Cytomegalovirus: pathogen, paradigm, and puzzle

Abstract

A century of progress Human CMV infection is extremely common.A recent analysis in the United States revealed an overall 50% seroprevalence among adults (1), but rates in some populations are even higher.For example, approximately 90% of Mexican-Americans in the United States are seropositive by age 50, as are 88% of stem cell transplant patients in Italy and 96% of individuals in southern Brazil (1-4).Despite its high worldwide prevalence, CMV infections are generally inapparent, except in newborns and immunocompromised individuals, for whom they can cause life-threatening disease affecting many organ systems.The virus was first detected in newborns during the early 20th century, when multiple reports described large cells in the urine of children with an often fatal systemic infection referred to as cytomegalic inclusion disease (5).The midcentury development of cell culture methods enabled propagation of CMV, but its detection in clinical specimens often required weeks of cultivation.Rapid diagnostic testing by centrifugation-enhanced inoculation combined with detection of CMV antigens in the 1980s was a transforming advance, enabling the diagnosis of CMV to be made in a clinically useful time frame (6).The emergence of effective antivirals in the 1990s was opportune, as CMV-associated disease was increasing in parallel with the AIDS epidemic and use of solid organ transplantation (SOT) and HSC transplantation (HCT).During the past two decades, further advances in diagnosis and treatment have greatly improved our ability to control CMV disease, but the virus still accounts for substantial morbidity, mortality, and cost.Along with these clinical advances, remarkable progress has been made in understanding the molecular biology of CMV.Application of the nucleic acid and protein analytic methods led to an appreciation of the extraordinary complexity of CMV, a point solidified by the landmark report of the first complete sequence of a CMV strain in 1990 (7). Refinements to methods for studying CMV gene function have continued to reveal myriad mechanisms underlying CMV's evolutionary success.However, understanding the pathogenesis of CMV diseases remains an enormous challenge, in large part because the virus only grows in human cells and it differs substantially from even its primate-infecting cousins (8-10).Here we summarize the current understanding of CMV biology and disease.The topic is too broad to cover completely in this for-mat, so the interested reader may wish to consult more comprehensive reviews (11, 12), as well as new reports that are certain to appear in the months and years ahead. CMV replication: insights but limitations from the labHuman CMV is the prototypic member of the β herpesvirus subfamily, which also includes human herpes viruses 6 and 7 and many animal CMVs.Its DNA genome is approximately 230 kb in size, the largest among known human viruses, and consists of unique long (UL) and unique short (US) segments, each of which is flanked by inverted repeats (RL and RS; Figure 1).Most of the approximately 200 genes encode proteins, but some express only noncoding RNAs, including approximately 14 microRNAs (miRNAs; refs.13-15).The central portion of the UL region contains clusters of core genes that have homologs in other herpesviruses, such as DNA polymerase, glycoprotein B (gB), and glycoprotein H (gH), whereas the remainder of the genome contain genes primarily found only in β herpesviruses or unique to human CMV (16, 17).In fact, considerable variation has been detected even among human CMV isolates (18).By convention, CMV genes are named by their position within the genome, although some also have additional descriptive names.For example, UL54 (the 54th gene in the UL region, according to the original report of the CMV, strain AD169, sequence; ref. 7) is the DNA polymerase gene.Propagation of CMV in cell culture has been an essential research tool but has definite limitations, in part because passage in the lab selects for mutants adapted for growth in this unnatural setting (19).As a result, commonly used lab strains have multiple mutations, deletions, and rearrangements (20).For example, the end of the UL region in isolates of the lab strains Towne and AD169 lacks approximately 13 kb of DNA, encoding 19 genes that are present in the Toledo strain (21, 22).Remarkably, inactivating mutations in the RL13 gene are detectable by sequence analyses of the viral genome immediately upon propagation of virus in cell culture (23), which suggests that this gene is necessary for success of the virus in humans but strongly inhibitory to replication in cell culture.The fact that most, if not all, prior laboratory-based studies have used only RL13 mutant viruses reinforces the importance of confirming results of laboratory studies with observations from the clinic.Advances in sequencing technology are sure to improve our understanding of the genetic variation of CMV without confounding artifacts that