Human Cytomegalovirus Genome - Human Cytomegalovirus - page 5

fibroblasts, as discussed below. The fifth clinical isolate, Merlin, was subjected
to three serial passages in fibroblasts before sequencing of uncloned DNA (Dolan
et al. 2004). FIX (Hahn et al. 2002) is a derivative of VR1814 (Grazia Revello et
al. 2001), which was isolated from a pregnant woman with a primary HCMV
infection; TR is a ganciclovir-resistant ocular isolate from an AIDS patient with
retinitis (Smith et al. 1998); PH was isolated from a bone marrow transplant
patient (Rice et al. 1984; Fish et al. 1995); Toledo (Quinnan et al. 1984) and
Merlin (Tomasec et al. 2000) were isolated from the urine of a congenitally
infected child. Thus, the sequenced isolates are derived from a variety of clinical
settings. Not surprisingly, these viruses each exhibit the same open reading frame
(ORF) organization (Fig. 1, clinical strain). For historical reasons discussed
below, the ORFs in the U L domain at the conventional left end of the map are
named RL1-14, and they are followed by UL1-151, IRS1, US1-34 and TRS1.
Two viral genes span from repeated to unique domains. IRS1 is coded across the
junction of the repeated IR S sequence and the unique U S domain, whereas the TRS1
ORF spans the junction of TR S and U S . As a result, the two encoded proteins have
nearly identical (differing by one amino acid) amino-terminal halves coded by the
repeated sequences and unique carboxy-terminal halves. The advantage to the virus
of this curious gene organization is not apparent.
Several strains of HCMV were developed as vaccine candidates. To attenuate these
viruses, clinical isolates were serially passaged in cultured fibroblasts. After serial pas-
sage, the vaccine candidates proved to be different in both their biological and genomic
properties. As they were selected for rapid replication in fibroblasts, they simultane-
ously lost the ability to efficiently enter and replicate in numerous cell types susceptible
to the parental virus, such as epithelial cells, endothelial cells, smooth muscle cells and
macrophages. Their inability to enter these cells results from mutations in ORFs that
encode constituents of a glycoprotein complex (gH-gL-pUL128-pUL130-pUL131;
Wang and Shenk 2005b) present in the virion envelope. These passaged viruses also
contain changes in additional genes that constrict their host range (see the chapter by

Clinical strain conventional map
RL1-14 IRS1 TRS1

UL1-151 UL1-151 US1-34 US1-34

AD169 lab strain conventional map

TRL1-14 IRL14-1 IRS1 TRS1

UL1-132 UL1-132 US1-34 US1-34

10 30 50 70 90 110 130 150 170 190 210 230 kb

Fig. 1 HCMV ORF organization. To p , conventional ORF map of clinical isolates. ORFs are
organized using the genome isomer that was originally employed as the conventional map for
AD169. Arrows portray the relative orientation of the RL1-14, UL1-151, IRS1, US1-34 and TRS1
gene segments. Bottom , conventional map of AD169. The RL region is repeated and designated
TRL and IRL in clinical isolates