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Department of Ophthalmology & Visual Sciences, Washington University School of Medicine, St. Louis, Missouri
| ABSTRACT |
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| Introduction |
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Few reports have analyzed global gene expression patterns in primary melanomas. Bittner et al. (6) identified a gene expression cluster, largely in metastatic melanoma cell lines, that correlated with invasive behavior in vitro. Tschentscher et al. (7) identified a correlation between gene expression profile and monosomy of chromosome 3 in uveal melanomas, but a relationship between gene expression and patient survival was not reported. Here, we present results of gene expression profiling in primary uveal melanomas from patients with long-term clinical follow-up. This study represents the largest number of primary melanomas of any site that have been analyzed for gene expression to date. Surprisingly, these tumors clustered naturally into two classes that correlated strongly with risk of metastasis. Furthermore, we provide evidence that clinical predictive testing may be feasible using this novel molecular signature.
| Materials and Methods |
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analysis, t test, and Pearson correlation coefficient as appropriate. Survival analysis was performed using Kaplan-Meier life table analysis on MedCalc Software (version 7.2.0.2). Survival was defined as the elapsed interval from date of eye removal to date of last follow-up or melanoma-related death.
Preparation of RNA.
For fresh tumors and normal melanocytes, total RNA was obtained using TRIzol (Invitrogen, Carlsbad, CA) and purified using RNeasy kits (Qiagen, Valencia, CA) according to the manufacturers instructions. RNA quality was assessed on the Bioanalyzer 2100 (Agilent, Palo Alto, CA). For paraffin-embedded samples, 10 10-µm sections were obtained from tissue blocks, and melanomas were dissected from the eye. Tumor tissue was deparaffinized with xylene, rehydrated with ethanol washes, and incubated overnight at 60°C in lysis buffer with proteinase K. Total RNA was isolated by phenol/chloroform extraction. For array studies, cDNA was generated by subjecting total RNA to reverse transcription, linear amplification, and in vitro transcription to generate biotinylated cRNA targets that were hybridized to Affymetrix Hu133A and B GeneChips according to manufacturer protocols. Chips were checked for quality assurance parameters and normalized for mean overall expression, and probe sets were analyzed for significance using Affymetrix software. For PCR analysis, cDNA was generated using RETROscript kit (Ambion, Austin, TX) and SuperScript II Reverse Transcriptase (Invitrogen) according to manufacturer instructions.
Gene Expression Microarray Analysis.
Gene expression values were subjected to log10 transformation and scalar normalization by the mean value. Principal component analysis was performed using Spotfire DecisionSite 7.0 software. Self-organizing maps were generated using GeneCluster2 software (10)
available online.1
Hierarchical clustering was performed by a centroid linkage algorithm available on Dchip software (11)
available online.2
Genes that discriminated tumor classes were identified by comparing median gene expression in each class by signal-to-noise algorithm available on GeneCluster2 software. Significance was determined for each gene by permutation of the data set 103
times and comparison to the absolute signal-to-noise ratio. Predictive models were generated with GeneCluster2 software using k nearest neighbor and weighted voting algorithms, followed by 4-fold cross-validation to evaluate model performance (12)
. Significance of predictor models was determined using the Fisher Test available on GeneCluster2 software. Bioinformatic analysis was performed using PubMed and LocusLink and the Affymetrix web sites.3
, 4
PCR and Analysis of Masked Samples.
Real-time PCR was performed using Lux Fluorogenic primer kit (Invitrogen) and an ICycler thermocycler (Bio-Rad, Hercules, CA) according to manufacturer instructions. PCR was used to analyze seven class-discriminating genes and glyceraldehyde-3-phosphate dehydrogenase as a control (see Supplemental File 1 for primer sequences). Results were analyzed with ICycler software. The
CT values (differences between logarithms of expressions of glyceraldehyde-3-phosphate dehydrogenase and gene of interest) generated by ICycler software were shifted globally to generate a minimum value = 1. To normalize for differences in initial RNA concentration, expression values were then divided by the mean expression value for each tumor.
Comparative Genomic Hybridization Microarray Analysis.
