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Journal of Virology, March 2007, p. 2950-2956, Vol. 81, No. 6
0022-538X/07/$08.00+0 doi:10.1128/JVI.02745-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Departments of Neurology,1 Microbiology, University of Colorado Health Sciences Center, Denver, Colorado 802622
Received 13 December 2006/ Accepted 28 December 2006
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Human tissue. Both the left and the right trigeminal ganglia (TG) and brain were removed from subjects who were not immunocompromised before death and who, at autopsy, did not exhibit cutaneous signs of recent herpesvirus infection. Table 1 lists the clinical features of the subjects from whom TG or brain (approximately 5 cm3 of frontal lobe containing both gray and white matter) were removed. Nerve roots were cleaned, flash-frozen in liquid nitrogen and stored at 70°C until individually powdered in liquid nitrogen.
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TABLE 1. Clinical features of humans from whom ganglia or brains were removed
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cDNA was synthesized from 100 ng of total or poly(A)-containing RNA. RNA in 12 µl of water was added to 8 µl of 2.5x RT buffer (1x RT buffer is composed of 50 mM Tris-HCl [pH 8.5], 30 mM KCl, 8 mM MgCl2, 20 U of RNase inhibitor, 0.5 mM concentrations of each deoxynucleoside triphosphate, and 10 µM oligo-d[T18]). Each complete cDNA reaction was spiked with either 1 µl of reverse transcriptase (20 U/µl; Transcriptor; Roche Diagnostics, Mannheim, Germany) or 1 µl of sterile water. The 20-µl reaction mixtures in sealed 96-well microtiter plates were vortexed briefly and centrifuged at 1,000 x g for 2 min, followed by incubation at 25°C for 10 min, 50°C for 60 min, 85°C for 5 min, and 4°C for 20 min.
Real-time PCR (fluorescence-based simultaneous amplification and product detection) and data analysis. Reactions were performed in a 20-µl volume of 1x qPCR Mastermix (VWR, West Chester, PA) containing 900 nM concentrations of each primer, 250 nM probe, and 7 to 20% of each cDNA sample using the 7500-Fast real-time PCR system (Applied Biosystems, Foster City, CA). Amplification conditions consisted of initial denaturation at 95°C for 10 min, followed by 40 two-step cycles of 15 s at 95°C and 1 min at 60°C. Fluorescence resulting from probe degradation was recorded during the 60°C extension cycle and analyzed using the SDS program (Sequence Detection Software; Applied Biosystems). The SDS-determined cycle threshold (CT) is defined as the cycle at which the fluorescence value intersects the threshold value, where the threshold value is set at 10-fold above the background fluorescence and is within the logarithmic phase of PCR amplification. VZV DNA standards were included in each real-time assay and yielded a consistent inverse relationship between the CT value and the amount of input template DNA. Linear regression analysis was performed by using SigmaPlot (Systat, Point Richmond, CA). Table 2 lists the TaqMan primer and probe sequences used (Integrated DNA Technologies, Coralville, IA). Each sample was analyzed in duplicate. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH; accession number BC_013310) was used as a cellular transcriptional control, whereas neurofilament heavy subunit (NFH-200; accession number NM_021076) was used as a neuron-specific transcriptional control (1). Residual cellular or viral DNA present in RNA samples was quantitated by PCR analysis of the RNA. In most samples, no residual DNA was detected, and the CT value was set at the final cycle number. Where residual DNA was detected (CT < 40), a modified two-tailed t test was used to analyze the results. For each sample containing residual DNA, the mean and standard deviation of DNA and cDNA copies were determined. In each assay, the mean copy number was three times greater than the largest standard deviation, assuring the significance of the cDNA copy number.
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TABLE 2. TaqMan primers and probes
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FIG. 1. Efficiency of VZV DNA quantitation. VZV DNA diluted at 1 to 10,000 copies was mixed with 100 ng of herring sperm DNA and amplified in duplicate using TaqMan primers specific for VZV genes 21, 29, 62, 63, and 66. Each primer set amplified 1 to 10,000 copies of VZV DNA with similar efficiency (R2 = 0.95). The data represent average CT values obtained with various VZV DNA copy numbers for all genes.
