions, budding, and viral release. About 60% of the virion dry weight made up of proteins, 35% of lipids, around 3% carbohydrate. The reverse transcriptase made up of 1771 amino acid protein, gp70 surface 586 aa protein, Pr95 911 aa protein, and Pr78 657 aa protein. Based on its structure, the M-PMV is sensitive to formaldehyde, high temperature (heat), and detergents. M-PMV contains two types of virus particles. One found in the cytoplasm and the other was found extracellularly. The intracytoplasmic particles (A-type) are small, ring-shaped structures, and 70 μm in diameter. The virions commonly found in a cluster in the cytoplasm and enveloped of the plasma membrane at the cell surface. The immature particles bud intracellular and are not considered to be infectious. Upon completing budding, immature particles undergo the maturation process (D-type) to acquire infectivity. The extracellular mature particles are about 125 nm in diameter, while the nucleoid and core-shell are central cylindrical structures separated by a space of about 8-10 nm.
Genome structure
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