Human cytomegalovirus (HCMV) clinical isolates display genetic polymorphisms in multiple genes. Some authors have suggested that those polymorphisms may be implicated in HCMV-induced immunopathogenesis, as well as in strain-specific behaviours, such as tissue-tropism and ability to establish persistent or latent infections. This review summarises the features of the main clustered HCMV polymorphic open reading frames and also briefly cites other variable loci within the viral genome. The implications of gene polymorphisms are discussed in terms of potentially advantageous higher fitness obtained by the strain, but also taking into account that the published data are often speculative. The last section of this review summarises and critically analyses the main literature reports about the linkage of strain specific genotypes with clinical manifestations of HCMV disease in different patient populations affected by severe cytomegalovirus infections, namely immunocompromised subjects and congenitally infected newborns.

Previously, we identified the glycoprotein gO gene, UL74, as a hypervariable locus in the human cytomegalovirus (HCMV) genome [Virology 293 (2002) 281]. Here, we analyze gO from 50 isolates from congenitally infected newborns, transplant recipients, and HIV/AIDS patients from Italy, Australia, and UK. These are compared to four gO groups described from USA transplantation patients [J. Virol. 76 (2002) 10841]. Phylogenetic analyses identified seven genotypes. Divergence between genotypes was up to 55% and within 3%. Discrete linkage was shown between seven hypervariable gO and gN genotypes, but not with gB. This suggests interactions, while gN and gO are known to form complexes with distinct conserved glycoproteins gM, gH/gL, respectively, both are involved in fusogenic entry and exit. Codon-based maximum likelihood models showed evidence for sites of positive selection. Further analyses of disease relationships should take into account these newly defined gO/gN groups.

Human cytomegalvirus (HCMV) ORF UL73 is a polymorphic locus, encoding the viral glycoprotein gpUL73-gN, a component of the gC-II envelope complex. The previously identified gN genomic variants, denoted gN-1, gN-2, gN-3 and gN-4, were further investigated in this work by analysing a large panel of HCMV clinical isolates collected from all over the world (223 samples). Sequencing and phylogenetic analysis confirmed the existence of the four gN genotypes, but also allowed the identification of a novel subgroup belonging to the gN-3 genotype, which was designated gN-3b. The number of non-synonymous (d N ) and synonymous (d S ) nucleotide substitutions and their ratio (d N /d S ) were estimated among the gN genotypes to evaluate the possibility of positive selection. Results showed that the four variants evolved by neutral (random) selection, but that the gN-3 and gN-4 genotypes are maintained by positive selective pressure. The 223 HCMV clinical isolates were subdivided according to their geographical origin, and four main regions of gN prevalence were identified: Europe, China, Australia and Northern America. The gN variants were found to be widespread and represented within the regions analysed without any significant difference, and no new genotype was detected. Finally, for clinical and epidemiological purposes, a rapid and low-cost method for genetic grouping of the HCMV clinical isolates was developed based on the RFLP revealed by SacI, ScaI and SalI digestion of the PCR-amplified UL73 sequence. This technique enabled us to distinguish all four gN genomic variants and also their subtypes.

A fundamental step in the efficient production of human cytomegalovirus (HCMV) progeny is viral egress from the nucleus to the cytoplasm of infected cells. In the family Herpesviridae, this process involves alteration of nuclear lamina components by two highly conserved proteins, whose homologues in HCMV are named pUL50 and pUL53. This study showed that HCMV infection induced the mislocalization of nuclear lamins and that pUL50 and pUL53 play a role in this event. At late stages of infection, both lamin A/C and lamin B showed an irregular distribution on the nuclear rim, coincident with areas of pUL53 accumulation. No variations in the total amount of nuclear lamins could be detected, supporting the view that HCMV induces a qualitative, rather than a quantitative, alteration of these cellular components, as has been suggested previously for other herpesviruses. Interestingly, pUL53, in the absence of other viral products, localized diffusely in the nucleus, whilst the co-expression and interaction of pUL53 with its partner, pUL50, restored its nuclear rim localization in distinct patches, thus indicating that pUL50 is sufficient to induce the localization of pUL53 observed during virus infection. Importantly, analysis of the nuclear lamina in the presence of pUL50-pUL53 complexes at the nuclear boundary and in the absence of other viral products showed that the two viral proteins were sufficient to promote alterations of lamins, strongly resembling those observed during HCMV infection. These results suggest that pUL50 and pUL53 may play an important role in the exit of virions from the nucleus by inducing structural modifications of the nuclear lamina. INTRODUCTIONWhilst herpesvirus genome packaging and capsid formation occur in the nuclear compartment, all subsequent steps of maturation take place in the cytoplasm, the final site of viral particle assembly. The most widely accepted model for herpesvirus nuclear egress suggests that capsids leave the nucleus through budding events at the nuclear envelope (Mettenleiter, 2004;Mettenleiter et al., 2006;Severi et al., 1988;Skepper et al., 2001;Stackpole, 1969), which consists of two leaflets, the inner and the outer nuclear membrane (INM and ONM, respectively) separated by the perinuclear space; nuclear capsids acquire a temporary envelope at the INM, which is subsequently lost by fusion with the ONM, allowing access to the cytosol. However, before crossing the nuclear envelope, virions have to overcome a massive obstacle underlying the INM, represented by the nuclear lamina, a thick meshwork of proteins associating in a highly organized structure. The main components of the nuclear lamina are the lamins, which, based on their expression patterns, properties and location, can be divided into two main classes: A-type lamins, including lamin A, AD10, C and C2; and B-type lamins, including lamin B1, B2 and B3 (Broers et al., 1997(Broers et al., , 2006Rzepecki, 2002;Stuurman et al., 1998). Several studies have shown that herpesvirus infection causes structural and bioch...

Clinical isolates of human cytomegalovirus (HCMV) show differences in tissue tropism, severity of clinical manifestations and ability to establish persistent or latent infections, characteristics that are thought to be related to genomic variation among strains. This work analysed the genomic variants of a new HCMV polymorphic locus, open reading frame (ORF) UL73. This ORF encodes the envelope glycoprotein gpUL73 (gN), which associates in a high molecular mass complex with its counterpart, gM, and induces a neutralizing antibody response in the host. Detailed sequence analysis of ORF UL73 and its gene product from clinical isolates and laboratory-adapted strains shows that this glycoprotein is highly polymorphic, in the N-terminal region in particular. gpUL73 hypervariability is not randomly distributed, but the identified genomic variants are clearly clustered into four distinct genotypes (gN-1, gN-2, gN-3 and gN-4), which are not associated with the gB subtype.Human cytomegalovirus (HCMV) is a ubiquitous betaherpesvirus with a broad spectrum of infectivity, as documented by the several different cell types that can be infected in vitro and by the multiple organ tropism observed during infection in vivo. Differences in tissue tropism, severity of clinical manifestations and ability to establish persistent or latent HCMV infections are thought to be related to genomic variability among strains (Brown et al., 1995 ;Fries et al., 1994 ; Meyer-Konig et al., 1998 a, b). The sequence similarity between different isolates varies depending on the region analysed (Bale et al., 1993) ; highly variable regions have been found in the HCMV genome.