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FIG. 2. Quantitative PCR analysis of VZV transcripts in VZV-infected MeWo cells. Duplicate dilutions of total RNA from VZV-infected cells were reverse transcribed, and cDNAs corresponding to VZV genes 21, 29, 62, 63, and 66 were quantitated. The regression line slopes were similar in all cases (0.97 ± 0.03).
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FIG. 3. Abundance of VZV transcripts in VZV-infected MeWo cells. Total RNA extracted from VZV-infected cells was diluted and reverse transcribed, and the number of cDNAs corresponding to VZV genes 21, 29, 62, 63, and 66 was quantitated and normalized to 1 µg of input RNA.
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FIG. 4. Integrity of RNA extracted from human ganglia. Total RNA extracted from the left (L) and right (R) TG from subjects 1 to 8, the brains from subjects 15 and 16, and from uninfected (U) and VZV-infected (V) MeWo cells was resolved by microfiltration. Distinct 28S and 18S rRNA bands are seen throughout except in samples from subjects 5 and 15. M, molecular weight markers.
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FIG. 5. Real-time PCR analysis of human ganglia and brain for cell transcripts. mRNA () and cDNA ( ) from the left and right TG of subjects 1 to 14 and the brains from subjects 15 and 16 were amplified with GAPDH-specific primers DNA (A) or neurofilament heavy 200-kDa protein-specific DNA (B), and the CT values were determined.
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TABLE 3. VZV cDNA copies per microgram of human TG mRNA
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FIG. 6. Prevalence (A) and abundance (B) of VZV transcripts in latently infected human TG. The left and right TG of 14 individuals were examined for the presence of transcripts corresponding to VZV genes 21, 29, 62, 63, and 66. For the prevalence data, VZV gene 63 transcripts were the most frequently detected, followed by gene 66, gene 62, and gene 29; no gene 21 transcripts were detected in any of the subjects. The average abundance (panel B) of transcripts in each positive TG normalized to 1 µg of input mRNA was highest for open reading frame (ORF) 63, followed by gene 29, gene 66, and gene 62.
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FIG. 7. Relative abundance of VZV gene 21, 29, 62, 63, and 66 transcripts in individual trigeminal ganglion. The 18 TG that were found to contain VZV gene 29, 62, 63, or 66 transcripts were selected for further analysis. For each TG the number of transcripts mapping to each VZV gene was compared to that of the most abundant virus gene. Thus, for 16 of the 17 individual ganglia in which VZV gene 63 transcripts were the most abundant, the number of VZV gene 29, 62, and 66 transcripts is <10% the number of VZV gene 63 transcripts. ORF, open reading frame.
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An ISH-based study found transcripts corresponding to three additional VZV genes in latently infected human ganglia. Kennedy et al. (15) reported VZV gene 4 transcripts in ganglia from 3 of 13 (23%) subjects, VZV gene 18 transcripts in 4 of 15 (27%) subjects, and VZV gene 40 transcripts in 1 of 5 (20%) subjects. Although ISH can detect minute amounts of transcript as well as its cellular location, this technique depends on investigator interpretation of a signal produced by grains on cells. Our sequence analysis did not find transcripts corresponding to VZV genes 4 or 40 (5); thus, future studies will continue to search for those transcripts as well as for VZV gene 18 transcripts.