Genomic DNA was prepared from 10 fresh tumor specimens using the Wizard Genomic DNA purification kit (Promega, Madison, WI) and used for comparative genomic hybridization, which was performed as a service provided by the Microarray Shared Resource at the Comprehensive Cancer Center, University of California, San Francisco, using a microarray-based platform containing a genome-wide collection of genomic contigs, as described previously (13)
. A log2 average raw ratio of > 0.5 was used as the threshold for significant DNA copy number deviations.
| Results and Discussion |
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44,690 probe sets. Hypothetical genes and expressed sequence tags were removed, and the remaining 34,382 probe sets were filtered to exclude those with a median significance level of P > 0.005 as determined by Affymetrix software. This filtering process resulted in 3075 highly significant genes for additional analysis. Unsupervised analysis was performed using principal component analysis, which exhibited distinct clusters of 14 and 11 tumors (Fig. 1A)
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0.25 and a significance level of P
0.01 (1% permutation level). This filtering process resulted in 62 discriminating genes, including significant gene clusters on chromosome 3 (P = 0.002) and 8q (P = 0.004) that were down-regulated and up-regulated, respectively, in class 2 tumors (Fig. 1, B and C)
0.3 and a significance level of P
0.001 (0.1% permutation level), most of the genes on chromosomes 3 and 8q were excluded (Fig. 1C)
Molecular Classes Correlate with Cytologic Severity.
We next analyzed the tumor classes for correlation with clinical and pathological features (Supplemental File 3). The only feature to demonstrate an association with tumor class was advanced patient age, a known risk factor for metastasis (15)
. Mean age was 56 years in class 1 and 73 years in class 2 (P = 0.008). To rule out the possibility that the melanoma gene expression patterns were solely a function of age rather than intrinsic biological differences between tumor classes, we examined the expression of 26 discriminating genes in normal uveal melanocytes from 8 patients ranging in age from 29 to 75 years (mean, 63 years). The melanocytes were analyzed using Affymetrix GeneChips in the same manner as the tumor samples described above. There was no correlation with age for any of the genes (P range, 0.15 to 0.97), suggesting that age alone is unlikely to account for the observed gene expression patterns.
Interestingly, the classification correlated poorly with melanoma cell type as recorded on pathology reports. However, because these tumors display a spectrum of cytologic features from well-differentiated spindle cells to poorly differentiated epithelioid cells (2) , we obtained a more quantitative estimate of tumor cytology by ranking the melanomas from lowest to highest proportion of epithelioid cells, as described previously (9) . Cytologic rank exhibited high interexaminer reproducibility (r = 0.8, P < 0.0001) and correlated strongly with molecular classification (P < 0.0001); class 1 corresponded to lower-grade spindle melanomas, whereas class 2 corresponded to the higher-grade melanomas with more epithelioid cells.
As Few as Three Genes Are Required for Accurate Class Prediction.
To identify a small set of genes that accurately predict class label, we analyzed the 26 discriminating genes using weighted voting and k nearest neighbor prediction algorithms. The overall model performance of each model was specified by the number of errors generated by 4-fold cross-validation (12)
. The best results were obtained using weighted voting, and subsequent analyses were performed using this algorithm. As few as three genes predicted correctly the class labels of all tumor samples with no errors, and there was no improvement in confidence by inclusion of more than three genes (Fig. 2A)
. One of the optimal three-gene sets (PHLDA1, FZD6, and ENPP2) demonstrated a significance of P = 3.5 x 10-5 by Fisher test and was chosen as the gene expression signature for additional analysis (Fig. 2B)
. The microarray expression values of the three signature genes, as well as four other top classifiers, were validated by real-time PCR analysis of RNA samples from 12 of the aforementioned tumors. All seven genes exhibited strong correlation between PCR and microarray expression values (P range, 0.01 to <0.001; Supplemental File 1).
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Note: Microarray experiments and survival analysis were performed with the assistance of the Siteman Cancer Center Multiplexed Gene Core Facility and Cancer Registry, respectively.
Requests for reprints: J. William Harbour, Box 8069, 660 South Euclid Avenue, Washington University School of Medicine, St. Louis, MO 63110. Phone: (314) 747-3315; Fax: (314) 747-5073; E-mail: harbor{at}vision.wustl.edu
1 Internet address: http://www-genome.wi.mit.edu/cancer/software/software.html. ![]()
2 Internet address: http://www.dchip.org. ![]()
3 Internet address: http://www.ncbi.nlm.nih.gov. ![]()
4 Internet address: http://affymetrix.com/index.affx. ![]()
Received 5/19/04. Revised 8/17/04. Accepted 8/31/04.
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