Caliciviruses

The application of electron microscopy to the examination of stool samples from patients with diarrhoeal disease resulted in the discovery of the viruses associated with gastroenteritis. Whilst rotaviruses and enteric adenoviruses have obvious and distinguishable morphological features there was considerable confusion about the role of 'small round viruses' found in stool samples from patients with nonbacterial gastroenteritis. An interim classification scheme described in 1982

FIG. 1. Electron microscopic appearance of (a) NLVs and (b) SLVs. NLVs have a ragged edge and amorphous structure whereas SLVs display the classic cup-shaped morphology from which caliciviruses derive their name. Scale bar = 100 nm. (Fig. 1a courtesy of Mr P. Pead, Fig. 1b reproduced courtesy of Mrs B. Cosgrove and Prof R. Madeley.)

FIG. 1. Electron microscopic appearance of (a) NLVs and (b) SLVs. NLVs have a ragged edge and amorphous structure whereas SLVs display the classic cup-shaped morphology from which caliciviruses derive their name. Scale bar = 100 nm. (Fig. 1a courtesy of Mr P. Pead, Fig. 1b reproduced courtesy of Mrs B. Cosgrove and Prof R. Madeley.)

allowed the clear differentiation of these viruses (Caul & Appleton 1982). In this scheme two distinctive morphological groups of human viruses (now known to be caliciviruses), both approximately 30—35 nm in diameter were recognized. The small round structured viruses (currently known as 'Norwalk-like viruses' or NLVs) have an amorphous structure with a ragged outer edge (Fig. 1a) and the 'classic' caliciviruses ('Sapporo-like viruses' or SLVs) display the true cup-shaped structures from which the calicivirus family derives its name (Fig. 1b). SLV infections are rarely diagnosed and are usually found in symptomatic young children whereas NLV infections are common in all age groups.

Caliciviruses possess a single-stranded, positive-sense RNA genome approximately 7400—7800 nucleotides in length (excluding the polyadenylate tail). Calicivirus genomes have a characteristic arrangement of their open reading frames (ORFs) that clearly distinguishes them from the genomes of picornaviruses (Clarke & Lambden 1997). The 5' region of the genome encodes a large nonstructural polyprotein that precedes a single viral capsid protein ORF. At the 3' terminus of the genome there is a small ORF that encodes a basic protein. Calicivirus replication can be distinguished from picornavirus replication by the production of a subgenomic RNA species that serves as the main template for the synthesis of the capsid protein.

Phylogenetic analysis of genome sequences has divided the calicivirus family into four distinct genera (Green et al 2000). These genera map with some of the biological properties of the viruses: for example, caliciviruses have long been associated with veterinary diseases especially involving marine mammals and respiratory infections of cats (Vesivirus genus) and more recently with hepatic diseases in lagomorphs (Lagovirus genus). However, viruses naturally infecting humans are found only in two of the genera: the NLVs and the SLVs. These generic names are themselves only temporary until more suitable and appropriate terms can be derived. Very little progress was made in the study of the human viruses until the early 1990s because they cannot be grown in cell culture and there is no reliable animal model. The application of modern molecular techniques to study virus genome structure and organisation has led to the production of new reagents that have provided further insights into the molecular biology of these viruses. Our aim is to review the most important advances in the molecular biology of the human caliciviruses with particular emphasis on genome structure and virus gene expression.

0 0

Post a comment