A possible explanation for our lack of detection of VZV gene 21 transcripts is provided by quantitative data for each individual ganglion shown in Fig. 7. In most TG analyzed, the amount of VZV gene 29, 62, and 66 transcripts relative to those of VZV gene 63 was <10%. Also, among the ganglia that contained VZV gene 63 transcripts, the detection of VZV gene 29, 62, and 66 transcripts appeared random. For example, in addition to VZV gene 63 transcripts, three ganglia also contained VZV gene 66 transcripts, two also contained VZV gene 29 and 62 transcripts, one also contained VZV gene 62 and 66 transcripts, one also contained VZV gene 29 and 66 transcripts, one also contained VZV gene 29, 62, and 66 transcripts, one also contained VZV gene 29 transcripts, and one also contained VZV gene 62 transcripts, whereas seven contained only VZV gene 63 transcripts. Thus, there seems to be two sets of latent VZV gene transcripts. The first contains only the abundant VZV gene 63 transcript. The second contains low-abundance VZV gene 29, 62, and 66 transcripts. It is possible that previously detected VZV gene 21 transcripts (3, 4, 5, 15) lie within the class of low abundance latent VZV transcripts. An alternative interpretation is that VZV gene 63 is always transcribed during latency, whereas VZV genes 21, 29, 62, and 66 are transcribed in cells undergoing early stages of virus reactivation. For example, spontaneous reactivation of latent herpes simplex virus type 1 has been reported in the TG of latently infected mice (11), which results in an unexpected pattern of herpes simplex virus type 1 gene expression (17, 18). This hypothesis is consistent with low-frequency spontaneous reactivation of virus, which results in the transcription of VZV genes 62, 66, 29, and/or 21.
The relative abundance (in decreasing order) of VZV genes 63, 29, 21, and 66 transcripts in VZV-infected cells determined by quantitative PCR (Fig. 3) is consistent with results of transcriptional array analysis of VZV (strain Ellen)-infected monkey kidney (BSC-1) cells in culture (8). The lower abundance of VZV gene 62 transcripts determined by quantitative PCR than that found by transcriptional array analysis could reflect a difference in virus strain (Dumas-like versus Ellen) or cell type (MeWo versus BSC-1). Alternatively, the slight discrepancy could be attributed to a 3' bias observed in cDNA when synthesis is primed with oligo(dT). To test this assertion, poly(A)+ RNA extracted from VZV-infected MeWo cells was primed with either oligo(dT) or a mixture of oligo(dT) and random hexanucleotides, and the VZV gene 62 transcripts were analyzed by transcriptional array analysis or quantitative PCR. Array analysis demonstrated that when cDNA synthesis was primed with oligo(dT), only 17% of VZV gene 62 cDNA extended to the 5' terminus. However, when cDNA synthesis was primed with a mixture of oligo(dT) and random hexanucleotides, the amount of full-length VZV gene 62 transcripts increased to 44%. Under the same cDNA synthesis conditions, the TaqMan primers or probe used here showed only a modest difference in the amount of VZV gene 62 copies present when the synthesis was primed with oligo(dT) (1.7 ± 0.2 x 108 copies) compared to the number of VZV gene 62 copies present when synthesis was primed with the oligo(dT) and random primers mixture (3.1 ± 0.03 x 108 copies) (data not shown).
Long oligonucleotide-based microarrays were used to determine the transcriptional pattern of VZV (strain Dumas) in virus-infected MeWo cells (16). The relative abundance (in decreasing order) of VZV genes 29, 21, 66, and 62 is consistent with our current quantitative PCR analysis of VZV-infected MeWo cells. The major discrepancy between the long oligonucleotide-based array analysis and the current quantitative PCR analysis is the relative abundance of VZV gene 63 transcripts. While Kennedy et al. (16) found the amount of VZV gene 63 transcripts to be between that of VZV genes 66 and 62, our quantitative analysis found that the number of VZV gene 63 transcripts was higher. As was the case for VZV gene 62 transcripts analyzed by PCR-based arrays (8), the long-oligonucleotide array analysis also showed target gene bias (specifically VZV genes 62 and 63 [16]) with probe made from oligo(dT)-primed cDNA.
Finally, our analysis of the prevalence and abundance of VZV gene transcripts in latently infected human ganglia extends previous studies by analyzing transcripts from all sequence-verified, latently transcribed VZV genes. In addition, our quantitative analysis has identified two classes of latently transcribed VZV genes. The abundant class consists of only VZV gene 63 and the less abundant class consists of VZV genes 21, 29, 62, and 66. The repeated detection and high abundance of VZV gene 63 transcripts in latently infected ganglia suggests that VZV gene 63 may be more important for the maintenance of virus latency than the less abundantly transcribed and randomly detected VZV genes 21, 29, 62, and 66.
Published ahead of print on 27 December 2006. ![]()
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