Biology Reference
In-Depth Information
novo viral protein synthesis are defined as immediate early genes. Early gene
expression requires de novo protein synthesis but not viral genome replication.
All other genes are designated as late genes and their expression occurs after viral
genome replication.
Work in our lab and the Nelson lab has addressed the kinetics of HCMV
miRNA expression. In general, HCMV miRNA levels within the infected cell are
observed to increase over the course of the viral replication cycle. Specifically,
HCMV miRNA expression is dependent on protein synthesis but independent of
viral genome replication and as such, HCMV miRNAs are characterized as early
genes (Dunn 2005; Grey 2005). Exceptions include mir-UL22A* (referred to as
mir-UL22A-3p in Fig. 1) and mir-UL70. Our experiments show that mir-UL22A*
is expressed in cells treated with a protein synthesis inhibitor (Dunn et al. 2005;
Grey et al. 2005). This suggests that mir-UL22A* is expressed with immediate
early kinetics. Mir-UL70, like mir-UL22A*, is expressed in the absence of protein
synthesis (Grey et al. 2005). Curiously however, it was not expressed in the
absence of viral genome replication and so does not fall into any of the previously
described kinetic classes. miR-UL148D kinetics have not been examined. Mir-
US33, reported by Tuschl and colleagues, could not be confirmed by experiments
(Grey et al. 2005). A caveat about ascribing kinetics to HCMV miRNAs is that
pre-mir-UL36 was shown to accumulate in cells treated with a protein synthesis
inhibitor (Grey et al. 2005). Data on other pre-miRNAs in the presence of a protein
synthesis inhibitor was not presented. The accumulation of pre-miRNA, and thus
the absence of mature miRNA, may be due to the paucity of components in the
miRNA biogenesis pathway due to inhibition of protein synthesis. Alternatively,
the accumulation of pre-miRNA in cells treated with protein synthesis inhibitors
may be due to the absence of host/viral cofactors important for the processing of
pre-miRNAs.
Another issue of interest is that most of the HCMV strains used in the studies
of HCMV encoded miRNAs were laboratory strains that had been passaged mul-
tiple times. Thus, to examine the possibility that miRNA expression from labora-
tory strains of HCMV may have been altered by their history of extensive
passaging, we performed a limited comparison of miRNA expression between the
laboratory Towne strain and a HCMV clinical isolate that had undergone very
limited passaging in tissue culture (Dunn et al. 2005). Qualitatively speaking, the
miRNA expression from the laboratory strain is comparable to that observed in the
clinical strain.
Tissue-Specific HCMV miRNA Expression
In animals, miRNA expression is characterized by tissue specific expression pat-
terns. HCMV lytically replicates in a wide variety of cell types both in vivo and in
vitro. Viral gene expression has been shown to vary depending on host cell type
(Yang et al. 2006; Goodrum et al. 2002, 2007) and work in our lab has identified a
